minnesota river basin turbidity tmdl scenario report · of 90 mg/l is exceeded under scenario 4...

Minnesota River Basin Turbidity

TMDL Scenario Report

(Including Information that Supports the Lake Pepin Turbidity and Excess Nutrient TMDL)

Prepared for:

Minnesota Pollution Control Agency St. Paul, Minnesota

Prepared by:

December 8, 2009

3200 Chapel Hill-Nelson Hwy, Suite 105 • PO Box 14409 Research Triangle Park, NC 27709

Tel 919-485-8278 • Fax 919-485-8280

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Minnesota River Scenario Report FINAL – December 8, 2009

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Table of Contents List of Tables ................................................................................................................. iii List of Figures ...................................................................................................................v Executive Summary........................................................................................................... ix 1 Introduction...................................................................................................................1

1.1 Water Quality Impairments............................................................................................................1 1.2 The Minnesota River Basin Model ................................................................................................4 1.3 Use of the Model to Support TMDLs ............................................................................................5

2 Preliminary Scenarios...................................................................................................7 2.1 Preliminary Scenario Components.................................................................................................7

2.1.1 Scenario 1 ................................................................................................................................7 2.1.2 Scenario 2 ..............................................................................................................................10 2.1.3 Scenario 3 ..............................................................................................................................11 2.1.4 Scenario 4 ..............................................................................................................................13

2.2 Results of Preliminary Scenarios .................................................................................................18 2.2.1 TSS Results............................................................................................................................18 2.2.2 Analysis of Loading Sources .................................................................................................34 2.2.3 Phosphorus and Sediment Export at Jordan...........................................................................49

3 TMDL Targets Scenario (Scenario 5) ........................................................................53 3.1 TMDL Targets .............................................................................................................................53

3.1.1 TSS Surrogates ......................................................................................................................53 3.1.2 Margin of Safety ....................................................................................................................54 3.1.3 Flow Duration Analysis .........................................................................................................54

3.2 TMDL Targets Scenario Setup ....................................................................................................55 3.2.1 Scenario 5 Baseline Setup......................................................................................................55 3.2.2 Additional Management Measures ........................................................................................56

3.3 TMDL Targets Scenario Results..................................................................................................57 3.3.1 TMDL Targets Scenario Results − Tributary Watersheds.....................................................57 3.3.2 TMDL Targets Scenario Results – Minnesota River Mainstem............................................68 3.3.3 Total Sediment Loads by Major Watershed ..........................................................................79 3.3.4 Mass Export from the Minnesota River at Jordan .................................................................80 3.3.5 TSS Loading by Land Use for TMDL Targets Scenario .......................................................82 3.3.6 Sediment Loading by Land Use for TMDL Targets Scenario ...............................................82

3.4 Sensitivity Analysis......................................................................................................................85 4 Summary .................................................................................................................93 5 References .................................................................................................................95


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List of Tables Table 1-1. Waterbody Segments Impaired by Turbidity Addressed by the Minnesota River Model .....4 Table 2-1. Scenario 1 Development.........................................................................................................7 Table 2-2. Scenario 1 Changes to Wastewater Phosphorus Discharges ..................................................9 Table 2-3. Scenario 2 Development.......................................................................................................10 Table 2-4. Scenario 3 Development.......................................................................................................11 Table 2-5. Scenario 4 Development.......................................................................................................14 Table 2-6. Flow-Weighted TSS Concentrations by Season for Baseline Calibration Conditions

and Scenarios 1 through 4....................................................................................................19 Table 2-7. Blue Earth-River near Mankato, Percentage of Time the Surrogate TSS Standard

of 90 mg/L is Exceeded under Scenario 4 ...........................................................................27 Table 2-8. Blue Earth River near Rapidan, Percentage of Time the Surrogate TSS Standard

of 90 mg/L is Exceeded under Scenario 4 ...........................................................................28 Table 2-9. Blue Earth at Good Thunder, Percentage of Time the Surrogate TSS Standard

of 90 mg/L is Exceeded under Scenario 4 ...........................................................................29 Table 2-10. Chippewa River at Montevideo, Percentage of Time the Surrogate TSS Standard

of 50 mg/L is Exceeded under Scenario 4 ...........................................................................29 Table 2-11. Cottonwood River at New Ulm, Percentage of Time the Surrogate TSS Standard

of 70 mg/L is Exceeded under Scenario 4 ...........................................................................30 Table 2-12. Hawk Creek at Sacred Heart, Percentage of Time the Surrogate TSS Standard

of 50 mg/L is Exceeded under Scenario 4 ...........................................................................31 Table 2-13. Le Sueur River at Rapidan, Percentage of Time the Surrogate TSS Standard

of 90 mg/L is Exceeded under Scenario 4 ...........................................................................31 Table 2-14. Redwood River at Redwood Falls, Percentage of Time the Surrogate TSS Standard

of 70 mg/L is Exceeded under Scenario 4 ...........................................................................32 Table 2-15. Yellow Medicine River at Granite Falls, Percentage of Time the Surrogate TSS Standard

of 50 mg/L is Exceeded under Scenario 4 ...........................................................................33 Table 2-16. Watonwan River at Garden City, Percentage of Time the Surrogate TSS Standard

of 90 mg/L is Exceeded under Scenario 4 ...........................................................................33 Table 2-17. Statistics for Daily Average TSS Concentrations at the Mouth of the Le Sueur

under Baseline, Scenario 4, and Scenario 4 with Increased Critical Shear Stress ...............47 Table 2-18. Statistics for Daily Average TSS Concentrations at the Mouth of the Le Sueur ..................48 Table 2-19. Phosphorus Export (lbs/yr), Minnesota River at Jordan, by Water Year..............................49 Table 2-20. Sediment Export, Minnesota River at Jordan, by Water Year (tons)....................................50 Table 3-1. TSS Surrogate Concentrations for Minnesota River.............................................................53 Table 3-2. 95th Percentile Flows (cfs) at USGS Gages Determined by MPCA .....................................54 Table 3-3. Scenario 5 Results for Blue Earth River near Mankato ........................................................63 Table 3-4. Scenario 5 Results for Blue Earth River at Rapidan.............................................................63 Table 3-5. Scenario 5 Results for Blue Earth River at Good Thunder...................................................64 Table 3-6. Scenario 5 Results for Chippewa River at Montevideo........................................................64 Table 3-7. Scenario 5 Results for Cottonwood River at New Ulm........................................................65 Table 3-8. Scenario 5 Results for Hawk Creek at Sacred Heart ............................................................65 Table 3-9. Scenario 5 Results for Le Sueur River at Rapidan ...............................................................66 Table 3-10. Scenario 5 Results for Redwood River at Redwood Falls ....................................................66 Table 3-11. Scenario 5 Results for Watonwan River at Garden City ......................................................67 Table 3-12. Scenario 5 Results for Yellow Medicine River at Granite Falls ...........................................67


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Table 3-13. Scenario 5 Results for Minnesota River from Chippewa River to Stony Run Creek ...........74 Table 3-14. Scenario 5 Results for Minnesota River from Minnesota Falls Dam to Hazel Creek...........74 Table 3-15. Scenario 5 Results for Minnesota River from Timms Creek to Redwood Creek .................75 Table 3-16. Scenario 5 Results for Minnesota River from Beaver Creek to Birch Coulee......................75 Table 3-17. Scenario 5 Results for Minnesota River from Cottonwood River to Little Cottonwood......76 Table 3-18. Scenario 5 Results for Minnesota River from Swan Lake Output to Minneopa Creek ........76 Table 3-19. Scenario 5 Results for Minnesota River at Mankato ............................................................77 Table 3-20. Scenario 5 Results for Minnesota River from Shanaska Creek to Rogers Creek .................77 Table 3-21. Scenario 5 Results for Minnesota River from Rush River to High Island Creek .................78 Table 3-22. Scenario 5 Results for Minnesota River at Jordan................................................................78 Table 3-23. Summary of Changes in Average Upland Sediment Delivery Rates (tons/ac/yr)

by Land Use for TMDL Targets Scenario ...........................................................................83 Table 3-24. Summary of Changes in Total Sediment Loading (tons/yr) by Source for

