a summary report of sediment processes in chesapeake bay ... · u.s. department of the interior...

U.S. Department of the InteriorU.S. Geological Survey

A Summary Reportof Sediment Processesin Chesapeake Bay and WatershedWater-Resources Investigations Report 03-4123

Cover. Image provided by ORBIMAGE. © Orbital Imaging Corporation and processing by NASAGoddard Space Flight Center.

U.S. Department of the InteriorU.S. Geological Survey

A Summary Reportof Sediment Processesin Chesapeake Bay and Watershed

edited by Michael Langland and Thomas Cronin

Water-Resources Investigations Report 03-4123

New Cumberland, Pennsylvania2003

U.S. DEPARTMENT OF THE INTERIORGALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEYCharles G. Groat, Director

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S.Government.

For additional information Copies of this report may bewrite to: purchased from:

District Chief U.S. Geological SurveyU.S. Geological Survey Branch of Information Services215 Limekiln Road Box 25286, Federal CenterNew Cumberland, Pennsylvania 17070 Denver, Colorado 80225-0286Email: [email protected] Telephone 1-888-ASK-USGS

ii

Executive summary, by Michael Langland, Thomas Cronin, and Scott Phillips . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Sediment and suspended solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Suspended sediment, water clarity, and submerged aquatic vegetation . . . . . . . . . . . . . . . . . . . . . . . . 4Watershed sources and transport of sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Sediment sources and transport to the bay and tributaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Shoreline erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Natural processes and variability in sediment transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Sediment deposition and storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17List of sediment workgroup members and affiliation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Workgroup members . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Chapter 1. Introduction, by Thomas Cronin and Michael Langland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Sediment workgroup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Report objectives and scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Terminology for sediment and total suspended solids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23TSS variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23TSS, light, and SAV. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Chapter 2. Watershed sediment sources, by Allen Gellis, Sean Smith, and Steven Stewart . . . . . . . . . . . . . . . 29

Upland sediment sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29Urban sediment sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Channel corridor sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Chapter 3. Watershed sediment transport, by Sean Smith, Michael Langland, and Robert Edwards . . . . . . . . 34

Channel hydraulics and sediment transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Channel morphology and hydraulics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Channel shaping processes and sediment flux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Channel sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Suspended-load and wash-load transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Bedload transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Reach-specific sediment-transport characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Chapter 4. Watershed sediment deposition and storage, by Julie Herman, Clifford Hupp, andMichael Langland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Upland storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42Floodplain and banks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43Reservoirs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45Susquehanna River reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

CONTENTS

Page

C O N T E N T S | i i i

Chapter 5. Estuarine sediment sources, by Thomas Cronin, Jeffrey Halka, Scott Phillips,and Owen Bricker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Estimates of major sediment sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49Watershed sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Shoreline erosion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51Oceanic input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56Internal sources of sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58Direct atmospheric input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Chapter 6. Estuarine sediment transport, deposition, and sedimentation, by Thomas Cronin,Lawrence Sanford, Michael Langland, Debra Willard, and Casey Saenger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Sediment transport pathways. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61Estuarine turbidity maxima zone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61Influence of climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Long-term processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64Centennial, decadal, and interannual time scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65Short-term extreme events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Deposition and sedimentation rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67Sediment resuspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Chapter 7. Integrated approaches to sediment studies, by Sean Smith, Julie Herman,Thomas Cronin, Gregory Schwarz, Michael Langland, Kenn Patison, and Lewis Linker. . . . . . . . . . . . . . . . . . 80

Sediment budgets: Watershed and estuary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80Watershed components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80Estuarine budgets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82Model-derived sediment estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Spatially Referenced Regression Model (SPARROW) for Sediment . . . . . . . . . . . . . . . . . . . . . 89Chesapeake Bay Watershed Model (WSM). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91Chesapeake Bay Water Quality Model (WQM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Sediment reduction controls (best-management practices) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

CONTENTS—Continued

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FIGURES

Page

Executive Summary

Figures 1-4. Maps showing:1. Location of Chesapeake Bay watershed and estuary . . . . . . . . . . . . . . . . . . . . . . . . 22. Concentrations of total suspended solids in winter and spring, 1992

and 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53. Comparison of historical (1880-present) and long-term sediment flux at

core sites in Chesapeake Bay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84. Major pathways of sediment transport in Chesapeake Bay . . . . . . . . . . . . . . . . . . 9

5. Pie charts showing relative contributions of sediment sources to the estuary withfastland (above tidal water) erosion and with fastland and nearshore (below tidalwater) erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6. Graph showing sources of fine-grained sediment from different sources based onliterature (right half) compared to model-generated loads (left half) . . . . . . . . . . . . . . 11

7. Map showing vulnerability of low-lying regions around Chesapeake Bay to futuresea-level rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

8. Graph showing comparison of coarse- (sand, gravel) and fine-grained (silt, clay)components of shoreline sediments from different studies . . . . . . . . . . . . . . . . . . . . . . 14

9. X-radiograph of 400-cm long sediment core from central Chesapeake Bay off LittleChoptank River mouth, approximately 11 m water depth . . . . . . . . . . . . . . . . . . . . . . . 16

Chapter 1.

1.1. Map showing location of Chesapeake Bay watershed and estuary . . . . . . . . . . . . . . . . . . 22

1.2. Graph showing mean monthly concentrations of total suspended solids (TSS) attwo CBP monitoring sites for shallow (0.5 m), near-surface (2-4 m), anddeep (24.5-31m) water depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

1.3. Map showing concentrations of total suspended solids in winter and spring, 1992and 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

1.4. Photograph of suspended fine sediment flocs from a site in upper Chesapeake Bayduring October 2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Chapter 2.

2.1. Graph showing land-use history and sediment yield from the Potomac River Basinin the northeastern United States, from the late 1700s to the 1960s, projectedto approximately 2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Chapter 3.

3.1. Sketch showing flow regimes affecting stream-channel and floodplain corridors . . . . . . 35

3.2. Diagram showing relations between profile location, sediment flux, and channelincision, defined as dz/dt = (1/γs) (dG/dx) + i, where dz/dt = change inchannel bed elevation with time, dG/dx = change in bedload transport withdistance downstream, γs = specific gravity of sediment . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.3. Graph showing changes in channel-bottom sediment sizes in the Fall Zone nearWashington, D.C.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

C O N T E N T S | v

FIGURES—Continued

Page

3.4. Cross sections showing floodplain stratigraphy observed by Jacobsen and Coleman,partitioned into three defining periods of sedimentation. . . . . . . . . . . . . . . . . . . . . . . . 41

Chapter 4.

4.1. Graph showing sedimentation rates from tree-ring and clay pads along selectedChesapeake Bay tributaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

4.2. Map showing location of three hydroelectric dams and reservoirs in the LowerSusquehanna River Basin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

4.3. Graph showing change in Conowingo reservoir sediment–storage capacity,1929-1996. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Chapter 5.

5.1. Pie charts showing relative contributions of sediment sources to the estuarywith fastland erosion and with fastland and nearshore erosion. . . . . . . . . . . . . . . . . . . 50

5.2. Location map of the nine River Input Monitoring (RIM) Sites. . . . . . . . . . . . . . . . . . . . . . . 52

5.3-5.4. Graphs showing:5.3. River Input Monitoring station sediment data, 1985 to 2000.

(A) Average annual suspended-sediment load (log scale) and(B) average annual sediment yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.4. Combined annual suspended-sediment loads and relation toannual flow for the Susquehanna, Potomac, and the James Riversnear the Fall Line. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

5.5. Diagram showing relation between fastland (above tide) erosion and nearshore(below tide) erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Chapter 6.6.1-6.2. Maps showing:

6.1. Major pathways of sediment transport in Chesapeake Bay . . . . . . . . . . . . . . . 626.2. General location of turbidity maxima (dark areas) for the major

tributaries and the bay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

6.3-6.4. Graphs showing:6.3. Graph showing proportion of ragweed (Ambrosia) pollen in core

MD99-2209 showing the peak in ragweed between 201-241 cmdepth corresponding to maximum agricultural and timberproduction land clearance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

6.4. Graph showing age-depth model for core MD99-2209 showing series ofradiocarbon ages (calibrated to years before 1950) and 2 sigma errorbars. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

6.5-6.6. Maps showing:6.5. Comparison of historical (1880-present) and long-term sediment

flux at core sites in Chesapeake Bay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 756.6. Estimates of sediment flux at different core sites in the Chesapeake

Bay, calculated by determining the amount of sediment lying abovethe peak in ragweed pollen and converting to mass . . . . . . . . . . . . . . . . . 76

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FIGURES—Continued

Page

Chapter 7.

7.1. Diagram showing watershed sediment, sinks and sources . . . . . . . . . . . . . . . . . . . . . . . . . 81

7.2. Graph showing fine-grained sediment sources from different sources based onliterature compared to model-generated loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

7.3. Schematic of a nested basin defined by upstream and downstream monitoringstations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

7.4-7.5. Graphs showing:7.4. Modeled sediment-solids loads above and below the Fall Line . . . . . . . . . . . . 927.5. Relative proportion of light attenuation by component for major

bay segments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

7.6-7.7. Maps showing:7.6. Location of estuary model segment number as used in the water-quality

model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 957.7. Estuarine areas that benefit more from sediment controls than from

nutrient controls in the watershed and tidal tributaries . . . . . . . . . . . . . . 96

TABLES

Table 2.1. Construction site sediment loadings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.2. Post-development urban watershed sediment sources . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.3. Baltimore County Storm Water Management Module (SWMM) pollutantload results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.1. Mean sediment deposition rates for Coastal Plain rivers . . . . . . . . . . . . . . . . . . . . . . . . . 44

6.1. Summary of sedimentation rates in Chesapeake Bay and tributaries from selectedpublished studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

6.2. Summary of chronological and sedimentary rate data for Chesapeake Bay . . . . . . . . . . 69

7.1. Sediment budget data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

7.2. Suspended sediment source loads in the Chesapeake Bay estuary and itssub-estuaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

7.3. Composition of bank solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

7.4. Sediment reductions for various best-management practices simulatedin the Watershed Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

C O N T E N T S | v i i

ADPC Acoustic Doppler Current ProfilerBMP Best Management PracticeCBEMP Chesapeake Bay Estuary Model PackageCIMS Chesapeake Information Management SystemCBP Chesapeake Bay ProgramETM Estuarine Turbidity MaximumGIS Geographic Information SystemKGS Kodak Grey ScaleLIDAR Light Detection and Ranging (technique used in remote sensing)MAR Mass Accumulation RateMWGOC Metropolitan Washington Council of GovernmentsNASQAN National Stream Quality Accounting NetworkNAWQA National Water Quality Assessment ProgramNESCI National Estuary Sediment Contaminant InventoryNID National Inventory of DamsNLCD National Land Cover setNRCS Natural Resources Conservation ServiceNRI National Resources InventoryRADM Regional Acid Deposition ModelRIM River Input Monitoring ProgramRF1 River Reach File 1SAV Submerged Aquatic VegetationSDR Sediment Delivery RatioSPARROW Spatially Referenced Regression on Watershed AttributesSRBC Susquehanna River Basin CommissionSSC Suspended Sediment ConcentrationSTATSGO State Soil Survey Geographic databaseSWGP Sediment WorkgroupSWMM Storm Water Management ModuleTMDL Total Maximum Daily LoadTSS Total suspended solids concentrationTVSS Total Volatile Suspended SolidsUSCOE U.S. Army Corps of EngineersUSEPA U.S. Environmental Protection AgencyUSGS U.S. Geological SurveyUSLE Universal Soil Loss EquationWQM Water Quality ModelWSM Watershed Model

ACRONYMS AND ABBREVIATIONS

v i i i | C O N T E N T S

Multiply By To obtain

Length

inch (in) 25.4 millimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

yard (yd) 0.9144 meter

Area

square mile (mi2) 2.590 square kilometer

Volume

gallon (gal) 3.785 liter

acre-foot (acre-ft) 1,233 cubic meter

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second

cubic foot per second per square mile[(ft3/s)/mi2]

0.01093 cubic meter per second per squarekilometer

cubic foot per day (ft3/d) 0.02832 cubic meter per day

gallon per minute (gal/min) 0.06309 liter per second

gallon per day (gal/d) 0.003785 cubic meter per day

gallon per day per square mile[(gal/d)/mi2]

0.001461 cubic meter per day per squarekilometer

million gallons per day (Mgal/d) 0.04381 cubic meter per second

million gallons per day per square mile[(Mgal/d)/mi2]

1,461 cubic meter per day per squarekilometer

Mass

pound, avoirdupois (lb) 0.4536 kilogram

ton, short (2,000 lb) 0.9072 megagram

ton per acre 0.0002242 metric ton per square meter

ton per day (ton/d) 0.9072 metric ton per day

ton per day per square mile[(ton/d)/mi2]

0.3503 megagram per day per squarekilometer

ton per year (ton/yr) 0.9072 megagram per year

ton per year (ton/yr) 0.9072 metric ton per year

CONVERSION FACTORS AND ABBREVIATED WATER-QUALITY UNITS

C O N T E N T S | i x

Abbreviated water-quality units used in report:g/m2, grams per square metermg/L, milligrams per liter

Density

pound per cubic foot (lb/ft3) 16.02 kilogram per cubic meter

pound per cubic foot (lb/ft3) 0.01602 gram per cubic centimeter

Temperature

degree Fahrenheit (°F) °C=5/9.(°F-32) degree Celsius

Multiply By To obtain

CONVERSION FACTORS AND ABBREVIATED WATER-QUALITY UNITS—Continued

x | C O N T E N T S

A Summary Report of Sediment Processesin Chesapeake Bay and Watershed

EXECUTIVE SUMMARY

by Michael Langland, Thomas Cronin,and Scott Phillips1

The Chesapeake Bay, the Nation's largestestuary, has been degraded because of diminishedwater-quality, loss of habitat, and over-harvestingof living resources. The bay was listed as animpaired water body in 2000 under the CleanWater Act because of excess nutrients and sedi-ment. Water-quality standards must be met in thebay by 2010. The Chesapeake Bay Program (CBP),a multi-jurisdictional partnership, completed anagreement called Chesapeake 2000 that revises andestablishes new restoration goals through 2010 inthe bay and its watershed. In the agreement,improving water quality is identified as one of themost critical elements in the overall protection andrestoration of the Chesapeake Bay and its tributar-ies (fig. 1). Therefore, the authors of the report triedto extract, discuss, and summarize importantaspects of sediment and sedimentation that aremost relevant to the CBP and other sedimentrelated-issues with which resources managers areinvolved. Many of these most important aspectsare underlined throughout the report. The first ofmany important concepts is that excess sediment isone of the most important contributors todegraded water quality and has adverse effects oncritical habitats (submerged aquatic vegetation(SAV) beds) and living resources (shellfish andfinfish) in Chesapeake Bay and its watershed.

Sediment is solid material (soil and rockfragments) transported and deposited by wind,water, or ice; chemically precipitated from solu-tion; or secreted by organisms. Sediment sus-pended in the water column consists of solidparticulate organic and inorganic material(Chapter 1). This material can reduce water clarityand increase light attenuation such that light pene-tration commonly is below the thresholds needed

to support healthy SAV. SAV beds constitute animportant biological resource in estuaries. Thesebeds influence the physical, chemical, and biologi-cal conditions of the estuary and provide criticalhabitat for many other species, in addition to theirphotosynthetic activity that produces organicmaterial used by other plants and animals. In addi-tion, SAV provides shelter and substrate for manyinvertebrate species including commercial shellfishand finfish. SAV also can contribute to improvedwater quality through uptake of nutrients duringthe SAV growing season, when excess nutrientlevels can lead to excessive algal growth, increasedturbidity, and oxygen depletion.

In the Chesapeake 2000 agreement, the CBPrecognized that interim SAV restoration goals setin 1993 had not been met and that a new acceler-ated program of protection and restoration wasneeded. The strategy for SAV restoration isdescribed in detail in a recent report submitted tothe CBP Implementation Committee. As part of theeffort to protect and restore SAV and meet water-quality standards in the bay, the CBP has commit-ted to correcting the sediment and nutrient prob-lems in the bay and its tidal waters. The goal of thiscommitment is the removal of the bay from the listof impaired watersheds by the year 2010. To dothis, the CBP is committed to developing sedimentand nutrient allocations for major basins within thebay watershed. The allocations would be used bythe jurisdictions to revise nutrient- and sediment-reduction goals. Watershed-management plansthat address the protection, conservation, and res-toration of stream corridors, riparian forest buffers,and wetlands would be developed to meet the pro-posed goals. The CBP is also in the process ofexamining new and innovative management plansin the estuary itself and along the coastal zones ofthe bay that may decrease sediment influx andimprove water quality. These commitments requireinformation about sediment sources, transport,composition, and deposition in various parts of thebay and its watershed to formulate sediment-reduction management strategies. Specifically,information is required to develop sediment-1 U.S. Geological Survey.

E X E C U T I V E S U M M A R Y | 1

Figure 1. Location of Chesapeake Bay watershed and estuary.

2 | E X E C U T I V E S U M M A R Y

reduction strategies in 2003, to evaluated theirinitial effectiveness in 2005-06, and to assesswhether water-clarity standards have been met by2010.

In addition to its effect on water clarity,excess sediment can have other adverse effects onecosystems. For example, sediment can carry toxiccontaminants and pathogens that may negativelyaffect fisheries and other living resources. Exces-sive sedimentation also can degrade the vitality ofoyster beds and other bottom-dwelling organismsin the bay and affect commercial shipping and rec-reational boating by accumulating in shippingchannels. In the bay watershed, sediment is listedas the primary cause of impairment in manystreams where it can severely degrade stream habi-tat and decrease benthic populations. Understand-ing estuarine and fluvial sedimentary processes iscritical for improving water quality and livingresources in the bay and should provide improvedmanagement of stream corridors and protection oferoding coastal zones in the watershed.

A Sediment Workgroup (SWGP) was createdin April 2001 under the auspices of the NutrientSubcommittee. It was recognized that reliable andup-to-date information on sediment processes inthe bay and its watershed was widely dispersed inthe literature and not readily accessible to the CBPand management community. This report presentsthe results and conclusions of the efforts of theSWGP; the highlights are given in this ExecutiveSummary.

Sediment and Suspended Solids

A variety of conceptual and technicalapproaches have been used to study the origin,transport, and fate of particulate material in theChesapeake Bay system. Sediment is a termdescribing particulate material. In estuaries likeChesapeake Bay, sediment consists largely ofwater-borne detrital material (pebbles, sand, mud)and varying amounts of particulate organic mate-rial. Over time, sediment may accumulate to formclastic rocks (conglomerate, sandstone, shale).However, most sediments deposited during thepast 8,000 years in Chesapeake Bay are still uncon-solidated. Sediment deposited during the last fewcenturies in the bay still contains more than 50-per-cent water content in pore spaces between sedi-mentary particles. The organic fraction of sedimentcollected from the bottom of the bay indicate sedi-

ment consists of 1-3 percent organic material; therest consists of inorganic mineral material andlesser amounts of shell material.

Sediment in the bay usually is studied byobtaining water column samples, bottom samples,and (or) sediment cores. These samples are thensubjected to a variety of physical, chemical, andbiological analyses depending on the scope andpurpose of the research. Geologists describe sedi-ment in terms of grain size, texture, mineralogy,and other characteristics. From the standpoint ofwater clarity, one of the most important character-istics of bay sediment involves the distinctionbetween fine-grained sediment, which refers to theclay (less than 1/256-mm diameter) and silt (1/256to 1/16-mm diameter) -sized fractions, and coarse-grained sediment, which refers to the sand (1/16 –2-mm diameter) and pebble (2-64 mm diameter) -sized fractions. This fine/coarse distinction isimportant because most coarse material is trans-ported along the bottom of rivers and the bay andhas little effect on light penetration. In contrast,fine-grained sediment commonly is in suspensionand, depending on its abundance, grain-size distri-bution, and degree of aggregation, can influencelight penetration.

In contrast to research on sediment that hasaccumulated on the bottom of the bay and its trib-utaries, hydrologists and biologists commonlyinvestigate particulate material suspended in thewater and collected in a water sample (Chapter 1).This particulate material is referred to as eithertotal suspended solids (TSS) or suspended-sedi-ment concentration (SSC). These two measure-ments are used to quantify the concentrations ofsuspended solids in a water sample (Gray and oth-ers, 2000), and both are given in milligrams perliter. SSC is measured as the dry weight of totalsediment in a sample divided by the amount ofwater-sediment mixture in the sample. TSS is mea-sured by several methods, usually by taking a sub-sample of known volume from the originalsuspended-sediment sample, drying the sediment,and dividing by the known volume. Most sus-pended-solids measurements cited in this reportrefer to TSS.

The relation between fine-grained sedimentloads (mass per unit time) to the bay and TSS con-centrations in bay waters is not well understood.This is particularly true in terms of the chemicalcomposition (organic versus inorganic), grain-sizedistribution, and aggregation state. The relationbetween rates of fine-grained sediment accumula-


tion on the bottom of the bay and TSS concentra-tions in the water column is not clear. However,available information suggests that relatively highsediment accumulation characterizes regions ofhigh turbidity such as the Estuarine TurbidityMaximum (ETM) zone in the northern bay. In thisreport, every effort was made to assimilate resultsderived from disparate studies of sediment andTSS into a consistent and meaningful context.

Suspended Sediment, Water Clarity,and Submerged Aquatic Vegetation

The amount of light reaching SAV living inshallow waters of Chesapeake Bay is influenced bymany factors (Chapter 1). The most importantproperties in the water column are water color(usually discussed as dissolved organic carbon),concentrations and size distributions of TSS, andchlorophyll a. Collectively, these constituentsdecrease the amount of light reaching the leaf sur-face of SAV relative to their presence in the watercolumn. Water column TSS consists of organicmaterial, referred to as total volatile suspendedsolids (TVSS), and inorganic ‘mineral’ matter.Because TVSS consists of organic components ofwater (phytoplankton, heterotrophic plankton,bacteria, and particulate organic material), its rela-tive contribution to TSS is related to nutrient con-centrations and algal abundance. The inorganicmineral component of TSS, which commonly com-prises greater than 50 percent of total TSS, gener-ally consists of fine-grained silts and clays.Therefore, inorganic sediment plays an importantrole in the degradation of water clarity in the bay.The relative abundance of inorganic sediment isrelated to various physical processes such as riverdischarge, tidal and wave erosion, estuarine circu-lation, and currents. Additional factors involvinginorganic sediment abundance include local andregional geology, geomorphology, and land uses.

In addition to TSS and chlorophyll a in thewater column, epiphytes and other organic andinorganic material accumulate on the SAV leaf sur-faces. This accumulation decreases light penetra-tion, which is necessary for photosynthesis.Epiphyte abundance on SAV is itself influenced bynutrient loading and algal abundance. The amountof inorganic material settling on leaf surfaces isinfluenced by mineral sediment in the water col-umn. Light attenuation by material settling on SAVsurfaces is related closely to processes taking placein the water column.

Comprehensive analyses of TSS spatial andtemporal variability in the Chesapeake Bay and itstributaries have yet to be carried out. Nonetheless,information available in the Chesapeake Bay Infor-mation Management System (CIMS) indicates ahigh degree of temporal and spatial variability inTSS concentrations. Bay-wide spatial variability inTSS concentrations during winter and spring sea-sons during relatively dry (1992) and wet (1993)years is shown in figure 2. These plots were con-structed using seasonally averaged TSS data andspatial contouring analyses. They show two mainfeatures of bay TSS variability:

• High winter TSS concentrations during1992 near the mouth of the bay reflectocean-source sediments (see Chapter 3).

• High TSS concentrations in the northernbay and in the larger tributaries, especiallyduring 1993, reflect high turbidity in theETM zones (see Chapter 4).

Spatial analysis of TSS is useful to illustratethe complexity of TSS in the bay and allows forcomparison with model-generated water-qualityinformation and maps of SAV census and distribu-tion data. Studies to date have suggested that timeand space scales of sediment transport in the sys-tem can be quite short/small. Additional data onthe shorter term, smaller scale variability of TSSwould help to formulate and test a more accuratesediment-transport model for the bay. Animproved model could lead to a better understand-ing of the relations between sediment sources andsuspended-sediment distributions. Further discus-sion of TSS variability is given in Chapter 1.

An additional complexity not reflected inthe TSS patterns is that the grain size characteris-tics of TSS also can influence the amount of lightattenuation because of different optical characteris-tics of different material. Additional research onTSS, especially studies that determine TSS size andcomposition, is necessary for a better understand-ing of the relative proportions and physical charac-teristics of inorganic and organic sediments andtheir sources throughout the bay system.

In summary, the literature indicates that thephysical and biological processes governing inor-ganic and organic sediment production, transport,and deposition are complex and related to oneanother. For example, management efforts toreduce nutrient loadings also might help improvewater clarity by affecting the levels of particulatematerial in the water column and algal epiphyte


Figure 2. Concentrations of total suspended solids in winter and spring, 1992 and 1993. (Total suspended-solids data from Chesapeake Bay Program, Annapolis, Md., Chesapeake Information Management System.)


growth on SAV plants. Future research programsand management strategies to control chlorophylla and nutrient loadings could be coordinated withefforts aimed at reducing the concentration of inor-ganic sediment. The cumulative effects of organicand inorganic material on light attenuation need tobe considered in management plans.

It also is clear, however, that the physicalprocesses governing the introduction, transport,re-suspension, and deposition of inorganic sedi-ment are distinct from biological production ofparticulate material driven by nutrient concentra-tions and primary production. Primary productionis defined as organisms, such as algae, that convertsolar energy to organic substances through themolecule, chlorophyll. Primary producers serve asa food source for higher organisms. Consequently,management practices aimed at reducing nutrientconcentrations may not be sufficient to reduce theinfluence of inorganic sediment on water clarity.Allocations for inorganic-sediment reduction ulti-mately will be distinct from those for nutrients. Inaddition, the spatial variability in sediment sourceand the physical processes influencing inorganicsediment transport and deposition need to be con-sidered. The remaining sections in the executivesummary and following chapters in the reportdescribe these processes and their relevance towater clarity.

Watershed Sources and Transport of Sediment

A large proportion of sediment that entersthe Chesapeake Bay is derived originally from ero-sion in the bay watershed. Erosion from uplandland surfaces and erosion of stream corridors(banks and channels) are the two most importantsources of sediment coming from the watershed.Sediment erosion is a natural process influenced bygeology, soil characteristics, terrestrial habitatcover (land cover), topography, and climate.

Some generalizations can be made abouterosion, sediment yield (mass per unit area perunit time), and land use in the bay watershed(Chapters 2 and 3).

• For the entire Chesapeake Bay region,river basins with the highest percentage ofagricultural land use have the highestannual sediment yields, and basins withthe highest percentage of forest cover havethe lowest annual sediment yields.

• Urbanization and development can morethan double the natural background sedi-ment yield; the increase in sediment yieldis highest in the early development stages.

• After development is completed, erosionrates are lower; however, sediment yieldfrom urbanized areas can remain highbecause of increased stream corridor ero-sion due to altered hydrology.

• One study in an urban setting estimated2/3 of the sediment in the water columnwas from streambanks and 1/3 was fromupland erosion.

• Other activities also influence upland ero-sion. For example, mining for coal andminerals, although in decline in the water-shed from historical levels, still contributesfine particles from “reworked” piles to riv-ers. This can increase sediment yieldsabove background levels.

• Most of the sediment yield from the water-shed to the bay is transported duringbankfull conditions, which take place onaverage every 1-2 years, and during rela-tively large storm events. Hence, sedimentinput to the bay potentially can be affectedby large-scale patterns of climate change.

Despite these generalities, one of the mostimportant conclusions drawn by the SWGP wasthat the relative contribution of upland sedimentand the sediment stored in stream corridors hasnot been quantified in the bay watershed. Suchinformation is important to formulate effectivesediment-reduction strategies.

Another important conclusion involves thehistorical changes in the generation and delivery ofsediment from watershed sources to the estuary.Natural pre-colonization erosional processes havebeen severely disrupted since the 17th century as aresult of land-use practices. During the 18th and19th centuries, the amount of land cleared for agri-culture and timber production was extensive. Dur-ing this time, 70-80 percent of the original forestcover was cleared. This land disturbance increasederosion rates in the bay watershed, leading togreater amounts of sediment transport from theland surface toward the bay and its tributaries. Thetrend toward deforestation peaked in the late 1800sand was reversed during the 20th century whenreforestation increased. Erosion rates, in theory,should have decreased during this period. How-


ever, urbanization and the remobilization of previ-ously eroded sediments may have contributed tocontinued high erosion rates during the past fewdecades.

Quantitative region-wide data on decadaltrends in erosion over the past few centuries arelacking. The rates of erosion can be inferred fromlong-term changes in sediment mass accumulationobtained from geological studies of sediment coresin the bay and tributaries. These studies indicate afour- to five-fold increase in sediment mass accu-mulation in some parts of the bay since the 1800s(fig. 3). However, in contrast to areas experiencinglarge increases in sediment loads, other regionsexperienced little or no change in post-colonizationsediment rates. This indicates that the effect of landclearing on sediment accumulation was not uni-form throughout the bay system and variedaccording to watershed histories. In addition,physical processes controlling erosion and deposi-tion in the bay itself may vary.

The substantial lag time between upland andstream-channel erosion and eventual transport anddeposition into critical bay habitats is not well doc-umented. Much of the sediment eroded fromcleared land during colonial times may still bestored in upland areas and in stream corridors.These storage areas include riparian areas and res-ervoirs, small tidal tributaries, and lowland flood-plain zones. It is unknown what proportion ofsediment eroded during land clearance is stored inchannels and tributaries and what proportion actu-ally has reached the bay. This temporarily storedsediment—sometimes referred to as “legacy sedi-ment”—will ultimately make its way to the bay.However, it may take decades or longer, depend-ing on its location in the watershed and future cli-matic and hydrologic factors. Therefore, futureimprovements in water clarity may take years todecades following implementation of land-usechanges in the watershed. For this and other rea-sons addressed below, the CBP may want to con-sider land-based practices nearer the tidal portionsof the bay and its tributaries and additional man-agement strategies both along and in the baycoastal zones to help meet water-clarity goals by2010.

Sediment Sources and Transport to the Bayand Tributaries

The primary sources of fine- (clay and silt)and coarse (sand and gravel) -grained sedimentinto the main bay are input from the main rivers inthe watershed, input from smaller tributaries andstreams, erosion from shorelines and coastalmarshes, ocean input at the mouth of the bay, andinternal biogenic production of skeletal andorganic material. A generalized map of pathwaysfor sediment movement is shown in figure 4; majorsediment sources to the bay are shown in figure 5.On the basis of these figures and additional infor-mation discussed below, five generalities aboutsediment movement and sources can be made(Chapters 5 and 6).

1) Although estimates of the relative contri-butions of different sediment sources in any partic-ular region vary among different authors, itgenerally is agreed that in the northern bay, theSusquehanna River is by far the dominant sourceof sediment influx; in the southern bay, shorelineerosion and influx from the ocean is the dominantsource; and in the central bay, the majority of sedi-ment influx comes from shoreline erosion or is pro-duced internally by biological processes. Mostsediment entering the bay from the SusquehannaRiver is trapped by the ETM zone, which is aregion of high turbidity in the northern bay (seebelow).

2) For rivers on the western shore, watershedinputs are the primary source of sediment deliv-ered to tidal fresh regions of tributaries. As in themain stem, there is an ETM zone upstream in thelarger tributaries. For regions of western shore trib-utaries downstream of the ETM zone, and in mostEastern Shore rivers, coastal plain tributaries andshorelines are more important sources of sediment.Implication for the tidal tributaries could be tofocus on sediment sources in the watershed to helpimprove clarity in the tidal fresh zones and focusmore on fastland (above tidal water) and nearshore(below tidal water) sources of sediment to improveclarity downstream of the ETM zone in each tribu-tary.

3) Export from tributaries to the main stembay is a complex subject with differing opinionsexpressed in the literature. Many researchers havesuggested that much of the sediment transportedinto the major tidal tributaries from major rivers,smaller tributaries, and shoreline erosion is depos-ited in the tributaries. Other researchers, however,


Figure 3. Comparison of historical (1880-present) and long-term sediment flux at core sites in ChesapeakeBay (determined by methods and data described in Chapter 6, table 6.1).


Figure 4. Major pathways ofsediment transport in Chesa-peake Bay (from Hobbs andothers, 1990). (Note, thethickness of arrows does notequate to amount of masstransported.)


have suggested substantially more export of sedi-ment out of tributaries and into the bay than gener-ally is believed. This is especially true duringextreme weather events, such as Tropical StormAgnes, or sustained periods of high freshwaterinflow, when a substantial amount of sediment canbe exported into the main stem bay. Obtainingquantitative data on this issue would requireextensive field studies.

4) Whereas northern and southern bay sedi-ment sources are dominated by the SusquehannaRiver and shoreline and ocean input, respectively,the sources of sediment entering the central bay areless well known. Early studies suggested as muchas 18 and 22 percent of suspended material in thecentral bay came from skeletal material andorganic production, respectively, and as much as52 percent came from shoreline erosion. A numberof studies, using geochemical tracers in sediments,satellite images, buoys, and other methods, pro-vide evidence that fine-grained material may betransported southward out of the ETM zone andnorthward from ocean sources into the central bayregion. This material may play an important role inmany critical SAV regions. However, quantitativeestimates of the relative proportion of fine-grainedsediment transported southward and northwardinto the central bay compared to local shorelineerosion and biogenic production remains one ofthe uncertainties of sediment transport within thebay proper.

5) Little or no sediment is exported from thebay to the adjacent ocean except during extremeclimate events causing high freshwater inflowfrom the watershed. This reflects the overall sedi-ment trapping nature of the entire bay system.