TMDL Targets Scenario ......................................................................................................83 Table 3-25. Sensitivity Analysis of Sediment Loading (tons/yr) and Percent Reduction

Associated with Groups of Management Options, 2001-2006 ............................................87 Table 3-26. Differences in Average Annual Sediment Load (tons/yr) Due to Omission

of a Group of Management Options, 2001-2006 .................................................................88 Table 3-27. Average Number of Days per Year Greater than TSS Surrogate Concentration f

or TMDL Scenario and TMDL Scenario with Specific Groups of Management Options Omitted, 2001-2006 ...............................................................................................89

Table 3-28. Change in Average Number of Days per Year in Excess of TSS Surrogate Concentration Due to Omission of a Group of Management Options, 2001-2006..............90


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List of Figures Figure 1-1. Base Map of the Minnesota River Watershed ........................................................................2 Figure 1-2. Segments of the Minnesota River Basin Listed for Turbidity Impairments ...........................3 Figure 2-1. Modeled Land Use for Existing Conditions and Scenario 4.................................................18 Figure 2-2. Seasonal Flow-Weighted Average TSS, Chippewa River at Montevideo............................21 Figure 2-3. Seasonal Flow-Weighted Average TSS, Yellow Medicine River at Granite Falls...............21 Figure 2-4. Seasonal Flow-Weighted Average TSS, Hawk Creek at Sacred Heart ................................22 Figure 2-5. Seasonal Flow-Weighted Average TSS, Redwood River at Redwood Falls ........................22 Figure 2-6. Seasonal Flow-Weighted Average TSS, Cottonwood River at New Ulm............................23 Figure 2-7. Seasonal Flow-Weighted Average TSS, Watonwan River at Garden City ..........................23 Figure 2-8. Seasonal Flow-Weighted Average TSS, Blue Earth River at Good Thunder.......................24 Figure 2-9. Seasonal Flow-Weighted Average TSS, Blue Earth River at Rapidan.................................24 Figure 2-10. Seasonal Flow-Weighted Average TSS, Blue Earth River near Mankato............................25 Figure 2-11. Seasonal Flow-Weighted Average TSS, Le Sueur River at Rapidan ...................................25 Figure 2-12. Seasonal Flow-Weighted Average TSS, Minnesota River at Mankato ................................26 Figure 2-13. Seasonal Flow-Weighted Average TSS, Minnesota River at St. Peter.................................26 Figure 2-14. Seasonal Flow-Weighted Average TSS, Minnesota River at Jordan....................................27 Figure 2-15. Blue Earth River near Mankato, Scenario 4 Simulated TSS ................................................28 Figure 2-16. Blue Earth River at Rapidan, Scenario 4 Simulated TSS .....................................................28 Figure 2-17. Blue Earth River at Good Thunder, Scenario 4 Simulated TSS ...........................................29 Figure 2-18. Chippewa River at Montevideo, Scenario 4 Simulated TSS ................................................30 Figure 2-19. Cottonwood River at New Ulm, Scenario 4 Simulated TSS ................................................30 Figure 2-20. Hawk Creek at Sacred Heart, Scenario 4 Simulated TSS.....................................................31 Figure 2-21. Le Sueur River at Rapidan, Scenario 4 Simulated TSS........................................................32 Figure 2-22. Redwood River at Redwood Falls, Scenario 4 Simulated TSS ............................................32 Figure 2-23. Yellow Medicine River at Granite Falls, Scenario 4 Simulated TSS ...................................33 Figure 2-24. Watonwan River at Garden City, Scenario 4 Simulated TSS..............................................34 Figure 2-25. Sediment Load Sources - Blue Earth River above Rapidan River........................................35 Figure 2-26. Sediment Loading Rates for Blue Earth above Rapidan River.............................................35 Figure 2-27. Sediment Load Sources - Chippewa River at Montevideo ...................................................36 Figure 2-28. Sediment Loading Rates for Chippewa River at Montevideo ..............................................36 Figure 2-29. Sediment Load Sources - Cottonwood River at New Ulm ...................................................37 Figure 2-30. Sediment Loading Rates for Cottonwood River at New Ulm ..............................................37 Figure 2-31. Sediment Load Sources - Hawk Creek at Sacred Heart........................................................38 Figure 2-32. Sediment Loading Rates for Hawk Creek at Sacred Heart ...................................................38 Figure 2-33. Sediment Load Sources - Le Sueur River at Rapidan...........................................................39 Figure 2-34. Sediment Loading Rates for Le Sueur River at Rapidan ......................................................39 Figure 2-35. Sediment Load Sources - Redwood River at Redwood Falls ...............................................40 Figure 2-36. Sediment Loading Rates for Redwood River at Redwood Falls...........................................40 Figure 2-37. Sediment Load Sources - Watonwan River at Garden City..................................................41 Figure 2-38. Sediment Loading Rates for Watonwan River at Garden City .............................................41 Figure 2-39. Sediment Load Sources - Yellow Medicine River at Granite Falls ......................................42 Figure 2-40. Sediment Loading Rates for Yellow Medicine River at Granite Falls .................................42 Figure 2-41. TSS Average Concentrations for Le Sueur River Segments, 1993-2006 .............................43


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Figure 2-42. Percentage of TSS Observations Greater than 90 mg/L for Le Sueur River, 1993-2006 .....43 Figure 2-43. 95th Percentile TSS Concentrations for Le Sueur River, 1993-2006 ....................................44 Figure 2-44. Index of Importance of Channel versus Upland Processes in Le Sueur River

Sediment Load (at Mouth) under Scenario 4 .......................................................................45 Figure 2-45. Histogram Showing Results of Increased Critical Shear Stress Values in the

Bluff Reaches of the Le Sueur .............................................................................................47 Figure 2-46. Index of Importance of Channel versus Upland Processes in Le Sueur River

Sediment Load (at Mouth) after Implementing Increases in Critical Shear Stress in Channel Reaches and Reductions in Load from Developed Land...................................48