To obtain quantitative data on the sedimentpathways and sources discussed above, the SWGPcompiled available data on the relative contribu-tions of fine-grained sediment loads into regions ofthe bay and in certain tributaries based on some ofthe more comprehensive research papers. Thesedata are presented in figure 5 and expressed asmass and percent contribution. Coarse-grainedsediments (sand and gravel) are not considered inthis analysis, although data on coarse sediment isextensive in the literature. These data are furtherclassified and plotted by source and compared tosediment mass contributions from different water-sheds as estimated by the CBP Water QualityModel Scenario from 2000 (fig. 6).

Six potential sources of sediment shown infigure 6 are described as follows: (1) Riverineinput is defined as suspended sediment trans-ported by the major rivers entering the bay andusually measured by monitoring stations near theFall Line Zone. (2) Tributary input is defined assediment entering the tidal parts of major tributar-ies from smaller rivers draining the Eastern andWestern Shore. (3) Shoreline sources are defined assediment derived mainly from bank and headlanderosion, although low-lying coastal marshes alsocontribute. (4) Biogenic sediment has two compo-nents—skeletal material and particulate organicmaterial, both produced by organisms. Biogenicsediment commonly is not measured in studies ofsediment flux in the bay. Therefore, values for theproportion of biogenic material commonly are notavailable. (5) Import of sediment signifies sedi-ment imported from the main stem into larger trib-utaries. (6) The last source of sediment is theimport into the bay through its mouth from oceansources. This sediment ultimately comes from the

Figure 5. Relative contributions of sediment sources to the estuary with fastland (above tidal water) erosion (left)and with fastland and nearshore (below tidal water) erosion (right). (Based on data in chapter 7, table 7.2, andU.S. Army Corps of Engineers, 1990.)

1 0 | E X E C U T I V E S U M M A R Y

EX

EC

UT

IVE

SU

MM

AR

Y|

11

odel-generated loads (left half)
Figure 6. Sources of fine-grained sediment from different sources based on literature (right half) compared to m(Based on table 7.2 in chapter 7).

continental shelf and coastal regions of the south-ern Delmarva Peninsula and is subject to complexdepositional and erosional patterns in the baymouth region.

Several important conclusions can be drawnfrom the data in figure 6:

• Susquehanna River sediment dominates inthe north.

• Oceanic-source sediment is the dominantsource in the southern bay, although thistotal includes an unknown amount of sedi-ment eroded from shorelines and perhapssome sediment exported from major riv-ers. A further breakdown of this large loadof sediment requires more detailed analy-sis.

• Different tributaries have different relativecontributions from riverine, shoreline, bio-genic, and oceanic sources.

• In different parts of large tributaries suchas the Potomac and James, the relative pro-portion of shoreline and riverine sedi-ments vary in upstream and downstreamregions. This reflects the trapping of river-ine sediments by the ETM zone and thediminished influence of riverine sourcesfurther downstream in a major tributary.

• Shoreline sources of sediment are numeri-cally important in the Choptank and Rap-pahannock tributaries and to a lesserextent in the Potomac and York Rivers.

Although the comparison of empirical andmodel-generated sediment loads is illustrative,caution is urged because of different definitions ofregions in the two data sets. Moreover, the valuesin figure 6 do not distinguish resuspended sedi-ment, which might overwhelm the loading ofnewly introduced sediment in some regions undercertain conditions (see below). Further data-modelevaluation might minimize the discrepancies of theshoreline erosion loads by using more recent andspatially detailed estimates of shoreline erosion.

The improved database available from theliterature on sources and transport of sediment invarious regions of the bay and its tributaries sug-gests additional modifications to sediment-man-agement strategies in the future may beconsidered. For example, the water-quality modelfor Chesapeake Bay may be used to guide sedi-ment-reduction strategies. Currently, the model

considers load estimates from the watershedmodel calibrated from TSS data collected at theRiver Input Monitoring sites, estimates of sedi-ment inputs from below the River Input sites, andfrom estimates of shoreline sediment input basedon estimates determined by Ibison and others(1992). In the future, it will be most important tointegrate refined sediment-source estimates notonly for shoreline and riverine input, but also forbiogenic and oceanic sources of sediments intoChesapeake Bay. In addition to factoring in allpotential sources of sediment influx and resuspen-sion, modeling simulations may begin to integrateknowledge of the spatial and temporal variabilityin shoreline erosion summarized below and dis-cussed in detail in Chapter 5 of this report.

Shoreline Erosion

The contribution of shoreline erosion to totalsuspended sediment deserves special comment forseveral reasons (Chapter 5). First, shorelines areretreating because of the relatively rapid rate ofsea-level rise (1.3 ft for the last century) (Croninand others, 2000) in the Chesapeake Bay and Mid-Atlantic coast. This rate is twice that of the world-wide average and is the result of regional land sub-sidence and ocean warming. Although estimates ofthe future rate of sea-level rise caused by globalwarming include an extremely high degree ofuncertainty, most experts expect an acceleration ofsea-level rise. This acceleration implies greatercoastal submergence and perhaps shoreline ero-sion in low-lying regions of the Chesapeake Bayarea. The regions vulnerable to sea-level rise overthe next century are shown in figure 7.

A second critical aspect of shoreline erosionis that most research indicates the relative contri-bution of shoreline erosion is variable, and may beas high as 80 percent or more of the total fine-grained sediment load in the central part of themain stem, south of the bay ETM zone, and in thecentral regions of large tidal tributaries. Becausethe Bay Program Water-Quality Model currentlyassumes one value (uniform rate) from fine-grained shoreline erosion, it will be important infuture model development and implementation ofmanagement actions to take into account variabil-ity in shoreline loads.

The third important aspect of shoreline ero-sion involves potential management efforts toreduce total sediment input into the bay system.As discussed below and in Chapter 4 of this report,


sediment derived from the watershed upland andstream channels can take years to decades orlonger to actually reach the lower tidal tributariesand the main stem of the bay. Although the transittime is not known precisely, it is clear that theimplementation of management practices in thewatershed most likely will not have an immediateeffect on bay water clarity because of sediment-transport processes. In contrast, managementactions to protect and maintain the extensiveshorelines of the bay system may have a moreimmediate effect on decreasing sediment loadsinto parts of the estuary.

It is important to remember that, althoughexcess sediments may be detrimental for SAVgrowth, a certain amount of suspended sediment isnecessary for the health of other systems in the bayand its tributaries. For example, sediment is criticalto maintaining the elevation of tidal wetlands. Animportant source of sediment for these wetlands isfrom overbank flooding (i.e. suspended sedimentin the riverine and/or estuarine waters). Sus-pended sediment in littoral cells is also a naturalsource of material for beach progradation (beachgrowth) in some areas.

Figure 7. Vulnerability of low-lying regions around Chesapeake Bay tofuture sea-level rise. Future sea-level projections are uncertain butareas shown in red are most vulnerable to submergence and/or stormsurges (from Titus and Richman, 2001).

E X E C U T I V E S U M M A R Y | 1 3

Finally, three important points about shore-line erosion require emphasis. First, shoreline ero-sion can occur in both “fastland” (above tidalwater) and nearshore (below tidal water) zones.Fastland erosion accounts for one-third and near-shore erosion accounts for two-thirds of the esti-mated shoreline erosion. However, most studieshave focused on quantifying fastland erosioninputs. The rate for fastland erosion is used in theWater-Quality Model (model used by the Chesa-peake Bay Program to simulate water-quality con-ditions in the estuary). Therefore, shoreline erosionmay be underestimated because of the non-inclu-sion of the nearshore component.

The second point involves grain size, whichstrongly influences light attenuation. In Water-Quality Model simulations, about 33 percent of thetotal shoreline contribution to suspended sediment

was considered to be sand- and gravel-sized (andthus not usually suspended). These assumptionsare based on the work of Ibison and others (1992).The literature on grain size for sediments depos-ited in the bay and source sediments that outcropalong the bay margins is extensive. Data from threeof the more extensive studies are compared to thefine- and coarse-grained breakdown used in CBPmodel simulations (fig. 8). The figure shows thatthe Ibison and others (1992) coarse/fine ratio issimilar to that obtained by Hobbs and others (1992)for the Maryland part of the bay. However, shore-line sediment from the southern bay (Byrne andothers, 1980) and from the tidal part of the Poto-mac River (Miller, 1987) has a relatively greaterproportion of coarse sediment than in the Ibisonand others (1992) estimates.

Figure 8. Comparison of coarse- (sand, gravel) and fine-grained (silt, clay) components of shorelinesediments from different studies. (Sources: Ibison and others, 1992; Byne and others, 1980; Hobbs andothers, 1992; and Miller, 1987.)


The third and one of the more importantconclusions of the SWGP is that shoreline erosionof banks and coastal marshes is a large source offine-grained suspended sediment. However,amounts vary greatly depending on the region andlocation. Given that shoreline erosion is likely tobecome an increasing source of sediment if the rateof future sea-level rise accelerates, shoreline-pro-tective measures may be an important componentin future management actions.

Natural Processes and Variabilityin Sediment Transport

Several natural physical processes exert astrong influence on the transport, resuspension,and deposition of sediment in the bay and its tribu-taries (Chapter 6). The variability stemming fromthese processes poses a degree of complexity andadditional challenges for managers in Federal,State, and local government agencies in develop-ing management strategies to improve water clar-ity. The most important processes includeprecipitation and river discharge associated withclimatological variability, wind-generated waveand tidal current sediment resuspension in theestuary, and tidal- and current-generated sedimentinput from the ocean near the mouth of the bay.

Climatological processes operate over vari-ous timescales and are responsible for brief,intense weather events (hurricanes and storms)and seasonal, year-to-year, and decadal changes inrainfall, river discharge, and sediment loads. Forexample, it has been estimated that TropicalStorms Agnes (1972) and Eloise (1975) transported40 million tons of sediment into the estuary. Thisamount is about equivalent to the amount of sedi-ment normally transported from the entire water-shed in 10 years. Such events also can lead tosouthward export of sediment out of the ETM zonefrom the northern bay into the central bay. Simi-larly, strong storm events can reverse the long-termpattern of bay-to-tributary import of sediment andcause sediment to be exported from major tidaltributaries into the main stem of the bay.

Over intermediate timescales (seasonal todecadal), climatological research using availablerecords and paleoclimate data obtained from sedi-ment cores and tree-rings indicate that over thepast few centuries, seasonal and multi-yeardroughts alternate with relatively wet periods.This natural variability leads to cyclic-like changesin bay salinity, sediment transport, deposition, and

composition as illustrated in the cyclic pattern offine- and coarse-grained sediment from a regionoff the Little Choptank River (fig. 9). This interan-nual climate-driven variability in sediment charac-teristics occurs in conjunction with changes innutrient loads and changes in biogeochemicalcycling. These changes have had strong effects onthe living resources of the bay. In particular, inter-annual climate variability influences the produc-tivity, biogenic production, phytoplanktondynamics, and, ultimately, water clarity and SAVpopulations of the bay.

Another natural process that influenceswater clarity is sediment resuspension, especiallyin the ETM zone and shallow waters. Resuspen-sion involves complex processes controlled mainlyby wind-driven wave action, density-driven (salin-ity) estuarine circulation influenced by freshwaterdischarge from the Susquehanna River and othertributaries, and by tidal processes to a lesser extent.Biological processes in the water column furthercomplicate inorganic sediment movement con-trolled by these physical processes. The productionof organic particulate matter becomes mixed withinorganic material, influencing light attenuationand physical settling rates. Resuspension of bot-tom sediment produces an enormous mass of sus-pended matter that can diminish water clarity. Forexample, estimates of total suspended load in thedeeper parts of the main stem bay in the ETM zonecan reach 135,000 metric tons per day, compared tocombined daily suspended matter load input fromthe Susquehanna River, shoreline erosion, and bio-genic sources of only 4,400 metric tons. Resuspen-sion of fine-grained material is most prominent inthe deeper parts of the bay where fine-grained sed-iment is dominant. Once in suspension, however,fine-grained sediments can influence shallowwater habitats if it is circulated by wind-drivencurrents and transported to margins of the bay.Thus, the potential contribution of sediment resus-pension processes in deeper water may need to beconsidered when managing shallow-water SAVhabitats.

Land-based management actions mayreduce sediment loads in rivers. However, extremeweather events, climate variability, and tidal resus-pension of sediment will continue to effect waterclarity even if sediment delivery from the land isreduced. Although these natural processes them-selves cannot be controlled, efforts to better under-stand the role that climate and other physicalprocesses play in TSS generation are extremely


Figure 9. X-radiograph of 400-cm long sediment core from central Chesapeake Bay off Little Choptank Rivermouth, approximately 11 m water depth. Alternating light and dark colors represent climate and hydrology-driven changes in sedimentation, including changes in source, grain size, and biogeochemistry (From T.Cronin, U.S. Geological Survey and J. Hill, Maryland Geological Survey).


important for ultimately improving probability-based model prediction of TSS loading from differ-ent sediment sources. Modeling sediment loadsand secondary resuspension under various riverdischarge extremes and under specified wave andtidal conditions is one approach that might be con-sidered. In addition to more data-model researchefforts, “in-situ” management practices to reduceresuspended sediment should be investigated as ameans to improve water clarity. Investigationscould include, but not be limited to, breakwaters toreduce wave energy, planting of SAV beds, estab-lishment of oyster beds, and protection and rees-tablishment of filter feeders.

Sediment Deposition and Storage

Unlike some estuaries that export sedimentto the ocean, the Chesapeake Bay has been a sedi-ment trap since sea-level rise flooded the formerSusquehanna River Valley about 8,000 years ago.Sediment eroded from uplands and stream corri-dors, transported from the ocean, eroded fromshorelines, and produced by biological processeshas ultimately settled, been deposited, and buriedby successive layers of sediment in the main stemof the bay and in depositional regions in the tribu-taries (Chapters 4 and 6). Over geological times-cales, sediment in depositional areas may beeroded and redeposited; over shorter timescales,most sediment transported to the bay over the pastfew millennia is effectively stored and no longercontributes to suspended material. Understandingwhere, when, and how sediment settles in the baysystem—that is, where the sediment “sinks” areand how sediment is resuspended prior to “perma-nent” burial—is a critical part of understandingthe overall sediment budget in the bay. The impor-tant regions of sediment storage in the main stemof the bay, including the ETM zone, in streamchannels, and in smaller tributaries are presentedhere.

Generally, coarse-grained sediment (sand)blankets Susquehanna Flats in the northernmostbay, the flanks of the bay, and much of the southernbay. Fine-grained sediments (silts and clays) blan-ket most of the deeper parts of the bay, includingthe main channel and the Tangier and PocomokeSound channels. Similar depth-related grain-sizepatterns are found in the major tidal tributariesand reflect the winnowing of fine-grained sedi-ment from coarse-grained material and transportfrom shallow to deeper water. The greatest thick-nesses of sediment that have accumulated since the

bay formed consist of fine-grained silts and claysdeposited in the main channel of the bay and chan-nels in the larger tidal tributaries. However, asshown in figure 3, sediment mass accumulationvaries temporally, prior to and since colonization,and spatially in different regions of the bay system.In some regions of the modern Chesapeake Bay, nosediment is accumulating and these regions areundergoing net long-term erosion. Stratigraphicinconformities recognized in the geophysical andsedimentary record of the bay provide evidence formany periods of erosion over the past 8,000 years.These historical records indicate long-term shiftsfrom net sediment accumulation to erosion andvice-versa over periods of hundreds to thousandsof years. These long-term depositional and ero-sional patterns are caused by changes in estuarinecirculation and other factors that are as yetunknown.

As discussed above, in the main stem bay,the ETM zone acts as a barrier for southward sedi-ment transport of material introduced into the bayfrom the Susquehanna River and thus is an impor-tant site of sediment deposition. Similar sedimenttrapping and deposition occurs in the ETM zonesof other tidal tributaries. In the area upstream ofthe ETM zone, in the tidal fresh zone, the contribu-tion of sediment from watershed sources will besignificant. Processes operating in the ETM zonecan maintain areas of high sediment concentra-tions in the water column before settling. The highconcentrations result in local degradation of waterclarity. The location of the ETM zone in each tidaltributary depends on tributary-specific processesand will vary seasonally and yearly with freshwa-ter flow from the watershed. Although the short-term dynamics of sediment accumulation in theETM zone are understood fairly well, long-termshifts in the position of the ETM zone and the tim-ing of sediment transport out of the ETM zone arenot well known. Downstream of the ETM zone, agreater contribution of sediment likely comes fromlocal shoreline and marsh erosion, shallow waterresuspension, and input from the bay.

Significant sediment is deposited and storedin river channels and floodplains adjacent to thebay. Most of this stored sediment in these regionsprobably is derived from upland erosion duringextensive land clearance during the 18th and 19thcenturies. This “legacy’ sediment is not completelyunderstood in terms of the volume of sediment instorage or what has reached the bay. Although sed-iment stored in river channels and tributaries will


ultimately reach the bay (and thus this storage istemporary), it is likely that this transport will takeyears, decades, or even centuries, depending onfuture land uses and climatological and hydrologi-cal conditions. Therefore, management actions inthe watershed may improve water-quality condi-tions in the estuary, but there may be substantialperiod of time before the results occur.

In summary, sediment accumulation variesspatially and temporally in response to many fac-tors. On the basis of current understanding of sedi-mentary processes in the bay system, there willlikely be a “lag time” of years to decades or longerbetween the implementation of a watershed best-management practice, a reduction in sedimentload to the Bay, and ultimate deposition of sedi-ment in the bay bottom. This conclusion does notmean that sediment reduction in watershedregions will not have a positive effect on waterquality. On the contrary, land-use changes canhave a rapid effect on stream-water quality in thelocal area, and management strategies to restorelight conditions in the tidal fresh zone above theETM zone will be more dependent on sedimentreductions from the watershed. What is uncertain,however, is the effect “downstream” of watershedmanagement. Additional research into the lagtimes between historical land clearance and sedi-ment loads would improve our understanding ofhow such future land-use changes will affectaquatic habitats.

List of Sediment Workgroup Membersand Affiliation

Lead editors

Michael Langland, HydrologistU. S. Geological Survey215 Limekiln RoadNew Cumberland, PA 17070

Thomas M. Cronin926A National CenterU.S. Geological SurveyReston, VA 20192

Lead reviewers

Kenn PatisonPa. Department of Environmental ProtectionDivision of Conservation Districts and Nutrient

ManagementP.O. Box 8465Harrisburg, PA 17015-8464

Bruce WardlawU.S. Geological SurveyEastern Earth Surface Processes Team, MS 926A12201 Sunrise Valley DriveReston, VA 20192

Editorial assistant

Kimberly N. ColliniChesapeake Research ConsortiumChesapeake Bay Program Office410 Severn Avenue, Suite 109Annapolis, MD 21403


E X E C U T I V E S U M M A R Y / 1 9

Workgroup Members (alphabetical)

Name Agency

Michael Bowman Virginia Department of Conservation and RecreationOwen Bricker* U.S. Geological SurveyGrace Brush Johns Hopkins UniversityKimberly Collini Chesapeake Research ConsortiumThomas Cronin* U.S. Geological SurveyLee Currey Maryland Department of the EnvironmentRobert Edwards* Susquehanna River Basin CommissionAllen Gellis* U.S. Geological SurveyNormand Goulet Northern Virginia Regional CommissionJerry Griswold Natural Resources Conservation ServiceAmy Guise U.S. Army Corps of EngineersJeffrey Halka* Maryland Geological SurveyJulie Herman* Virginia Institute of Marine ScienceLee Hill Virginia Department of Conservation and RecreationClifford Hupp* U.S. Geological SurveyMartha Jonas U.S. Army Corps of EngineersTimothy Karikari District of Columbia Department of HealthJurate Landwehr U.S. Geological SurveyMichael Langland* U.S. Geological SurveyLewis Linker* U.S. Environmental Protection AgencyMatthew Monroe West Virginia Department of AgricultureKenn Patison* Pennsylvania Department of Environmental ProtectionFrank Payer Pennsylvania Department of Environmental ProtectionScott Phillips* U.S. Geological SurveyBrian Rustia Metropolitan Washington Council of GovernmentsNancy Rybicki U.S. Geological SurveyLawrence Sanford* University of Maryland, Center for Environmental ScienceGregory Schwarz* U.S. Geological SurveyGary Shenk U.S. Environmental Protection AgencyKelly Shenk U.S. Environmental Protection AgencySean Smith* Maryland Department of Natural ResourcesSteven Stewart* Baltimore Co. Dept. of Environmental Protection & Resource

ManagementJeffrey Sweeney U.S. Environmental Protection AgencyDebra Willard* U.S. Geological Survey

* indicates contributing author to the report.

With additional contributions from non-work group members.


Acknowledgments

This report relied heavily on contributionsfrom cited literature, contributed unpublishedwork, and information from presentations at Sedi-ment Workgroup meetings. The authors wish toexpress their gratitude to workgroup memberswho did not contribute to “authorship” explicitly,and also to nonmembers whose contributionsaided in the completion of the report. DhirenKhona, Cheryl Vann, Wayne Newell, NancyRybicki, Jurate Landwehr, Casey Saenger, all fromthe U.S. Geological Survey, Charles Hobbs of theVirginia Institute of Marine Science, James Titus,Richard Batiuk, Gary Shenk and Lewis Linker ofthe U.S. Environmental Protection Agency, DavidJasinski of the University of Maryland, GraceBrush of the Johns Hopkins University, Christo-pher Spaur of the U.S. Army Corps of Engineers,and Thomas Beauduy of the Susquehanna RiverBasin Commission. Finally a special acknowledge-ment to Thomas Simpson of University of Mary-land, chair of the Nutrient Subcommittee for hisinvolvement in the creation of the Sediment Work-group and providing guidance and goals fromwhich to work from.

ABSTRACT

The Chesapeake Bay, the Nation's largestestuary, has been degraded because of diminishedwater quality, loss of habitat, and over-harvestingof living resources. Consequently, the bay waslisted as an impaired water body due to excessnutrients and sediment. The Chesapeake Bay Pro-gram (CBP), a multi-jurisdictional partnership,completed an agreement called “Chesapeake 2000”that revises and establishes new restoration goalsthrough 2010 in the bay and its watershed. Thegoal of this commitment is the removal of the bayfrom the list of impaired waterbodies by the year2010. The CBP is committed to developing sedi-ment and nutrient allocations for major basinswithin the bay watershed and to the process ofexamining new and innovative management plansin the estuary itself and along the coastal zones ofthe bay. However, additional information isrequired on the sources, transport, and depositionof sediment that affect water clarity. Because theinformation and data on sediment processes in thebay were not readily accessible to the CBP or tostate, and local managers, a Sediment Workgroup(SWGP) was created in 2001.

The primary objective of this report, there-fore, is to provide a review of the literature on thesources, transport, and delivery of sediment inChesapeake Bay and its watershed with discussionof potential implications for various managementalternatives. The authors of the report haveextracted, discussed, and summarized the impor-tant aspects of sediment and sedimentation thatare most relevant to the CBP and other sedimentrelated-issues with which resources managers areinvolved. This report summarizes the most rele-vant studies concerning sediment sources, trans-port and deposition in the watershed and estuary,sediments and relation to water clarity, and pro-vides an extensive list of references for those want-ing more information.

CHAPTER 1. INTRODUCTION

by Thomas Cronin and Michael Langland

The Chesapeake Bay is one of the largest andmost productive estuarine systems in the world.The Chesapeake Bay “main stem,” defined by tidalzones, is approximately 195 mi long and 3.5 to35 mi wide, and has a surface area of nearly4,400 mi2. The main stem is entirely within Mary-land and Virginia. Nearly 50 rivers, with thou-sands of tributary streams and creeks, drain theapproximately 64,000 mi2 forming the ChesapeakeBay Basin. The basin contains more than150,000 stream miles in the District of Columbiaand parts of six states: New York, Pennsylvania,Maryland, Virginia, West Virginia, and Delaware(fig. 1.1). Nine rivers, including the Susquehanna,Patuxent, Potomac, Rappahannock, York (consistsof the Mattaponi and Pamunkey), James, Appo-mattox, and Choptank (fig. 1.1), contribute approx-imately 90 percent of the bay’s mean annual fresh-water inflow of 69,800 ft3/s (U.S. Army Corps ofEngineers, 1977). The Susquehanna River, the larg-est river entering the bay, drains nearly 43 percentof the 64,000-mi2 basin and normally contributesabout 50 percent of the freshwater reaching thebay.

Background

The Chesapeake Bay has been degradedbecause of water-quality problems, loss of habitat,and over-harvesting of living resources. The Ches-apeake Bay was listed as an impaired water bodyin 2000 under the Clean Water Act because ofexcess nutrients and sediment and it must meetFederal regulatory water-quality standards by2010. The Chesapeake Bay Program (CBP), a multi-agency partnership, completed Chesapeake 2000,a new agreement that revises and establishes resto-ration goals for the next 10 years in the bay and itswatershed. In the agreement, improving waterquality is identified as the most critical element inthe overall protection and restoration of the Chesa-peake Bay and its tributaries. Part of the degrada-tion in water quality is caused by excess sedimentin the water column and its adverse effects on theliving resources and associated habitat.

During the last 30 years, excess sediment hascaused significant reductions in submergedaquatic vegetation (SAV); covered filter-feedingbenthic organisms, thereby affecting their vitality;and delivered chemical constituents and patho-gens associated with sediment to the bay, affecting

fisheries and other living resources. Water clarityand sediment problems are not unique to the estu-ary and its tidal tributaries; many stream habitatsin the watershed also are affected by these prob-lems.

Sediment Workgroup

To establish and implement sediment-reduc-tion measures and to improve water-quality mod-eling efforts to understand the potential effect ofmanagement policies, the CBP required informa-tion on the sources, transport, and deposition ofsediment that is affecting water clarity. Because theknowledge and data on sediment processes werenot readily accessible to the CBP or to state andlocal managers, a Sediment Workgroup (SWGP)was created in 2001. The SWGP consists of Federal,State, and local government scientists and manag-ers and university researchers with various back-grounds and expertise relevant to sedimentaryprocesses.

Since its inception, the SWGP convenedmonthly to examine sediment-related issues, tohear invited speakers from the scientific commu-nity, to prioritize research needs, and to develop aset of management implications based on theSWGP findings. In the early stages of the SWGPefforts, a provisional outline of a summary reportwas decided upon and various workgroup mem-bers were charged with writing chapters or parts ofchapters on topics of their expertise. Because of theinherent interdisciplinary nature of sedimentaryprocesses in the bay and its watershed, expertise inhydrology, geology, biology, physical oceanogra-phy, environmental science, meteorology and cli-matology, among other topics, was required.Consequently, the meetings among SWGP mem-bers fostered a unique, though sometimes chal-lenging, exchange of ideas on sedimentation froma wide variety of perspectives. To our knowledge,such an interdisciplinary investigation of sedimen-tary processes has never before been undertakenfor Chesapeake Bay.

Report Objectives and Scope

The primary objective of this report is to pro-vide a review of the literature on the sources, trans-port, and delivery of sediment in Chesapeake Bayand its watershed with discussion of the potentialimplications for management actions. Because theChesapeake Bay has been one of the most intenselystudied estuaries over the past 50 years, it would

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Figure 1.1. Location of Chesapeake Bay watershed and estuary.

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be impossible in a single document to fully reviewand critically discuss all aspects of sediment pat-terns and processes in the region. Therefore, theauthors of the report have extracted, discussed,and summarized important aspects of sedimentand sedimentation that are most relevant to theCBP and other sediment-related issues with whichresources managers are involved. Many of thesemost important aspects are underlined throughoutthe report. In producing this document, the SWGPmembers drew on peer-reviewed publishedliterature, research in progress, the opinions ofinvited experts, and certain “grey” literaturereports containing valuable information. Theextensive bibliography in this report serves as aresource for those readers seeking more informa-tion on methodology and results.

The writing and editing process of the finaltext involved a large degree of subjectivity in termsof the scope and treatment of each topic. Becausethe SWGP was created with the mandate by theCBP to help provide input to the managementcommunity, the report does not provide a compre-hensive review of all available literature, and gapsexist in the coverage of certain topics. Nonetheless,the report summarizes those studies most relevantto concerns about sediment and water clarity andprovides references for those wanting more infor-mation.

Following the Executive Summary, there arechapters on Watershed Sediment Sources, Trans-port, and Deposition; Estuarine Sediment Sources,Transport, Deposition; and Sedimentation, Inte-grated Approaches to Sediment (sediment budgetsand modeling), and a Bibliography.

Terminology for Sedimentand Total Suspended Solids

Sediment is solid material transported anddeposited by wind, water, or ice, chemically pre-cipitated from solution, or secreted by organisms.In estuaries like Chesapeake Bay, sediment consistslargely of water-borne detrital material (pebbles,sand, mud), including varying amounts of particu-late organic material. Over time, sediment settlesto the bottom and accumulates to form clasticrocks (conglomerate, sandstone, shale). However,most sediment deposited during the past 8,000years in Chesapeake Bay is still unconsolidatedand those sediments deposited during the last fewcenturies still contain more than 50-percent watercontent in pore spaces between sedimentary parti-

cles. The organic fraction of sediment collectedfrom the bottom of Chesapeake Bay consists of1-3 percent organic material, the rest consists ofinorganic mineral material and varying amounts ofshell material.

Geologists refer to sediments in terms ofgrain size, texture, mineralogy, and other charac-teristics. Throughout this report, an important dis-tinction is made between fine-grained sediment,which refers to the clay- (less than 1/256-mmdiameter) and silt- (1/256- to 1/16-mm diameter)sized fractions, and coarse-grained sediment,which refers to the sand- (1/16 to 2-mm diameter)and gravel- (2 to 64-mm diameter) sized fractions.This fine/coarse distinction is important becausemost coarse material is transported along the bot-tom of rivers and the bay and has little effect onlight penetration. In contrast, fine-grained sedi-ment commonly is found in suspension and vari-ably blocks light penetration depending on itsabundance, grain-size distribution, and degree ofaggregation (flocculation).

Hydrologists commonly refer to sedimentusing terminology that reflects their interest in thetotal amount of suspended material in a watersample. Total suspended solids (TSS) and sus-pended sediment concentration (SSC) are twomeasurements of the concentration of suspendedsolids in a water sample (Gray and others, 2000).Both measurements usually are given in milli-grams per liter. SSC is measured as the dry weightof total sediment in a sample divided by theamount of water-sediment mixture in the sample.TSS can be measured by several methods. It usu-ally is measured by taking a subsample of knownvolume from the original suspended-sedimentsample, drying the sediment, and dividing by theknown volume. Most TSS measurements cited inthis report refer to data obtained with this method.

TSS Variability

Data from CBP monitoring sites and thepublished literature show that the relative propor-tion of inorganic and organic components of TSSvary seasonally and interannually, due mainly tovariability in freshwater inflow, and vary spatially,depending on proximity to shoreline, oceanic, andriverine sources of inorganic sediment. Gallegosand Moore (as cited in Batiuk and others, 2000)used CBP data from 1994 to 1996 to show thatwhen TSS concentrations are high (greater than50 mg/L), the organic component of TSS consti-

C H A P T E R 1 | 2 3

tutes an average of 18 percent of TSS; and whenTSS concentrations are low to moderate (less than50 mg/L), the organic component varies from 0 to90 percent. The organic component of TSS wasdominant (greater than 50 percent) only when TSSconcentrations were low (less than 10-15 mg/L).

Annual variability in TSS in the bay and itstributaries can be illustrated by plotting 15 years ofmonthly TSS data from two CBP stations in thebay. TSS data from 1985 to 2000 for surface anddeep water at station CB4.4 in the central mainstem bay and for surface (0.5 m) and near-surface(2-4 m) depths in Pocomoke Sound are shown infigure 1.2. These data show the following featuresof TSS variability:

• On average, surface TSS values are higher(10 to greater than 50 mg/L) at thePocomoke Sound EE3.3 site compared tothe main stem CB4.4 site (4 to 10 mg/L).

• Deep-water TSS maxima occur in winter atthe CB4.4 monitoring site, reaching greaterthan 80 mg/L during some years, but only20 to 40 mg/L during others.

• Summer deep-water TSS minima are rela-tively invariant from year to year; thesefeatures are observed in other TSS recordsfrom the central main stem bay.

• Surface (0.5 m) and deep-water (24.5-31 m)TSS records are not correlated with oneanother at the CB4.4 site

• Surface (0.5 m) and near-surface (2 to 4 m)TSS records at the EE3.3 site generally aresimilar to each other.

Spatial and seasonal variability in TSS isshown in figure 1.3 where CBP data for winter andspring seasons are mapped for relatively wet(1993) and dry (1992) years. These maps, preparedby D. Khona (University of Florida and USGS) andT. M. Cronin (USGS), show high winter concentra-tions of TSS in 1992 near the mouth of the bay,reflecting ocean-source sediments. High TSS con-centrations were observed in the northern bay, andin the larger tributaries, especially during 1993,reflecting high turbidity in the estuarine turbiditymaximum zones. Such spatial analyses are usefulto identify regions of relatively high TSS (20 togreater than 80 mg/L), which, if the organic toinorganic ratios reported in Batiuk and others(2000) hold, represent regions where the inorganiccomponent of TSS is high (greater than 70 percent).

The evidence that the major component ofTSS is inorganic mineral sediment at moderate tohigh TSS values, and the fact that TSS is a majorcause of light attenuation, has significant implica-tions for water quality management in general,and efforts to improve water clarity in particular.These facts imply reducing chlorophyll a and theorganic component of TSS through nutrient reduc-tion will only partially address the causes ofdiminished water clarity. To fully address the issueof water clarity in Chesapeake Bay, detrital sedi-ment—that is, “suspended solids” introduced bythe influx of mineral clays, silts, and sand-sizedparticles into aquatic systems, must also be takeninto account. Therefore, most of the current reportfocuses on processes and patterns of detrital sedi-ment erosion, deposition, and re-suspension inChesapeake Bay, its tidal tributaries, and its water-shed that influence TSS concentrations and, ulti-mately, SAV and critical habitats.

In addition to sediment introduced fromexternal sources, and the volatile organic compo-nent of TSS, particulate material also is producedin the water column and on the bay bottomthrough the biological secretion of hard skeletonsby diatoms (siliceous), dinoflagellates (organic-walled cysts), and calcareous organisms (foramin-ifera, ostracodes, mollusks). These “biogenic” com-ponents of sediment range in size from a fewmicrons to greater than 1 mm in diameter. Theabundance of biogenic material is influencedstrongly by nutrient influx and productivity.Regardless of their origin, once shell-producingorganisms die, their skeletons behave like otherfine-grained particulate material and settlethrough the water column. They either becomeincorporated into sediment accumulating on thebay bottom or, like inorganic clays and silts,become subject to resuspension and transport bytides and currents. Thus, biogenic material contrib-uting to TSS and diminished water clarity also isdiscussed.

The degree of suspended particle aggrega-tion is also important for many aspects of sus-pended-particle dynamics. Studies (Fugate andFriedrichs, 2002; Sanford and Halka, 1993; Sanfordand others, 2001; Schubel, 1971) have separatedChesapeake Bay suspended sediment into twopopulations: a relatively unaggregated, slowly set-tling background suspension, and a highly aggre-gated, rapidly settling population that is main-tained in the water column by resuspension.Aggregated particles are considerably larger and

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Figure 1.2. Mean monthly concentrations of total suspended solids (TSS) at two CBP monitoring sites forshallow (0.5 m), near-surface (2-4 m), and deep (24.5-31m) water depth. (Plots show interannual and seasonalvariability in TSS.)

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Figure 1.3. Concentrations of total suspended solids in winter and spring, 1992 and 1993. (Total suspendedsolids data from Chesapeake Bay Program, Annapolis, Md., Chesapeake Information Management System.)

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settle faster than their constituent sediment parti-cles. They are commonly made up of a broad dis-tribution of particle sizes and chemicalcompositions, and can be strongly influenced byboth biological and physical processes (Hill andothers, 2001; Kranck and others, 1993; Schubel,1971; Zabawa, 1978). Little is known about aggre-gation/disaggregation dynamics in the bay, but itis known that large aggregates are less efficient atblocking light than small aggregates or unaggre-gated fine sediment particles (fig. 1.4) (Gardnerand others, 1985; Sanford and others, 2001;Zaneveld and others, 1979).

A wide variety of methods have been used tostudy sediments and sedimentary processes inChesapeake Bay. These include bathymetric sur-veys, geophysical surveys of accumulated sedi-ment on the bay floor; Light Detection andRanging (LIDAR) surveys of shorelines; AcousticDoppler Current Profiler (ADCP) surveys; satelliteimagery; sediment core analyses; sedimentarygeochemistry; short-lived radioisotopic analyses ofmass accumulation rates; geochemical tracers ofsediment source; photogrammetric and carto-graphic analysis of coasts; mineralogical analysisof sediment; and analysis of TSS concentration and

composition. Each study had its own objectives,which in most cases were not directly concernedwith issues of water clarity and SAV health. Thereader is urged to consult the original literature fordetails of methodologies and conclusions.

TSS, Light, and SAV

The amount of light reaching SAV in shallowwaters of Chesapeake Bay is influenced by manyfactors. The most important properties in the watercolumn are water color, and concentrations of TSSand chlorophyll a. Water column TSS consists oforganic material, referred to as total volatile sus-pended solids (TVSS) and inorganic ‘mineral’ mat-ter. Because TVSS consists of organic componentsof water (phytoplankton, heterotrophic plankton,bacteria, and particulate organic material), its rela-tive contribution to TSS is related to nutrient con-centrations and algal abundance. The inorganicmineral component of TSS generally consists offine-grained silts and clays and the abundance ofmineral sediment is related to various physicalprocesses such as river discharge, tidal and waveerosion, estuarine circulation, and currents, as wellas geology, geomorphology, land-use, and otherfactors.

Figure 1.4. Photograph of suspended fine sediment flocs from a site in upperChesapeake Bay during October 2002. The image was obtained with a particle-imaging system consisting of a low-light video camera and a collimated light beam.The width of the image is approximately 1 centimeter and the depth of field isapproximately 1 centimeter. Flocs and particles smaller than 0.003 centimeter(30 microns) are not resolved (photo credit, Larry Sanford, University of Maryland,Center for Environmental Science, 2003).

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The organic and inorganic components ofTSS vary in their relative proportion spatially andtemporally in complex and only partially under-stood ways. In general, at typical measured levelsof TSS (less than 50 mg/L), more than 50 percent(usually 60 to greater than 90 percent) of the TSSloading consists of inorganic material; at relativelyhigh TSS concentrations, TVSS approaches anaverage of about 18 percent of the TSS. Only at rel-atively low TSS values (less than 10-15 mg/L) doesTVSS consist of more than 50 percent of the TSS.These patterns suggest that the inorganic fractionof TSS, i.e., fine-grained sediment ultimatelyderived from riverine, shoreline, and oceanicsources, and reintroduced into the water columnthrough resuspension of bottom sediment, plays amajor role in light attenuation.

In addition to TSS and chlorophyll a in thewater column, epiphytes and other organic andinorganic material accumulated on the SAV leafsurfaces decrease the amount of light penetrationnecessary for photosynthesis. Because epiphyteabundance on SAV is itself influenced by nutrientloading and algal abundance, and the amount ofinorganic material settling on leaf surfaces is influ-

enced by mineral sediment in the water column,light-attenuation by material settling on SAV sur-faces is related closely to processes in the watercolumn.

In summary, the literature on TSS, light, andSAV relations indicates complex physical and bio-logical processes governing inorganic and organicsediment production, transport, and deposition.The physical processes that govern the introduc-tion, transport, re-suspension, and deposition ofinorganic sediment are distinct from biologicalproduction of particulate material driven by nutri-ent concentrations and primary production. How-ever, these processes are related to one anothersuch that efforts by water-resource managers toreduce nutrient loadings might also help toimprove water clarity by affecting the levels of par-ticulate material in the water column and algal epi-phyte growth on SAV. Future research programsand management strategies to control chlorophylla and nutrient loadings could be coordinated withefforts aimed at reducing the concentration ofmineral sediment and thus take into account thecumulative impacts of organic and inorganic mate-rial on light attenuation.

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CHAPTER 2. WATERSHED SEDIMENTSOURCES

by Allen Gellis,1 Sean Smith,2

and Steven Stewart3

Watershed sediment sources can be sepa-rated into sediment originating in upland regions,sediment from urban areas, and sediment erodedfrom channel corridors. In this section, these sub-jects are briefly discussed, although it should beemphasized that the processes controlling sedi-ment flux in the watershed are strongly interre-lated. Additional discussion of sediment sourcesand transport is given in Chapter 7 in the contextof developing quantitative sediment budgets.

Upland Sediment Sources

Upland sediment refers to material erodedfrom hillslope surface areas adjacent to stream cor-ridors. Upland regions include forests, rangeland,agriculture (cropland and pasture), rural, andurban areas. Land-surface characteristics stronglyinfluence the sediment flux from a particularwatershed region, and thus it is important tobriefly discuss sediment removal from upland sur-faces.

Soil from land surfaces is eroded throughdetachment of material by either water (raindropimpact and runoff) or wind (eolian). Soil erosionby water is the dominant transport mechanismfrom upland sources and commonly is expressedquantitatively and incorporated into a sedimentbudget (Leopold and others, 1966; Dietrich andDunne, 1978; Swanson and others, 1982; Gellis andothers, 2001). Sediment removed and transportedfrom upland sources typically is reported as a yieldover time (tons per square mile per year or squarekilometers per year).

In a classic paper on sediment derived fromland surfaces, Wolman and Shick (1967) discussedpost-colonial land-use change in the northeasternUnited States and its effect on sediment yield(fig. 2.1). Wolman and Shick proposed that in thelate 1800s, when forestland was converted to agri-culture, sediment yields increased from100 tons mi-2 (35 metric tons km-2) to 600 tons mi-2

(210 metric tons km-2). During the 1960s, many

rural areas near cities became urbanized resultingin another increase in sediment from constructionactivity when sediment yields exceeded2,000 tons mi-2 (35,000 metric tons km-2).

Several other studies provide estimates ofsediment yields from land surfaces in the Chesa-peake Bay watershed region. Guy and Ferguson(1962) reported yields of 25,000 to 50,000 tons mi-2

(8,750 to 17,500 metric tons km-2) from construc-tion areas near Washington D.C. Roberts andPierce (1976) proposed that the Patuxent Rivermore than doubled its sediment yield afterurbanization (983 tons mi-2; 344 metric tons km-2

compared to pre-urbanization values of408 tons mi-2; 143 metric tons km-2). In a detailedstudy of the Western Run Basin (60 mi2) north ofBaltimore, Costa (1975) estimated that land clear-ing for agriculture caused 34 percent of erodedsediment to be transported through the basin and66 percent was retained in storage. Of the66 percent of sediment in storage, 21 percent wasdeposited on floodplains and 79 percent wasretained on hillslopes as colluvium and sheetwashdeposits. Costa found that channels initiallyresponded to the increased sediment load byaggrading. As sediment loads decreased as a resultof decreasing agricultural practices and soil con-servation, stream channels began to incise andscour of stream channels became an importantsource of sediment.

Brown and others (1988) used 10Be (an iso-tope of beryllium) to estimate the erosion in48 basins of the eastern United States, including10 basins that drain to the Chesapeake Bay. 10Be isa cosmogenic isotope produced in the atmosphereand deposited on the earth’s surface during precip-itation. Interpretations of basinwide erosion werebased on an erosion index defined as the ratio ofthe amount of 10Be leaving a basin to the amountdeposited on it. The highest rates of erosion wereobserved in the Piedmont streams, and the lowestrates were observed in Coastal Plain streams, dueto differences in land use and stream gradients.The Piedmont has had two centuries of farmingthat disturbed the topsoil and led to high erosionof sediment with higher concentrations of 10Be.Annual pre-colonization sediment yields for thePiedmont were estimated to be 34.3 tons mi-2

(12 metric tons km-2), a value that closely matchesvalues from modern undisturbed basin sedimentyields (Brown and others, 1988).

1 U.S. Geological Survey.2 Maryland Department of Natural Resources.3 Baltimore County Department of Environmental

Protection and Resource Management.

C H A P T E R 2 | 2 9

Coal-mining activities also can contributefine particles to fluvial systems and thus canincrease natural sediment yields 30 to 40 timesabove background levels (Biesecker and others,1968). Since 1907, coal separation by wet methodscarried fine waste to the nearest rivers, contribut-ing between 12 and 18 percent of the total fine sed-iment load (Biesecker and others, 1968).

Reed and Hainly (1989) compared the effectsof coal mining on sediment yield in mined andunmined areas of Pennsylvania between 1978 and1982. Sediment yields in an unmined basin domi-nated by agriculture were 0.48 ton acre-1 (1.1 met-ric tons hectare-1) but only 0.0036 ton acre-1

(0.0081 metric ton hectare-1) in a forested basin.A mined area had a sediment yield of 5.5 tonsacre-1 (12.3 metric tons hectare-1). Installation of asediment-retention pond below the mined areareduced the sediment yield to 0.14 ton acre-1

(0.31 metric ton hectare-1). In two other minedareas, sediment yield below sediment-retentionponds was 0.19 and 0.30 ton acre-1, respectively(0.42 and 0.67 metric ton hectare-1). Reclamation ofvegetation on the two mined sites reduced sedi-ment yield to 0.037 ton acre-1 (0.083 metric tonhectare-1) and 1.0 ton acre-1 (2.24 metric tonshectare-1). Since the 1960s, sediment discharge hasbeen decreasing in many rivers in Pennsylvaniabecause of decreased mine activity and stricter reg-ulations (Williams and Reed, 1972).

In a study of sediment yields for the Susque-hanna River Basin, Williams and Reed (1972) notedthat the range in sediment yields was related to

topography (slope), geology, glacial history, andland use. Soils derived from sandstones in theAppalachian Plateau Physiographic Province, withits extensive forest cover, had low sediment yields.In mined areas of the Appalachian Plateau, sedi-ment yields were surprisingly low. Internal drain-age and depressions left from mining were cited asthe causes for this low sediment yield. In minedareas of the Valley and Ridge Physiographic Prov-ince, sediment yields were high compared tounmined areas of the Valley and Ridge. The lowestsediment yields were in the sections of the Valleyand Ridge Physiographic Province that are under-lain by limestone. Internal drainage, presumably ofkarst systems, was cited as the cause for the lowsediment yields in the limestone terrain.

Langland and others (1995) used suspended-sediment data for rivers draining the ChesapeakeBay watershed to examine the influence of landcover on TSS and SSC. They found that the largestmedian SSC was in the Upper Potomac RiverBasin, and the maximum SSC was in the Susque-hanna River Basin. Correlations of annual sedi-ment yields computed with a log-linear multiple-regression model to land use indicated that basinswith the highest percentage of agriculture had thehighest sediment yields and basins with the high-est percentage of forest cover had the lowest sedi-ment yields.

For the York River system, a series of sedi-ment budgets for 11 nested sub-watersheds rang-ing in size from 65 to 6,900 km2 were compared toexamine distribution of sediment load as a func-

Figure 2.1. Land-use history and sediment yield from the Potomac River Basin in the northeastern UnitedStates, from the late 1700s to the 1960s, projected to approximately 2000 (from Wolman and Shick, 1967).

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tion of watershed size (Herman, 2001). The studyshowed that in low-relief Coastal Plain water-sheds, sediment budgets are influenced more bythe characteristics of the river system than by sub-watershed size. Upland erosion was the majorsource of sediment in the Pamunkey River; bankerosion was the major source in the MattaponiRiver. Upland storage was the major sink for bothtributaries. This study also showed that little sedi-ment from the upper watershed reached the estu-ary, and at the river mouth, the net movement ofsediment is from the bay into the estuary.

In summary, sediment yield from uplandregions of the bay watershed vary greatly becauseof the differences in land-use characteristics, geo-morphology, and climatology of the region.

Urban Sediment Sources

Urban sediment sources change during thecourse of urbanization. Initial sediment sources areassociated with land-surface disturbance activitiesfrom construction. After development sites havebeen stabilized, the mass of sediment delivered tothe stream system is reduced. Sediment wash-offfrom post-development commonly is less than thepre-development condition. The sediment fromimpervious urban areas is associated with dry andwet atmospheric deposition, deterioration of roadand built surfaces; and deterioration of vehiclesand other human artifacts. Sediment also may beproduced from pervious surfaces depending onhow effectively the pervious surfaces are main-tained. The construction process results in theinstallation of impervious surfaces and the com-paction of pervious soils by heavy equipment.Both these processes result in increased runoff withthe potential for increased streambed and channel-bank erosion.

Construction-site runoff is the largest con-tributor of sediment in developing urban areas(U.S. Environmental Protection Agency, 1993). Esti-mates of uncontrolled construction-site sedimentloadings range from 7.2 to 1,000 (tons acre-1) year-1.A summary of a range of studies in USEPA (1993)is shown in table 2.1. Sediment controls are esti-mated to be approximately 60 to 70 percent effec-tive in trapping sediment from construction sites;erosion controls were 80 to 90 percent effective(Caraco, 1995). However, sediment traps are moreeffective at removing coarse-grained particles thanfine-grained particles. Schueler and Lugbill (1990)found that particle-size distribution became finerin a comparison of inflow particle size to outflow

particle size. This would indicate that althoughmuch of the sediment from a construction site canbe trapped on-site through best-management prac-tices, the sediment that is released will be finergrained.

Stabilization after construction and the even-tual coverage of pervious surfaces with imper-vious material result in a decrease in sedimentdelivered from the watershed to the stream sys-tem. The installation of stormwater best-manage-ment practices results in trapping of sedimentparticles before delivery to the stream system.

A number of studies have looked at the rela-tion between urban land uses and sediment.Dreher and Price (1995) reported the relationbetween land use and sediment delivery in poundsper inch of rain in Illinois. Their results are pre-sented in table 2.2 with an extrapolation to 40 in. ofrain, which is the long-term annual mean for cen-tral Maryland. They calculated an enrichment ratioby comparing the extrapolated sediment load foreach land use to the sediment load for the wood-land/wetland land-use category. As can be seenfrom table 2.2, land-use categories with high levelsof impervious area (industrial, commercial, high-ways, and high-density residential) had the high-est sediment loadings and consequently thehighest enrichment ratios. However, their studydid not determine whether the source of the sedi-ments was from watershed wash-off from impervi-ous surfaces, watershed erosion of pervioussurfaces, or stream channel erosion.

Watershed management plans have beenprepared for a number of watersheds in BaltimoreCounty, Md. The results from the Storm WaterManagement Module (SWMM) pollutant loadmodel for two of the studies (Loch Raven Water-shed and Patapsco River Watershed) are presentedin table 2.3. The sediment pollutant loads arehigher for land uses with higher impervious areacoverage.

Table 2.1. Construction-site sediment loadings(from USEPA, 1993)

Sediment loading(tons per acre per year)

Reference

35.6 to 1,000 York County Soil and WaterConservation District, 1990

30 Franklin County, FL, date unavailable30 to 200 Wisconsin Legislative Council, 199135 to 45 MWCOG, 1987

50 to 100 Washington Department ofEcology, 1989

C H A P T E R 2 | 3 1

Using the SWMM, the Baltimore County Lit-tle Gunpowder Falls Water Quality ManagementPlan provided an estimate of the amount of sedi-ment attributable to washoff from the watershedand the amount attributable to stream channel ero-sion. For the watershed as a whole approximatelytwo-thirds of the sediment load was the result ofchannel erosion and not watershed sediment con-tribution. This is consistent with the findings ofTrimble (1997), where stream-channel measure-ments from 1983 to 1993 in San Diego Creek indi-cated that two-thirds of the sediment yield was theresult of channel erosion.

During urbanization, streams undergo threestages—an initial aggradation phase where sedi-ment from construction activities results in sedi-ment deposition in the stream channel; an earlyerosion phase where fine sediments gradually areremoved, exposing gravel and cobble and thechannel cross section increases; and a late erosionphase where the channel down cuts and widensalong its entire reach. Miller and others (2000) mea-sured channel change for historical cross sections

in Watts Branch, in the Piedmont Province ofMontgomery County, Md. Their studies showedthat between 1972 and 1993, the streambedaggraded because of the deposition of sedimentfrom construction. This was accompanied by chan-nel widening and an increase in cross-sectionalarea between 1993 and 1999. Hammer (1972) exam-ined the changes caused by urbanization in thePiedmont of southeastern Pennsylvania and foundthat an increase in discharge is accompanied bystream channel widening that takes place over a 10to 20-year period. Robinson (1976) studied streamsin the Piedmont of Maryland and concluded thaturbanization increased channel area approxi-mately two times and width/depth ratios 1.7 timesthose of rural channels. He postulates that it takesat least 15 years for a stream to reach a new equilib-rium form following development.

Effective land-use planning and sedimentcontrol can help reduce the impacts of the aggrada-tion phase on the streams. Stormwater manage-ment with peak and volume control, preferablynear the source, will help reduce the impacts of the

Table 2.2. Post-development urban watershed sediment sources (Dreher and Price, 1995)

Land-use category

Sediment delivery

Pounds per inchof rain

Pounds per40 inches of rain

Enrichmentratio

Milligramsper liter

Industrial 16.18 647.3 28.53 120Commercial/institutional 14.52 580.6 25.59 80Low-density residential 4.53 181.1 7.98 100High-density residential 8.17 326.8 14.40 90Vacant 1.36 54.4 2.40 60Open land/urban park 1.14 45.4 2.00 50Highway/arterial road 10.90 436.0 19.22 80Agriculture 3.40 136.1 6.00 150Woodland/wetland .57 22.69 1.00 50Railroad 3.68 147.3 6.49 80

Table 2.3. Baltimore County Storm Water Management Module (SWMM) pollutant load results

Land-use category

Sediment loads

Loch RavenStudy

Enrichmentratio

Patapsco RiverStudy

Enrichmentratio

Commercial/industrial 446.3 9.25 718.8 10.58Low-density residential 158.1 3.28 155.1 2.28Medium-density residential 213.3 4.43 285.9 4.20High-density residential 279.3 5.82 410.9 6.04Open land/urban park 140.3 2.91 135.6 1.99Crop land 366.4 7.60 361.6 5.32Pasture 243.8 5.06 306.9 4.51Forest 48.2 1.00 68.0 1.00

3 2 | C H A P T E R 2

erosion phase. Beyond these measures, effectivebuffer creation and management and stream resto-ration also are available tools for stream protectionand improvement.

Channel Corridor Sources

The channel corridor refers to the channelbed, banks, and floodplain areas of a stream. Inecological terminology, the channel corridor iscalled the riparian zone. Streambank erosionoccurs in channel corridors through the directremoval of banks and beds by flowing water, typi-cally during periods of high flow. The meandering(side to side) movement of a stream is a naturalprocess whereby streams adjust their channelshape in response to flows over long periods oftime. Geomorphologists commonly use the term“equilibrium” to characterize the size and shape ofa stable channel and the amount of sediment “nat-urally” generated within a basin. Lane (1955) sug-gested that the energy of a stream is a function ofthe speed and volume of water, and this energymust be in balance with the size and volume ofsediment transported by the stream. Anthropo-genic land disturbance (clearing of land, urbaniza-tion, channelization) severely alters this naturalequilibrium. In practical terms, this means that ifeither the volume (increased runoff) or velocity(steeper slopes) of water increases, the increase instream energy will increase the sediment-carryingcapacity of the stream. The usual source for thisadditional sediment from increased stream energycomes from the stream channel (bed and banks),which undergo erosion.

After the equilibrium of a stream is dis-rupted, a series of events take place that aredescribed by the Channel Evolution Model(Simon, 1989). In this model, disruption causes thechannel to cut deeper and increases water storage,which in turn increases stream velocity. Thisincrease in velocity results in streambank erosion, awidened stream channel, and the development ofnew floodplain at a lower elevation in the streamchannel. After the process of downcutting hasbegun, it will continue to downcut upstream untila grade control (bedrock, culvert) is reached oruntil the stream once again reaches equilibrium.

Despite the development of explanations forthe form and adjustment of stream channels in theChesapeake Bay watershed, relatively little site-specific information on stream adjustment and itsrelation to bank erosion and sediment flux is avail-able. Even less data are available on the quantifica-tion of sediment loss and particle-size transport as

a result of bank erosion. However, available stud-ies indicate a wide range in erosion rates, from afew inches per year in a “naturally” stable streamto as much as 5 ft per year in areas of the Piedmont.For example, in urbanized watersheds in the Pied-mont areas of Pennsylvania, streambank erosioncan exceed sediment accumulation and bankrebuilding, resulting in the enlargement of thechannel (Hammer, 1972). However, Leopold (1973)observed a decrease in channel cross-sectional areain the Piedmont of Maryland during a period ofintense development in the watershed. This is, per-haps, an indication of sediment accretion. Pizzutoand others (2000) more recently observed thaturbanized channels were approximately 26 percentlarger in cross-section area than rural channels inthe Piedmont of Pennsylvania.

Although the contribution of sediment fromstreambank erosion may be a significant sedimentsource in many streams in the watershed, the per-centage of “unstable” streambanks in the baywatershed is not known. Several promising lines ofresearch may address this lack of information. Forexample, measurement of cosmogenic isotopes canprovide estimates of bank sediment in terms of itspercentage contribution to load of total sediment.Modeling studies also have potential to determinebank erosion and sediment transport derived fromsediment particle-size data from the bank andfloodplain. In one such study, the USCOE analyzedfloodplain sediments from previously sampledand flow-gaged USGS sites in the SusquehannaRiver Basin and found that most of the bank mate-rial in the lower Piedmont areas is composed offine sands and silts that can be easily eroded andtransported as suspended material (Megan Jones,U.S. Army Corps of Engineers, oral commun.,2003).

The diversity of topographic and geologicconditions within the watershed and the complex-ity of hydraulic conditions in natural channelscommonly limit the use and applicability of infor-mation from site-specific study areas for broaderwatershed-wide applications. As a result, model-ing the effect of channel adjustments on sedimentsupply to channels has limited predictive value. Insummary, quantitative estimates of how streamrestoration and other best-management practicesinfluence streambank erosion and resulting sedi-ment delivery to the tidal estuaries and the mainstem bay remain imprecise.

C H A P T E R 2 | 3 3

CHAPTER 3. WATERSHED SEDIMENTTRANSPORT

by Sean Smith,1 Michael Langland,2

and Robert Edwards3

This chapter provides an overview of thephysical processes associated with stream-channeladjustment in the Chesapeake Bay watershed, andthe relation between the processes of adjustmentand sediment flux. A brief explanation of differ-ences in channel appearance and behavior withinthe watershed are discussed to provide perspectiveon the conditions that are capable of generatingchanges in the rates and magnitudes of sedimentmovement to the Chesapeake Bay.

Channel Hydraulics and Sediment Transport

The Chesapeake Bay watershed contains avariety of landscapes including steeply slopedmountains of the Appalachian, Valley and Ridge,and Blue Ridge Physiographic Provinces, dissectedlandscapes of the Piedmont and western CoastalPlain Physiographic Provinces, and flat areas onthe Delmarva Peninsula (Langland and others,1995). Stream channels have different characteris-tics in each of these regions that reflect the influ-ence of the long-term geologic processes thatcreated the dominant topographic and sedimen-tary environments. The appearance, stability, andmodes of channel adjustment differ in each of thephysiographic settings. Consequently, responses tochanges in land use vary across the ChesapeakeBay watershed, resulting in different changes inthe flux of sediment through channel networks.The inconsistency in channel adjustment andrelated sediment-transport dynamics requires thatthe approaches used for stream-stabilizationprojects related to sediment management be partlycustomized to address specific hydraulic and geo-morphological conditions.

Channel Morphology and Hydraulics

Stream and river channels are landform ele-ments that have their dimensions and patternsgoverned by water flow and sediment supply.A stream reach can be described using three differ-

ent perspectives—cross section, longitudinal view,and planform views. Different dimensional mea-surements are associated with each perspective.

The channel “cross section” dimension gov-erns the width and depth of the flow area, whichaffects flow velocities. Collectively, the width,depth, and flow velocity comprise the hydraulicgeometry of the channel, which has a direct rela-tion to sediment transport. Several attempts havebeen made in the bay watershed and similar set-tings in the mid-Atlantic to characterize the rela-tions between stream channel cross-sectionaldimensions and flow characteristics using thehydraulic geometry framework initially proposedby Leopold and Maddock (1953). These haveincluded the investigation of Kolberg and Howard(1995) on the hydraulic geometry of Piedmontchannels, the analysis of the factors affectingdownstream changes in cross-sectional morphol-ogy in the Valley and Ridge Physiographic Prov-ince of central Pennsylvania by Pizzuto (1992), asurvey of Maryland Piedmont and Coastal Plainchannels by Prestegaard and others (2000), and thesurvey of bankfull discharge and channel charac-teristics in the Piedmont in Maryland by McCand-less and Everett (2002). Although the flow-conveyance characteristics of streams are depen-dent partly on the sediment concentrations andsupply, only the approach used by Pizzuto directlyconsidered sediment discharge as an independentvariable. This limits the utility of the other investi-gations because trends associated with down-stream changes in channel conditions cannot befully explained without sediment information.

The “longitudinal” profile also governs flowcross-sectional area and velocity through its rela-tion to the energy gradient, which is approximatedby the slope of the water surface in the down-stream direction. Within a channel reach, the slopegoverns the force and power of the water flow,which determines the capability to transport sedi-ment. At the scale of an entire drainage network,profiles usually are sloped more steeply in head-water areas than at basin outlets. Geomorphicanalysis of longitudinal profile characteristics ofstreams in the Valley and Ridge, Blue Ridge, Pied-mont, and Coastal Plain Physiographic Provincesin the Chesapeake Bay watershed has beenattempted (Hack, 1957). However, systematictrends between the downstream progression of theprofile and channel bottom sediment characteris-tics are difficult to resolve in many river networksbecause of localized changes in geology.

1 Maryland Department of Natural Resources.2 U.S. Geological Survey.3 Susquehanna River Basin Commission.

3 4 | C H A P T E R 3

Natural stream and river channels that areformed by water flow and sediment deposition arecomposed of an active channel (bankfull flow) andan adjacent floodplain (flood flow) (fig. 3.1). Theprimary flow regimes in streams and rivers can bepartitioned into:

• base flows that originate as slow releasesof ground water or surface water fromponds in the absence of precipitation andhave little capacity to transport sediment;

• bankfull flows that fill channels up to thetops of their banks, (these flows have beenfound to be important determinants of thechannel dimensions because they can bethe most “effective” conveyors of sedimentover extended time periods (Wolman andMiller, 1960; Leopold and others, 1964;Dunne and Leopold, 1978)); and

• flood flows that over top streambanks.(These flows affect channel stabilitythrough dramatic erosion and sedimenttransport in brief periods of time (Bakerand others, 1988; Grover, 1937; Smith,1997; Smith and others, 1999). The flood ofJanuary 1996 provided an example of therole of floods in sediment movement intothe bay, transporting approximately17 times the amount of sediment normallydelivered to the Chesapeake Bay in thesame month (Zynjuk and Majedi, 1996)).

Channel-Shaping Processes and Sediment Flux

Two questions related to stream channelsand sediment remain difficult to answer in theChesapeake Bay watershed:

• When can a stream channel be consideredstable?

• What discharges have the greatest influ-ence on the channel form?

The term “equilibrium,” more accuratelystated as “steady-state equilibrium,” commonly isused to describe the condition under which theaverage shape and dimensions of a stream aremaintained over a period of time, such as severaldecades or a century (Schumm, 1977). Channelchanges can occur within a stream in equilibriumin response to changes in sediment supply, butthey are localized in a reach and last for relativelyshort periods of time. Short-term widening andcontraction of channels in response to erosion anddeposition of sediment during floods are examplesof this variability, as observed by Costa (1974) inthe Piedmont following Hurricane Agnes in 1972.Wolman and Gerson (1978) also described changesin the channel width following flooding in Bais-man Run and the Patuxent River in Maryland.

The perpetuation of an equilibrium channelcondition requires consistent watershed conditions(Carling, 1988). Watershed changes that alter thefrequency and magnitude of water and sedimentdischarges make it difficult to maintain consistentchannel conditions over time (Werrity, 1997). Virtu-ally all the watersheds draining to the Chesapeake

Figure 3.1. Flow regimes affecting stream-channel and floodplain corridors (Modified from Smithand others, 2000).

C H A P T E R 3 | 3 5

Bay have experienced numerous changes in landuse over the past century. As a result, few, if any,stream networks have experienced steady-stateequilibrium conditions since European coloniza-tion.

The shape and dimension of a stream chan-nel is influenced collectively by the frequency,magnitude, and velocity of the flows passingthrough the channel, the type and amount of sedi-ment supplied to the channel, and the structuralcharacteristics of the stream channel bed andbanks. In the absence of structural controls, thecapacity of the reach to convey supplied sedimentis an important factor affecting channel shape anddimension. Channel reaches in the bay watershedthat have received excessive sediment loads fromagricultural fields or urban construction activitiesmay not be capable of transporting all the suppliedmaterials. This can result in the temporary buildup of sediments and braided conditions with mul-tiple bars and channels. Conversely, many chan-nels that have received increases in flow withoutsimultaneous increases in sediment supply havedegraded because of a net export of sediment.

“Bankfull discharge,” ”dominant discharge,”and “effective discharge” are terms used by engi-neers and geomorphologists to describe the flowsthat have the greatest influence on the channeldimension. Each has a direct or indirect relation tothe frequency and magnitude of sediment trans-port. The concept of relating a single discharge toan optimized condition of sediment flux and chan-nel stability has become a popular focus for thedesign of stream-channel restoration projects in theChesapeake Bay watershed. Procedures for calcu-lating effective sediment discharge have been pub-lished by the USCOE; however, the over-simplification of the relation between sedimentdischarge and channel dimensions to a single dis-charge limits applicability with broad-scale use inthe development of channel designs (Biedenharnand others, 2000).

The patterns of channel migration across avalley also relate to sediment flux through a reach.In naturally meandering channels, bank erosion onthe outside of a meander bend can be compensatedby the accumulation of deposited sediment (bankrebuilding) on the inside of the bend. If the rates oferosion and accumulation are similar, the channelwill change its position but maintain its cross-sec-tional dimension. This condition is sometimestermed “dynamic equilibrium” because it is a form

of stability. However, increased flows anddecreased sediment loads disturb this equilibrium.Areas on the inside of a bend that normallyaggrade with sediment can experience net erosion,thereby resulting in an apparent straightening ofthe channel centerline and an increase in the chan-nel width and average depth. The effect of channelstraightening caused by increases in flows anddecreased sediment supplies can be observed inmany urbanized areas such as the Washington,D.C., and Baltimore metropolitan areas. Channelsin urbanized Piedmont settings in Pennsylvaniahave been characterized by lower sinuosity thanthose in rural areas (Pizzuto and others, 2000).

The process through which streams becomestraightened often is related to channel widening.In unprotected urbanized watersheds, increasedstreambank erosion can exceed sediment accumu-lation and bank rebuilding, resulting in theenlargement of the channel. These trends weredocumented by a survey of channels in urbanizingwatersheds in Piedmont areas of Pennsylvania(Hammer, 1972). However, a decrease in channelcross-sectional area was observed by Leopold(1973) in the Piedmont of Maryland during aperiod of intense development in the watershed.Wolman and Shick (1967) previously had devel-oped a model of stream response to land-usechanges that characterized changing sediment fluxand associated channel adjustment in Piedmontchannels near Baltimore. The changes identifiedincluded stable conditions under fully forestedwatershed conditions, aggradation in response toforest clearing for agriculture, degradation of chan-nels as agricultural land goes fallow, pronouncedaggradation during urban construction, andremoval of accumulated sediment following thetermination of construction as channels adjust tothe reduced sediment supply and urban stormflows.