Figure 2-47. Phosphorus Export, Minnesota River at Jordan, Water Years 2001-2006............................50 Figure 2-48. Sediment Export, Minnesota River at Jordan, Water Years 2001-2006 ...............................51 Figure 3-1. Scenario 5 Results for Blue Earth River near Mankato ........................................................58 Figure 3-2. Scenario 5 Results for Blue Earth River at Rapidan.............................................................58 Figure 3-3. Scenario 5 Results for Blue Earth River at Good Thunder...................................................59 Figure 3-4. Scenario 5 Results for Chippewa River at Montevideo........................................................59 Figure 3-5. Scenario 5 Results for Cottonwood River at New Ulm........................................................60 Figure 3-6. Scenario 5 Results for Hawk Creek at Sacred Heart ............................................................60 Figure 3-7. Scenario 5 Results for Le Sueur River at Rapidan ...............................................................61 Figure 3-8. Scenario 5 Results for Redwood River at Redwood Falls ....................................................61 Figure 3-9. Scenario 5 Results for Watonwan River at Garden City ......................................................62 Figure 3-10. Scenario 5 Results for Yellow Medicine River at Granite Falls ...........................................62 Figure 3-11. Seasonal Flow-Weighted Average TSS for Scenario 5, Blue Earth River at Mankato ........68 Figure 3-12. Scenario 5 Results for Minnesota River from Chippewa River to Stony Run Creek ...........69 Figure 3-13. Scenario 5 Results for Minnesota River from Minnesota Falls Dam to Hazel Creek...........69 Figure 3-14. Scenario 5 Results for Minnesota River from Timms Creek to Redwood Creek .................70 Figure 3-15. Scenario 5 Results for Minnesota River from Beaver Creek to Birch Coulee......................70 Figure 3-16. Scenario 5 Results for Minnesota River from Cottonwood River to Little Cottonwood......71 Figure 3-17. Scenario 5 Results for Minnesota River from Swan Lake Output to Minneopa Creek ........71 Figure 3-18. Scenario 5 Results for Minnesota River at Mankato ............................................................72 Figure 3-19. Scenario 5 Results for Minnesota River from Shanaska Creek to Rogers Creek .................72 Figure 3-20. Scenario 5 Results for Minnesota River from Rush River to High Island Creek .................73 Figure 3-21. Scenario 5 Results for Minnesota River at Jordan................................................................73 Figure 3-22. Total Sediment Load by Major Watershed, Existing Baseline Conditions...........................79 Figure 3-23. Total Sediment Load by Major Watershed, Scenario 5 ........................................................79 Figure 3-24. Comparison of Total Suspended Solids Load Results for All Scenarios

(Water Years 2001-2006) ....................................................................................................80 Figure 3-25. Comparison of Total Phosphorus Load Results for All Scenarios .......................................81 Figure 3-26. Total Phosphorus Load by Major Watershed, Existing Baseline Conditions .......................81 Figure 3-27. Total Phosphorus Load by Major Watershed, Scenario 5 Conditions ..................................82 Figure 3-28. Average Annual Sources of Sediment Load to the Minnesota River under

Existing Baseline Conditions...............................................................................................84 Figure 3-29. Average Annual Sources of Sediment Load to the Minnesota River under

TMDL Targets Scenario Conditions....................................................................................84 Figure 3-30. Load Gain Associated with Omitting Specific Management Groups, 2001-2006................88


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Figure 3-31. TSS Average Concentrations, Le Sueur River Segments 1993-2002, for Existing Conditions, TMDL Scenario, and Sensitivity Analyses Omitting a Specific Group of Management Options............................................................................................91

Figure 3-32. Percentage of TSS Concentrations Greater than 90 mg/L, Le Sueur River Segments 1993-2002, for Existing Conditions, TMDL Scenario, and Sensitivity Analyses Omitting a Specific Group of Management Options ...........................................................91


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Executive Summary Segments of the Minnesota River and its tributaries are impaired by elevated turbidity. Turbidity is a measure of light scattering or cloudiness of water, and elevated turbidity inhibits healthy plant growth on the river bottom, impedes the ability of aquatic organisms to find food, and can choke the gills of fish.

To protect the legally designated use of the Minnesota River and tributaries to support healthy aquatic life, the State has specified a numeric limit or water quality standard for turbidity. This water quality standard has been exceeded in 18 waterbody segments of the Minnesota River watershed. These waterbody segments thus do not achieve their designated use for the support of aquatic life. In such cases, the Clean Water Act requires the State to estimate a Total Maximum Daily Load (TMDL), which is the maximum amount of pollutant load that is consistent with the affected waterbody segments meeting water quality standards and supporting their designed uses.

Turbidity, as an index of light scattering, is not itself representable as a mass load of pollutants. The Minnesota Pollution Control Agency (MPCA) has, however, demonstrated that elevated turbidity in the Minnesota River system is caused primarily by loads of suspended inorganic sediment. MPCA has developed concentration targets for suspended sediment that are consistent with achieving the water quality standard for turbidity. Suspended sediment – unlike turbidity – is a measurable mass quantity. MPCA must therefore develop a TMDL to specify the appropriate limits on sediment loading that will enable compliance with the turbidity water quality standard in the basin.

Sediment load to the Minnesota River and its tributaries derives from a variety of sources, including surface erosion from fields and other land uses, ravines and gullies that cause mass loss of soil, erosion of the stream banks and beds, and the collapse of bluff faces in areas where tributary streams descend into the old glacial river valley. Sorting out the relative importance of these different sources under a variety of conditions is a difficult task – but one that is essential to formulate a useful solution that targets load reductions appropriately. To accomplish this, two things are needed: a mathematical simulation model that represents the response of the system to various management options, and multiple lines of evidence to help constrain the source representation and assure that it is reasonable and appropriate.

The mathematical simulation model is the Hydrologic Simulation Program – FORTRAN (HSPF), a comprehensive, EPA-supported model of watershed hydrology and pollutant generation and transport operating at an hourly time step. An HSPF model of the Minnesota River watershed was previously developed to represent rainfall-runoff and nutrient/oxygen demand transport and reaction processes in the watershed. For the current effort, the existing HSPF model was significantly refined to better represent sediment load generation, transport, and channel hydraulic properties. (Channel hydraulics are important because they determine both sediment transport capacity and channel bed/bank erosion processes.)

Multiple lines of evidence were used to ensure that the simulation model provides a reasonable representation of real world processes. The simulation model was successfully calibrated (and validated) to observed flows and suspended concentrations at multiple locations in the basin. Representation of instream observations is not, by itself, sufficient to ensure that the relative importance of different sources of load is correctly captured in the simulation. It is important to guard against incorrect interpretations that provide an acceptable net result by virtue of compensating errors that attribute too great an importance to one source category while undervaluing others. To better constrain the model, MPCA collected radioisotope data that distinguish between sediment sources in recent contact with the atmosphere (e.g., surface erosion from fields) and those that derive from deeper, buried sediment (e.g., ravines, bluffs), along with other evidence on rates of bluff collapse and channel incision. These multiple lines of evidence indicate that approximately one-third of the sediment load in the Minnesota


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River derives from surface erosion, one-third from ravines, and one-third from channel bed, bank, and bluff contributions. The calibrated model successfully represents the source attribution information.

Calibration and validation of the flow and water quality simulation model is documented in the earlier Model Calibration and Validation Report (Tetra Tech, 2008). The present report uses the calibrated model to investigate what types and amounts of load reductions would be needed to meet the water quality standard.

Past efforts at improving land management in the Minnesota River basin have focused on reducing soil losses from cropland. While this is an important effort – and one that has achieved considerable success – these surface loads constitute only about one-third of the total sediment load transported in the Minnesota River. Full compliance with the water quality standards can only be achieved if the other two-thirds of the loads (from ravines and bed, bank, and bluff processes) are addressed as well. Controlling these additional load sources is a difficult challenge – but one that is not divorced from upland management as it is the pattern of runoff flows – particularly the enhancement of high flows caused by drainage management – that has exacerbated the non-surface contributions to surface loads.

This Scenario Report describes the development of five major model simulation scenarios constructed to hone in on a strategy that would achieve full compliance with water quality standards. The first three scenarios focus on traditional techniques of upland management at levels generally thought to be acceptable to the majority of stakeholders in the basins. The results predict that these efforts, even if fully implemented, would fall far short of attaining the turbidity standards – in large part because the two-thirds of the load that is not from surface erosion is not addressed to a significant degree.