Progressive channel incision is anothermode of adjustment that commonly occurs in stre-ambeds composed of easily erodible materials.Channels cut downward when the export of sedi-ment from a reach exceeds the sediments importedinto a reach (fig. 3.2). This condition creates a loca-tion of high sediment supply from a localizedstream reach that persists until the gradient of thechannel is reduced to a level that no longer pro-motes a net erosion of materials. Incision processescommonly involve the upstream progression of aheadcut. Wolman (1987) has described such pro-cesses and their relations to sediment flux in a

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second-order stream channel in the PiedmontPhysiographic Province of Maryland. Observa-tions included the movement of a headcut approx-imately 18 m upstream over 3 years, episodicmovements of gravels, and the transport of themajority of suspended sediments in episodic high-flow events during the monitoring period.

First- and second-order channels in theWestern Shore of the Coastal Plain are particularlyprone to downcutting because they are steepenough to generate erosive flows and are com-posed of highly erodible materials, such as uncon-solidated sand. These headwater channels receivelimited sediment contributions to compensate forchannel erosion, which promotes vertical down-cutting. This is particularly characteristic of urban-ized watersheds. Unfortunately, the contribution ofsediment from erosion in the headwaters of theCoastal Plain and Piedmont to sediment loading inthe Chesapeake Bay has not be quantified for any

time scale. A complication in developing such anestimate is that stream maps accurately delineatingsmall first-order channels prone to incision are notreadily available for the Chesapeake Bay water-shed.

Specific geomorphic processes associatedwith incision can vary with the landscape settingand climatic conditions. Steeply sloped first- andsecond-order channels in the Appalachian Plateau,Valley and Ridge, and Blue Ridge PhysiographicProvinces can generate high-energy flows; how-ever, bedrock prohibits down-cutting by erosionover short time scales. Hillslope processes thatmove large amounts of sediment over short timeperiods, such as debris flows and landslides, canalter channels on steep slopes during extreme pre-cipitation events. In the Rapidan River Basin inVirginia, floods and debris flows in 1996 provided

Figure 3.2. Relations between profile location, sediment flux, and channel incision, defined as [dz/dt = (1/γs)(dG/dx) + i,] where dz/dt = change in channel bed elevation with time, dG/dx = change in bedload transport withdistance downstream, γs = specific gravity of sediment (Modified by S. Smith from Richards and Lane, 1997).

C H A P T E R 3 | 3 7

evidence of such events; however, these events arerare in most of the Chesapeake Bay watershed(Gori and Burton, 1996).

Channel Sediments

Sediment sizes generally are divided intoclays (less than 0.004 mm), silts (0.004-0.062 mm),sands (0.062-2 mm), gravels (2-64 mm), cobbles(64-256 mm), and boulders (greater than 256 mm).To some extent, lithology and transport mecha-nisms determine the shapes of the particles, whichcan range from spherical to platy. The variability oflithologic conditions throughout the ChesapeakeBay watershed create a diversity in the size distri-butions, densities, and shapes of sediment beingtransported downstream towards the Coastal Plainrivers and Chesapeake Bay (Smith and others,2000). Changes in the grain-size distribution onand within the channel bed also can occur. Thesechanges usually are characterized by a reduction inthe median grain size with distance downstream inlarge drainage networks. Changes in bed grainsizes also can occur over relatively short distances,as observed by Prestegaard and others (2000) in thereach of Northwest Branch traversing the FallZone near Washington, D.C. (fig. 3.3).

Individual sediment particles move either byremaining in suspension in the water column or byrolling, skipping, or hopping along the bottom ofthe channel as “bedload” (Vanoni, 1975; Yorke andHerb, 1978; Meade and others, 1990). The part ofthe total sediment load moving in the water col-umn can be further partitioned into the “sus-pended load,” which is characterized by aconcentration that decreases with elevation abovethe channel bed, and “wash load,” which has ahomogeneous distribution through the water col-umn (Leopold and others, 1964; Vanoni, 1975).

Suspended-Load and Wash-Load Transport

Sediment moving in suspension is entrainedin response to flow velocities and turbulence.Material in suspension generally consists of parti-cles of fine sand, silt, and clay. Because suspendedsediment concentrations depend on grain size andflow velocity, the rates of removal of fine sedimentsfrom suspension from the water column can takeextended periods of time and require very lowflow velocities. Hence, the management of finesediment after it is brought into suspension can bedifficult or impossible with standard best-manage-ment practices, including sediment ponds.

Figure 3.3. Changes in channel-bottom sediment sizes in the Fall Zone near Washington, D.C. (Modifiedfrom Prestegaard and others, 2000).

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Suspended-sediment transport commonly isevaluated using sediment concentration and veloc-ity profiles that describe conditions through thewater column (Leopold and others, 1964; Garcia,1999). The sediment profiles are represented as afunction of the ratio of the sediment concentrationat different levels through the water column to thesediment concentration near the bed. Developmentof computer simulations of suspended-sedimenttransport is complicated because of complex flowphenomenon associated with turbulence and dis-tortions. The same problems make the collection ofsuspended-sediment data difficult, particularly ifthe hydraulic condition at a sampling location iscomplicated by changing roughness distributionsor asymmetry in the cross-sectional geometry(Gray, 2002). Suspended-sediment information col-lected in the Chesapeake Bay watershed com-monly is collected as TSS, which includes bothinorganic and organic materials. This method maymisrepresent the total sediment transport in somewatersheds (Gray and others, 2000).

Bedload Transport

Similar to suspended load, sediment trans-ported as bedload is driven by the force of thewater flow. The amount of force required to initiatemotion of a particle depends on its size, density,shape, and position relative to other particles.Although estimating bed load transport accuratelyis difficult, relations between bedload transportand water flow have been examined by numerousauthors (Vanoni, 1975; Wohl, 2000). Approachesused to predict bedload transport usually arebased on a well-known set of governing equationsfor the conservation of flow, mass, and momentumand represented using constitutive relations devel-oped through experimentation that describe howsediments with different characteristics respond towater flows (Vanoni, 1975; Wohl, 2000; Middletonand Wilcock, 1994; Meade and others, 1990).

Little bedload information is available in theChesapeake Bay watershed and no programmaticefforts are planned to gather data. However, bed-load transport can be important in localized areasbecause of its relation to channel stability andadjustments. Bedload generally is less than20 percent of the total sediment transferred fromcontinental uplands to the coastal margins (Yorkeand Herb, 1978, Vanoni, 1975). However, bedload-transport rates and magnitudes can significantlyaffect channel hydraulic geometry and stability ingravel-bed rivers. This also can affect the total sed-

iment yield from a reach, including fine sedimentsmoving in suspension. The influence of bedloadtransport on channel stability and TSS yield can beparticularly important in several unique areas ofthe Chesapeake Bay watershed where large quanti-ties of gravel and cobble materials are suppliedfrom bedrock or upstream areas. The northernareas of the Appalachian Plateau where past gla-cial activity has deposited large amounts of graveloverburden materials are one such location. TheFall Line between the Piedmont and Coastal PlainPhysiographic Provinces is another geomorphi-cally unique location where the bedload move-ments of coarse gravels supplied in pulses canpromote large localized changes in channel flowconveyance capacities over short time periods(Smith, 1997).

The frequency and magnitude of the trans-port of coarse sand and gravels has potential impli-cations for stream and river channel stability, thedevelopment of stable channel engineering proto-cols, and assessments of aquatic habitat (Bieden-harn and others, 2000). Many formulas forestimating gravel transport as bedload have beendeveloped from experimentation using single-sized sediment. These experimental approacheshave, for the most part, evaluated mixed-sized sed-iments as simple percentages of sand. However,changes in the sorting of mixed-sized sedimentsduring transport affect transport rates for gravels.This may have relevance to the effect of construc-tion sediments on total sediment transport inurbanizing watersheds (Wilcock, 1998; Wohl,2000).

Watershed changes that directly alter thesupply of sediments naturally transported as bed-load can include channel engineering and dam-construction activities. Changes indirectly influ-encing bedload transport conditions can includeincreases in the frequency and magnitude of dis-charges associated with forest clearing and urban-ization, and truncated peak flows from largereservoirs with large storage capacities for waterand sediment.

Field data necessary to estimate bedloadtransport are difficult to collect because bedloadsampling requires sampling bottom materialsacross the width of a channel, commonly duringhigh-flow conditions (Edwards and Glysson,1988). Estimates can be developed using experi-mentally derived bedload transport functions;however, the error can very high. A variety of

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channel bottom and bar sediment sampling tech-niques have been developed to estimate bottomsediment grain size, which serves as an indepen-dent variable in most bedload transport functions(Wolman, 1954, Wohl, 2000). Some of these sam-pling methods have been popularized for use instream-channel assessment exercises conducted bymany agencies and their consultants in streammanagement and channel-rehabilitation projectsthroughout the country, including the ChesapeakeBay watershed (FISRWG, 1998).

Although bedload is a relatively minor per-centage of the total sediment load delivered to theChesapeake Bay, bedload-transport characteristicsof the gravel-bed rivers flowing through the baywatershed influence the total sediment load,including fine sediment transported in suspension.Measured bedload-sediment data would be help-ful to evaluate total sediment loading, to performstability assessments in gravel-bed streams, and todevelop stream-channel stabilization designs. Todate, very few attempts have been made in the baywatershed to collect bedload information in con-junction with stream channel assessment or reha-bilitation projects conducted in the past decade(Mallonee and others, 2002; McCoy and others,1997). Therefore, almost no information exists onthe relation between bedload and total sedimentyield to the Chesapeake Bay.

Reach-Specific Sediment-TransportCharacteristics

Sediment materials are transported from thefirst-order channels in the headwaters of water-sheds to the higher-order channels and the basinoutlets. However, the rate of transport for the grainsizes supplied by a watershed is not constant overtime or consistent through a drainage network.The amount of sediment that moves through areach depends on the amount of sediment input,

the magnitude, frequency, and duration of flows,and the hydraulic geometry of the channel. Forexample, incising channel reaches associated withlow order (headwater) streams commonly releasemore sediment than they receive. This sedimentcan become stored in bars and adjacent floodplainsin downstream reaches that receive more sedimentthan they can transport. In some localized areas,such as the “Fall Line” border between the Pied-mont and Coastal Plain Physiographic Provinces,sediment from upstream is conveyed through areach in pulses that coincide with high flows(Smith, 1997).

Sediment stored in stream-channel bar for-mations and on the floodplain can provide a his-tory of changing land-use activities. Distinctchanges to the morphology of the active channeland floodplain have been observed in response toalterations in watershed hydrologic conditions andsediment supply (Jacobsen and Coleman, 1986).Three defining periods of sedimentation wereidentified by Jacobsen and Coleman (1986) in rela-tion to observed floodplain strata (fig. 3.4).

Pre-settlement period: The Piedmont flood-plains were formed over long periods of time bythe settling of fine sediment in the wooded areasadjacent to active stream channels.

Agricultural period: Widespread establish-ment of farming caused dramatic increases in sedi-ment supply and the deposition of significantlayers of sediment in the floodplain over a rela-tively short time period.

Very recent period: Reduced agriculturalactivities and improved sediment control havedecreased sediment supply from over-landsources. Stored sediment in floodplain deposits arereworked, resulting in the downstream transportof fine sediments and the reworking of coarse sedi-ments into bar deposits.

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Figure 3.4. Floodplain stratigraphy observed by Jacobsen and Coleman, partitioned into three definingperiods of sedimentation (Modified from Jacobsen and Coleman, 1986).

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CHAPTER 4. WATERSHED SEDIMENTDEPOSITION AND STORAGE

by Julie Herman,1 Clifford Hupp,2

and Michael Langland2

Sediments eroded from the land surface arestored in the Chesapeake Bay watershed in threeprimary places: upland surfaces, in reservoirsbehind dams, and in floodplain riparian regions.This section discusses studies relevant to the stor-age of sediment that has been eroded but has notyet reached the bay or its tributaries. A discussionof legacy sediments, defined as those sedimentseroded during extensive colonial land clearanceand temporarily trapped in the watershed, is pre-sented in Chapter 6 on Deposition and Sedimenta-tion Rates.

Upland Storage

This section reviews recent papers onupland sediment storage. Upland sediment stor-age refers to sediment that has been mobilized inupland regions and redeposited on the upland sur-face before reaching a stream. This type of sedi-ment is referred to as colluvium, and its storagemay constitute a large percentage of materialeroded from uplands. Although colluvial sedimentcan be difficult to measure, its residence time inupland storage and its delivery to stream coursesmay have important implications for water-qualitymanagement.

Colluvium is deposited at the base of hills-lopes, at field edges, in buffers, and in swales anddepressions (isolated wetlands). Evidence of theimportance of sediment storage and remobiliza-tion as a source of sediment in drainage basins ofvarious scales is increasing (Walling, 1988). Water-shed studies have shown that a large percentage ofthe total sediment eroded is stored as colluvium.During a study on Coon Creek, Wis. (360 km2

drainage area), Trimble (1981) found that 38 to63 percent of upland-source sediment was depos-ited as colluvium. In four large (>1,000 km2) drain-age basins in the Piedmont of North Carolina,colluvial storage was estimated to be 71 to81 percent of mean annual sediment production(Phillips, 1991b). In Western Run, a Piedmontwatershed in Maryland, Costa (1975) found that52 percent of sediment eroded from agricultural

lands was stored as colluvium. In a series of nestedsub-watersheds in the York River watershed (Pied-mont and Coastal Plain Physiographic Provincesof Virginia), 57 to 74 percent of upland erosion wasstored as colluvium (Herman, 2001).

Buffers tend to decrease the velocity of over-land flow and trap colluvial sediment by deposi-tion. Buffers may be grassy, forested, or zoned andusually are at field edges or in riparian zones. Inareas with lower slopes, such as the Coastal PlainPhysiographic Province, buffers appear to be moreeffective (Dillaha and Inamdar, 1996). In forestedriparian areas, more than 50 percent of the sedi-ment eroded in cultivated fields was depositedwithin 100 m of the field margins (Cooper and oth-ers, 1987). Fine particles also may enter the soilprofile with infiltrating water (Dillaha andInamdar, 1996).

A technique commonly used to estimateupland storage is called the sediment deliveryratio (SDR). The SDR is the ratio of sediment reach-ing a basin outlet compared to the total erosionwithin the basin (Walling, 1983). The portion ofmobilized sediment that is not delivered to astream channel remains on the upland as collu-vium. The following discussion of SDRs includesonly the transport pathway from upland erosion tostream edge. Values for SDRs range from 0 to morethan 1 and commonly are found to decrease prima-rily with an increase in drainage area. A ratio inexcess of 1 implies that delivered load exceedsgross erosion and that additional stored sedimentis being mobilized (Walling, 1983; Novotny andChesters, 1989).

The Chesapeake Bay Program watershed-modeling effort assumes that basins between 13and 259 km2 have ratios that vary between 0.1 and0.22, respectively, and 0.18 is used as a constantSDR from field to edge of stream for the sub-water-sheds (L. Linker, U.S. Environmental ProtectionAgency, oral commun., 1996). Values for VirginiaAgricultural Pollution Potential Database (VirGIS)SDRs in the York River watershed range from 0.01to 0.96; the mean is about 0.31 for crop and pastureland and 0.06 for all land uses (crop, pasture, for-est). For comparison, the SDR for the Yadkin/PeeDee River system in North Carolina (47,900 km2) is0.039 (Phillips, 1991a).

The SDR concept has limitations (Walling,1983; 1994). Considerable uncertainty surroundsthe methods for calculating SDRs, and there is nogenerally applicable predictive equation. Walling

1 Virginia Institute of Marine Science.2 U.S. Geological Survey.

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(1983) cites examples of proposed delivery ratioequations, all of which relate 'larger-scale' catch-ment properties (such as basin area and basinrelief) to sediment delivery. No equations wereavailable using parameters that define the land-surface pathway over which sediment-laden waterflows, such as surface roughness and soil perme-ability.

Another problem is that sediment deliveryalso may be discontinuous over temporal and spa-tial scales. Sediment eroded in the headwaters maybe stored, while sediment remobilized from down-stream is transported out of the basin (sedimentdecoupling) (Phillips, 1995), making SDR estimatesinaccurate. In smaller basins, there is less opportu-nity for sediment storage so the SDR may not be assusceptible to the lag time. Spatial diversity oftopography, land use, and soil conditions illus-trates the problems of spatial lumping and theattempts to represent sediment delivery of a water-shed with a single number. Therefore, SDRs shouldbe used with caution (Novotny and Chesters,1989).

A strategy to partially rectify these concernsis to apply the delivery ratio concept on a distrib-uted basis using a grid of square cells (Walling,1983; VirGIS reports). In one approach, VirGISused a first-order exponential function that wasassumed to approximate the amount of sedimentmoved from a cell to a receiving stream. The equa-tion includes the influence of vegetative cover andthe steepness and length of the flowpath (VirGISreports). Because 'correct' estimates virtually areimpossible, VirGIS calculated an SDR that gener-ally reflects expected trends (Shanholtz, 1988).Another method is to calculate gross erosion foreach cell and then sequentially route sedimentdownslope through adjacent cells towards a chan-nel, with a proportion of material being redepos-ited along the transport pathway until a final edge-of-stream value is obtained. Distributed deliveryratios were developed for total suspended solidsfrom trapping efficiencies of vegetated filter strips,but their results overestimated total sediment load(Levine and others, 1993). Although the distrib-uted approach possesses certain merits, in practiceit may offer little advantage over a lumped methodbecause of uncertainties in assigning deliveryterms to individual cells (Walling, 1983).

Several other methods hold promise forquantifying colluvial storage. For example,Cesium-137, a short-lived radioisotope has been

used to examine relatively recent sediment redis-tribution on agricultural fields (Fredericks and Per-rens, 1988). Long-term (decadal to historic timescales) redistribution of sediment also can beexamined by measuring changes in soil morphol-ogy, especially the truncation and accretion of soilprofiles. For example, Phillips and others (1999)examined the fluvial, aeolian, and tilling processesthat redistribute soil in a small watershed in NorthCarolina. They discovered that sediment wasdeposited immediately downslope from convexi-ties, forming thin fan deposits at toe slopes, indepressions, and at the borders of fields.

Floodplain and Banks

The Coastal Plain of the southeastern andmid-Atlantic United States is characterized by abroad, frequently inundated low-gradient floodplain. These riparian systems have received con-siderable ecological study but distinctly lesshydrogeomorphic study. Data on quantitative pro-cess linkages among hydrology, geomorphology,and ecology remain largely undocumented.Although sometimes heavily affected by land use,these flood plains and their bottomland hardwoodsystems remain a critical landscape element for themaintenance of water quality by trapping and stor-ing large amounts of sediment and associated con-taminants. Nearly 90 percent of all sediment istrapped for varying periods of time along streamsbefore reaching saltwater (Meade and others,1990). Thus, these flood plains are the last place forsediment storage before entering critical estuarinenursery areas for fish and wildlife.

Jacobsen and Coleman (1986) outlined aflood plain-development model that describedmorphological changes in Piedmont alluvial chan-nels flowing through stored (legacy) sedimentdeposits. They concluded that current changesincluded the erosion of the fine floodplain sedi-ments and storage of coarser materials in lagdeposits that developed into channel bar forma-tions. Localized storage reaches also have beenidentified by Smith and Prestegaard (Sean Smith,Maryland Department of Natural Resources, oralcommun., 2003) within geomorphic transitionareas, such as the boundary between the Piedmontand Coastal Plain Physiographic Provinces. Therelevance of localized changes in sediment convey-ance within drainage networks is watershed-spe-cific. Misinterpretations can promote improperriver-corridor management strategies (Prestegaardand others, 2000).

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Considering management timeframes, mostbottomlands, especially those on the Coastal Plain,exhibit net aggradation through sediment deposi-tion from two initially distinct sources: (1) runofffrom adjacent uplands (riparian buffer) and(2) streamflow during inundation of bottomlands(riparian retention). Geomorphic analyses (Leo-pold and others, 1964; Jacobson and Coleman,1986; Kleiss, 1996) verify that riparian retention ofsediment is a common and important fluvial pro-cess. Unfortunately, retention time of sedimentmay be the most poorly understood and generallyunquantified aspect of sediment budgets (R.B.Jacobson, U.S. Geological Survey, written com-mun., 1996). Johnston (1991) found only four pub-lished accounts of vertical accretion rates or massaccumulation for mineral fines in the United Statesfor any type of wetland. More recently, wetlandvertical accretion rates were reported by Hupp(2000) for West Tennessee, eastern Arkansas, SouthCarolina, North Carolina, and along tributaries tothe Chesapeake Bay in Maryland and Virginia(table 4.1).

Researchers are investigating several tribu-taries of the Chesapeake Bay in an effort to under-stand sediment and associated contaminantstorage and transport pathways in various hydro-geomorphic settings. Extensive riparian wetlandswithin the Coastal Plain regions of the bay maytrap as much as 70,000 kg yr-1 of sediment along a2-km reach (Hupp and others, 1993). Several moni-toring sites have been established along the

Chickahominy, Pamunkey, Mattaponi, Piankatank,Patuxent, Choptank, and Pocomoke Rivers, as wellas other smaller tributaries. In addition to monitor-ing, long-term tree-ring data and short-term artifi-cial marker horizons are being used to documentnet sediment deposition rates. Radioisotopic tech-niques also are being applied to track sedimentsources and estimate poorly understood sedimentretention times.

Initial results from both short- and long-termdata indicate that substantial amounts of sedimentare deposited at all the monitoring sites at ratesexceeding 1 mm yr-1 (fig. 4.1). Sedimentation ratesare highest where alluvial (brownwater) streamsreceive runoff from either agricultural or urbanareas with high loads of suspended sediment. TheChoptank and Pocomoke Rivers, which originateon the Coastal Plain and Delmarva Peninsula, haverelatively high sedimentation rates for blackwater(highly organic) rivers, however, the sedimentloads usually are low. These rivers would normallyhave tea-stained, but generally clear water color.However, because these rivers experience consid-erable channelization, sediment has been mobi-lized from drainage ditches and the main channel.Therefore, these rivers act more like pipelines thanrivers that have functioning riparian areas.

It may seem intuitive that as sediment-ladenflow leaves the main channel and enters a forestedwetland, velocities slow because of the hydrauli-cally rough nature of the forested bottom (also a

Table 4.1. Mean sediment deposition rates for Coastal Plain rivers

[Data from dendrogeomorphic analyses. The Cache River was investigated twice in differentstudies and locations.]

River Type

Mean sedimentdeposition rate

(millimetersper year)

Authorship and date

Hatchie, Tennessee Alluvial 5.4 Bazemore and others (1991)Forked Deer, Tennessee Alluvial 3.5 Bazemore and others (1991)Chicahominy, Virginia Alluvial 3.0 Hupp and others (1993)Obion, Tennessee Alluvial 3.0 Bazemore and others (1991)Patuxent, Maryland Alluvial 2.9 Schening and others (1999)Cache, Arkansas Alluvial 2.7 Hupp and Schening (1997)Roanoke, North Carolina Alluvial 2.3 Hupp and others (1993)Cache, Arkansas Alluvial 1.8 Hupp and Morris (1990)Wolf, Tennessee Alluvial 1.8 Bazemore and others (1991)Mattaponi/Pamunkey, Virginia Alluvial 1.7 Schening and others (1999)Coosawhatchie, South Carolina Blackwater 1.6 Hupp and Schening (1997)Choptank, Maryland Blackwater 1.5 Schening and others (1999)Pocomoke, Maryland Blackwater 1.5 Hupp and others (1993)

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dramatic increase in wetted perimeter) andconsequently sediment is deposited (Kleiss, 1996).Yet, until recently, few attempts have been made toquantify sediment deposition in any wetlandsystem. Even fewer published accounts describingfactors affecting local variation in deposition rateare available (Hupp and Schening, 1997; Hupp,2000). The amount of suspended fine materialavailable strongly influences deposition potential.Variation in local elevation across a bottomlandand correlated length of hydroperiod (length oftime flow is in wetland) also have been cited asimportant factors affecting deposition rate (Huppand Morris, 1990; Hupp and Bazemore, 1993;Kleiss 1993, 1996). Several other interrelated factorsmay play an important role in local sedimentdeposition, including flow velocity, distance offlowpath from main channel, hydraulic connectionto main channel, internal flowpaths, ponding(typically in backswamps or behind levees),roughness from standing vegetation and largewoody debris, and beaver activity.

In summary, the trapping of sediment andassociated contaminants in the riparian and flood-plain zones of lowland (Coastal Plain) tributaries isa major water-quality function of these systems.This function will play an increasingly importantrole in the retention of sediment entering the bay.Activities such as channelization, which limits theamount of contact between streamflow and the

riparian zone, will compromise the natural abilityof the streams to retain sediment and contami-nants.

Reservoirs

The large numbers of dams and impound-ments that have been built in the bay watershedhave a significant effect on river sediment loads.Dams interrupt the “natural” down-river flow ofsediment. Although most water eventually isreleased downstream, sediment is effectively cap-tured behind dams. In fact, many reservoirs trap atleast half the sediment annually flowing into themuntil reaching sediment storage capacity (Meadeand others, 1990). After a reservoir reaches its sedi-ment-storing capacity, sediment loads flowingdownstream “through” the reservoir will increaseand approximately equal that amount transportedinto the reservoir.

Sedimentation in any reservoir can be evalu-ated using bathymetric data or direct calculationwith consideration of trapping efficiency. Costa(1975) estimated that one-third of total sedimentserosion in Loch Raven Reservoir Basin (located inMaryland) since European colonization left thebasin and two-thirds was still in storage. He basedthese conclusions on an analysis of the bathymetricconditions in the reservoir at the downstream endof the drainage network. Reservoir sedimentationalso was measured by Ortt and others (2000) in the

Figure 4.1. Sedimentation rates from tree-ring and clay pads along selected Chesapeake Baytributaries. Rates from clay pads may be exaggerated by lack of compaction and non-decayedorganic material.

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Gunpowder River watershed and by Ocean Sur-veys, Inc. (1997) in the Upper Patuxent River, bothof which are in the Piedmont Physiographic Prov-ince of Maryland. Accretion rates measured inLoch Raven and Pretty Boy Reservoirs average 1.4to 1.5 cm yr-1, resulting in a total accumulation of10,100 and 8,740 m3 yr-1(Ortt and others, 2000).Ocean Surveys, Inc. found that a third reservoir inMaryland (Tridelphia) was accumulating approxi-mately 50,000-m3 yr-1 since its construction in 1942.In all three studies, the relation between reservoirsedimentation and watershed sediment yield wasnot formally developed.

Susquehanna River Reservoirs

During floods, large amounts of sedimentand nutrients are transported into the reservoirsystem, and, along with sediments and nutrients

already trapped in the reservoirs, are available fordeposition, resuspension, scour, and transportdownstream. However, scour of sediment fromreservoirs during floods increases the storagecapacity of the reservoirs. For example, the threemost recent floods—June 1972, September 1975,and January 1996—removed about 36 million tonsof sediment from three reservoirs in the lower Sus-quehanna River Basin (Langland and Hainly,1997).

The largest dams in the bay watershed are inthe lower reaches of the Susquehanna River.A reservoir system, consisting of Lake Clarke, LakeAldred, and Conowingo Reservoir, forms behindthree consecutive hydroelectric dams (fig. 4.2). SafeHarbor Dam, built in 1931, forms Lake Clarke.

Figure 4.2. Location ofthree hydroelectric damsand reservoirs in the LowerSusquehanna River Basin.

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Holtwood Dam, built in 1910, forms Lake Aldredand is the smallest of the three reservoirs.Conowingo Dam, built in 1928, is the largest andfurthest downstream reservoir.

Since their construction, the reservoirs havebeen filling with sediment and sediment-associ-ated nutrients. The upper two reservoirs havereached capacity and generally no longer traplarge amounts of sediments and nutrients. How-ever, Conowingo Reservoir has not reached capa-city and currently is trapping about 2 percent ofnitrogen, 40 percent of phosphorus, and 50-70 percent of suspended sediment that would oth-erwise be discharged to the Chesapeake Bay(Langland and Hainly, 1997).

Long-term discharge records are kept by thepower-plant operators. Since 1985, continuous dis-charge and water-quality data has been collected atConowingo and Marietta by the USGS and the Sus-quehanna River Basin Commission (SRBC). In1990, 1993, and 1996, the USGS determined thewater depth to sediment in the reservoirs in aneffort to calculate the remaining sediment-storagecapacity in the reservoir system and to estimatewhen the reservoirs will reach sediment-storagecapacity (Langland and Hainly, 1997). The 1996data collection followed a major flood in the Sus-quehanna River Basin. These studies showed thatalthough the Conowingo Reservoir is not yet fullof sediment, little space remains to be filled. Thecross-sectional areas of available space for nutrientand sediment storage have changed from 1928 to1996 and the probable cross-sectional area whenthe reservoir is at full sediment-storage capacity isshown in figure 4.3. From the upper end of the res-ervoir to about 28,000 ft upstream from the dam,the reservoir has very little sediment-storagecapacity remaining; the capacity from 28,000 ftdownstream to the dam was reduced greatlybetween 1928 and 1996. As a result of scour duringthe January 1996 flood, storage capacity in theConowingo Reservoir increased approximately1,600 acre-ft, which is equivalent to 2.4 million tonsof sediment. About 29,000 acre-ft remain to befilled, or about 42 million tons of sediment can bedeposited, before the sediment-storage capacity isreached (area in red, fig. 4.3).

Estimating the time remaining until the res-ervoir reaches sediment-storage capacity is diffi-cult because the amount of sediment transportedand deposited in the reservoirs depends on factorssuch as land-use and management practices, rain-fall, and large storm events. Despite these uncer-tainties, the reservoirs currently are estimated toreach storage capacity in 20-25 years. When thisoccurs, the amount of sediment and nutrientstransported to the bay by the Susquehanna Riverwill equal that amount delivered into the reservoirsystem. If all other conditions remain constant, andassuming 60-percent sediment trapping efficiency,this will result in a 2-percent average annualincrease in the nitrogen load, a 40-percent averageincrease in the phosphorus load, and a 150-percentaverage annual increase in the suspended-sedi-ment load. After capacity has been reached, agreater increase in the annual loads of nutrientsand sediment transported to the Chesapeake Baywill take place during major scour-producing floodevents. A task force, commissioned by the Susque-hanna River Basin Commission, composed of sci-entists, individuals from private industry, andlawmakers, recently addressed the issue of sedi-ment retention in the reservoirs and published alist of recommendations dealing with sedimentissues in the watershed and bay (SusquehannaRiver Basin Commission, 2000).

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Figure 4.3. Change in Conowingo reservoir sediment–storage capacity, 1929-1996.

CHAPTER 5. ESTUARINE SEDIMENTSOURCES

by Thomas Cronin,1 Jeff Halka,2

Scott Phillips,1 and Owen Bricker1

This chapter describes the sources of sedi-ments entering the Chesapeake Bay estuary fromvarious riverine, oceanic, biogenic, shoreline, andatmospheric sources. The chapter begins with anintegrated look at contributing sources to the estu-ary, then proceeds into more detailed descriptionsof the individual sources. Because of the integratednature of the watershed-bay system, many of theprocesses described here are necessarily related tothose covered in the prior chapter on watershedsedimentary processes. Following this section onsediment sources, the chapter describes sedimenttransport to and within the estuary, deposition(storage) and sedimentation in the estuary, andsecondary resuspension of sediment in the bay andits tributaries.

Estimates of Major Sediment Sources

The major sources of fine-grained sedimentto the estuary are from external and internalsources. The three major external sources includethe above-Fall Line watersheds, below-Fall Linewatersheds, and oceanic inputs. Two major inter-nal sources are from shoreline erosion and biogenicproductivity. Few complete studies on the estuarythat summarize the relative contribution of thesesources have been conducted. The best availabledata to assess the approximate relative contribu-tion of these sources on an estuary-wide basis,without accounting for spatial or temporal vari-ability or transport within the estuary, have beencompiled and are presented in this chapter.

The Fall Line watershed inputs were derivedfrom the River Input Monitoring Station data forthe 1989–99 period and average inputs totaled4.27 million metric tons per year (Langland andothers, 1999). It was assumed that the great major-ity, if not all, of the sediment supplied at the FallLine was fine-grained silts and clays. The water-sheds included in this total were the Susquehanna,Potomac, James, Patuxent, Mattaponi, Pamunkey,Appomatox, and Choptank. Tributaries on theEastern Shore that were not represented include

the Chester, Nanticoke, Wicomico, and PocomokeRivers. On the Western Shore, the major tributarynot included was the Patapsco River.

The below-Fall Line loads have been esti-mated for few tributaries including the Potomac(Miller, 1987), Choptank (Yarbo and others, 1981;1983), Rhode (Pierce and Dulong, 1977) and Patux-ent (Roberts and Pierce, 1974; 1976) Rivers. Theamount from these four studies totaled 0.9 millionmetric tons per year, all of which was assumed toconsist of fine-grained sediments. No attempt wasmade in this effort to estimate below-Fall Lineloads from tributaries that had not been studied(see Watershed Model discussion in Chapter 7).

Oceanic input of fine-grained sediment wasestimated from the works of Schubel and Carter(1976) and Hobbs and others (1992). The formerutilized a conservative salt-budget but did notactually measure sediment input. The latter deter-mined the deficit of deposited sediment through-out the bay on the basis of comparisons ofhistorical bathymetry and ascribed the differenceto oceanic input, 14 percent of which was deter-mined to be fine-grained sediments. Examiningthese two studies produced a total estimate of theoceanic input at 1.14 million metric tons per year offine-grained sediments to the bay.

Input of sediment from fastland (abovewater, mean tide) shoreline erosion and associatednearshore (below water, mean tide) erosion (dis-cussed in detail later in this chapter) was estimatedusing data from the USCOE Shoreline ErosionStudy (1990), which summarized the results sepa-rately for Maryland (Kerhin and others, 1988) andVirginia (Byrne and others, 1982). Estimates of therelative amounts of fine-grained and coarse-grained components of shoreline erosion werederived from the shoreline studies in Virginia(Ibison and others, 1990; 1992). Details of the anal-yses can be found in the Shoreline Erosion sectionof this chapter. Fastland erosion alone was deter-mined to supply 3.60 million metric tons per yearof fine-grained sediment from the shorelines of thebay; including the associated nearshore erosionincreased this number to 8.42 million metric tonsper year.

The relative contribution of these major sedi-ment source components to the entire ChesapeakeBay estuary is shown in figure 5.1. It is recognizedthat there are areas where data is lacking, includ-ing lack of river input monitoring measurementsfor all tributaries, relatively few below the Fall Line

1 U.S. Geological Survey.2 Maryland Geological Survey.

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tributary suspended sediment estimates, internalbiogenic productivity data are lacking, and oceanicand shoreline inputs are only roughly estimated.Biogenic productivity has been shown to be signif-icant in certain areas of the bay in certain seasons(Biggs, 1970) and needs to be more fully addressedin future studies. These results also may notinclude the effects of extreme climatic events.However, these results indicate the major sourcesof fine-grained sediments and will provide usefulinformation in devising management strategies. Asshown in figure 5.1, the above-Fall Line and shore-line erosion inputs are the dominant contributorsof fine-grained sediment, together supplying 79 to86 percent of the total load. Fastland shoreline ero-sion (36 percent) and Fall Line Riverine input(43 percent) are roughly sub-equal if associatednearshore erosion inputs are not included (leftgraph). However, if the nearshore component ofshoreline erosion is included in the total, the shore-line contribution dominates all other sources withan input of 57 percent of the total (right graph).

Although shoreline protection measuresmay be an important component of future manage-ment strategies, it may be difficult to measurablyreduce the input because of the dispersed nature ofthe source and the difficulties in reducing the near-shore erosion component. Construction of harderosion-control devices in areas of high fastlanderosion rates may increase erosion in the adjacentnearshore area because of wave reflection andrefraction, thus compounding the problem of sedi-ment supplied from shoreline erosion.

Watershed Sources

Sediment eroded from the watershed isdelivered to the bay and its tidal tributariesthrough river transport. Much of the sedimenttransported in the large rivers (Susquehanna, Poto-mac, and James) initially is deposited near the FallLine, which refers to the zone where a change intopography separates the Piedmont PhysiographicProvince from the Coastal Plain. This zone gener-ally represents the limit of the tidal influence of riv-ers and tends to coincide with the area where theharder crystalline rocks of the Piedmont Physio-graphic Province and the softer unconsolidatedrocks of the Coastal Plain Physiographic Provinceoverlap. The large amount of deposition near theFall Line occurs due to an abrupt reduction instream gradient (lesser slope so less stream veloc-ity) and influence from tides of the bay. Above theFall Line, rivers are typified by a high load of inor-ganic suspended sediments and at times are thedominant source of TSS that results in reducedwater clarity in the upper tidal tributaries of thebay. In contrast, many rivers that originate belowthe Fall Line in the Coastal Plain PhysiographicProvince, where the stream gradient is low, trans-port relatively smaller amounts of mineral sedi-ments but contain high levels of dissolved organicmaterial. Of the sediment generated in watersheds,up to 80-percent is trapped for a period of timealong streams before reaching saltwater (Costa,1975; Trimble, 1981; Herman, 2001). This sedimentmay take years to centuries to be transported to thebay because of continual deposition and resuspen-sion in stream corridors.

Figure 5.1. Relative contributions of sediment sources to the estuary with fastland (above tidal water) erosion (left)and with fastland and nearshore (below tidal water) erosion (right). (Based on data in chapter 7, table 7.2, and U.S.Army Corps of Engineers, 1990)

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In the mid-1980s, the USGS Chesapeake BayRiver Input Monitoring (RIM) Program was estab-lished to quantify loads and long-term trends inconcentrations of nutrients and suspended mate-rial entering the tidal part of the Chesapeake Baywatershed from its nine major tributaries (fig. 5.2).From 1985 to 2000, the two rivers with the highestannual sediment load were the Potomac River(1,932,000 tons) and the Susquehanna River(1,154,600 tons) (fig. 5.3A). During this period, thetwo rivers with the highest sediment yield werethe Rappahannock River (329 tons mi-2) and thePotomac River (167 tons mi-2) (fig. 5.3B). Monthlyand annual trends are estimated for sediment con-centration and load using samples collected once amonth and during storm events. Sediment concen-tration and streamflow are then used in a statisticalmodel to estimate the load of sediment enteringthe tidal reach of each river.

The long-term annual average of suspendedmaterial contributed by the nine RIM basins isapproximately 4.3 million tons yr-1. About 90 per-cent of this material came from the three largestrivers (Susquehanna, Potomac, and James)(fig. 5.4). Quantity of streamflow is the dominantfactor transporting and delivering suspendedloads from the watersheds to the estuary. Deliv-ered loads varied from 12.2 million tons yr-1 (1996)to 0.71 million ton yr-1 (2001). A severe drought,which occurred over much of the bay watershedfrom 1999 to 2001, resulted in an annual averageload of 780,000 tons yr-1, about 80 percent belowthe normal long-term average. Although the totalloads in 1990-2001 were less because of lower rain-fall and lower streamflow, the relative contributionof the major rivers was similar to the long-termaverage (Langland and others, 1995). Long-termtrends in monthly loads (1985-2001) at the nineRIM sites indicate there had been no significantchange. However, if the influence of streamflow isremoved, the trend in monthly concentrationshows a significant decrease at three sites and asignificant increase at two sites.

Currently, there are no long-term monitoringdata to estimate the load contributed from areasbelow the Fall Line. Computer simulations com-pleted using the Chesapeake Bay watershed Model(discussed in Chapter 7) estimates an additional 1million ton yr-1 of sediment enters the estuary fromthe unmonitored Coastal Plain region, with minorinputs from the Piedmont region below the FallLine monitoring stations.

Shoreline Erosion

Chesapeake Bay formed in response to risingsea level following the last major advance of thePleistocene glaciers. Approximately 18,000 yearsago, ocean levels in the mid-Atlantic region wereapproximately 400 ft lower than at present and byapproximately 8,000 years ago had reached a levelsufficient to begin inundation of the deeply incisedvalley cut by the Susquehanna River and tributar-ies (Colman and others, 1990; 2001; Cronin andothers, 2000). Continued sea-level rise first floodedthe deep narrow river valleys and then the sur-rounding gently sloping lands of the Coastal PlainPhysiographic Province. As the water level rose inthe bay, erosion of the unconsolidated sedimentsalong its shorelines contributed to the expansion ofthe estuary. Because sea level continues to rise inthe bay region at a rate of approximately 1.0 to1.4 ft per century, and because the rate may accel-erate in the future as a result of global warming,shoreline erosion in response to the rising sea levelis an important process ongoing in the bay.

The immediate cause of shoreline erosion isthe action of waves on the sediments along theshore. Without a rising sea level, a dynamic equi-librium state would be reached in which shorelineerosion would decrease dramatically from thepresent rate. However, when sea level is rising, theaction of waves reaches further inland over time tocontinue the process of shoreline erosion.

Erosion of the shorelines results in an imme-diate introduction of sediment to the estuarinewaters. Shoreline erosion usually is described interms of its location. The relation between fastlanderosion and associated nearshore erosion is shownin figure 5.5.

The rate of erosion at any particular locationis dependent on a number of factors that includeland use, sediment composition, orientation of theshoreline, bathymetry of the offshore region, andthe local wind fetch for generation of waves. Therelative importance of these factors in determiningthe erosion rate is difficult to assess for the bay as awhole because each factor is highly variable bothspatially and temporally. Historical shorelineshave been mapped in Maryland and Virginia anderosion rates derived from these shorelines inte-grate the dominant processes that have driven ero-sion at a particular location.

Historical erosion rates mapped in Maryland(Conkwright, 1975) and Virginia (Byrne andAnderson, 1977) were used by the USCOE in 1990

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Figure 5.2. Location map of the nine River Input Monitoring (RIM) Sites.

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Figure 5.3. River Input Monitoring station sediment data, 1985 to 2000. (A) Average annualsuspended-sediment load (log scale) and (B) average annual sediment yield. Most annual loadswere computed on the basis of suspended sediment.

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Figure 5.4. Combined annual suspended-sediment loads and relation to annual flow for theSusquehanna, Potomac, and the James Rivers near the Fall Line.

Figure 5.5. Relation between fastland (above tide) erosion and nearshore (below tide) erosion.

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to estimate the magnitude of the erosion rate bay-wide (U.S. Army Corps of Engineers, 1990) and todevelop an estimate of a bay-wide sedimentbudget (Hobbs and others, 1992). The Marylanderosion rate maps (2003) currently are beingupdated to include a recent (about 1990) shorelinealong with the historical shorelines. These mapswill provide updated estimates of shoreline changefor the Maryland part of the bay.

The USCOE (1990) report indicated thatshoreline erosion was not limited to fastland ero-sion but also included an envelope of sedimentthat is eroded to the base of wave action in thenearshore zone off the eroding shoreline. Analysisof historical bathymetric data by the USCOE indi-cated that this depth of erosion extended out to anaverage present-day (2003) 8-ft water depth.Including these nearshore sediments in the volumederived from fastland shoreline erosion increasedthe overall total from 4.7 × 106 yards3 yr-1 to11.0 × 106 yards3 yr-1. Using similar bulk densityvalues and percentages of sand as for fastland ero-sion, the total yield to the estuarine waters fromthe fastland and nearshore erosion increases to12.6 × 106 metric tons yr-1, of which 8.4 × 106 metrictons is fine-grained silt plus clay. With fastlandshoreline protection, nearshore erosion will con-tinue because of wave action. Hardening of theshoreline with bulkheads has been shown toincrease erosion of the nearshore bottom throughreflection of wave energy.

Values for bulk density of eroding shorelineshave differed in various reports produced for thebay. Kerhin and others (1998) utilized densitiesobtained from the Maryland Department of Trans-portation that ranged from 1.67 g cc-1 for silt claysto 1.92 g cc-1 for sands; the average was 1.78 g cc-1.Byrne and others (1982) and Hobbs and others(1992) reported using values that were consistentwith those of Kerhin and others (1998). The shore-line erosion studies of Ibison and others (1990;1992) cited soil scientists in using a bulk densitymeasurement of 1.5 g cc-1 to convert shoreline ero-sion volumes to mass. Biggs (1970) used mineralgrain densities in calculating a sediment budget forthe northern Chesapeake Bay, which at 2.65 to2.72 g cc-1 would overestimate the percentage ofsediment derived from fastland shoreline erosion.Direct measurements of dry bulk densities at anumber of sites in the Maryland portion of theChesapeake Bay yielded an average value of1.5 g cc-1 (Hill and others, oral commun., 2003).Marsh sediments have much lower dry bulk den-

sity values that center at approximately 0.5 g cc-1

(Anderson and others, 1977, Stevenson and others,1985). The range in values reported for bulk-density measurements is due in part to naturalvariability that may be confounded by measure-ments made on different levels in the strata andmeasurement of simple bulk density rather thandry bulk density. Moisture incorporated in the sed-iment, while adding mass, is not relevant to themeasurement of sediment yield. The Ibison andothers (1990; 1992) value of 1.5 g cc-1 for bank ero-sion is approximately mid-range in the valuesreported, and is corroborated by the direct drymeasurements of Hill and others (oral commun.,2003). This value currently is being used in the CBPmodeling effort. It thus represents a reasonableaverage value for fastland shoreline density. Usingthis value, the resulting load to the ChesapeakeBay from shoreline erosion is 5.4 × 106 metric tonsyr-1 (more discussion in chapter 7).

However, the Bay Model uses a constantshoreline erosion rate, allowing calibration to sedi-ment concentrations on the scale of the Chesa-peake Bay estuary, but it is not spatially variableand does not account for nearshore erosion. Inaddition, to compensate for lack of a resuspensionsimulation, modeled shoreline loads remain in sus-pension an unrealistic length of time. Therefore,the model may be underestimating the variableinput from shoreline load and could be overesti-mating the influence of shoreline erosion on waterclarity in the shallow-water zone.

Not all shoreline erosion is detrimental. It isalso important to note the necessary and beneficialfunctions of sediments within estuaries. Sedimentis critical to maintaining the elevations of tidal wet-lands, particularly in response to sea level rise. Animportant source of sediment to salt marshes isoverbank flooding, which generally delivers sus-pended fine sediments to the marsh substrate.Coarse material of upland origin, and suspendedcoarse sediment in littoral cells are responsible forthe development and maintenance of bay dunesand beaches. Dunes, beaches, and wetlands arecritical habitats for a diverse array of estuarineflora and fauna. Depending on the grain size of theeroding shoreline, the introduced sediments canprovide valuable habitat in the form of sandybeaches, or conversely, fine-grained clays that canremain suspended for long periods in the waterwith consequent negative effects on the ecosystem.The wave action also serves to transport sedimentsalong the shoreline in a down-drift direction, and

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can resupply beaches along the shore. In somelocations in the bay, the rate of supply of coarsersediments can produce areas where the shorelinehas accreted over recent, relatively short time peri-ods. However, erosion is the dominant effect of thecombined action of rising sea level and waves.

Sediment derived from shoreline erosionincludes not only fine-grained silts and clays thatcan remain suspended in the water column forextended periods and result in significant lightattenuation, but also includes sand- and gravel-sized components that settle at the base of theeroding shore-forming beaches. These beaches canserve to buffer the shoreline from the encroachingwaves and rising seas while providing potentiallyvaluable habitat that commonly is lacking in theChesapeake. Thus, protecting the shorelines fromerosion may have the unintended consequence offurther eliminating critical habitat in the bay. InVirginia, the eroding shorelines consist of approxi-mately 33-percent sand and gravel (Ibison and oth-ers, 1990; 1992). In Maryland, the percentagereported in mass calculations (35 percent) was sim-ilar (Kerhin and others, 1988). Recently, additionalsampling in Maryland yielded an average bankcomposition of 47-percent sand and gravel (Hilland others, oral commun., 2003)). Thus, althoughestimates vary, at least one-third of fastland shore-line erosion contributes to nearshore sandy sedi-ments (fig 5.5) that have little influence on lightattenuation, although two-thirds, or 3.6 x 106 met-ric tons yr-1, has the potential to remain suspendedin the water column.

Oceanic Input

Sedimentation in the southern part of Chesa-peake Bay has been the subject of numerousdetailed studies over the past 40 years. In thesouthern bay, large quantities of sediment arederived from inflow from the Atlantic Ocean conti-nental shelf through the mouth of the bay becauseof tides and ocean currents and from coastal ero-sion of headlands along the bay margins (Harrisonand others, 1967; Meade, 1969; Meade, 1972). Themouth of the bay is characterized by complex sedi-mentary processes that result from variations inthe tidal prism, fluvial input to the estuary, stormconditions in the estuary and in the ocean, andmutually exclusive ebb- and flood-dominatedchannels (Ludwick, 1975). Estimates of sedimentinflux through the mouth have relied on bottom-sediment sampling (Byrne and others, 1980), long-term averaging from geological and geophysical

studies (Colman and others, 1988), mineralogicaldata (Berquist, 1986), and short-lived radioisotopicstudies of sediment cores (Officer and others,1984). This section discusses those aspects of sedi-mentation in the southern bay most relevant toissues of water clarity.

Studies of long-term sedimentation in thesouthern bay indicate that subsurface Holocenesediment is filling the former Susquehanna Riverchannel (Colman and others, 1988). This suggeststhat the majority of sediment entering the baythrough the mouth has been, and continues to be,relatively coarse sands. The historical southwardprogradation of the southern tip of the DelmarvaPeninsula completely covering the pre-HoloceneSusquehanna River channel at the mouth of thebay (Colman and others, 1990) attests to the south-ward movement of large quantities of sand alongthe Atlantic Coast of the peninsula. These sandsnot only extended the peninsula tip over the earlierlocation of the incised Susquehanna River channel,but sub-surface bedforms reveal the movement oflarge quantities around the peninsula tip and intothe bay (Colman and others, 1988; Colman andHobbs, 1987).

Analysis of successive bathymetric surveysconducted from the mid-1800s to the mid-1900sand analyses of bottom sediments show significantaccumulations of sediment in the region of themouth relative to other parts of the bay (Byrne andAnderson, 1977; Byrne and others, 1980; Kerhinand others, 1988; Hobbs and others, 1990; 1992).These studies suggest that the volume of sedimentthat has accumulated in the bay during the 1840-1940 period cannot be accounted for solely fromshoreline erosion, biogenic production, and river-ine input. The volume of sand-sized sedimentexceeded the available sources by a factor ofbetween 2.7 and 7.6. The range is dependent on thelevels of confidence that were ascribed to thebathymetric changes observed in comparing thehistorical surveys. Most of this difference in thesand-sized fraction of quantifiable sediment was inthe Virginia part of the bay. Finer-grained mudsexceeded quantifiable sources by a factor of 2.4, avalue less than that for sands, but still large (seebelow). Consequently, Hobbs and others (1990)concluded that input of ocean-source sedimentfrom the adjacent Atlantic Ocean into the mouth ofthe bay must be a significant source of the totalsediment deposited in the bay. Examination of rel-atively long-term Holocene (10,000 year) deposi-tional records for the main stem of the Chesapeake

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Bay also indicates that very large sediment vol-umes have been deposited in the bay mouth areaand northward to the southern end of TangierSound (Colman and others, 1992). These data indi-cate that the greatest sediment volume is associ-ated with the bay mouth. This suggests that,averaged over the Holocene, the continental shelfhas been a more significant source of sediment tothe bay than the Susquehanna River and otherwatershed tributaries (Colman and others, 1992).

Although sand is the predominant sedimenttype in the southern bay, the transport of fine-grained sediment northward from the southernregions and from the main stem bay into largertributaries cannot be dismissed. In a comprehen-sive survey of the distribution, physical properties,and sedimentation rates in the Virginia part of thebay, Byrne and others (1980) reached the followingconclusion:

“… channels leading to the James and York tribu-taries are mud as are the entrance channels andbasis of the embayments of Mobjack, andPocomoke and Tangier Sounds.”

They also concluded that:

“The deposition patterns suggest that there isappreciable advection of fine sand from the bay-mouth region to at least 35 kilometers up the bay.The area of deposition is argued to occur as a con-sequence of net up-bay estuarine circulationthrough the deep channel along the easternshore.”

Byrne and others (1980) also commented onthe discrepancy between the sediment budget ofSchubel and Carter (1976), based on salt flux calcu-lation, which could not account for the large vol-ume of sediment deposited since the 1840s.Schubel and Carter had proposed that there is netimport of sediment from main stem to larger tribu-taries:

“If indeed the tributaries are sinks for materialstransported from the bay, then the apparent discrepan-cies between bottom accumulation and the previous esti-mates of source strength are enlarged. If the tributariesare sources rather than sinks, and if the bay mouth is astronger source than hitherto estimated, then the orderof magnitude discrepancy for silt and clay accumulationwould be reduced” (emphasis by original authors).

This conclusion suggests that significantamounts of finer-grained material is entering thebay from its mouth and that the sub-estuary riversare a potential source of fine sediment to the bay

(see also Hobbs and others, 1990). Evidence thatfiner-grained particles derived from the southernbay, possibly from oceanic sources, reach even far-ther up the bay was discussed in Hobbs and others(1990) who, quoting the work of Halka, concludedthat:

“silts are transported much farther up-estuarythan had previously been reported.”

Other evidence supports the idea thatalthough sand-size material dominates the surfacesediments in the southern bay, fine-grained claysand silts also are accumulating in some areas at arapid rate. In Chapter 2 of this report, theextremely high TSS loads during the winter of 1992near the mouth of the bay indicated a large poten-tial source for transport northward. Officer andothers (1984) reviewed sediment flux rates for theentire bay on the basis of lead-210 dating of sedi-ment cores and determined that sediment massaccumulation rates in the southern bay equaledthose of the northern bay where SusquehannaRiver inflow dominates as a sediment source.Officer and others found that southern bay massaccumulation rates ranged from 0.1 to0.8 g m-2 yr-1. Studies of drift buoys also show thatsurface currents are capable of carrying fine-grained sediments from the bay mouth region farto the north. Harrison and others (1967) showedthat bottom drifters released on the shelf have beenrecovered as far north as Tangier Sound, suggest-ing that suspended material has the potential fortransport relatively far up the bay in the landward-flowing denser saline water.

In summary, sediment in the southern bay isderived mainly from the adjacent ocean with anunknown contribution from shoreline erosionalong the bay margins. These sources contribute torelatively high long-term sedimentation rates inthe southern main stem bay and in adjacentsounds and embayments. Although much of thesediment deposited in the southern bay is sand-sized, part is composed of clay and silt-sized mate-rial and there also is good evidence for its signifi-cant net up-estuary transport. Because thismaterial has the potential to influence water clarityin the shallow-water bays and sounds of the bay,further study of sediment transport and depositionin the southern bay may be beneficial.

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Internal Sources of Sediment

This section describes (1) biogenic produc-tion of shell material by phytoplankton andbenthos in the bay and (2) particulate matter gen-erated internally in the Pocomoke River Basin.Brief mention also is given to the role of SAV insedimentation.

Biogenic sediments generated within Chesa-peake Bay itself can be defined broadly as anymaterial consisting of the remains of organismsgenerated within the estuary by skeletal formationor organic production. This would include diatomsiliceous skeletal material, dinoflagellate cysts, cal-careous shells of benthic organisms (mainly fora-minifera, ostracodes, mollusks), sponge spicules(siliceous), fish scales and bones (mainly phos-phatic), and organic matter from in-situ. Diatoms,for example, can constitute 5-10 percent of dry sed-iments, and calcareous shells can constitute asmuch as 5 percent. Biogenic suspended matter ofmost concern in terms of water quality can beviewed as those components in the water column,mainly phytoplankton (diatoms and dinoflagel-lates) and zooplankton. Historically, increasingturbidity in the bay, due in part to biogenic sus-pended matter, has been hypothesized as a contrib-uting factor to the decline in SAV for at least20 years (Orth and Moore, 1983).

A review of the literature on biogenic com-ponents of sediment in Chesapeake Bay can besummarized in two seemingly contradictory con-clusions. First, in a comprehensive review of sedi-ment characteristics in the bay and its tributariesthat provided quantitative estimates of sedimentsources and budgets, Nichols and others (1991)concluded that biogenic production and consump-tion were “neglected since they are usually small.”If one accepts this conclusion, and in light of thelack of biogenic sediment data in most previousstudies of Chesapeake Bay sediments, it would atfirst appear that in-situ generated suspended mat-ter is not quantitatively significant in the overallsediment budget of the bay.

Second, in one of the few studies to considerthe composition of suspended sediments in thebay, Biggs (1970) concluded that skeletal materialand organic production contributed 18 and22 percent, respectively, to suspended matter in themid-bay. In the northern bay, these values wereonly 2 percent, being overwhelmed by riverineinput from the Susquehanna River. Biggs did notconsider the southern bay. Extensive literature

published since the studies of Nichols and othersand Biggs suggests that biogenic material is animportant component of suspended matter in thebay and has probably become more important inthe past few decades, at least in many areas.

Overall organic productivity (driven bynutrient influx, including silica) has increased sub-stantially during the 20th century. This assertion isbased on trends in chlorophyll a (Harding andPerry, 1997), biogenic silica (Cooper and Brush,1991; Colman and Bratton, oral commun., 2003),diatom floras (Cooper, 1995), dinoflagellates (Wil-lard and others, 2003), carbon and nitrogen iso-topes (Bratton and Colman, written commun.,2003), and organic biomarkers (Zimmerman andCanuel, 2000). Much of this increase has takenplace since the Biggs study in the 1960s. This sug-gests the biological component of suspended mat-ter in the bay is in all likelihood progressivelyincreasing, although seasonal and interannualvariability is great. Biological processes play animportant role in the production, transport, andfate of particulate sediment within and down-stream of the Estuarine Turbidity Maximum (ETM)zone (discussed in Chapter 6) of the bay and itslarge tributaries (Kemp and Boynton, 1984; Fisherand others, 1988) together with tidal resuspensionand other processes (Sanford and others, 1991).Organic-inorganic coupling greatly affects particlesettling time that, together with physical processes,will determine whether material is deposited in theETM zone, advected laterally, or transporteddownstream of the turbidity maximum zone. Ulti-mately, these processes affect water quality in largeparts of the northern bay and under certain condi-tions the mid-bay. Moreover, biotic processes pro-duce organic carbon, which modulated by regionalphysical processes (mainly river discharge, sedi-ment grain size), influences the amount of carbonburial in bay sediments (Hobbs, 1983).

Although SAV beds themselves are not adirect source of sediment, they can influence sedi-mentary processes in coastal ecosystems. Amongtheir potential effects in Chesapeake Bay and itstributaries, SAV can slow water velocity, increaseparticulate settling rates, improve water clarity,control sediment dynamics, and effect nutrientcycling and water chemistry. Thus, SAV has beenreferred to as “biological engineers.” Because plantbiomass varies seasonally, it is likely there is a tem-poral pattern to SAV-influenced processes and themagnitude of the effect of these processes shouldvary with SAV abundance and distribution. Little

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research has been carried out on these influences inthe Chesapeake Bay. Further research that leads toan understanding of these estuarine and hydro-geomorphological processes would be beneficial.

Ground water of the Pocomoke River Basinis rich in reduced iron, particularly in theNassawango subbasin where bog iron depositsalong the flood plain of Nassawango Creek werestripped in the mid-1800s to supply an iron smelternear the town of Snow Hill. The rate of bog-ironformation was so rapid that areas could be re-stripped in a matter of a few years. Bog iron is stillforming in this area, and in other parts of thePocomoke Basin. Ferrous iron concentrations inexcess of 20 ppm has been measured in groundwater. When this water emerges at the surface or isdischarged into the river system, it rapidly oxi-dizes to an amorphous particulate iron oxyhydro-xide that in time crystallizes to form the mineralgoethite.

The iron in this system is important for atleast two reasons. First, iron strongly sorbs phos-phorus and many trace metals. Early reports on thecomposition of the Nassawango bog ore indicatethat it commonly contained up to 10 percent phos-phorus, which made the pig iron smelted from thisore brittle upon cooling (Singewald, 1911). Second,the iron precipitating in the rivers increases turbid-ity, which reduces light penetration to rootedaquatic vegetation and effects other organisms bycoating gills and interfering with oxygen transfer.The first effect will play a role in the behavior andcycling of phosphorus in the system; the secondeffect will impact biota in the system.

In the fall of two very dry years (1999 and2001), a USGS study found the rivers in the centralpart of the Pocomoke River Basin appeared quiteturbid (visual examination) although there hadbeen no storms to wash sediment-laden runoff intothe rivers. Samples of the particulate matter creat-ing the turbidity were iron-rich and displayed aweak x-ray diffraction pattern of goethite. Someorganic material, probably algae, also seemed tocontribute to the turbidity, but this has not yet beeninvestigated. Whatever the mix of materials thatcaused the turbidity, they were generated inter-nally in the rivers and were not contributed byrunoff.

If all the sediment erosion and runoff wereeliminated in the Pocomoke River Basin, it wouldhave no effect on the turbidity generated by theiron oxyhydroxide mechanism. Any practice rec-

ommended to reduce suspended sediment in thesewaters must take internally generated “sediment”into consideration. Best-management practices forsediment control in the watershed will probablyhave little effect on the turbidity generated byinternal processes.

In summary, in-situ biological processes arefueled by external nutrient influx, modulated byclimate and river discharge variability, and influ-enced by estuarine circulation, tides, and wind.Well-documented temporal trends of the past cen-tury in organic production, phytoplankton ecol-ogy, riverine nutrient and sediment influx,although not usually considered in analyses of baysediment, suggest that biological components ofChesapeake Bay sediment are even more impor-tant than they were 40-50 years ago. Althoughquantitative estimates of the relative contributionof in-situ biogenic material in various regions ofthe bay cannot be made with certainty on the basisof current data, it is likely that efforts to reducenutrient influx would improve water clarity byreducing biogenic sediment.

Direct Atmospheric Input

Direct atmospheric input of particulate mat-ter to the Chesapeake Bay and tributary surfacewaters is not anticipated to be a major contributorto the total sediment load to the system. The mag-nitude of sediment particulate input has been esti-mated from conversations with and informationprovided by Dr. Joel Baker (University of Mary-land Center for Environmental Science, oral com-mun., 2003). This work stemmed from preliminarysediment-budget calculations performed for Balti-more Harbor and the estuarine part of the PatapscoRiver, as part of a cooperative program examiningcontaminants in the harbor bottom sediments(Baker and others, 1997).

Particulate matter in the atmosphere that isdeposited on the surface of the bay and its tributar-ies can be separated into two components: wetfalland dryfall. Suspended particulates in rainwater(wetfall) is estimated to be 1 mg L-1 or 1 g m-3.With rainfall averaging about 1 m yr-1, the directinput of particulates to surface waters is1 g m-3 yr -1. Dryfall is estimated in the followingmanner. The concentration of particulate matter inthe atmosphere is estimated to be 10 µg m-3.Assuming an aerodynamic mass median diameterof 1 µm, these particles settle at a rate of approxi-mately 0.1 cm sec-1. Multiplying the concentration

C H A P T E R 5 | 5 9

by the settling velocity by the number of secondsper year yields a value for particulate delivery tosurface waters of 0.31 g m-3 yr-1. Because a part ofthe atmospheric particulates are soluble, withammonium sulfate being the dominant aerosolspecies, the dry deposition flux that contributes tosuspended load of surface waters is approximately0.15 g m-3 yr-1. The total particulate load deliveredby the atmosphere is estimated to be1.15 g m-3 yr-1.

A data set from an urban location may not bedirectly applicable to areas of the bay that are bor-dered by forests, suburban housing, or farmedfields. However, the concentrations utilized proba-bly represent a reasonable median between, forexample, forests that deliver little to the atmos-pheric load and recently tilled fields that on windydays may provide a substantial amount of dryfallto adjacent water bodies. The total atmosphericload is assumed to provide a reasonable approxi-mation of direct sediment input to the bay and itstributaries.

The total surface area of the main stem of theChesapeake Bay is approximately 6.5 x 109 m, andthe total surface area of the bay plus the tidal tribu-taries is 11.5 x 109 m (Cronin, 1971). Thus, the totalflux of atmospheric particulates to the main stembay is 7.5 x 109 g yr-1, and to the bay and the tidaltributaries is 1.3 x 1010 g yr-1 (8.0 x 103 and 1.4 x 104

metric tons yr-1, respectively). A simple method tocharacterize the magnitude of these terms is tocompare them to the suspended sediment loadsupplied by the Susquehanna River, which hasbeen well characterized by the USGS as part of theChesapeake Bay RIM Program. The mean annualsediment load supplied by the Susquehanna Riverbetween 1979 and 2001 was 1.2 x 1012 g yr-1

(Michael Senus, U.S. Geological Survey, oral com-mun., 2003). Thus, the estimated total wetfall anddryfall associated atmospheric particulate loadrepresents 0.5 percent of the suspended sedimentload supplied by the Susquehanna River.

6 0 | C H A P T E R 5

CHAPTER 6. ESTUARINE SEDIMENTTRANSPORT, DEPOSITION,

AND SEDIMENTATION

by Thomas Cronin,1 Lawrence Sanford,2

Michael Langland,1 Debra Willard,1

and Casey Saenger1

This chapter discusses the transport, deposi-tion, and sedimentation (accumulation) of sedi-ment in the estuary. Climatic variability andextreme meteorological events exert a strong influ-ence on sediment movement in the estuary overdifferent time scales by affecting river discharge,estuarine circulation, and salinity. Sediment depo-sition is inherently related to sediment erosion,transport, and re-suspension and involves com-plex processes operating over different time scales.Although the Chesapeake Bay is one of the moststudied estuaries in the world, there are manyunknown aspects to sedimentation processes in thebay. The discussion and papers cited provide avaluable source of information about the processescontrolling sedimentation. These processes are sointerwoven that the discussion of one cannot beseparated easily from the others. Therefore, somediscussion involves multiple processes and is over-lapped intentionally.

Sediment Transport Pathways

The major pathways of sediment transportin the northern, central, and southern bay and themajor tributaries are shown in figure 6.1, which ismodified from Hobbs and others (1990). The path-ways have been determined from a wide variety ofstudies of estuarine circulation and stratigraphy,and are based on sedimentation rates from sedi-ment cores, modern and historic sediment compo-sition and distribution of sediment, totalsuspended solids concentration data, and othersources. This figure also shows information on sed-iment sources to the bay (Chapter 5). It features thefollowing major characteristics of sedimentsources, transport, and deposition:

• a large input of sediment from the oceannear the mouth of the bay,

• a net southward flow of sediment downthe axis of the northern and central mainchannel (mostly derived from the Susque-hanna River),

• an influx of sediment from eroding head-lands along the margins of the bay,

• sediment transport from the main stembay into major tributaries (except duringextreme high flow events when sedimentcan be exported into the bay).

Most researchers agree on these broad pat-terns that reflect the major physical processes (cli-mate, currents, tides, and density-driven estuarinecirculation) and topographic and geomorphiccharacteristics of the bay region, which govern sed-imentation in the estuary (see reviews by Hobbsand others, 1990; Halka, 2000).

Although figure 6.1 indicates major trans-port pathways, it should be emphasized that finer-scale patterns of sediment transport and deposi-tion can vary greatly depending on the region,location, and time scale of the study. Reasons forthis variability and the effects of this variabilitywill be discussed in the remaining sections of thischapter. Also note that on figure 6.1, there is nocorrelation between the size of the arrows and theamount of sediment transported.

Estuarine Turbidity Maxima Zone

The northern main stem bay and larger tidaltributaries have an Estuarine Turbidity MaximaZone (ETM zone)—a region where fine-grainedparticulate material is “trapped,” deposited, andsometimes resuspended and redeposited (fig. 6.2).The ETM zone is a result of complex physical pro-cesses (freshwater inflow, tidal and wave-drivencurrents, gravitational circulation), particle floccu-lation, and biogeochemical processes (Sanford andothers, 2001). Within the ETM zone, high rates offine particle introduction from the watershedtogether with the physics of estuarine circulationmaintain an area of high concentrations of sus-pended sediment and reduced light availability.