The fourth model scenario takes bold steps to decrease the remaining load sources, including aggressive measures to increase the Conservation Reserve Program, reduce hydraulic energy that erodes ravines, and stabilize the stream channels in the bluff areas.

Scenario 4 is predicted to provide significant reductions in sediment loads to the Minnesota River, but is still not sufficient to meet water quality standards. This scenario does, however, point the way forward to efforts that would be needed for full compliance. Extensive diagnostic evaluations of the results of Scenario 4 were then used to design Scenario 5, the TMDL Targets Scenario, which is predicted to achieve the specified objectives. Most importantly, Scenario 5 incorporates extensive channel rehabilitation efforts to reduce bed, bank, and bluff loads, in addition to large reductions in surface and ravine loads of sediment.

The simulation model predicts that, to achieve full compliance with water quality standards, the total load of sediment delivered to the larger rivers in the Minnesota River watershed would need to be reduced from current levels by approximately 90 percent. This includes similar reductions in all three of the major source categories – upland erosion, ravines, and bed, bank, and bluff processes. The needed reductions are obtained through combinations of the following practices:

• Increase in the area of pasture, Conservation Reserve, and perennial crop lands, • Increased adoption of conservation tillage, • Elimination of surface tile drain inlets, • Reduction in ravine erosion through use of drop structures on tile drain outlets, • Detention of the first inch of runoff from cropland near the source area, • Infiltration of the first inch of runoff from urban land in MS4 areas, • Reduction in sediment load from urban land outside MS4 boundaries, • Reduction in rates of bluff collapse, and • Rehabilitation of channels to reduce bed and bank erosion in the bluff reaches.


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1 Introduction The purpose of this document is to describe applications undertaken with the calibrated Minnesota River Watershed Model to support the Minnesota River Turbidity TMDL Project. This report builds upon the previously published Minnesota River Basin Model Calibration and Validation Report (Tetra Tech, 2008), which documents the development of the model. Specifically, the model is used in this report to predict and evaluate the efficacy of a variety of management strategies that have been proposed to achieve water quality standards for turbidity.

1.1 WATER QUALITY IMPAIRMENTS The Minnesota River begins at Big Stone Lake on the South Dakota border, and from there flows 335 miles southeast to Mankato and then northeast to join the Mississippi at Fort Snelling (Figure 1-1). The watershed covers 16,770 square miles. Most of the watershed was originally native prairie and pothole wetlands. Now it is part of the Corn Belt, with the majority of the land area converted to corn-soybean rotation and other types of agriculture. For many parts of the watershed conversion to agriculture required enhancement of drainage through ditches and subsurface tile drains.

Many portions of the Minnesota River drainage exhibit high levels of turbidity. Turbidity of water is a measure of light scattering, and is caused by suspended and dissolved matter, such as clay, silt, organic matter, algae, and stains due to dissolved organic compounds. In the Minnesota River, increased turbidity is primarily due to total suspended solids (TSS). TSS is the concentration of suspended material in the water as measured by the dry weight of solids filtered out of a known volume of water. TSS can include sand, silt, clay, plant fibers, algae, and other organic material. Inorganic sediment in water typically makes up most of the TSS and turbidity under conditions when turbidity is elevated; however, organic matter can be the dominant contributor to turbidity at other times, particularly under low flow conditions.

It is important to note that turbidity is a measure of light scattering, not light attenuation, although the two phenomena are closely related. In addition to scattering, attenuation is also caused by absorption of light by dissolved organic compounds. Such compounds play an important role in limiting light in aquatic systems, but are not effectively measured by standard turbidity meters.

Turbidity and light absorption together limit light penetration and inhibits healthy plant growth on the river bottom. With elevated turbidity, aquatic organisms may have trouble finding food, gill function may be affected, and elevated amounts of sediment associated with turbidity can cause spawning areas and other habitat to be covered. To address these problems, Minnesota water quality regulations establish a limit or water quality standard for turbidity. In class 2B waters (such as the Minnesota River), the water quality standard for turbidity is 25 NTUs. “NTUs” stands for Nephelometric Turbidity Units, which is a standardized measure of light scattering in a water sample.

Eighteen waterbody segments in the Minnesota River basin are listed as impaired by turbidity and will require the development of Total Maximum Daily Loads (TMDLs) (Figure 1-2 and Table 1-1).


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Figure 1-1. Base Map of the Minnesota River Watershed


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Figure 1-2. Segments of the Minnesota River Basin Listed for Turbidity Impairments

Source: (MPCA, 2005)


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Table 1-1. Waterbody Segments Impaired by Turbidity Addressed by the Minnesota River Model

Segment MPCA Assessment Unit

Minnesota River; Chippewa R to Stoney Run Cr 07020004-501

Yellow Medicine River; Spring Cr to Minnesota R 07020004-502

Minnesota River; Timms Cr to Redwood R 07020004-509

Minnesota River; Minnesota Falls Dam to Hazel Cr 07020004-515

Hawk Creek; Spring Cr to Minnesota R 07020004-587

Chippewa River; Watson Sag Diversion to Minnesota R 07020005-501

Redwood River; Ramsey Cr to Minnesota R 07020006-501

Minnesota River; Shanaska Cr to Rogers Cr 07020007-501

Minnesota River; Blue Earth R to Shanaska Cr 07020007-502

Minnesota River; Cottonwood R to Little Cottonwood R 07020007-503

Minnesota River; Swan Lk Outlet to Minneopa Cr 07020007-505

Minnesota River; Beaver Cr to Birch Coulee 07020007-514

Cottonwood River; JD #30 to Minnesota R 07020008-501

Blue Earth River; Le Sueur R to Minnesota R 07020009-501

Blue Earth River; Rapidan Dam to Le Sueur R 07020009-509

Watonwan River; Perch Cr to Blue Earth R 07020010-501

Le Sueur River; Maple R to Blue Earth R 07020011-501

Minnesota River; Rush R to High Island Cr 07020012-503

The same loading of sediment that causes increased turbidity in the Minnesota River also affects water quality downstream, particularly in Lake Pepin, an impoundment of the Mississippi River. The Minnesota River is estimated to contribute about 80 to 90 percent of the sediment load to Lake Pepin (Kelley et al., 2006). Lake Pepin is subject to accelerated infilling due to upstream sediment loads along with turbidity problems. Sediment-associated nutrients, such as phosphorus, move along with the sediment. At low flows, these increased nutrient loads accelerate algal growth in Lake Pepin.

1.2 THE MINNESOTA RIVER BASIN MODEL To support the development of TMDLs in the basin, Tetra Tech developed a dynamic water quality simulation model. The Minnesota River Basin Model consists of 10 linked Hydrologic Simulation


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Program – FORTRAN or HSPF models (Bicknell et al., 2001), addressing eight major subwatersheds plus the Middle Minnesota and Lower Minnesota River mainstem from Lac qui Parle to Jordan, MN.

HSPF is a comprehensive modeling package developed by EPA for simulating water quantity and quality for a wide range of organic and inorganic pollutants from complex watersheds. In HSPF, a subwatershed is typically conceptualized as a group of various land uses all routed to a representative stream segment. Several small subwatersheds and representative streams may be networked together to represent a larger watershed drainage area. Various modules are available and may be readily activated to simulate various processes, both on land and in-stream.

Land processes for pervious and impervious areas are simulated through water budget, sediment generation and transport, and water quality constituents’ generation and transport. Hydrology is modeled as a water balance in multiple surface and soil layer storage compartments. Interception, infiltration, evapotranspiration, interflow, groundwater loss, and overland flow processes are considered and are generally represented by empirical equations. Sediment production is based on detachment and/or scour from a soil matrix and transport by overland flow in pervious areas, whereas solids buildup and washoff is simulated for impervious areas. It includes agricultural components for land-based nutrient and pesticide processes and a special actions block for simulating management activities. HSPF also simulates the in-stream fate and transport of a wide variety of pollutants, such as nutrients, sediments, tracers, dissolved oxygen/biochemical oxygen demand, temperature, bacteria, and user-defined constituents.

Development of HSPF models for the Minnesota River basin has proceeded over many years, and was originally focused on the simulation of flow and nutrients. Recently, the model has been updated to represent conditions in the basin through 2006. A number of specific enhancements were made to better refine the suspended sediment calibration, most significantly including the use of existing flood models to improve estimates of channel shear stress and better characterization of river corridor sources. A variety of lines of evidence, including radioisotope analyses, were used to ensure that the model provides an accurate attribution of pollutant loads to different source classes, including surface washoff, development of ravines, channel erosion and deposition processes, and point source loads. Additional information from new smaller-scale modeling efforts was also incorporated. These efforts have resulted in the creation of an improved model of demonstrated predictive ability that meets MPCA’s needs for completing the turbidity and nutrient TMDLs.

The revised model was recalibrated for flow, sediment, total phosphorus, and total nitrogen. The model development, calibration, and validation process is described in detail in the Model Calibration Report (Tetra Tech, 2008). As described in that report, the model achieves desired quality criteria and provides a high quality representation of the generation, fate, and transport of flows, sediment, and pollutant loads in the watershed.

1.3 USE OF THE MODEL TO SUPPORT TMDLS Following the acceptance of the model, the next step is to apply the model to determine what management actions can be taken to achieve water quality standards. This report documents the application of the model to a variety of scenarios. Each scenario represents a combination of specific management activities. The model application then enables examination of the efficacy of the scenario in achieving management goals. A total of five major scenarios were run, in addition to the baseline simulation; however, many of these scenarios involved a variety of different sub-scenarios. The first four scenarios were designed primarily to test the sensitivity of responses to various management alternatives proposed by stakeholders. The fifth scenario explicitly seeks to achieve compliance with water quality standards, building upon the information obtained from the first four scenarios.

Section 303(d) of the Clean Water Act (CWA) as amended by the Water Quality Act of 1987 and EPA’s Water Quality Planning and Management Regulations (Title 40 of the Code of Federal Regulations


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(CFR), Part 130) requires states to identify waterbodies which are not meeting water quality standards applicable to their designated use classifications. Section 303(d) also requires that Total Maximum Daily Loads (TMDLs) be developed for all waterbodies for which technology based limits alone are not sufficient to achieve water quality standards (USEPA, 1991).

A TMDL represents the total load of a pollutant that can be discharged to a waterbody and still meet the applicable water quality standards. The TMDL can be expressed as the total mass or quantity of a pollutant that can enter the waterbody within a unit of time. In most cases, the TMDL determines the allowable load for a constituent and divides it among the various contributors in the watershed as wasteload allocations (i.e., allocations to permitted point source discharges) and load allocations (i.e., allocations to nonpoint source). The TMDL also accounts for natural background sources and provides a margin of safety.

Determination of the allowable load portion of the TMDL is a straightforward process where numeric water quality criteria or targets are available. In essence, the load must be limited to amounts such that, when divided by flow, the concentration criterion is not exceeded. For Minnesota River turbidity, MPCA has defined specific surrogate objectives for total suspended solids (TSS) concentrations that will achieve the turbidity standard in each impaired reach ranging from 50 to 100 mg/L (see Section 3.1). The gross amount of reduction from existing loads needed to achieve standards can be estimated directly from observed data, prior to application of the simulation model. MPCA is following this approach to determine the allowable load through use of a load-duration analysis. The load-duration analysis does not, however, explicitly identify the source of the load or how the needed level of control may be achieved. The simulation model thus provides the tool for determining allocations – how the total allowable load can be partitioned to individual sources – and for evaluating implementation strategies – demonstrating what specific management actions can be used to achieve the needed load reductions.

Additional loading targets for the export of sediment and nutrient load from the Minnesota River at Jordan will be defined via the downstream Lake Pepin TMDL effort, as the Minnesota River is believed to provide the largest portion of the loads that affect Lake Pepin. The State will need to meet the most stringent reductions determined by both the Minnesota River and Lake Pepin TMDLs. While the Lake Pepin TMDL has not yet been finalized, it appears at this time that achieving turbidity standards within the Minnesota River will provide the more stringent reduction requirements.


7

2 Preliminary Scenarios Scenarios 1 through 4 evaluate the simulated responses of the system to a variety of management actions and other anticipated changes and are grouped as preliminary scenarios. The resulting information was then used to construct a scenario that achieves TMDL targets (Scenario 5).

This section summarizes the development of and results from the four preliminary scenarios. Section 2.1 describes the development of each of these scenarios. Results are then summarized in Section 2.2. Particular attention is paid to the results of Scenario 4, including detailed diagnostics of the causes and sources of remaining excursions of the water quality targets for TSS.

2.1 PRELIMINARY SCENARIO COMPONENTS

2.1.1 Scenario 1 Scenario 1 was designed to represent “continuous change.” It generally represents a continuation of current trends in land use and management practices, and is expected to achieve a small but steady course of incremental improvements. The focus was on two areas: current trends in crop production and management and permitting requirements for Municipal Separate Storm Sewer Systems (MS4s) draining urban lands.

The components included in Scenario 1 are summarized in Table 2-1. Additional changes to wastewater discharges are summarized in Table 2-2.

Table 2-1. Scenario 1 Development

Scenario Component Modeling Approach Implementation

Land Use:

16 to 18 percent reduction in CRP acreage per USDA estimates due to ethanol.

Small increase in pasture acres due to regional increases in livestock.

General changes in land use are addressed by repopulating the land use table in the Schematic block. Spreadsheet tools were created to facilitate this.

Set to 17% reversion of CRP to row crop.

As CRP includes land converted from tillage to pasture, the “small increase in pasture” is assumed to be represented in the net CRP conversion (e.g., the difference between 17% and 18%)

Cropping System

50 percent of land coming out of CRP goes into corn and 50 percent into soybeans. Check estimates from Food and Agricultural Policy Research Institute.

50 percent of acres of land over 3 percent slope use crop residue of 30 percent or greater.

Regarding nutrient management “Follow recommendations of U of M, local cooperatives, or crop consultants.”

The model already represents cropland as corn-soy rotation. This assumption can’t be easily changed.

Increasing 30% residue on higher slope lands is done by calculating the fraction in the higher slope category for each subbasin and adjusting the conservation tillage area accordingly.

As stated.

Implement as stated.

Assume no changes to nutrient generation rates; however, nutrient load will change with changes in erosion and runoff


8


Wastewater Discharges

1 mg/l for facilities upgrading or expanding.

1 mg/L P limit is simulated by changing the ext sources linkage to represent P as a fixed concentration multiplier on effluent flow.

List of facilities below. Some changes are also necessary to address facilities that have recently received limits other than 1 mg/L. Those with current 1 mg/L limits are also simulated as a function of flow as their earlier loads were higher.

Urban Stormwater

Treat the first ½ inch of runoff

Generalized “treatment” of urban runoff is approximated by inserting scaling factors into the mass link block to reduce sediment and nutrient loads. Explicit simulation of specific stormwater treatment options, such as detention ponds, is not done for Scenario 1, so impacts on hydrology are not included.