In the Chesapeake Bay, the main ETM zoneis north of Baltimore and was the focus of a num-ber of studies in the late 1960s and early 1970s thatdefined the ETM zone (Schubel, 1968a, 1968b;Schubel and Biggs, 1969; Schubel and Kana, 1972).From these studies, it was determined the mainETM zone of the Chesapeake Bay is associatedwith the input of sediment and freshwater from

1 U.S. Geological Survey.2 University of Maryland, Center for Environ-

mental Science.

C H A P T E R 6 | 6 1

Figure 6.1. Major pathways ofsediment transport in Chesa-peake Bay (from Hobbs andothers, 1990). (Note, thethickness of arrows does notequate to amount of masstransported.)

6 2 | C H A P T E R 6

the Susquehanna River. Recent study of the upperChesapeake Bay ETM zone led Sanford and others(2001) to conclude that asymmetrical tidal resus-pension and transport are primarily responsiblefor the maintenance of the ETM zone at the limit ofsaltwater intrusion. This is in contrast to the earlierChesapeake studies that ascribed the formation ofthe ETM zone primarily to gravitational circulationpatterns but is quite similar to recent studies ofETM zones in other locations. However, all thestudies have confirmed the importance of resus-pension processes to the maintenance of high con-centrations of suspended sediment and associated

light attenuation. Without the effects of tidal resus-pension, the rapidly settling aggregates of fine par-ticles would remain on the bottom.

Each of the tributary systems have an associ-ated ETM zone near the upstream limit of saltwa-ter intrusion. Examples have been noted in theRappahannock (Nichols, 1977), the Potomac(Knebel and others, 1981), and the York (Lin andKuo, 2001) Rivers. Analysis of Chesapeake Baywater-quality monitoring data sets for the Patux-ent and Choptank Rivers indicate the appearanceof similar turbidity maxima zones in these smaller

Figure 6.2. General location of turbidity maxima (dark areas) for the major tributaries and thebay (Marsha Olsen, U.S. Environmental Protection Agency, written commun., 2002).

C H A P T E R 6 | 6 3

sub-estuaries (Larry Sanford, unpub. data, 2003).Some tributaries have more than one ETM zone(Lin and Kuo, 2001), probably because of multipleconvergent transport zones. An additional com-plexity, which is not well understood, is the poten-tial for seasonally varying sediment trapping in theETM zone (Sanford and others, 2001).

Extreme events like flooding and droughtinvolving the mobilization and transport of sedi-ment in the watershed influence the movement ofthe ETM zone up and down in the tidal estuariesand the main stem of the bay. The ETM zone typi-cally is near the freshwater/saltwater interfacewhere a large percentage of riverine sediment istrapped. Major storms and increased river dis-charge deliver higher sediment loads to the estuaryand can shift the location of the ETM zone severaltens of kilometers downstream, allowing sedimentto “escape” the usual ETM zone. However, themajority of fine-grained river-borne sediment istrapped in ETM zones and only escapes the upperreaches of the northern Bay and upper parts oftributaries during extreme hydrologic events.These limited results must be regarded as tenta-tive, however, because most studies have focusedon relatively short time scales (days to weeks, witha few interannual comparisons).

Influence of Climate

Climatic variability and extreme meteorolo-gical events strongly influence sediment and nutri-ent transport to the Chesapeake Bay and its tribu-taries. During periods of high precipitation andabove-mean river discharge, climate-driven pro-cesses can lead to scouring of sediments frombehind dams, increases in suspended solids in thewater column, shifts in the position of ETM zones,export of sediment from ETM zones, increasedmean turbidity, and other changes in sedimentaryprocesses. Conversely, during periods of droughtsuch as that seen during late 2001-early 2002, rela-tively low freshwater flows can contribute to highwater clarity and relatively robust growth in SAVbeds. This section briefly describes climate pro-cesses operating over long term (millennial), inter-mediate (centennial, decadal, interannual), andshort-term (extreme events, seasonal) time scalesderived from a selective review of the literature.A more detailed review of the general subject ofclimate-driven changes in freshwater flow to estu-aries and its physical, chemical, and biologicaleffects was provided by Albers and others (writtencommun., 2003).

Long-Term Processes

Geophysical surveys and stratigraphic andradiocarbon studies of long sediment cores havedocumented the long-term history of ChesapeakeBay. These studies show that the bay formed dur-ing the early Holocene period, about 8,000 yearsbefore present (yr BP), when a rising sea levelflooded the ancient Susquehanna River Valley(Colman and others, 1988; Cronin and others,2000). Relative sea level has continued to risebecause of subsidence of the mid-Atlantic regiondue to post-glacial isostatic adjustment (Ellisonand Nichols, 1976; Peltier, 1996; Kearney, 1996; Col-man and others, 2001; Bratton and others, 2002,Cronin and others, 2003).

Since its inception, the bay has been the siteof sediment accumulation, which reaches a totalthickness of 25-30 m in some parts of the mainchannel of the bay. On the flanks of the bay,Holocene sedimentary sequences consist mainly ofsandy sediments and are relatively thin because ofthe winnowing action of waves, currents, and tidesthat tend to transport fine-grained sediment fromthe flanks to the deeper channel.

Long sediment cores have penetratedthrough the entire Holocene sequence into fluvialsediments. Coupled with geophysical surveys,these records indicate that complex long-term sedi-mentation patterns characterize parts of the bay.For example, some regions have experienced sev-eral thousand years of relatively continuous depo-sition, followed by no net sediment deposition forseveral thousand years. These stratigraphic uncon-formities represent either periods of non-deposi-tion or deposition followed by substantial periodsof submarine erosion. It is difficult to explain thisintermittent Holocene sedimentation over millen-nial time scales. Most likely, climatological factorsmay influence erosion and sediment transportfrom rivers and geologic processes, such as litho-logic composition of pre-Holocene sediments, pre-Holocene topography inherited from late-Pleis-tocene low sea level, and submarine gravity slidingand tidal currents, govern long-term sedimenta-tion patterns and rates. Unconformities have beenrecognized in sediments deposited in various areasduring the early (8,000-10,000 yr BP), middle(~3,000-6,000 yr BP), and late Holocene (3,000 yrBP to present). Even in cores containing a fairlycontinuous record of deep channel sedimentationover several millennia, significant lithologicchanges in the sediment composition are common.

6 4 | C H A P T E R 6

These most likely signify changes in sedimentsource, currents, tides, and circulation, or other fac-tors.

Rising sea level during the past 8,000 years(see also Sediment Rates section below) also hashad an effect on sediment sources and transport toChesapeake Bay because of its influence on cliffand coastal marsh erosion. In general, rising sealevel leads to submergence of land areas and hasthe potential to increase the proportion of sedi-ment derived from coastal erosion relative to sedi-ment from riverine, oceanic, and biogenic sources.Over the past century, the rate of relative sea-levelrise in the Chesapeake Bay region has been abouttwice the global rate because of regional land sub-mergence caused by post-glacial isostatic adjust-ment. Estimates of future rates of global sea-levelrise from natural and anthropogenic causes (suchas global warming) are subject to large uncertainty.However, despite this uncertainty, the bay area willcontinue to experience submergence and large-scale coastal erosion over the next century becauseof continuing regional subsidence. This meansthat, without management of coastal zones, a large,perhaps increasing, contribution of future sedi-ment will come from shoreline sources.

Centennial, Decadal, and Interannual Time Scales

Climate processes operating over centennial,decadal, and interannual time scales are the subjectof intensive research programs aimed at distin-guishing natural climate variability and its causesfrom human-induced 20th century climate change(“global warming”). The causes of regional climatevariability are still poorly understood. Severalstudies of the sedimentary record in the bay havedemonstrated that the mid-Atlantic region in gen-eral, and Chesapeake Bay in particular, have expe-rienced quasi-cyclic oscillations in precipitationand temperature over the past few millennia.

Stratigraphic and paleoecological evidencefrom changes in salinity-sensitive foraminiferaindicate salinity oscillations of 10-15 ppt in themesohaline region at 11-m water depth during thepast 500 years (Cronin and others, 2000). Fourteen“wet-dry” cycles, including 16th and 17th century“mega-droughts” could be recognized thatexceeded 20th century droughts in their severity.Wet periods occurred nearly every 60-70 years,lasting less than 20 years; the mean annual rainfallwas 25-30 percent above normal, and freshwaterdischarge was about 50 percent greater than dur-

ing droughts. Climatological processes (increasedprecipitation and river discharge) also are partiallyresponsible for broad ecosystem degradation andgreater hypoxia since the 1960s. This also mayreflect the combined effects of lower dissolved oxy-gen (DO) and decreased water clarity (Karlsen andothers, 2000). Although the direct effect of thesewell-documented climatological processes on sedi-mentation are uncertain, the paleoecological pat-terns suggest a multi-decadal pattern of improvedwater clarity during relatively dry intervals, anddecreased clarity during extended wet periods.These climate-driven estuarine changes are super-imposed on long-term ecosystem response to landclearance and nutrient influx (Brush, 1984; Cooperand Brush, 1991; Willard and others, 2003).

Decadal to centennial oscillations in Chesa-peake Bay temperatures have been discovered onthe basis of the trace element chemistry in fossilostracode shells from well-dated sediment cores(Cronin and others, 2003). These studies indicatedchanges in mean spring-summer water tempera-tures of several degrees Celsius during the Medi-eval Warm Period (800-1400 AD) and Little Ice Age(1400-1900 AD) and that 20th-century temperatureextremes exceeded those of the past 2,200 years.Although not directly related to sedimentary pro-cesses, these results suggest the mid-Atlanticregion is sensitive to natural and anthropogenicclimate variability over time scales pertinent tolong-term water-quality management.

Interannual and decadal quasi-cyclic oscilla-tions of sedimentation are recorded in physical,chemical, and biological records in many sedimentcores. For example, physical and chemical sedi-mentary changes are evident in oscillations of sedi-ment color measured using Kodak Grey Scaleimagery (KGS). Alternating light and dark sedi-mentary layers reflect changes in sedimentgeochemistry and grain size related to hypoxia andmethane production (Hill and others, 1992). Thesechanges have been identified in a number of longsediment cores in the main stem of the bay(T. Cronin, U.S. Geological Survey, written com-mun., 2003). At a site off the mouth of the LittleChoptank River, a nearly completed integratedstudy conducted by the USGS and Maryland Geo-logical Survey has documented episodic (cycles of~ 2-4 and 10-15 years) changes in sediment sourceusing KGS, cesium-137, and micropaleontology(T. Cronin and J. Hill, oral commun, 2003). Thedata imply that over the past century, year-to-yearchanges in sediment source are a characteristic of

C H A P T E R 6 | 6 5

at least this region of the bay. During wet years,greater proportions of land surface and coastalmarsh sediments are deposited; during relativelydry years, sediment from riverine input and (or)re-suspension dominates.

There is a large body of literature using thebay’s paleoecological and geochemical recordrecovered from sediment cores to documentchanges in the bay ecosystem caused by post-colo-nization land clearance, and more recently, fertil-izer application and elevated nutrient influx (seeCooper and Brush, 1991; Karlsen and others, 2000,Colman and Bratton, 2003; Cronin and Vann, 2003).Most of these studies have focused on issuesrelated to DO levels in the bay. Although the effectof land-use activities on DO is severe and well doc-umented, the effect of climate variability is also ofgreat significance for sediment flux and tempera-ture variability. This is because even the mostambitious restoration efforts will not return thewatershed to pre-colonization conditions. Addi-tional investigations of the influence of climatevariability on sediment flux into and within thebay would lead to further understanding of thisissue.

Short-Term Extreme Events

Extreme meteorological events such asstorms and hurricanes can lead to flooding, peri-ods of extremely high TSS loading, and massivesediment transport. Flooding mobilizes, trans-ports, and delivers large amounts of sediments in arelatively short time, potentially resulting in bothimmediate and lasting environmental damage.Perhaps the most well-studied extreme event wasHurricane Agnes in June 1972 that has been cred-ited with initiating a decline in the SAV popula-tions of the bay that, according to someresearchers, have yet to recover in certain regions(Davis and Laird, 1976; Orth and Moore, 1983). TheAgnes-induced floods of June 1972 in the Susque-hanna River Basin had nearly the same peak dis-charge as floods caused by events in January1996—about 1 million ft3 s-1(410 million gal min-1)(Langland and Hainly, 1997). The total streamflowdischarge was approximately double for the June1972 flood compared to the January 1996 flood; thesediment load during the 1972 flood was triple the1996 sediment load (30 million tons compared to10 million tons, respectively). More sediment wasavailable and transported to the upper bay becauseof increased land disturbance in summer (1972)compared to winter (1996).

Monitoring records of suspended sedimentare useful indicators of extreme events in relationto more typical “average hydrological” conditions.From 1979 to 2001, the average annual sedimentload delivered to the Chesapeake Bay from themajor sub-basins at the Fall Line was estimated tobe approximately 4 million tons. (During theperiod 1979 to 1984, the loads represented the com-bined input from the three largest rivers (Susque-hanna, Potomac, and James). During this timeperiod, the average annual sediment load trans-ported was more than doubled (10 million tons) onfour occasions, 1979, 1984, 1985, and 1996, totalingapproximately 60 millions tons of sediment. In1972, an estimated 30 million tons of sediment wasdelivered to the upper bay. These extreme eventsall were related directly to tropical storms thatdelivered unusually large amounts of rainfall overlarge land areas. An exception was 1996, whenheavy rains, deep snow pack, and above normalwinter temperatures combined to create anunusual sequence of events resulting in high flowsand scouring of reservoir sediments in the lowerSusquehanna River. The scoured sediments werethe dominant source of the transported sediment(Langland and Hainly, 1997). Conversely, theannual sediment load was nearly half (2.5 milliontons) the long-term average on eightoccasions: 1981, 1991-92, 1995, 1997, 1999-2001.The majority of the watershed had a precipitationdeficit since 1999. The average annual sedimentload for 1999-2001 was less than 1 million tons,one-fifth of the annual long-term sediment load.

The greatest effects of extreme events likeflooding and drought involve the mobilization andtransport of sediment in the watershed and themovement of the ETM zone up and down in thetidal estuaries and in the main stem of the bay(fig. 6.2). The ETM zone typically is near the fresh-water/saltwater interface where a large percentageof riverine sediment is trapped. Major storms andincreased river discharge deliver higher sedimentloads to the estuary and can shift the location ofthe ETM zone several tens of kilometers down-stream, allowing sediment to “escape” the ETMzone. Some tributaries have more than one ETMzone. The most upstream ETM zone probablyreflects the riverine source; the downstream ETMzone would be dominated by shoreline erosion,resuspension, and fine-sediment input from thebay. An additional complexity, however, is a sea-sonality to storm-induced sedimentary trapping inthe ETM zone (Sanford and others, 2001).

6 6 | C H A P T E R 6

In summary, episodic events, interannualand decadal variability, and long-term changes inmean climatic conditions are all characteristics ofnatural processes affecting the bay system. In lightof evidence that humans have altered the globalclimate system, producing anomalous climatebehavior in many regions, it is important to obtaina better understanding of the regional climatebehavior and its effect on the bay. Although statis-tics on trends in streamflow and precipitation canindicate the probability of an extreme, short-termevent taking place during a given time span, theactual prediction of a time and place is very impre-cise at best, if not impossible. However, improvedpredictability of interannual climate variabilityassociated with El Nino-Southern Oscillation nowis applied routinely when trying to solve agricul-tural and environmental problems. Recentadvances have been made in regional climatemodels for the mid-Atlantic (Crane and Hewiston,1998; Jenkins and Barron, 1996; Najjar, 1999), andthese have been applied to predictions of futuresalinity change in the bay (Gibson and Najjar,2000). Similar efforts are needed to understandfuture changes in sediment flux that will resultfrom more frequent extreme climate events andgreater interannual variability. Coupled with betterunderstanding of the patterns and causes of pastclimate and sediment variability, predicting futurechanges in regional mid-Atlantic climate usingregional climate models could be an importantcomponent of future research, planning, and man-agement.

Deposition and Sedimentation Rates

Most researchers define sedimentation rateas the linear accumulation of sediment in centime-ters per year and convert this rate into volumetricestimates of sediment flux, or mass accumulationrate (MAR), usually given in grams per squaremeter per year or grams per cubic centimeter peryear. These conventions are followed here and intables 6.1 and 6.2, and form the basis of discussionfor the remainder of this chapter. Table 6.1 pro-vides an abstracted summary of each publication’sregion of investigation, methodology, and esti-mated sedimentation rates. Table 6.2 is a selectivesummary of published studies covering manyregions in the bay area. Because of widely varyingfield and analytical methods, statistical techniques,and the temporal and spatial variability in sedi-mentation, it is recommended strongly that thereader examine the original papers of interest fordetails.

A variety of approaches have been used tostudy patterns and processes of bay sedimentation(table 6.1). These include the following methods:geophysical surveys (determining rates of the past7,500 years of estuarine sedimentation), bathymet-ric surveys (comparing 19th and 20th centurybathymetry), short-lived radioisotopes (137Cs,210Pb), other chemical markers (useful for the pastcentury), and pollen stratigraphy (mainly Ambro-sia, ragweed, pollen) correlated with land-usechanges (documenting post-colonization land-clearance rates). Monitoring methods not includedin tables 6.1 or 6.2 include instrumental measure-ment of total suspended solids in water samplesand satellite imagery and remote sensing.

Radiocarbon (14C) dating has been used toestimate sedimentation rates in Chesapeake Bay.However, measured radiocarbon ages are a func-tion of complex processes including cosmogenicradiocarbon production in the atmosphere, carboncycling in the hydrosphere and biosphere, and thepotential uptake of reservoir (“old”) carbon intodifferent types of carbonaceous material (such aswood, peat, total organic carbon, shells). Radiocar-bon ages that have not been calibrated to the globalcarbon cycle can provide inaccurate ages and, inthe case of many ages published for ChesapeakeBay, can lead to erroneous estimates of sedimenta-tion rates.

Three broad, well-supported conclusions canbe drawn from the data in tables 6.1 and 6.2. First,sedimentation rates are relatively high—on theorder of 0.1 to 1 cm yr-1—compared to those char-acteristic of other aquatic environments such asmost lakes, deep sea habitats, bays, and estuaries.Because the bay was flooded by sea-level riseabout 8,000-7,500 years ago, as much as 25-30 m ofsediment have accumulated in the main channel,and thick accumulations of sediment characterizethe marshes of the bay. High sedimentation rates inthe channel reflect in part the sediment-trappingcapability of partially mixed estuaries. This is theresult of several factors, some of which are dis-cussed below.

Second, sedimentation rates vary widelydepending on the region. For example, sedimenta-tion rates can easily vary five- to ten-fold (0.1 togreater than 1.0 cm yr-1) over small and large spa-tial scales. Spatial variability is evident especiallythroughout the main stem of the mid-bay andlocally in small sub-estuaries such as the RhodeRiver and tributaries entering the Potomac.

C H A P T E R 6 | 6 7

68

|C

HA

PT

ER

6

m; Cu, copper; Cs, cesium; TSS, total

Comments

ntinuity in Sedimentation

dic sedimentation noted.

onoghue (1990) for variations in Rhode River summary of other studies.

rates near mouth, highest upper est. andutaries, pre-colonial rates given.

ke down upper-mid-lower est. All post-landrance European rates. Rates in paper vary by, period, region, core.

post-European rates, vary location, tributary.

pre-colonial dates given.

entation rates vary temporally and spatially.rox. range given.

entation rate varies.

Table 6.1. Summary of sedimentation rates in Chesapeake Bay and tributaries from selected published studies

[m, meter; cm, centimeter; g, gram; yr, year; NA, not applicable; <, less than; >, greater than; Pb, lead; C, carbon; Pu, plutoniususpended solids; --, no data]

Core Reference LocationWaterdepth(m)

Mean annuallinearsediment

rate(cm / yr)1

Mean annualsediment flux(g / cm-2/yr)1

Timeinterval

Method

PC-6 Adelson and others (2001); Helz andothers (2000)

Main stem ofCalvert Cliffs

20 3.1-4.20.11

1.2-1.360.07

Post-colonialPre-colonial

14C (210Pb, Pu,Cu peak)

Disco

Gmss 16,Gmssc

Adelson and others (2001) Main stem offPatuxent

NA 2.84 1.07 Post-colonial 210Pb

Cornwell and others (1996) Mid-baymain stem

17.515

----

2.41.8

Post-colonial 210Pb

Roberts and Pierce (1976) Patuxent NA 3.70 5.5-7.45 1970s TSS

Hobbs and others (1992); Kerhin andothers (1988)

Baywide Various 0.64 NA 1840s-1950s Bathymetic survey

Officer and others (1984)(Summarizes Hirschberg andSchubel, 1979; Helz and others,2000; and Goldberg and others,1978)

North bayMid baySouth bay

4-328-334-13

------

0.3-1.20.1-0.30.1-0.8

20th century20th century20th century

210Pb, 137Cs,239/240Pu

Episo

Donoghue (1990) Rhode River NA 0.07-0.470.5-1.5

NA Pre-colonialPost-colonial

14C, 210Pb, 137Cs See Dand

Defries (1986); Brush and others(1982); Knebel and others (1981)

Potomac EastPotomac East

0.3-12.8 >0.1->0.470.21->1.14

2>0.05-0.190.09 ->1.0

Post-colonialPost-colonial

Pollen, 210Pb, 137Cs Lowertrib

Brush (1984, 1989) Furnace bayMagothyNanticoke

~1----

0.17-0.390.04-0.23

0.2

0.15-0.20 Post-colonialPost-colonialPost-colonial

PollenPollenPollen

X brocleatime

Brush (1984) Patuxent ~5 0.51 Post-colonial

Brush (1984) 10 Tributaries Various 20.17-0.35 0.14-0.18 Post-colonial Mean

Cooper and Brush (1991, 1993) Mid bay 9-15.1 0.17-0.31 Post-colonial 210Pb, 137Cs,14C, Pollen

Some

Khan and Brush (1994) Jue bay High marshintertidal

0.15-0.89 ~0.09-0.52 Post-colonial Pollen

Khan and Brush (1994) Patuxent Low marsh 0.18-0.7 0.09-0.56 Post-colonial Pollen

Brush and Davis (1984) Ware River 2.5-7.0 0.36-0.39 Post-colonial

Brush and Hilgartner (2000) 36 cores,8 tributaries

<1-2.2 0.02-3.0 Post-colonial Pollen, 14C SedimApp

Pasternak and Brush (2001) Bush River Various NA 1 Recent Monitoring

Halka (2000) Entire bay Various 3<0.1-0.5 ~0.04-0.2 Holocene Geophysical survey

Zimmerman and Canuel (2000) Mid-bay 15 1.0-1.67 0.477 Post-colonial 210Pb, 137Cs,Pollen

Sedim

Cutshall and others (1981) James River NA 1.0-19.0 NA 20th century 137Cs

Donoghue and others (1989) North bay NA 0.35 0.27 Post-colonial 137Cs

NOTE: See individual papers for methods, error bars. Values were rounded when multiple papers gave slightly different values.

1 When range is not given, values are means.2 1600-1700s.3 Mean ~0.15.

CH

AP

TE

R6

|6

9

Ta

[m

Meanannuallin. sed

ratepre-RW(cm/yr)

Post-RWsed. flux

(g/cm2 yr-1)

Pre-RWsed. flux

(g/cm2 yr-1)Comments/sources

IM 0.4293 0.3683 0.2318 Unconformity at300 cm / Croninand others (2000)

IM .0386 .3938 .0209 Cronin and others(2000)

IM .3284 .6262 .1773 NAO paper agemodel /

Cronin and others,(2000)

P -- .1941 -- Gravity core depth1.2 1

M -- .3145 -- Cs peak fromA. Zimmerman

-- .0850 -- From G. Smith, Md.DNR

M -- .3400 -- From A. Mannino,USGS

M -- .4108 -- From A. Mannino,USGS

P .1477 .0255 .0798 New data

L -- .7678 -- New data

P .0864 .0878 .0466 New data

P .0875 .0255 .0473 New data

P .0420 .1728 .0227 New data

P -- .6800 -- RW from short core /new data

P -- .5667 -- RW peak est fromforams / new data

P -- .1063 -- New data

P .5172 .6800 .2793 RW from short core /new data

ble 6.2. Summary of chronological and sedimentary rate data for Chesapeake Bay

, meters; cm, centimeters; g/cm2, grams per square centimeters; RW, Ragweed; --, no data; Cs, cesium; C, carbon]

Region CoreWaterdepth(m)

Latitude(degrees)

Longitude(degrees)

Datecored

Coringmethod

Corelength(cm)

Depthto

137Cspeak(cm)1

Depthto

rag-weedpeak(cm)2

Depthrange

ofrag-

weedpeak(cm)

Depthto

14C950yr

date(cm)

Age14Cdate

(yr BP)3

Meanannual

lin.sed. rate

post-RW

(cm/yr)

AGESCruise

MD99-2205 16.0 38 33.95N 76 26.66W June 21, 1999 Calypso 673 -- 130 -- 300 466 1.0833

AGESCruise

MD99-2207 25.0 38 01.83N 76 12.88W June 21 ,1999 Calypso 2,070 -- 139 20 173 950 1.1583

AGESCruise

MD99-2209 26.0 38 53.18N 76 23.68W June 22, 1999 Calypso 1,720 94 221 20 510 950 1.8417

atuxentTransect

PTXT-2-G 11.5 38 19.58N 76 23.54W Sept. 18, 1996 Gravity 116 36 68.5 8.5 -- -- .5708

ain stem AZM3 15.0 38 34.05N 76 26.76W March 1, 1996 Kasten 242 51 111 20 -- -- .9250

BachelorsPointCore 695

38 40.03N 76 10.648W Piston 300 -- 30 10 -- -- .2500

id-Bay CB MB-2(Parker)

25.1 38 33.079N 76 26.0297W June 6, 2001 Piston 494 -- 120 40 -- -- 1.0000

id-Bay CB-2207 26.1 38 12.822N 76 12.876W June 6, 2001 Piston 476 -- 145 15 -- -- 1.2083

otomac River DEFRIES-5-1 5.8 38 16.735N 76 49.722W Sept. 19-20,2001

Piston 210 -- 9 -- 139 950 .0750

ittleChoptank

LCHPT-1-P-4&5

14.9 38 31.493N 76 18.212W Piston 400 150 271 30 -- -- 2.2583

otomac River NKL-12-1 6.9 38 10.101N 76 43.168W Sept. 19-20,2001

Piston 200 -- 31 -- 107 950 .2583

otomac River NKL-16-1 7.4 38 06.334N 76 34.193W Sept. 19-20,2001

Piston 154 -- 9 -- 86 950 .0750

otomac River NOMBAY-1 5.2 38 08.898N 76 43.173W Sept. 19-20,2001

Piston 200 -- 61 -- 98 950 .5083

ocomokeSound

PC2B-1&2 7.9 37 53.433N 75 48.409W Sept. 18, 2001 Piston 480 -- 240 20 -- -- 2.0000

ocomokeSound

PC3B 11.4 37 50.741N 75 48.745W May 15, 2001 Piston 177 -- 200 20 -- -- 1.6667

ocomokeSound

PC4B 27.3 37 48.300N 75 50.301W May 15, 2001 Piston 122 -- 37.5 7.5 -- -- .3125

ocomokeSound

PC6B-1&2 15.3 37 44.913N 75˚ 52.333W Sept 18, 2001 Piston 476 -- 240 20 450 476 2.0000

70

|C

HA

PT

ER

6

P 0.1080 0.1303 0.0583 New age model /Cronin and others(2000)

P .1761 .0142 .0951 New age model /Cronin and others(2000)

P .2284 .2182 .1233 Cronin and others(2000)

P .5492 .2862 .2965 Cronin and others(2000)

P .0545 .0312 .0295 Uppermost E Hol14C date used /new data

P .0648 .0283 .0350 RW is <20cm depth /new data

P .0261 .3655 .0141 New data

Ta

[m

Meanannuallin. sed

ratepre-RW(cm/yr)

Post-RWsed. flux

(g/cm2 yr-1)

Pre-RWsed. flux

(g/cm2 yr-1)Comments/sources

arker Creek/Choptank

PRCK-1-P-1 10.7 38 32.8657'N 76 28.7112W Sept. 23, 1996 Piston 315 -- 46 6 141 950 0.3833

arker Creek/Choptank

PRCK-3-P 24.3 38 32.6359'N 76 25.6199W Oct. 9, 1996 Piston 452 -- 5 0 160 950 .0417

otomacTransect

PTMC-3 23.1 38 01.6118'N 76 13.1938W Sept. 25, 1996 Piston 452 29 77 6 278 950 .6417

atuxentTransect

PTXT-2-P-3&5 11.5 38 19.584'N 76 23.548'W June 20, 1998 Piston 400 -- 101 10 397 609 .8417

atuxentTransect

PTXT-3-P-2 22.5 38 20.0007'N 76 18.5801W Sept. 20, 1996 Piston 432 -- 11 5 59 950 .0917

atuxentTransect

PTXT-4-P 15.5 38 21.480N 76 20.251W June 1, 1999 Piston 500 -- 10 NA 67 950 .0833

otomac River WICO-1 9.8 38 16.769N 76 49.369W Sept. 19-202001

Piston 210 -- 129 -- 135 300 1.0750

1 Approximately 1963-64.2 1880 +/- 20 years Willard and others, 2003.3 Age in calendar years before present (1950) using CALIB 4.1 program.

ble 6.2. Summary of chronological and sedimentary rate data for Chesapeake Bay—Continued

, meters; cm, centimeters; g/cm2, grams per square centimeters; RW, Ragweed; --, no data; Cs, cesium; C, carbon]

Region CoreWaterdepth(m)

Latitude(degrees)

Longitude(degrees)

Datecored

Coringmethod

Corelength(cm)

Depthto

137Cspeak(cm)1

Depthto

rag-weedpeak(cm)2

Depthrange

ofrag-

weedpeak(cm)

Depthto

14C950yr

date(cm)

Age14Cdate

(yr BP)3

Meanannual

lin.sed. rate

post-RW

(cm/yr)

Third, independent researchers using differ-ent methods have produced generally similarquantitative estimates of sediment rate and flux,regardless of the scope of the study. This fact indi-cates a high degree of confidence that the valuesgiven in tables 6.1 and 6.2 are relatively accurateand support the conclusions of several priorresearchers (for example, Kerhin and others, 1988;Donoghue, 1990; Halka, 2000).

Three major hypotheses about the patternsof sedimentation in the Chesapeake Bay estuaryhave emerged over the past few decades largely onthe basis of historical patterns. These concern(1) sediment trapping in the ETM zone, (2) tribu-tary and bay “import export” of sediment, and(3) legacy sediments. These concepts are so perva-sive in the bay community that it is useful to sum-marize their basic tenets and mention someuncertainties surrounding them.

The northern main stem bay and larger tidaltributaries have an ETM zone—a region wherefine-grained particulate material is “trapped,”deposited, and, sometimes resuspended and rede-posited. It generally is believed that the majority offine-grained river-borne sediment is trapped inETM zones and only escapes the upper reaches ofthe northern bay (or upper parts of tributaries)during extreme hydrological events. Most studiesof the ETM zone have focused on relatively shorttime scales (sub-annual to interannual).

The ETM zone hypotheses explain manyaspects of observed suspended material in the bayand its tributaries; however, it should be noted thatother studies have indicated more sediment maybe “escaping” the ETM zone than previouslybelieved. For example, certain geochemical tracerdata indicate sediment has been transported overlonger time scales than current studies would indi-cate from the northern bay to at least to the mid-bay (Darby, 1990; Helz and others, 2000). In themain stem bay, Schubel and Pritchard (1986) esti-mated that the ETM zone migrates 40-55 km sea-ward during flood events, which would lead tosouthward export of a least some SusquehannaRiver sediment. On the basis of isotopic analyses ofsediments from the central main stem bay, Helzand others (2000) concluded that the source ofsome mid-bay sediment was the SusquehannaRiver. These studies suggest that sediment“escapes” the ETM zone, which is especially

important because the processes involve mainlyfine-grained suspended sediments of most concernfor water clarity.

The second hypothesis about sedimentationcan be referred to as the tributary “import-export”hypothesis. This idea holds that there is a netimport of sediment from the main stem bay intolarger tidal tributaries except during extreme highflow events when some sediment is exported fromtributaries to the main stem. The tributary import-export hypothesis is an idea that has not beentested in detailed field studies to the extent thatquantitative estimates of import-export can bederived for each tributary. In the RappahannockRiver estuary, Nichols (1977) indicated 10 percentof the sediment discharged by Hurricane Agnes in1972 escaped the Rappahannock River into the bay.In the York River system, the sediment load nearGloucester Point (about 10 km upstream of theestuary mouth) typically involves landward trans-port of sediment (J. Herman, Virginia Institute ofMarine Science, oral commun., 2003), whereas sea-ward transport is associated with episodic, ener-getic events such as storms and hurricanes (Gaoand Collins, 1997; Geyer and others, 2001). Satellitedata also show export of suspended material fromtributaries into the bay during relatively wet peri-ods (Stumpf, 1988). Bottom-sediment surveys inthe southern bay (Byrne and others, 1980) andother lines of evidence (Officer and Nichols, 1980;Lukin, 1983) also indicate the hypothesis is true.

The issue of import/export to and from trib-utaries also pertains to the issue of sedimentsources to the mid-bay, a region where input ofsuspended material from the north (mostly Sus-quehanna River inflow) and ocean-source sedi-ment from the south are thought to contributerelatively small proportions of the total sedimentflux. Although Officer and others (1984) concludedthat sedimentation rates in the central main stemof the bay were lower than those in the northernand southern bay, there is nonetheless a thick accu-mulation of Holocene sediment in many parts ofthe central bay. If northern and southern sourcesare minor in the mid-bay, then it is difficult toaccount for the thick accumulation of sediment inparts of the main stem of the central bay, even withlarge contributions from shoreline erosion. Exportfrom the northern ETM zone, import of fine-grained sediment from the southern bay, and sedi-ment export from tributaries all contribute to sedi-ment flux into the central bay, although the relativecontributions of each have not been quantified.