Apply to urban impervious area within designated Phase II MS4 boundaries. Assume the unspecified treatment has no impact on hydrograph. Assume the treated volume achieves reductions of 65% in TSS, 52% in TP, and 20% in TN. (The first two rates correspond to MN Stormwater Manual average removal rates for wet detention; the TN rate is based on current influent/effluent medians for wet detention in the International Stormwater BMP Database). Runoff in excess of ½ in with associated load is routed directly to stream.


9

Table 2-2. Scenario 1 Changes to Wastewater Phosphorus Discharges

Facility Watershed Representation

Blue Earth Blue Earth Simulate at 1 mg/L (current permit) Fairmont Blue Earth Simulate at 1 mg/L (current permit) Trimont Blue Earth Simulate at 1 mg/L (current permit) Benson Chippewa Simulate at 1 mg/L (current permit) Hancock Chippewa Current limit of 4 mg/L, but have frequently discharged

higher, so change to 4 mg/L as function of flow Farwell/Kensington Chippewa Has permit limit of 4 mg/L, but consistently less. No

changes to existing simulation Montevideo Chippewa Simulate at 1 mg/L (possible future) Clara City Hawk Simulate at 1 mg/L (possible future) Willmar Hawk Simulate at 1 mg/L (current permit) Eagle Lake Le Sueur Eliminate from model (discontinued during sim. period) Madison Lake Le Sueur Eliminate from model (to be eliminated, remove from

model) Waseca Le Sueur Simulate at 1 mg/L (pending permit) Arlington Lower Mainstem Simulate at 1 mg/L (probable permit) Belle Plaine Lower Mainstem Simulate at 1 mg/L (current limit) Dairy Farmers of America Lower Mainstem Simulate at permit limit of 4 mg/L (historically has been

greater than 4) Henderson Lower Mainstem Will be connected to Le Sueur, but continue in model

to represent flow contribution, using 1 mg/L Le Center Lower Mainstem Simulate at 1 mg/L (current limit) Le Sueur Lower Mainstem Simulate at 1 mg/L (current limit) Milton G. Waldbaum Lower Mainstem Simulate at 1 mg/L (current limit) Hillcrest Health Center Middle Mainstem Eliminate from model (discontinued during simulation

period) Lake Crystal Middle Mainstem Simulate at 1 mg/L (current limit) Mankato Middle Mainstem Simulate at 1 mg/L (current mass limit in place) Morgan Middle Mainstem Simulate at 1.22 mg/L (current mass limit in place) Morton Middle Mainstem Simulate at 1 mg/L (possible limit) New Ulm Middle Mainstem Simulate at 1 mg/L (pending permit) Renville Middle Mainstem Simulate at 1 mg/L (pending permit) Saint Peter Middle Mainstem Simulate at 1 mg/L (current limit) SM Beet Sugar Middle Mainstem Simulate at 0.75 mg/L (current limit) Madelia Watonwan Simulate at 1 mg/L (current limit)


10

2.1.2 Scenario 2 Scenario 2 is designed to represent “innovative change” and entails pursuing breakthrough ideas, such as modification of land use and cropping systems, to accelerate progress. Options in this category are considered challenging. The components included in Scenario 2 are summarized in Table 2-3.



Land Use:

Increase pasture/CRP/ perennial crops to 10% of the watershed. Target steepest sloping land. Achieve by reducing conventional tillage only. Changes to manured land also occur (see Cropping Systems)

General changes in land use are addressed by repopulating the land use table in the Schematic block.

CRP land is represented as pasture in the model. If existing CRP is > 10% of area, no change is made. Area needed to achieve 10% is calculated, removed from conventional tillage, and added to the subbasins with the higher average slopes.

Cropping System

75 percent of row cropland with slopes greater than 3 percent use crop residue of 30 percent or greater.

On cropland with slopes less than 3 percent, 50 percent of surface tile intakes protected (examples of protection include, but are not limited to, buffers, risers, rock inlets, etc.).

No-till, strip till, and other reduced tillage methods or perennial crops on land with over 12 percent slope.

Nutrient management: follow U of M fertilizer recommendations. Manure management plans adjusted to nitrogen; full implementation of plans with setbacks from sensitive areas.

Increasing 30% residue on higher slope lands is done by calculating the fraction in the higher slope category for each subbasin and adjusting the conservation tillage area accordingly.

Tile inlet protection is simulated by reducing Special Actions factor controlling rate of sediment influx to tile drains.

Results of changes in fertilizer and manure application rates alter model nutrient parameters. Parameters controlling nutrient load are adjusted in accordance with changes in application rates.

Fraction in conservation tillage is recalculated. Existing high residue fraction kept for slopes less than 3%; those greater than 3% brought up to 75%

Weighted analysis of tile inlet protection assumes surface inlets are three times as likely to occur on slopes less than 3 percent (this assumption was used in OE/DO TMDL). Protection results in 50% reduction in rate of sediment entering tiles.

Because area in > 12% slope is small (max of 5% in a subbasin) and is much less than area > 3% slope, these components are assumed to be included in the 75% target for conservation tillage on slopes > 3%.

Relies on Mulla et al. 2001 Table 21a. Reduce N and P fertilizer on conventional and conservation tillage cropland to agronomic rates. For manured land baseline is 2.1 times agronomic rates. Reduce commercial fertilizer on manured areas to recommended level in Table 21a. This requires expanding land area receiving manure by 2.11. Assume all P needs supplied by manure, resulting in a reduction to 39.9% of P loading from existing conditions (still well in excess of agronomic rates). Lower rate of manure application per acre provides corresponding reduction in fraction of organic matter load in excess of conventional tillage.


11


15% reduction in sediment from ravines due to use of drop structures

Ravine loading is simulated as a function of flow depth; applied to cropland only.

Reduce the multiplicative factor on ravine transport (KGER) by 15%. This serves to reduce loading particularly in those areas of steeper slopes and more erodible soils where ravine formation is most likely, such as portions of the Blue Earth and Le Sueur watersheds.


1 mg/l for all mechanical facilities.

P limit is simulated by changing the ext sources linkage to represent P as a fixed concentration.

Includes industrial discharges and stabilization ponds that are majors. If current limit is < 1 mg/L the lower limit is retained.

Urban Stormwater

Treat the first inch of runoff

Generalized “treatment” of urban runoff is done by routing flow from urban impervious surfaces to a special unit-area RCHRES that routes water and treats the first inch. Explicit simulation of specific stormwater treatment options, such as detention ponds, is not done for Scenario 2, so impacts on hydrology are not included.

Apply to urban impervious area within the MS4 boundaries. Assume the unspecified treatment has no impact on hydrograph. Assume the treated volume achieves reductions of 65% in TSS, 52% in TP, and 20% in TN. (The first two rates correspond to MN Stormwater Manual average removal rates for wet detention; the TN rate is based on current influent/effluent medians for wet detention in the International Stormwater BMP Database). Runoff in excess of 1 in with associated load is routed directly to stream.

2.1.3 Scenario 3 Scenario 3 represents “advancing change” − a higher level of effort that is intended to represent a transformation of current thinking. Management options are focused at the structural level and are perceived to be difficult, but potentially feasible, involving use of the best alternatives. Although these aggressive actions may appear onerous and difficult, they do not appear to be sufficient fully achieve water quality standards, as explained further below.

The components of Scenario 3 are summarized in Table 2-4.


Scenario Component Modeling Approach Approach/Information Needs

Land Use:

Increase pasture/CRP/ perennial crops to 20% of the watershed. Target steepest sloping land. Achieve by reducing conventional tillage only.