C H A P T E R 6 | 7 1

The third major hypothesis pertains to theeffect of large-scale deforestation from agricultureand timber production on sediment flux to the bay.This hypothesis commonly is expressed in the ideaof “legacy” sediment—a concept derived largelyfrom studies of fluvial systems. It implies that partof the sediment originating from cleared lands hasbeen trapped in transit in rivers and the upperparts of tributaries and has not yet reached thelower reaches of tidal tributaries or the Chesa-peake Bay itself.

Although land clearance in the ChesapeakeBay watershed has no doubt led to large-scale ero-sion, it is still not known what proportion of sedi-ment eroded since land clearance began remainstrapped in uplands and stream channels, and howmuch has been transported to the lower tidalreaches of tributaries or to the main stem of thebay. Several studies have concluded that sedimen-tation rates and fluxes have increased frombetween three- to tenfold as a result of extensive18th- and 19th-century land clearance. It has beensuggested that this human-induced historicalincrease in sediment flux is manifested in the highrates of sedimentation measured in the upper partsof tributaries where most of the eroded sediment isdeposited.

A possible indicator for depositional eventsis paleoecological evidence from the central bay forhistorical degradation in phytoplankton communi-ties (diatoms in Cooper and Brush, 1993;dinoflagellates in Willard and others, 2003). How-ever, some phytoplankton communities also areinfluenced strongly by eutrophication, and it is dif-ficult to separate the effects of increased turbidity.Except for the few studies of the paleoecology inthe main stem, field studies of sedimentation dur-ing the past few centuries have focused on rela-tively small sub-estuaries and include little or nodata on sediment accumulation downstream inmore distant regions (Defries, 1986). Moreover, asdescribed below, published pre-colonial rate com-parisons are based on poorly dated Holocene sedi-ments and should not be used for evaluatingtemporal variability in rates. Consequently, theeffect of land clearance on diminished water clarityand bay faunas and floras is uncertain.

One way to address the issue of land-useeffects on sedimentation in Chesapeake Bay is tocompare pre-colonial “natural” and historical(since 1880) sedimentation flux estimated fromsediment cores obtained on cruises between 1996

and 2002. As part of a larger study of sedimenta-tion in Chesapeake Bay, the USGS tested thehypothesis that land-use changes have led to large-scale increases in sedimentation. The analysis usedpollen stratigraphy of core samples and calibratedradiocarbon ages to evaluate rates of sedimenta-tion over the last thousand years within differentregions of the bay (table 6.1).

The peak abundance of ragweed (Ambrosia)pollen in sediment cores was used as an agemarker for the period from A.D. 1880 to 1900. Peakragweed abundance has been dated directly usingshort-lived radioisotopes in several sediment coresand it correlates with historical records of maxi-mum timber production and large-scale land clear-ance about from 1880 to 1900 A.D. (Brush, 1989;Willard and others, 2003). It should be emphasizedthat the ragweed peak used for dating is not thesame as the initial rise in ragweed pollen, which isa valuable time marker used in many studies todate the earlier 18th-century land clearance. Thetemporal variability in the percentage of ragweedpollen in core MD99-2209 from the channel off themouth of the Rhode River is shown in figure 6.3.The pre-historical value of about 1 percent rose tonear 15 percent during the ragweed period. Themidpoint of the ragweed maximum between 201-241 cm was used to calculate sediment flux sinceabout 1890 AD. There is some subjectivity in iden-tifying the ragweed peak due to variability in sedi-mentation at different core sites, and processes ofpollen transport and deposition. This event isnonetheless a useful time marker for obtaining afirst approximation of the mean annual sedimentflux during the past 100 years or so.

For each core, a linear age model for the sed-imentary sequence deposited prior to the ragweedpeak was developed using radiocarbon dates onmarine-estuarine mollusk and foraminiferal shells.Radiocarbon ages based on total organic carbonmaterial gave ages about 1500 – 2000 years too oldbecause of “old carbon” and were not used (Col-man and others, 2001). An age-depth model forcore MD99-2209 from the main channel off themouth of the Rhode River is shown in figure 6.4.The linear age model has a correlation coefficient(r2) of 0.98. The stratigraphic position of the maxi-mum proportion of ragweed pollen also is shown.It can be seen that the stratigraphic interval near540-550 cm core depth is dated at about 1000 yrB.P. Thus, for core MD99-2209, about 300 cm ofsediment accumulated between 1000 and1880 A.D. (between 550 and 260 cm core depth).

7 2 | C H A P T E R 6

Using these methods to estimate meanannual sedimentation rate, linear accumulationrates for the post-1880 and pre-1880 intervals wereconverted into mass flux for each sediment core.The sediment flux in 16 cores for which only his-torical sediment fluxes were calculated based onsediment thickness above ragweed peak is shownin figure 6.6. A comparison between pre- and post-1880 rates for 24 cores is shown in figure 6.5. Thepatterns seen in figure 6.5 confirm observationsfrom prior studies discussed above that historicalsedimentation rates vary by about an order ofmagnitude throughout the bay area (from less than0.1 to 0.8 g cm-2 yr-1). Some of the highest rateswere in Pocomoke Sound (PC6B), the northernmain channel (MD2209), and off the mouth of theLittle Choptank River (LCHPT-1-P-4&5). Some ofthe lowest rates were in the Potomac River tribu-tary (NKL-12-1 and NKL-16-1).

Some sites (for example, MD99-2209) haveabout a four-fold greater sediment flux during thelast century than during the prior 1,000 years(fig. 6.5). These results confirm the general conclu-sions of other studies of sediment cores for the cen-tral main stem discussed by Cooper and Brush(1991), Cronin and others (2000), and Colman andBratton (2003). However, at many sites the histori-cal rates have been roughly equal to or haveexceeded pre-historical (1000-1880 A.D.) rates.

There are several possible explanations ofthese results. The most likely explanation is thatprior paleoecological studies were, by design,focused on regions in the bay characterized byfairly continuous sedimentation and relativelyhigh sedimentation rates at least at interannualtime scales (bay sediments usually do not preserveseasonal patterns of sediment). High sedimenta-tion rates and continuous accumulation provide ahigh temporal resolution with minimal gaps in thestratigraphic record and are ideal for paleoecologyand the reconstruction of ecosystem history at dec-adal and centennial time scales. However, usingonly cores with continuous sedimentation andstratigraphy introduces a bias when evaluatingspatial patterns of sedimentation, because areas ofslow accumulation or erosion are excluded. Theresults presented in figures 6.5 and 6.6 clearlyinclude core sites where relatively little sedimenthas accumulated (or has accumulated and hassince been eroded) during historical time. The evi-dence suggests that pre-historical sedimentation atsome of these sites was as rapid as during histori-cal time.

Figure 6.3. Proportion of ragweed (Ambrosia)pollen in core MD99-2209 showing the peak inragweed between 201-241 centimeters depthcorresponding to maximum agricultural andtimber production land clearance (Modifiedfrom Willard and others, 2003).

C H A P T E R 6 | 7 3

Figure 6.4. Age-depth model for core MD99-2209 showing series of radiocarbon ages (calibrated toyears before 1950) and 2 sigma error bars.

7 4 | C H A P T E R 6

Figure 6.5. Comparison of historical (1880-present) and long-term sediment flux at core sites inChesapeake Bay (determined by methods and data described in table 6.1).

C H A P T E R 6 | 7 5

Figure 6.6. Estimates of sediment flux at different core sites in the Chesapeake Bay, calculated bydetermining the amount of sediment lying above the peak in ragweek pollen and converting to mass(based on data in table 6.2).

7 6 | C H A P T E R 6

Another possible factor is that the physicalprocesses, such as tides, currents, and estuarine cir-culation change, over decades to centuries alteringsedimentation. Erosion and deposition of sedimentis extremely dynamic and variable over seasonaland interannual time scales in some parts of Ches-apeake Bay such as the York River. Large-scaleshifts in sediment deposition and erosion alsooccur over millennial time scales in ChesapeakeBay as illustrated by the common occurrence ofstratigraphic unconformities representing tempo-ral gaps of several thousand years (Cronin andothers, 2000).

It is important to emphasize that this analy-sis was limited to sedimentation in the central bayregion at water depths of about 6-25 m and theserates cannot be extrapolated to shallower water orto the more proximal reaches of tributaries. None-theless, the results indicate that the effect of histor-ical factors on sediment and water clarity cannotbe extrapolated from one region to the entire bay.These results also highlight a need for focusedresearch on sediment flux, land use, and physicalprocesses in the most critical habitats and regionsof the bay.

Sediment Resuspension

Bottom sediments in the Chesapeake Baycan be resuspended in response to tidal currents,waves, and boating traffic and can be a significantsource of the sediment load in the water column,potentially increasing light attenuation. Theamount of sediment introduced to the water col-umn by resuspension is highly variable spatiallyand temporally. Moreover, the ways physical forc-ing mechanisms generate suspended sediment arecomplex, and the transport of the particles subjectto resuspension, including their settling rates andeventual redeposition on the bottom, is only par-tially documented. In different parts of the estua-rine system, the relative importance of the majormechanisms controlling resuspension can be sig-nificantly different. This section presents the cur-rent understanding of sediment resuspension inthe bay.

The importance of tidal resuspension in fine-sediment regions of Chesapeake Bay and its tribu-taries has long been recognized (Sanford andHalka, 1993; Schubel, 1968a; Schubel, 1969). Recentstudy of the upper Chesapeake Bay ETM zone ledSanford and others (2001) to conclude that asym-metrical tidal resuspension and transport prima-

rily are responsible for the maintenance of the ETMzone at the limit of salt intrusion. This is in contrastto early studies that ascribed the formation of theETM zone to gravitational circulation patterns.They also confirm the importance of resuspensionprocesses to the maintenance of this zone of highconcentrations of suspended sediment and associ-ated light attenuation. Without the effects of tidalresuspension, the rapidly settling aggregates offine particles would remain on the bottom. Tribu-tary estuaries of the Chesapeake Bay system alsoare characterized by one, or occasionally two, ETMzones (Lin and Kuo, 2001; Nichols, 1974). Tidalresuspension in the relatively sediment-starvedmid-estuary below the ETM zone is weaker butstill significant (Ward, 1985).

Wave-forced resuspension coupled withwave-induced shoreline erosion in shallow (lessthan 2 m deep) parts of the estuarine system gener-ally is understood to produce significant amountsof suspended sediment in the water column. How-ever, relatively few site-specific studies of this topichave been conducted to date (Wilcock, 1998).Those that are available are applicable only to aparticular location and time frame. Their resultscannot be extrapolated to the larger estuarine sys-tem, due, in part, to the variable geometry of theChesapeake Bay that results in both variable fetchand wide ranges in nearshore bathymetry. Fetchinfluences the ability of local winds to generatewaves; local variations in bathymetry influence thedirection and energy of waves approaching shal-low-water zones and shorelines.

In the relatively deeper waters of the Chesa-peake system, wave-forced resuspension may besignificant under storm conditions and can domi-nate the normal tidally induced resuspension sig-nal (Sanford, 1994; Ward, 1985; Wright and others,1992). The influence of wind-wave bottom shearstresses on sediment resuspension and subsequenttransport can be projected with advances in com-putational power and numerical modeling tech-niques (Lin and others, 1997; Lin and others, 2002).Computer simulations suggest that wind-gener-ated waves can produce significant bottom shearstress, resulting in sediment resuspension. Afterthe physical forcing associated with the storm-wave energy is reduced, the resuspended sedi-ments settle rapidly to the bottom. The sedimentsexhibit increased erodibility for some period oftime thereafter (Sanford, 1994), thus increasing thelikelihood of subsequent transport by lowerenergy tidal currents.

C H A P T E R 6 | 7 7

A similar dependence of bottom-sedimentgrain size with storm-wave bottom shear stress hasbeen observed in intermediate water-depths in theChesapeake Bay (Nakagawa and others, 2000). Inthat study, the bottom-sediment grain size wasrelated to strong wind events that occurred lessthan 5-percent of the time, not to the mean windspeed for the area. The results of these studiespoint to the importance of infrequent high-energyevents in sediment resuspension, transport, andeventual distribution on the bottom of the Chesa-peake Bay. In the vicinity of the bay mouth, long-period swell waves with increased bottom shearstress enter from the Atlantic Ocean and are likelyto resuspend more bottom sediment than stormwaves further up the estuary (Boon and others,1996; Wright and others, 1992). These higherenergy waves in the southern bay could also influ-ence the formation of the secondary turbidity max-imum in the York and similar southern bay sub-estuaries (C. Friedrichs and L. Schaffner, oral com-mun., 2003).

In an effort to examine the relative magni-tude of tidal resuspension as an instantaneoussource of TSS in the upper bay, L. Sanford (Univer-sity of Maryland, Center for Environmental Sci-ence, written commun., 2003) provided the SWGPwith estimates of the amount of sediment resus-pension that occurs on a daily basis in the northernChesapeake Bay. These estimates are summarizedbelow because of their significance to the questionof sediment resuspension. Note that these resultswere generated for the main ETM zone of the bay,not the entire estuary. The estimates include anestimated volume of water in the ETM zone (fromthe mouth of the Susquehanna River south toTolchester), the average background concentra-tions of suspended sediment, or that which ispresent irrespective of currents and bottom shearstress, and the resuspended sediment concentra-tion in that water volume. Using these values,

the TSS load in the ETM zone isestimated to be approxi-mately 135,000metric tons (MT) during maximum tidalresuspension. This includes 90,000 MTof resuspended TSS per tidal cycle and45,000 MT of back-ground TSS. Theestimated loading rate due to tidalresuspension is 180,000 MT per day, butthis material also is redepos-ited twiceper day. This can be compared to theestimated combined sediment input of4,400 MT per day to this area of the bay

from the Susquehanna River, shorelineerosion, and internal productivity of4,400 MT per day.

The relatively large value attributed to sedi-ment resuspension is due to multiplication of asmall number for suspended material per unit bot-tom area times the relatively large bottom area ofthe northern bay. A few caveats apply to these esti-mates. The estimates were based on only a smallnumber of study sites primarily in deeper watersof the ETM zone, such that the estimated total loadof resuspended material must be considered verypreliminary. It is not clear how much of the resus-pended deep-water sediment can be transportedlaterally into shallower areas of the estuary. Resus-pended sediments tend to be more aggregated andthus settle back to the bottom quickly, only to beresuspended again in the next tidal cycle. This con-tinued process of deposition followed by resuspen-sion results in the large total loads that arecalculated, but it also results in relatively short-lived peaks in resuspended sediment concentra-tion that are most pronounced near the bottom. Itshould be noted that the sediment concentrationsthat result from resuspension are not from newsediment being introduced to the system duringeach tidal cycle, but are instead a recycling of mate-rial already in place.

Despite the uncertainties, a major conclusionthat can be drawn from these estimates is that nor-mal bottom-sediment resuspension processescould be the dominant instantaneous source forthe suspended sediment load in the water column,when considered in a highly averaged spatial con-text.

In addition to natural processes of waves,currents, and tides, boating activity also can causesediment resuspension. A study of boat-wakeeffects on shore erosion in an area of high recre-ational boat use showed that boat wakes generatedless incident energy than normal wind-generatedwaves (Zabawa and Ostrom, 1980). The major fac-tor influencing shore erosion was a single stormevent during the study period, followed by windwaves associated with normal wind levels. Recre-ational boating undoubtedly has increasedthroughout the bay region since that study, but itremains unclear how significant the effect of boatwakes may be on resuspension in nearshore areas.It is possible that larger effects result from repeatedgeneration of boat wakes during periods of highrecreational vessel use, such as summer weekends.

7 8 | C H A P T E R 6

The effects from the passage of large commercialships has not been studied in the Chesapeake andcould be locally important because of the higherenergy waves produced by these ships. However,the relatively infrequent passage of these shipswould suggest that their importance is minimalrelative to wind-generated waves.

In summary, the ability to control resuspen-sion in the Chesapeake Bay that results from tidalcurrents and storm-generated waves is limitedbecause of the extremely widespread sedimentsource (for example, the entire bay bottom). How-ever, the processes that lead to sediment resuspen-sion and subsequent transport into sensitivehabitat zones need to be more fully understoodthrough direct measurement coupled with thedevelopment of computer models that simulateresuspension in response to known physical mech-anisms. With appropriate parameterizations repre-

senting sediment resuspension, deposition,consolidation, and bed armoring, these modelscould provide an understanding of where manage-ment actions can be most effective. The ability toreduce resuspension may be limited to in-situ prac-tices, such as breakwaters to reduce wave energyor the reestablishment of a significant populationof filter feeding oysters, that can be effective inremoving sediment from the water column. It hasbeen suggested that relatively high levels of resus-pendable sediment in the estuary may, in part, be alegacy of high sediment inputs from the watershedover the past few hundred years. Continued effortsto reduce sediment input from the watershedeventually will reduce the pool of resuspendablesediment in the estuary itself, although there willprobably be a significant time lag before any posi-tive benefits are noted.

C H A P T E R 6 | 7 9

CHAPTER 7. INTEGRATEDAPPROACHES TO SEDIMENT STUDIES

by Sean Smith,1 Julie Herman,2

Thomas Cronin,3 Gregory Schwarz,3

Michael Langland,3 Kenn Patison,4

and Lewis Linker5

Sediment Budgets: Watershed and Estuary

In this section, information related to thedevelopment of sediment budgets for the entirebay, a tributary, or a watershed are summarized.A sediment budget is a conceptual simplificationof the interactions between physical processesinvolved in the conveyance (movement) of sedi-ment downstream (Dietrich and others, 1980). Theconcept of developing a sediment budget is basedon the conservation of mass by accounting for sed-iment sources, sinks, and yield (output) within awatershed system (fig. 7.1). The fact that mass isconserved theoretically provides a strong con-straint on budgets by requiring that inputs, stor-age, and outputs be quantitatively balanced sothere is no unaccounted mass (Wilcock, 2002). Themass balance approach is used to compensate forthe inability to obtain physical data for every partof study area.

The construction of a budget is helpful fordeveloping linkages between erosion in uplandareas with subsequent sediment delivery down-stream (Trimble and Crosson, 2000). Sediment-budget information can be used to evaluate theeffects of natural and unnatural disturbances onsediment production and yield in the ChesapeakeBay watershed. In addition, sediment-budgetinformation can be used to predict the effects ofland-use changes in the watershed on sedimentyield, to determine best-management practices,and to guide the development of diagnostic toolsto formulate strategies for land-use planning.

Although the general idea of a watershedsediment budget is not new, there is an increasedawareness in recent years that the residence time ofsediments within a drainage basin may be animportant factor in determining the response of

river and estuarine systems to short-term land-usechanges (Phillips, 1991a). Improved understandingand quantification of the complex relationsbetween sediment source and storage componentsthrough the development of a sediment budgetalso enhances the ability to generate estimates ofwatershed sediment residence time. The temporaland spatial scales applied to a sediment budgetultimately will determine what factors govern theflux of sediment. For large spatial or long temporalscales, the process-based budgeting approach canbe used to evaluate the effect of long-term climaticchange on sediment production and yield to theChesapeake Bay. On smaller spatial and temporalscales, development of sediment budgets for smallwatersheds can be used to evaluate cumulativeand short-term effects of land-use modificationsand best-management practices in disturbed land-use settings.

Watershed Components

Watershed components of a sediment bud-get may be described as upland erosion, uplandstorage, wetland and (or) floodplain storage, chan-nel storage, streambank erosion, and load or yieldat the basin outlet. Many of these components andprocesses are discussed in previous chapters.Within a watershed, the function and roles of sedi-ment sources and sinks relative to total-basin sedi-ment yield can be highly variable, particularlywhen land use has changed significantly over time(Trimble, 1999). As a single flux term, large-scalewatershed sediment yield has limited valuebecause of the difficulties in establishing linkageswith well-defined processes and in determiningsediment residence times in the watershed. Parti-tioning the components of a budget into functionalzones, such as sediment production, transfer, andstorage areas, can improve the estimation of therelative influence of localized landscape settingsand land-use changes on overall sediment yield.

Development of techniques that facilitate therigorous evaluation of the sources and sinks in asediment budget is important for budget accuracy.However, assembling the data necessary for suchaccuracy is difficult (Walling, 1994). Balancing thebudget to obtain accurate estimates of watershedsediment yield requires that the error within eachcomponent be minimized. The benefits of reducingerror should be weighed against increased moni-toring costs and transferability of site-specificresults.

1 Maryland Department of Natural Resources.2 Virginia Institute of Marine Science.3 U.S. Geological Survey.4 Pennsylvania Department of Environmental

Protection.5 U.S. Environmental Protection Agency.

8 0 | C H A P T E R 7

Sediment budgets can be compiled for awide spectrum of spatial and temporal scales andthe relevance of the components directly relate tothe spatial and temporal scales under consider-ation. Spatial scales can range from small (channelreach and shoreline lengths of several to hundredsof meters) to watersheds draining small areas ofabout 0.5 to 1 mi2, to watersheds over 100 mi2, upto large basins the size of the Chesapeake Baydrainage basin. Sediment erosion is sometimesestimated with reference to 10,000 to 1 millionyears because of the relevance to the long-termevolution of modern topographic conditions.Chesapeake Bay sediment fluxes commonly areevaluated over time periods from 100 to10,000 years because of the relevance to sea-levelchanges, human effects under the current (2003)climatic conditions, and a management perspec-

tive. Sediment loadings from land disturbanceusually focus on short-term and instantaneousevents such as individual storms.

The physical processes controlling sediment-budget components also can vary with time. Thesechanges in physical processes can contribute tobiases in results when interpreting data sets thatspan different periods of time (Johnson, 1990).Temporal and spatial scales should be compatiblein a budget to produce robust results and practicalinterpretations (Campbell, 1992). Because anunderstanding of the present-day system is neededfor management purposes, budget componentscommonly are calculated on an annual basis usinginformation from decadal time scales. However,budgets based on annual averages are notdesigned to describe large sediment movementsduring relatively short time intervals (Kleiss, 1996).In the Chesapeake Bay watershed, sediment load-ings associated with European colonization and

Figure 7.1. Watershed sediment, sinks and sources (Modified from Jacobsen and Coleman, 1986).

C H A P T E R 7 | 8 1

land-clearing activities generally are discussed rel-ative to time scales of 1 to 3 centuries. The responseof the landscape to land clearing and the resultingincreases in sediment supply can be evaluated onthe basis of geomorphic changes, such as the pres-ence of incised channels, the aggradation of flood-plain areas, and infill of tidal estuaries. However,sediment remobilized from long-term storageareas can be wrongly identified as generated fromrecent erosion in upland areas within a basin(Campbell, 1992).

In general, a comprehensive understandingof sediment transport and fate is considered essen-tial for developing a sediment budget and design-ing and implementing effective plans for sedimentmanagement (Osterkamp and others, 1998; U.S.General Accounting Office, 1990). The accuracy ofsediment-flux estimates is compromised by inher-ent uncertainties in measuring sediment concen-trations and by the highly episodic nature ofsediment movements, particularly when evaluat-ing smaller basins. However, for annual or decadalflux estimates, the methods generally are reliable ifcalibrated with extended periods of data (Robert-son and Roerish, 1999). The Universal Soil LossEquation (USLE) (Natural Resources Conserva-tion Service, 1983) is an engineering methodwidely used for estimating sheet and rill erosion.Although receiving substantial support within theliterature, the mathematical assumptions of theUSLE recently have been questioned (Trimble andCrosson, 2000).

Conversely, relatively little direct evidence isavailable concerning the fate of sediment. Thecommon practice of quantifying sediment fatewith a sediment delivery ratio, estimated from asimple empirical relation with upstream basinarea, does not consider the relative importance ofindividual storage sites within a basin (Wolman,1977). Rates of sediment deposition (storage) inreservoirs and floodplains can be determined fromempirical measurement, but only a limited numberof sites have been monitored and net rates of depo-sition or loss from other potential sinks andsources is largely unknown (Stallard, 1998). In par-ticular, little is known about how much sedimentloss from fields ultimately makes its way to streamchannels and how much sediment subsequently isstored in or lost from the streambed (Meade andParker, 1985; Trimble and Crosson, 2000).

In summary, sediment source, storage, andyield components that collectively describe sedi-ment budgets have been quantified by variousmeans in the Chesapeake Bay watershed. How-ever, few comprehensive budgets, composed ofmultiple components and detailed field measure-ments, have been compiled. Estimation has beenrequired of one or more components that introduceerrors depending on the time scale, setting, andtime-period estimation. Integrated studies are lack-ing that investigate and quantify all components asa complete system at the same spatial and tempo-ral scales. In addition, the relations between small-watershed sediment processing and large-water-shed sediment yield have not been extensivelydocumented in each of the major tributaries drain-ing to the Chesapeake Bay. Therefore, it is difficultto correlate the effects of specific watershed land-use practices on sediment load to the estuary.

Estuarine Budgets

A sediment budget for the Chesapeake Baywould include inputs from the watershed(s) andestuarine components for shoreline erosion, bio-genic production, ocean, storage and re-suspen-sion, and tidal flux at the “outlet.” Variousresearchers have tried to quantify the flux of sedi-ment within the bay and its tributaries using awide range of methodologies. In reviewing this lit-erature on sediment budgets in the ChesapeakeBay system, an important methodological distinc-tion must be emphasized between those studiesthat address suspended particulate material in thewater column and those that address sediment thathas actually accumulated on the bay (tributary)floor. Suspended-sediment data, which includesUSGS Fall Line TSS load measurements (Langlandand others, 1999), and TSS monitoring data fromthe bay and tributaries, deals exclusively with par-ticulate material in the water column for a particu-lar time and region. Notable studies of suspendedparticulates that resulted in sediment budget infor-mation include well-known papers by Biggs(1970), Schubel and Carter (1976), and Nichols andothers, (1991). Although TSS studies may be ofimportance to water clarity and the SAV-TSS-lightissues discussed earlier in this report, they are notsufficient alone to construct a comprehensive sedi-ment budget.

In contrast, studies directly addressing surfi-cial modern bay-floor sediment accumulationinclude those by Ryan (1953), Donoghue (1990),Kerhin and others (1983; 1988), Byrne and others

8 2 | C H A P T E R 7

(1982), and Hobbs and others (1990; 1992). Studiesof temporal patterns of sediment deposition basedon long sediment cores and geochronological anal-yses also are relevant to sediment budgets and aredescribed in the section on Sediment Depositionand Sedimentation Rates discussed later in thischapter. Because they are a source of informationon long- and short-term sedimentary processes,sediment data from bottom samples and sedimentcores are of great utility to management efforts toimprove water clarity, and to issues of dredging,contaminants in sediments, and nutrient recycling.

Several comprehensive studies have expli-citly attempted to synthesize a bay-wide sedimentbudget. The original report should be consulted formethods and assumptions and detailed interpreta-tion. Ryan (1953) studied 200 sediment coresthroughout the bay and was one of the first todescribe the general character of sediments in themodern bay. Ryan showed that sands blanket thebay flanks and that fine-grained silts and clayscover the deeper main channel.

Another early sediment budget developedfor the bay used a simple, single-segment modelbased on salt-flux equations to compute sus-pended sediment and to estimate exchanges ofsuspended sediment between the bay and its tribu-taries and the bay and the ocean (Schubel andCarter, 1976). This study concluded that sedimentsources include the Susquehanna River(57 percent), shoreline erosion (32 percent), andsediment moving from the ocean into the mouth ofthe bay (12 percent). Schubel and Carter estimatedthe majority of sediment (92 percent) carried intothe bay was deposited in the bay itself; the remain-ing 8-percent was transported from the main stemof the bay into the tidal tributaries. Schubel andCarter concluded that the ETM traps most sedi-ment in the north, tributaries are net sinks of sedi-ment imported from the bay, and the bay is a sinkfor sediment imported from the ocean. As dis-cussed in the previous chapter, the net export ofsediment from the northern to central bay andfrom the tributaries to the main stem is a complex,unresolved issue in terms of the timing and quan-tity of sediment movement.

Similarly, the contribution of sediment fromcoastal marshes is a complex, commonly misun-derstood subject. Investigations of sediment flux inbrackish marshes include those by Kearney andWard (1986) and by Stevenson and others (1985).Stevenson and others (1988) calculated that brack-

ish estuarine and tidal freshwater marshes trapabout 5 to 11-percent of the annual sediment influxto the Chesapeake Bay. This would equate to2.6 × 106 tons (Officer and others, 1984). These esti-mates suggest a relatively small proportion of sedi-ment inputs in the bay are trapped by intertidalmarshes, contrary to the commonly perceived roleof marshes as depositional systems in estuaries.

The most comprehensive study aimed atdeveloping a bay-wide sediment budget is byHobbs and others (1990; 1992) (table 7.1). Buildingon the work of Ryan (1953), Biggs (1970), andSchubel and Carter (1976), Hobbs and others (1990;1992) compiled data from parallel studies in Mary-land (Kerhin and others, 1983, 1988) and Virginia(Carron, 1979; Byrne and others, 1982; Hobbs,1983) and produced maps and tables that quanti-fied net erosion and deposition of sedimentthroughout the bay. Unlike many other studiesbased on data from a longitudinal transect, or froma limited study area, this work produced a bay-wide sediment budget based on 3-dimensionaldata.

The approach of Hobbs and others (1990;1992) was to determine sediment erosion and dep-osition by comparing U.S. Coast and Geodetic Sur-vey hydrographic surveys carried out in the 1840swith more recent surveys made during the 1950s.Using the bathymetric differences between the twosurveys, total accumulation and erosion of sedi-ment was calculated in terms of volume and con-verted to mass (metric tons). The sediment budgetof Hobbs and others (1990; 1992) (table 7.1) pro-vides an excellent summary of average sedimentaccumulation and erosion over an approximately100-year period. Some of the most important con-clusions from their reports include:

• Between the 1840s and 1950s, net deposi-tion in Chesapeake Bay was between 1,049and 2,915 million metric tons.

• This total exceeds the sum of quantifiablesources by 2.7 to 7.6 times, most of whichis accounted for by ocean-source sands inthe southern part of the bay; the budget forinput and deposition of muds is balancedwithin a factor of 2.4.

• The Susquehanna River is a major sourceof fine-grained sediment to the upper bay.

C H A P T E R 7 | 8 3

• The proximal continental shelf provides alarge quantity of sand and suspended sed-iment (perhaps 40 percent of the net sedi-ment deposition in the bay).

• Fine-grained ocean-source sedimentsmight reach the mid-bay, which is farthernorth than previously thought to bedeposited.

As part of the National Estuary SedimentContaminant Inventory (NESCI), Nichols and oth-ers (1991) summarized sediment features for all themajor tributaries of the bay. Although not explic-itly a sediment budget study, the compilation byNichols provides a useful review of the literatureincluding data on sediment texture, sources, massbalance, and storage efficiency (proportion of

riverine input stored) in each tributary in the Ches-apeake system. The conclusions reached byNichols and others include:

• Shoreline erosion contributes proportion-ally more sediment in tributaries with lowriverine input.

• Shoreline erosion is more significant sea-ward in wider reaches of the bay becauseof exposure to wave action.

• Following erosion, winnowing and resus-pension processes sort fine and coarse sed-iment; fine sediment ultimately settlesfurther toward the channel because ofslower settling rates and tidal currents.

Table 7.1. Sediment budget data (modified from Hobbs and others, 1990)

[error, 95-percent confidence for predicted value; --, no data available]

Deposition in Maryland portion of the Chesapeake Bay, in millions of metric tons in 100 year period

Total Organic Inorganic Sand Silt Clay

Deposition 822.15 16.98 805.18 524.13 121.61 159.46Erosion 661.11 10.62 650.49 469.46 69.9 111.13Net 161.04 6.35 154.69 54.67 51.71 48.33

Mass of silt/clay deposited in Maryland portion of Chesapeake Bay (summary from different studies)

Biggs (1970) 83.8 million metric tons/centurySchubel and Carter (1976) 141 million metric tons/centuryKerhin and others (1988) (total) 281 million metric tons/centuryKerhin and others (1988) (net) 100 million metric tons/century

Deposition in Virginia portion of the Chesapeake Bay, in millions of metric tons in 100-year period

Value Standard Deviation Error

Sand 2,210.4 1,690.7 716.8Silt 329.6 305.9 110.2Clay 220.5 184.1 68.2

Total 2,760.8 2,180.8 895.2

Sedimentation in Chesapeake Bay, in millions of metric tons per 100-year period

Sources Sand Mud Total

Shoreline erosion, Maryland 74.0 137.0 211.0Susquehanna River suspended sediment -- 107.0 107.0Shoreline erosion, Virginia 40.0 2.5 42.5Biogenic silica, Virginia 0.8 -- 0.8Oceanic suspended sediment -- 22.0 22.0

Total 114.8 268.5 383.3

Deposition Value Standard Deviation Error

Sand 2,265.1 1,745.4 771.5Silt 381.2 357.6 161.9Clay 268.8 232.4 116.5

Total 2,915.1 2,335.6 1,049.9

8 4 | C H A P T E R 7

• Estuarine density-driven circulation influ-ences the fate of fine-grained sedimentonce it has entered the main stem bay sys-tem; the upper estuarine layers generallytransport sediment southward, and thelower layers transport sediment north-ward.

In addition to bay-wide sediment budgets,regional studies integrating bay tributaries withtheir watershed provide important linkagesbetween the aquatic system and adjacent landareas that are useful for land-use management.One example is a recent study of the York Riverand its watershed (Herman, 2001). A series of sedi-ment budgets were constructed for 11 nested sub-watersheds ranging in size from 65 to 6,900 km2 inthe Piedmont and Coastal Plain of Virginia. Thesewatersheds extended from the headwaters to theestuary mouth and were used to examine sedimentallocation as a function of watershed size. Dataspanning decadal time scales and loads were cal-culated on an average annual basis. The resultsfrom Herman (2001) showed that in these low-relief watersheds, sediment budgets are moreinfluenced by the river system (Mattaponi andPamunkey tributaries of the York River) than bysub-watershed size. Upland erosion was the majorsource of sediment in the Pamunkey River; bankerosion (including “shorelines” in the uppermosttidal zone) was the major source in the MattaponiRiver. Upland storage was the major sink for bothtributaries.