Changes to manured land also occur (see Cropping Systems)

General changes in land use are addressed by repopulating the land use table in the Schematic block. Spreadsheet tools are already available to assist with this.

CRP land is represented as pasture in the model. If existing CRP is > 20% of area, no change is made. Area needed to achieve 20% is calculated, removed from conventional tillage, and added to the subbasins with the higher average slopes.


12


Cropping System

75 percent of row cropland with slopes greater than 3 percent use crop residue of 30 percent or greater. In addition, these lands have a cover crop to increase the spring cover.

On cropland with slopes less than 3 percent, 50 percent of surface tile intakes protected (examples of protection include, but are not limited to, buffers, risers, rock inlets, etc.).

No-till, strip till, and other reduced tillage methods or perennial crops on land with over 12 percent slope.


30% reduction in sediment from ravines due to use of drop structures

Increasing 30% residue on higher slope lands is done by calculating the fraction in the higher slope category for each subbasin and adjusting the conservation tillage area accordingly (see slopes-analysis.xls).


Results of changes in fertilizer and manure application rates were simulated previously for nutrient TMDL development. Parameters controlling nutrient load are adjusted in accordance with changes in application rates.

Ravine loading is simulated as a function of flow depth. Applied to cropland only.

Fraction in conservation tillage is recalculated. Existing high residue fraction kept for slopes less than 3%; those greater than 3% brought up to 75%. Cover coefficients for cover crop based on analysis of RUSLE scenarios for Beaver Creek from Pete Cooper.

Same as Scenario 2.

Because area in > 12% slopes small (max of 5% in a subbasin) and is much less than area > 3%, these components are assumed to be included in the 75% target for conservation tillage on slopes > 3%.

Same as Scenario 2. Results in increase in manured land area.

Reduce the multiplicative factor on ravine transport (KGER) by 30% (actual factor varies).


0.3 mg/l for all mechanical facilities.

P limit is readily simulated by changing the ext sources linkage to represent P as a fixed concentration multiplier on effluent flow.

Includes industrials and stabilization ponds classed as majors and individually simulated.

Urban Stormwater

Infiltrate the first inch of runoff from both impervious and pervious urban surfaces.

Generalized “treatment” of urban runoff is done by routing flow from urban impervious surfaces to a special unit-area RCHRES that routes water and treats the first inch.

Apply to pervious and urban impervious area within the MS4 boundaries. Interpreted as weighted combination of runoff from impervious and pervious land producing first inch of runoff. Allow evaporation from infiltration basin. Infiltrated water volume directed to the stream at the rate of infiltration. Runoff in excess of 1 in with associated load is routed directly to stream with no treatment.

For the infiltrated fraction, assume that TSS and PO4 are reduced to regional groundwater concentrations of 5 and 0.06 mg/L. TN concentrations reduced 50%.


13

2.1.4 Scenario 4 The results of Scenario 3 demonstrated that additional management measures would be needed to achieve water quality standards. After presentation of the Scenario 3 results, participants at the Minnesota River Turbidity TMDL Stakeholder Committee meeting of July 24, 2008 developed a list of additional ideas for the next model scenario. Four important areas for additional focus included an effort to reduce sediment loading from ravines and gullies, water storage and upland drainage management, land use changes including conversion of agriculture to perennial crops, and reductions in bank and bluff erosion. Together these ideas formed the basis for Scenario 4 – “accelerating change.”

Potential ideas for reducing loading from ravines and gullies included energy dissipation, including installation of drop structures on tile drain outlets at the head of ravines, rerouting flow away from ravines, and physical rehabilitation.

Water storage and upland drainage management included a variety of ideas, many of which were aimed at retaining the first inch of water for 24 hours at or near the source, reducing erosive potential. This might be achieved through use of settling basins or constructed wetlands, culvert downsizing, modified ditch design, and changes to drainage management.

The land use changes considered included increases in Conservation Reserve Program conversions of crop to grassland, increasing crop residue cover, increasing use of reduced tillage or no-till practices, and conversion of some cropland to perennial crops.

Finally, results of the previous scenarios indicated that a significant portion of the observed excursions of sediment concentration targets was due to bank and channel erosion processes in the bluff areas. Therefore, some actions were also recommended to stabilize the stream banks and bluff areas, either through engineered solutions or through improved riparian plant cover.

Considerable effort was required to interpret these site-scale recommendations into components appropriate to the large-scale simulation model. The ways in which these ideas were incorporated into Scenario 4 are summarized in Table 2-5.


14



Land Use:

Increase pasture/CRP/ perennial crops plus forest to 20% of the watershed. Target areas near nickpoints, particularly in Blue Earth and Le Sueur. Achieve by reducing conventional tillage only. Increase Chippewa to 30%.

General changes in land use are addressed by repopulating the land use table in the Schematic block. Spreadsheet tools are already available to assist with this.

Setup same as Scenario 3 – except for Chippewa. New CRP is represented as perennial crops as in Scenario 3. Refocus assignment of new CRP to reaches near stream nickpoints by weighting toward the segments that contain the bluff areas

Cropping System

75 percent of row cropland with slopes greater than 3 percent use crop residue of 37.5 percent or greater. In addition, these lands have a cover crop to increase the spring cover.

All surface tile inlets are eliminated.


30% reduction in sediment from ravines due to use of drop structures, etc. 40% reduction in Blue Earth and Le Sueur

Increasing residue on higher slope lands is done by calculating the fraction in the higher slope category for each subbasin and adjusting the conservation tillage area accordingly.


Results of changes in fertilizer and manure application rates were simulated previously. Parameters controlling nutrient load are adjusted in accordance with changes in application rates.

Ravine loading is simulated as a function of flow depth, applied to cropland only.

Fraction in conservation tillage is same as in Scenario 3: Existing high residue fraction kept for slopes less than 3%; those greater than 3% brought up to 75% of land at 37.5%. Cover coefficients for cover crop based on analysis of RUSLE scenarios for Beaver Creek. Change residue percentage to 37.5%; because a winter cover crop is already included, this increases cover only for June.

Reduce sediment inflow to tile drains to nominal amount by changing SPECIAL ACTIONS factor from current value (varies by watershed, typically around 0.4) to 0.05. Decrease INTFW (rolled into Upland Drainage section)

Same as Scenarios 2-3. Results in increase in manured land area.

Reduce the multiplicative factor on ravine transport (KGER). Confirmed that this is accomplished by proportional reduction in KGER.

Upland Drainage Management

Controlled drainage on crop land with < 1% slope (5/15-9/15)

Two-stage ditch design

Store 1” runoff for at least 24 hours.

These factors are strongly inter-related. The third item may be largely achieved by implementing the first two. HSPF does not simulate local ditches directly, so address by increasing land surface storage capacity, which is a function of the product of surface roughness and slope length (NSURxLSUR). Controlled drainage is represented by decreasing interflow inflow (INTFW).

Increase NSURxLSUR: Due to limitations in allowed values in HSPF for NSUR, achieve this by increasing LSUR from 450 to 2350 (4750 in Blue Earth and Cottonwood) for LU 2,3,7 to achieve appropriate surface storage values of 1” max surface storage with INTFW reduced to 1.5. This increase is based on sensitivity analysis. Prorate summer INTFW to achieve 1.5 on lands <1% slope. For same lands, increase IRC to 0.95. Set SDOP to 0 so that transport capacity is not a function of stored water depth. Two-stage ditches are not explicitly modeled, but are assumed to be a part of the increased water storage capacity.