The York River study also indicated that lit-tle sediment from the upper watershed reached theestuary, indicating the Piedmont and Coastal Plainportions are “decoupled.” Decoupling defines aprocess where a significant portion of sedimenteroded from the upper or middle reaches of a basinis stored upstream and is not transported to thelower reaches of the basin. This results in increasedstreamflow energy and more sediment being mobi-lized and transported from downstream areas ofthe basin. As a result, management actionsdesigned to decrease upland erosion and theimplementation of buffers along streams to mini-mize the remobilization of colluvial storage mayhave limited effects farther downstream. There-fore, the improvement of water quality in the YorkRiver estuary may be largely independent of soil-conservation practices implemented extended dis-tances upstream. Water quality may be moreaffected by locally derived sediments near theestuary. The net movement of sediment at the

mouth of the river is from the bay into the estuary.This, in combination with sediment movementduring extreme storm events, implies that sedi-ment management strategies in the York Riverwatershed may also benefit from a regional focus.

Data from other studies provides additionalinformation on sediment flux to the bay from thetributaries and within the bay. Information thatwas compiled on sediment sources and budgetsfrom several studies is shown in figure 7.2. Thedata are reported in terms of contributions of sedi-ment from rivers, shoreline erosion, oceanicsources, tributaries, and in-situ biogenic produc-tion in metric tons per year. These data also aregiven in volumetric and relative percent contribu-tions in table 7.2. Other studies containing sedi-ment-budget information not included in thissummary are available for the Potomac River byMiller (1983) and Bennett (1983), the Rappahan-nock River by Lukin (1983), the Choptank River byYarbro and others (1983), the Anacostia by Scatena(1987), and the South River by Marcus and others(1993) and Marcus and Kearney (1989; 1991). Thefollowing is shown in figure 7.2:

• Susquehanna River sediment dominates inthe north.

• Oceanic-source sediment is the dominantsources in the southern bay, although thistotal includes an unknown amount of sedi-ment eroded from shorelines and perhapssome sediment exported from major riv-ers. A further breakdown of this large fluxof sediment requires more detailed analy-sis.

• Different tributaries have different relativecontributions from riverine, shoreline, bio-genic, and oceanic sources. Many studiesdid not include biogenic sediment and it islikely that biogenic material contributessubstantial amount to particulate materialin some regions.

• In different parts of large tributaries suchas the Potomac, the relative proportion ofshoreline and riverine sediments vary inupstream and downstream regions. Thisreflects the trapping of riverine sedimentsupstream and the diminished influence ofriverine sources bay-ward down a majortributary.

C H A P T E R 7 | 8 5

86

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PT

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7

el-generated loads (left half).
Figure 7.2. Fine-grained sediment sources from different sources based on literature (right half) compared to mod(Based on table 7.2 in Chapter 7.)

CH

AP

TE

R7

|8

7

T re in million metric tons per year (MT/yr)

[ pplicable]

Citation

del output (Lewis Linker, personal communica-tion, Watershed Model Phase 4.3Sediment load results, 2000Progress Scenario, 2003)

del output

del output

del output

del output

del output

del output

del output

ata, Kerhin and Halka (2000)

teral upper bayel sites from Feb.

Biggs (1970)

erson, 1977; Hobbsd others, 1984;

Nichols and others (1991)

arter, 1976; Kerhin others, 1982; Singe-yrne and Anderson,

Hobbs and others (1990)

ples, Byrne and Grant, 1978.

Byrne and others (1982)

nt analysis of up to to Nov. 1976.

Pierce and Dulong (1977)

samples at seasonal Roberts and Pierce (1974;1976)

and aerial photo- Miller (1987); Bennett (1983)

and aerial photo- Miller (1987); Bennett (1983)

others, 1992; Nichols,976; Officer andhers, 1981.

Nichols and others (1991)

ts. Herman (2001)

rs, 1981; Nichols and Nichols and others (1991)

m Sep 1979-Aug. Yarbro and others (1981; 1983)

able 7.2. Suspended sediment source loads in the Chesapeake Bay estuary and its sub-estuaries. Values a

Shaded areas represent model estimates; NC, not considered; --, no data; CB, Chesapeake Bay; mth, mouth; N/A, not a

EstuaryRiverine(above

Fall Line)

Shoreline(mainly

cliffs andheadlands)

Biogenic(not

measuredfor all

studies)

Import(fromBay to

tributary)

Ocean(import tosouthern

bay)

Tributaries1

(below fallline)

Sum Method

Susquehanna 900,000 NC NC NC NC 100,000 1,000,000 Chesapeake Bay Program Mo

Potomac 1,600,000 600,000 NC NC NC 200,000 2,400,000 Chesapeake Bay Program Mo

Rappahannock 200,000 400,000 NC NC NC 150,000 750,000 Chesapeake Bay Program Mo

York 100,000 550,000 NC NC NC 120,000 770,000 Chesapeake Bay Program Mo

James 1,100,000 450,000 NC NC NC 200,000 1,750,000 Chesapeake Bay Program Mo

West Shore Maryland NC 400,000 NC NC NC 200,000 600,000 Chesapeake Bay Program Mo

East Shore Maryland NC 1,500,000 NC NC NC 300,000 1,800,000 Chesapeake Bay Program Mo

East Shore Virginia NC 250,000 NC NC NC 50,000 300,000 Chesapeake Bay Program Mo

North Chesapeake 1,310,000 280,000 10,000 NC NC NC 1,600,000 Compiled from USGS gauge dothers, 1988; Biggs, 1970.

Central Chesapeake 33,000 275,000 206,000 NC NC NC 514,000 Bi-weekly sampling across 5 latransects and 6 deep chann1966-Jan. 1967.

Chesapeake2 1,550,000 600,000 NC NC 450000 NC 2,550,000 Compiled from Byrne and Andand others, 1990; Officer anSchubel and Carter, 1976.

South Chesapeake3 107,000 25,400 0 NC 1138400 NC 1,270,800 Complied from Schubel and Cand others, 1983; Byrne andwald and Slaughter, 1949; B1977.

South Chesapeake4 NC 423,000 12,520 NC -- NC 435,520 Complied from 2000 grab samAnderson, 1977; Jacobs and

Rhode5 NC NC NC NC NC 222 222 Gravimetric suspended sedime11 tributaries from Jan. 1974

Patuxent6 216,000 NC NC NC NC 49 216,049 55 mid-depth suspended sed. and characteristic intervals

Potomac-ChainBridge to mouth(historical)

1,350,000 150,000(230,000)

NC 10,000 NC 880,000 2,390,000 Comparison of shoreline mapsgraphs.

Potomac-301Bridge to mouth(historical)

440,000 100,000(170,000)

NC 10,000 NC 330,000 880,000 Comparison of shoreline mapsgraphs.

Rappahannock7 300,000 300,000 15,000 300,000 NC NC 915,000 Compiled from Hardaway and1977; Schubel and Carter, 1Nichols, 1980; Haven and ot

York 42,200 6,950 NC 910,000 NC NC 959,150 Quantified 11 sediment budge

James7 2,400,000 300,000 15,000 400,000 NC NC 3,115,000 Compiled from Haven and otheothers, 1991.

Choptank8 -80,000 340000 NC 36,000 NC 20,300 316,300 Monthly longitudinal cruises fro1980.

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that 86 percent of 656000000 Mt/y is sand.rne data not plotted in figure.

g average of total suspended material.A .

T es are in million metric tons per year(

ethod Citation

PERCENTAGES BY SEGMENTSusquehanna 90.00 NC NC NC NC 10.00 100.00

Potomac 66.67 25.00 NC NC NC 8.33 100.00

Rappahannock 26.67 53.33 NC NC NC 20.00 100.00

York 12.99 71.43 NC NC NC 15.58 100.00

James 62.86 25.71 NC NC NC 11.43 100.00

West Shore Maryland NC 66.67 NC NC NC 33.33 100.00

East Shore Maryland NC 83.33 NC NC NC 16.67 100.00

East Shore Virginia NC 83.33 NC NC NC 16.67 100.00

North Chesapeake 81.88 17.50 0.63 NC NC NC 100.00

Central Chesapeake 6.42 53.50 40.08 NC NC NC 100.00

Chesapeake 60.78 23.53 NC NC 15.69 NC 100.00

South Chesapeake 8.42 2.00 0.00 NC 89.58 NC 100.00

South Chesapeake4 NC 97.13 2.87 NC ? NC 100.00

Rhode NC NC NC NC NC 100.00 100.00

Patuxent 99.98 NC NC NC NC .02 100.00

Potomac-CB-mth 56.4 (54.6) 6.3 (9.3) NC .5 (.5) NC 36.8(35.6) 100.00

Potomac-Rt. 301-mth 50.0 (46.3) 11.4(17.9) NC 1.2 (1.1) NC 37.5(34.7) 100.00

Rappahannock 32.79 32.79 1.64 32.79 NC NC 100.00

York 4.40 0.72 NC 94.88 NC NC 100.00

James 77.05 9.63 0.48 12.84 NC NC 100.00

Choptank N/A 85.79 NC 9.08 NC 5.12 100.00

1 Miller’s study in the Potomac excluded sand.2 Only examined fine sediment, discusses biogenic sources but did not quantify.3 Riverine values are mud, 1138400 Mt/y ocean value is based on 220000 Mt/y + 918400 Mt/y based on Hobbs statement 4 Shoreline value is approximately 90 percent sand and is probably a conservative figure, ocean value represents sand. By5 Transport represents 1976 value.6 Total sediment input values are given, but are primarily riverine.7 Biogenic input published as <2.00E+04, and estimated at 1.5E+04, import value ignores resuspension.8 Negative value indicates a net export of riverine sediment, primary production not directly measure, but incorporated usin

verage calculated where range was given (Chesapeake/Biggs–Riverine, Rappahannock/Nichols–Import, York/Herman Shoreline)

able 7.2. Suspended sediment source loads in the Chesapeake Bay estuary and its sub-estuaries. ValuMT/yr)—Continued

EstuaryRiverine(above

Fall Line)

Shoreline(mainly

cliffs andheadlands)

Biogenic(not

measuredfor all

studies)

Import(fromBay to

tributary)

Ocean(import tosouthern

bay)

Tributaries1

(below fallline)

Sum M

• Shoreline sources of sediment are numeri-cally important in the Choptank and Rap-pahannock tributaries, and to a lesserextent in the Potomac River.

In summary, there are enormous scientificand technical challenges to constructing a realistic,quantitative sediment budget for the bay. The largearea covered by the bay and its watershed makesthe development of a bay-wide sediment budget adifficult undertaking with any method. Althoughsome integrated sediment-flux studies of smallertributaries and their watersheds resulted in sedi-ment budget estimates, these results cannot neces-sarily be extrapolated elsewhere in the bay becausesediment sources and processes are spatiallyextremely variable.

A sediment budget also is ultimately depen-dent on the time scale chosen. If a short time scaleis chosen, such as a single year, the complex tem-poral aspects of sedimentation, such as theunknown lag time from initial land-surface erosionuntil final deposition cannot be taken into account.Extreme episodic events that are of great impor-tance in sediment transport and deposition alsowould be neglected. Conversely, a long-term sedi-ment budget computing sediment flux over the8,000-year history of the bay may give realistic esti-mates of net sediment accumulation over millen-nia, but this would probably be of little use tomanagers concerned with improving bay waterquality.

Nonetheless, the literature provides a wealthof quantitative data on sediment flux from certainareas that could be of significant use in manage-ment efforts. A potential future need is an inte-grated study involving sedimentologists,hydrologists, and modelers to determine ways toapply the available data, to fill spatial gaps in thedata, and to validate bay sediment models againstempirical data.

Model-Derived Sediment Estimates

Various modeling approaches have beenused to understand and predict sediment flux inthe Chesapeake Bay system—the Spatially Refer-enced Regression Model (SPARROW) for sus-pended sediment, the Chesapeake Bay WatershedModel (WSM), and the Chesapeake Bay Water-Quality Model (WQM).

Spatially Referenced Regression Model(SPARROW) for Sediment

The SPARROW model is an effort to empiri-cally address the question of sediment fate andtransport on a national scale (G. Schwarz, U.S.Geological Survey, oral commun., 2002). TheSPARROW model was first used to estimate thedistribution of nutrients in streams and rivers ofthe United States and has subsequently been usedto describe land and stream processes affecting thedelivery of nutrients (Smith and others, 1997; Alex-ander and others, 2000; Preston and Brakebill,1999). The model makes use of numerous spatialdata sets, available at the national level, to explainlong-term sediment water-quality conditions inmajor streams and rivers throughout the UnitedStates. The model described here is intended toempirically evaluate regional-scale processesaffecting the long-term (decadal) transport of sedi-ment in rivers.

Suspended sediment has long been recog-nized as an important factor affecting waterresources. Besides its direct role in determiningwater clarity, bridge scour, and reservoir storage,sediment serves as a vehicle for the transport ofmany binding contaminants including nutrients,trace metals, semi-volatile organic compounds,and numerous pesticides (U.S. Environmental Pro-tection Agency, 2000a). Recent efforts to addresswater-quality concerns through the Total Maxi-mum Daily Load (TMDL) process have identifiedsediment as the single most prevalent cause ofimpairment in the Nation’s streams and rivers(U.S. Environmental Protection Agency, 2000b).

A comprehensive understanding of sedi-ment fate and transport is considered essential tothe design and implementation of effective plansfor sediment management (Osterkamp and others,1998; U.S. General Accounting Office, 1990). Sedi-ment sources are identified using sediment erosionrates from the National Resources Inventory (NRI)(Natural Resources Conservation Service, 2000)apportioned over the landscape according to 30-mresolution land-use information from the NationalLand Cover Data set (NLCD) (U.S. Geological Sur-vey, 2000). Over 76,000 reservoirs from theNational Inventory of Dams (NID) (U.S. ArmyCorps of Engineers, 1996) are identified as poten-tial sediment sinks. Other non-anthropogenicsources and sinks are identified using soil informa-tion from the State Soil Survey Geographic(STATSGO) database (Schwarz and Alexander,

C H A P T E R 7 | 8 9

1995) and spatial coverages representing surficialrock type and vegetative cover. The SPARROWmodel empirically relates these diverse spatialdatasets to estimates of long-term, mean annualsediment flux computed from concentration andflow measurements collected from 1985 to 1995from more than 400 monitoring stations. These sta-tions are maintained by National Stream QualityAccounting Network (NASQAN) (Alexander andothers, 1998), the National Water Quality Assess-ment (NAWQA) Program, and U.S. GeologicalSurvey District offices. The calibrated model isused to estimate sediment flux for over 60,000stream segments included in the River Reach File 1(RF1) stream network (Alexander and others,1999).

An important implication of the SPARROWmodeling approach adopted in this analysis is thatestimates of sediment production and loss arebased on measurements of in-stream flux. Otherancillary information, such as direct measurementsof long-term sediment storage and release fromreservoirs (Steffen, 1996) are incorporated into theanalysis by specifying additional equationsexplaining these ancillary variables.

The mean annual suspended-sediment fluxgenerated within and leaving a reach is referred toas the incremental reach flux. The flux consists oflong-term sediment load data and several hypoth-eses of sediment fate and transport. The estimationof long-term suspended-sediment load at a moni-toring station is based on the regression of the nat-ural logarithm of instantaneous suspended-sediment concentration on current and lagged val-ues of the natural logarithm of daily flow andother variables representing seasonal and trendeffects. If the station has concentration data col-lected more frequently than on a weekly basis, theregression model is modified to account for serialcorrelation. To be included in the analysis, a stationmust have at least 3 years of data between 1985and 1995.

The flexible mathematical structure of themodel is capable of accommodating a number ofhypotheses concerning sediment fate and trans-port. Sites of sediment storage can act as sedimentsources or sinks. A random coefficient form of themodel allows storage sites to serve as sources insome regions and sinks in others. Nonpointsources of sediment, such as soil, are distinguishedfrom sediment losses from storage (an alluvialplain) using the assumption that the former is a

primary process of weathering whereas the latter isa consequence of the accumulation of previouslyweathered material later released to streams underchanging hydraulic conditions. Accordingly, thepotential for storage loss in the model depends onthe extent of accumulated upstream soil erosiondue to weathering. The empirical validity of theUniversal Soil Loss Equation (USLE) estimate ofsoil erosion can be evaluated through statisticalhypothesis tests of the relevant coefficients. Alter-native measures of soil erosion also can be empiri-cally evaluated in the model by substitutingvariables serving as determinants of the USLE forthe USLE erosion estimate. Data on reservoir stor-age can be incorporated directly into the model byintroducing an additional storage equation.

To complete the model structure, individualreaches are combined to form a nested basin. Eachnested basin consists of reaches upstream from agiven monitoring station and below any monitor-ing station further upstream (if such stations exist)(fig. 7.3).

Preliminary Results.—There are manyimpediments to understanding sediment storagebecause few stream and reservoir sites are moni-tored and it is difficult to know where and to whatextent storage occurs in the basin—streambeds,floodplains, and (or) reservoirs for example. Basedupon the previous discussion, a preliminarySPARROW model was constructed for suspendedsediment in streams of the conterminous UnitedStates. The National model of sediment containsdata from over 600 stations from USGS NationalWater-Quality monitoring networks, numerousGIS spatial coverage of causative factors includingNRI, NLCD (National Land Cover Data-set), andSTATSGO, and RF1 stream network with over70,000 reservoirs from NID (National Inventory ofDams). The model structure is simple but flexibleand contains a sufficient number of monitoring sta-tions uniformly distributed nationally. The prelim-inary results show that the model agreesreasonably well with actual sediment data andcoefficients (explanatory variables) are interpret-able (G. Schwarz, U.S. Geological Survey, oral com-mun., 2002). Results also indicate that smallstreams, and not large streams, are sources of sedi-ment, reservoirs are large sinks of sediment, theNRI provides an incomplete estimate of erosion,wind erosion reduces sediment susceptible to ero-sion to streams by runoff, surface-water runoffincreases sediment erosion, and more permeable

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soils are less susceptible to erosion. Ideally, futuremodel runs may include revised data sets andincreased sediment data a-nd maps may be pro-duced of delivered sediments loads and yields to“edge of field” (erosion from land) and “edge ofstream” (erosion actually reaching the stream).

Chesapeake Bay Watershed Model (WSM)

The three cross-media models used for simu-lations of sediment in the Chesapeake Bay andwatershed include the Regional Acid DepositionModel (RADM), the Watershed Model (WSM,Phase 4.3), and the Chesapeake Bay Estuary ModelPackage (CBEMP, which will be referred to as theWater Quality Model or WQM) (Linker and Shenk,2000). The RADM is used to provide estimates ofthe deposition of air-borne nitrogen to the land andwater surface and is not discussed in this report.Additional information about RADM can beobtained at the Web site http://www.epa.gov/asmdner/radm.html/ . Simulation of suspended sedi-ment and total suspended solids transport fromthe watershed to the estuary is performed usingthe WSM. The subsequent effects of suspended sol-

ids on water clarity, SAV, and benthos are simu-lated using the WQM. In addition, the effects ofbenthos on suspended solids also are simulated.

The inputs of suspended sediment to the riv-erine system are calculated from each land unit inthe WSM (below). Using a set of empirical equa-tions, the detachment (erosion) of sediment fromthe soil matrix, movement of the eroded sedimentin surface runoff, and scour of sediment are simu-lated to predict suspended-sediment concentrationand load (Donigian and others, 1994).

There are two principal sources of erodedsediment in the WSM, raindrop detachment andagriculture tillage operations. Raindrop detach-ment includes variables for rainfall, energy, ante-cedent soil moisture, and percent of exposed soil.Raindrop detachment occurs throughout the year.Tillage operations from agricultural activities gen-erate sediment from the turning of soil and othercrop maintenance activities. Tillage operationsgenerally occur once or twice a year, and anamount of detached sediment is treated as aninstantaneous addition at time of tillage. Sedimentstorage is the amount of sediment eroded and

Figure 7.3. Schematic of a nested basin defined by upstream anddownstream monitoring stations. (F is the total sediment fluxgenerated within each nested basin, Lu is the upstream monitoredload, and Li is the sum of F and Lu leaving the basin.) (Schwarz andothers, 2001)

Land Processes River Processes

Erosionrate

Sedimentstorage

Transportfactors

Depositionand scour

Suspendedsediment

C H A P T E R 7 | 9 1

available for transport. Sediment storage is calcu-lated for each land use as a balance of sedimentattachment and detachment and washoff. Washoffof detached sediment is a function of antecedentsoil moisture and surface-water runoff. Parametersfor attachment and detachment and washoff areselected to match calculations of annual soil ero-sion from crops, pasture, and forest lands based onNational Resource Inventory (NRI) data applied tothe USLE. Gross erosion rates are reduced by adelivery ratio to represent deposition loss (storage)on the land.

Simulation of suspended sediments in riversis a mass balance of input, advection, scour, anddeposition. Scour and deposition of silt and clay issimulated on an hourly basis by comparing theshear stress calculated by the hydrology module toa critical shear stress. Other parameters are erod-ibility, settling velocity, and bed storage. Separateparameters can be used for silt and clay. Sand con-centration is simulated using a user-input powerfunction of carrying capacity.

Calibration of sediment is a mass-balanceapproach where:

Sediment mass balance =land surface inputs + scour – deposition

– advection downstream (1)

Specifically, the calibration is obtained by(1) setting consistent detached sediment valuesfrom field operations on the basis of crop use,(2) calibration of sediment wash-off from all landuses on the basis of the NRI, and (3) calibration toobserved sediment-concentration data at thewater-quality monitoring sites and adjustment ofscour and deposition parameters.

The RIM Program collects stream samplesfrom the most downstream non-tidal areas in theeight largest basins (Susquehanna, Potomac,James, Patuxent, Rappahannock, York-2 basins,and Appomatox). In addition, one site, Choptank,is sampled on the eastern shore. Using hydrologydata from 1985 to 1994, modeled total averageannual suspended-solids loads from the WSM areapproximately 4 million tons at the “Fall Line”River Input sites. This is in close agreement withthe total average annual long-term monitoringprograms estimated load. An additional 1.25 mil-lion tons are estimated by the WSM to be contrib-uted from land areas (about 15 percent of thewatershed) below the “Fall Line.” The contributionand variability of the modeled loads above andbelow the Fall Line Zone are shown in figure 7.4.The three largest rivers (Susquehanna, Potomac,and James), which represent about 90 percent ofthe total land area above the Fall Line, contributeabout 90-percent of the average streamflow, anddeliver the greatest amount of sediment to theestuary (Langland and others, 1995).

Figure 7.4. Modeled sediment-solids loads above and below the Fall Line. (From Chesapeake BayProgram Watershed Model v. 4.3.)

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Chesapeake Bay Water Quality Model (WQM)

The Chesapeake Bay WQM is a quantitativetool used to simulate the effects of the watershedand shoreline (or bank) contributions of suspendedsolids on water quality in the bay and its tributar-ies. The model is extremely complex. The model asit pertains to the sediment issues discussedthroughout this report will be presented brieflybelow. The reader is encouraged to pursue addi-tional details about the model, which can be foundin Cerco and others (2002) and at the CBP Model-ing Web site http://www.chesapeakebay.net/pubs/subcommittee/mdsc.

Watershed sediment sources from above andbelow the Fall Line derived from the bay WSM areused as input for the WQM. Model simulations arecarried out for all suspended sediment dischargesat an hourly time step and then compiled into adaily average for all river loads input into theWQM. River input sediment loads estimated bythe WQM at sites below the Fall Line are distrib-uted to the lateral cells—areas in the bay nearshorelines—on the basis of the relative watershedarea associated with each of the lateral cells.

Empirical data compiled from variousregions of the Chesapeake Bay are used to parame-terize the WQM for estimates of suspended solidsderived from shoreline erosion. Load estimatesfrom shoreline sources are estimated as long-termaverages expressed in volume or rate of mass peryear on the basis of the volume of eroded materialobtained from comparisons of topographic mapsor aerial photographs that usually span severalyears. Information contained in a report by theUSCOE (1990) and extensive measurements of thecomposition of eroded bank material for the majorVirginia tributaries (Ibison and others, 1992) serveas primary sources of information on shoreline ero-sion (table 7.3).

Estimating bank loads for the model requiresconsideration of the volume of eroded material,the composition of the material, and the fraction oferoded material reaching the water column. Thegrain-size distribution of eroded material reachingthe water column is an extremely important factorbecause sand and gravel sink rapidly and do notcontribute to light attenuation (discussed in chap-ters 1, 5, and 6). Because of the high spatial varia-tion in shoreline sediment sources and gaps in thedatabase, bank erosion is considered in the modelas a spatially and temporally uniform process.Loads to each surface cell are calculated as:

Bank load = (Length) (erosion rate)(fraction of silt/clay in total volume eroded)

(calibration factor to adjust bank loads)(associated nutrient/carbon concentration) (2)

A mean value for bank erosion of11.4 kg m-1 d-1 is used. This total contains anestimated average of about 37-percent coarse mate-rial (sand and gravel); the remainder is fine-grained material (table 7.3). Additional data fromMaryland indicate average bank compositions ofabout 50-percent sand (Hill and others, 2001). Themodel-generated fine-grained solids estimate of5.7 kg m-1 d-1 is a reasonable first approximation ofmean fine-grained suspended load from shorelines(table 7.3).

Calibration of the WQM involves taking theWSM model daily sediment loads and using a con-stant daily input of shoreline erosion loads consis-tent with reported shoreline erosion rates.Sediment loads are removed by regional adjust-ment of settling rates to achieve observed solidconcentrations in the water column consistent withthe tidal program monitoring data.

Table 7.3. Composition of bank solids (from Ibison and others, 1992)

Gravel(percent)

Sand(percent)

Silt(percent)

Clay(percent)

Average of allobservations,

in kilograms permeter per day

Mean 20.3 17.0 60.9 1.8 11.4Median 16.3 16.3 63.1 .1 8.55Standard deviation 16.0 14.1 26.0 .44 8.71Maximum 71.9 60.3 98.7 5.31 32.7Minimum .7 .1 1.6 0 .81Number of samples 255 255 255 255 44Model 5.7

C H A P T E R 7 | 9 3

Two settling parameters are used in themodel: a water column settling rate and a rateincorporating suspended sediments into the sedi-ment layer. In some regions, such as the turbiditymaximum and littoral zones, these settling ratesare adjusted to reduce the amount of sedimententering the sediment layer, providing a methodfor the WQM to simulate re-suspension of fine par-ticles.

Future research on sediment and water qual-ity might include efforts to integrate the WQMefforts with field studies of sediment sources andgrain size, with particular focus on spatial variabil-ity in shoreline loads.

The WQM also produces information show-ing the relative contribution to light attenuationfrom water color, algae and other organic material,and TSS (fig. 7.5). The inorganic component oflight attenuation (suspended sediment) is domi-nant in nearly all bay segments (fig. 7.5). Thesetypes of data are useful for examining possible dif-ferent sediment-reduction allocations and strate-gies. The spatial distribution of the modelsegments area are shown in figure 7.6. However,

the components of attenuation alone do not deter-mine the response to nutrient and solids-loadreductions. Of paramount importance is therequirement by USEPA to bring total attenuationbelow levels that support SAV and meet water-clarity goals. A more useful classification of theChesapeake Bay is to divide into regions subject to(1) nutrient control and (2) sediment-solids control(fig. 7.7). Regions subject to nutrient control areareas that meet living-resources criteria (Batiukand others, 1992) and areas in which criteria can bemet by reducing attenuation from organic matter.These correspond to areas in which attenuationfrom color and fixed solids is less than 2 m-1 forfreshwater species and less than 1.5 m-1 for otherspecies. Regions in which attenuation from colorand sediment solids exceeds 1.5 m-1 (saltwater) to2 m-1 (freshwater) will not support SAV absentreductions in fixed solids. This classification indi-cates SAV cannot be restored to large parts of themajor tributaries solely via nutrient reduction. Res-toration of SAV to the turbidity maximum of themain stem and to the headwaters of several minortributaries also requires sediment-solids reduc-tions (Cerco and others, 2002).

Figure 7.5. Relative proportion of light attenuation by component for major bay segments.(Segment locations shown on figure 7.6.)

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Figure 7.6. Location of estuary model segment number as used in the water-quality model (WQM).

C H A P T E R 7 | 9 5

Figure 7.7. Estuarine areas that benefit more from sediment controls (shaded area) than from nutrient controls(areas shown in yellow) in the watershed and tidal tributaries (Cerco and others, 2002).

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However, as has been discussed throughoutthis report, the bay ecosystem involves very com-plex physical and chemical processes; therefore,addressing nutrient or sediment issues alone mostlikely will not meet the water-clarity goals by 2010.In conclusion, because neither nutrient nor sedi-ment is completely dominant in terms of lightattenuation and resulting loss of water clarity, itmay be necessary for water-resource managers todevelop nutrient and sediment reduction strate-gies. These strategies will vary spatially and tem-porally on the basis of light attenuation factors andoverall cost/benefit analysis.

Sediment Reduction Controls(Best-Management Practices)

The CBP WSM (version 4.3) simulates TSSreductions resulting from the implementation ofbest-management practices. The simulation meth-ods for estimating TSS reductions include land-use

conversions, application of best-management-practice efficiencies, and a combination of land-useconversions and efficiencies (table 7.4).

Land-use conversion represents the conver-sion of one land use into another. Conversion of aland use with a high sediment-loading rate into aland use of lower sediment-loading rate is simu-lated as a reduction in sediment loads. An exampleof such a conversion would be planting riparianforest buffers on conventionally tilled agriculturalland. In this example, the model simulates thereduction as the difference between the conven-tionally tilled land sediment-loading rate minusthe forest loading times the number of acres con-verted. The difficulty with land-use conversion isthat the model assumes the land-use conversion isimmediate and complete. In the example of forestbuffers, the model assumes the newly planted sap-lings immediately function as a mature forest

Table 7.4. Sediment reductions for various best-management practices simulated in theWatershed Model (L. Linker, U.S. Environmental Protection Agency, written commun., 2002)

Best-management practiceSedimentreduction(percent)

Model use

Wetland restoration (high-till) 96 Land-use conversionWetland restoration (low-till) 84 Land-use conversionWetland restoration (hay) 80 Land-use conversionTree planting (high-till) 96 Land-use conversionTree planting (low-till) 84 Land-use conversiontree planting (pasture) 82 Land-use conversionLand retirement (high-till) 90 Land-use conversionLand retirement (low-till) 61 Land-use conversionLand retirement (hay) 53 Land-use conversionForest conservation (pervious urban) 76 Land-use conversionStreambank protection with fencing (pasture) 75 EfficiencyConservation tillage (high-till) 73 Land-use conversionForest buffers (high-till, low-till, hay) 70 Land-use conversion & efficiencyStormwater management (pervious, impervious urban) 65 EfficiencyTree planting (mixed open) 58 Land-use conversionGrass buffers (high-till, low-till) 53 Land-use conversion & efficiencyErosion and sediment control (pervious, impervious urban) 50 EfficiencyForest harvesting practices (forest) 50 EfficiencyFarm plans (high-till) 40 EfficiencyFarm plans (pasture) 14 EfficiencyFarm plans (low-till, hay) 8 EfficiencyStreambank protection without fencing (pasture) 40 EfficiencyAbandoned mine reclamation (exposed/urban) 17 Land-use conversionCover crops (high-till, low-till) 15 Efficiency

C H A P T E R 7 | 9 7

buffer. This results in the WSM model overestimat-ing the sediment-load reductions of the forestbuffer for the period it takes the buffer to reachmaturity equal to that of resident forest. Addi-tional data is needed to develop variable TSS effi-ciencies for land-use conversion accounting for thematurity of the conversion over time. For best-management practices that mature quickly or arequickly functional, such as grass buffers and wet-lands, there most likely would be minimal orinconsequential overestimation.

Application of best-management-practicepercent efficiencies represents the second methodutilized within the watershed model to simulateTSS reductions. These best-management practicesreduce the TSS load by a set percentage of eachacre treated or affected by the best-managementpractice. As an example, implementing a farm planon conventional cropland is estimated to reduceTSS loads by 40 percent for each acre under theplan.

At the time best-management-practice effi-ciencies were developed, limited data were avail-able on the effectiveness of best-managementpractices for reducing TSS loads. Consequently, theCBP decided to use an interim methodology forTSS reductions based on total phosphorus reduc-tions. For nearly all best-management practiceswith TSS reduction efficiencies (except storm-water management), the TSS reduction efficiencies

are set equivalent to the phosphorus reduction effi-ciency for the practice. This interim methodologyis based on the premise that sediment movementand transport is the primary mechanism for phos-phorus transport and that reduction in total phos-phorus results in a similar reduction in sedimentloss. Sediment load reductions also may be overes-timated by not varying the efficiencies of best-management practices for different storm eventsand accounting for design limitations, includingdesign “lifetimes.” The WSM assumes a constantreduction in sediment load for all flows, at alltimes. A reduction in efficiency usually resultsfrom higher flows and the capacity to store, treat,and hold sediment is lost over time.

The percent reduction efficiencies for totalphosphorus are based on a variety of informationsources depending on the particular best-manage-ment practice. These sources include scientific lit-erature, performance data, local site-specificstudies, and best professional judgment in somecases. At the time the efficiencies for the best-man-agement practices were agreed upon by the CBP, itwas acknowledged that new methodologies forestimating TSS reduction efficiencies should beevaluated. Additional data on sediment transportwould be helpful to define separate TSS reductionefficiencies for those best-management practicesthat will be considered for implementation bywater-resource managers to reach new sedimentgoals.

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