15


Bank and Bluff Erosion Decrease maximum scour to simulate bank stabilization

HSPF asymptotes to maximum scour rate (M) as flows rise above critical shear. Change M.

No changes to bluff collapse rates.

Reduce M silt and M clay by 20% for each reach; also KSAND. No changes made to critical scour values.


1 mg/l TP for all mechanical facilities.

P limit is readily simulated by changing the ext sources linkage to represent P as a fixed concentration multiplier on effluent flow.

Revert from Scenario 3 to Scenario 2 conditions. Includes industrials and stabilization ponds classed as majors and individually simulated. A separate run (Scenario 4A) is made with point source P loading turned off.

Urban Stormwater

Infiltrate the first inch of runoff from both impervious and pervious urban surfaces.

Generalized “treatment” of urban runoff is done by routing flow from urban impervious surfaces to a special unit-area RCHRES that routes water and treats the first inch.

Apply to pervious and urban impervious area within the MS4 boundaries. No change from Scenario 3. No reductions to developed lands outside MS4 boundaries

Baseflow Sediment Concentration

Remove “extra” sources.

Calibration for several basins requires adding a low-flow clay load associated with ground water (may represent instream activities such as gravel mining as well). These are considered manageable loads that can be reduced or removed.

Reduce those basins with higher values – e.g., any with concentration greater than 5 mg/L reduced to 5.

The various components of Scenario 4, taken together, would result in significant changes in the distribution of land uses in the Minnesota River watershed. These changes are summarized graphically in Figure 2-1 over the next several pages.

LeSueur River- Existing Land Use

Urban7%

Conventional Tillage39%

Conservation Tillage38%

Pasture/CRP4%

Manured Cropland7%

Marsh4%

Forest1%

LeSueur River- Scenario 4 Land Use


Forest1%

Manured Cropland11%

Marsh4%

Pasture/CRP19%

Urban7%



16

Redwood River- Existing Land Use


Manured Cropland7%

Urban7%

Forest1%

ConservationTillage26%

Marsh3%

Pasture/CRP9%

Redwood River- Scenario 4 Land Use


Urban7%

Pasture/CRP19%

Marsh3%

Manured Cropland14%

Forest1%

Conservation Tillage

25%

Watonwan River- Existing Land Use



Manured Cropland7%

Forest1%

Urban6%

Marsh3%

Pasture/CRP1%

Watonwan River- Scenario 4 Land Use

Forest1%

Manured Cropland14%

Marsh3%

Pasture/CRP19%

Urban6%



Yellow Medicine River- Existing Land Use


Forest1%

Urban5%

Marsh4%

Pasture/CRP12%

Manured Cropland5%


Yellow Medicine River- Scenario 4 Land Use

Marsh4%

Urban5%

Pasture/CRP19%

Forest1%

Manured Cropland11%




17

Blue Earth River- Existing Land Use



Manured Cropland

5%

Forest1%

Urban7%Marsh

3%

Pasture/CRP3%

Blue Earth River- Scenario 4 Land Use


Pasture/CRP19%

Marsh3%

Forest1%

Manured Cropland

10%

Urban7%


Chippewa River- Existing Land UseUrban

5%

Manured Cropland

4%

Forest5%


ConservationTillage34%Marsh

6%

Pasture/CRP12%

Chippewa River- Scenario 4 Land Use


Forest5%

Marsh6%

Pasture/CRP25%

ManuredCropland

8%


Urban5%

Cottonwood River- Existing Land Use



Urban6%

Manured Cropland

8%

Forest1%

Marsh3%

Pasture/CRP4%

Cottonwood River- Scenario 4 Land Use

Marsh3%


Forest1%

ManuredCropland

16%

Urban6%

Pasture/CRP19%



18

Hawk Creek- Existing Land Use

Forest1%



Manured Cropland

7%

Urban7%

Marsh3%

Pasture/CRP4%

Hawk Creek- Scenario 4 Land Use


Urban7%

Pasture/CRP19%

ManuredCropland

14%


Forest1%

Marsh3%

Figure 2-1. Modeled Land Use for Existing Conditions and Scenario 4

2.2 RESULTS OF PRELIMINARY SCENARIOS

2.2.1 TSS Results Under baseline (existing) conditions, TSS concentrations are frequently well above 100 mg/L, whereas the surrogate TSS concentrations needed to achieve turbidity standards range from 50 to 100 mg/L. Initial comparison among the preliminary scenarios was made on the basis of flow-weighted concentrations (that is, total load divided by total flow). This provides a useful basis for comparing the magnitude of load reductions, although it does not provide a direct indication of the frequency of occurrence of elevated concentrations. The method was further refined by reporting by individual seasons within the crop year (defined as December through November), in which the seasons are loosely defined as December -March (over-winter period), April – May (spring field preparation and planting), June-July (summer crop growth), August-September (crop maturity), and October-November (harvest and post-harvest field operations). Comparisons are made over crop years 2001-2005, which provides a representative mix of dry and wet years and is near in time to the 2001 land use that forms the basis of the calibrated model.

The resulting estimates are first summarized in tabular form (Table 2-6), and then graphically by major watershed (Figure 2-2 through Figure 2-14). Concentrations are elevated in various seasons, depending on the occurrence of large washoff events, but are generally much lower in the winter.

The table and graphs demonstrate that Scenarios 1 through 3 provide only modest reductions in flow-weighted mean TSS concentrations relative to existing baseline (calibration) conditions, for reasons that will be explored further below. Only with the aggressive actions of Scenario 4 is there a substantial lowering of TSS load and concentrations.


19

Table 2-6. Flow-Weighted TSS Concentrations by Season for Baseline Calibration Conditions and Scenarios 1 through 4

Baseline Calibration Scenario 1 Scenario 2 Scenario 3 Scenario 4

Season Season Season Season Season

Location Crop Year

Dec-Mar

Apr-May

Jun-Jul

Aug-Sep

Oct-Nov

Dec-Mar

Apr-May

Jun-Jul

Aug-Sep

Oct-Nov

Dec-Mar

Apr-May

Jun-Jul

Aug-Sep

Oct-Nov

Dec-Mar

Apr-May

Jun-Jul

Aug-Sep

Oct-Nov

Dec-Mar

Apr-May

Jun-Jul

Aug-Sep

Oct-Nov

2001 120.2 173.6 473.6 56.9 55.5 118.9 173.1 474.0 56.8 53.4 118.6 171.4 423.5 57.1 54.9 121.8 197.5 377.2 51.0 46.1 92.4 140.1 170.1 43.6 39.6 2002 95.6 150.8 304.8 212.8 247.1 94.8 150.0 303.3 209.6 246.9 92.2 144.9 261.6 195.2 226.0 89.1 141.5 237.5 185.7 218.7 69.5 108.5 144.6 124.1 146.5 2003 69.6 191.3 87.2 75.8 33.7 69.4 190.6 83.9 72.0 32.8 70.8 187.8 83.7 73.9 34.4 68.7 194.3 79.3 70.6 31.7 53.4 147.9 67.0 66.9 29.1 2004 64.1 545.1 467.2 602.0 106.3 63.6 543.9 466.3 603.6 105.8 62.0 483.6 436.8 576.0 100.8 59.0 442.1 436.6 540.8 108.7 48.8 203.2 279.0 307.4 117.0

Blue Earth - Mankato

2005 123.0 210.6 109.8 1,076.7 324.4 122.3 210.2 108.1 1079.5 323.6 121.8 202.6 108.6 951.8 282.1 121.0 223.0 113.0 802.7 272.8 103.6 182.4 94.4 319.8 125.2 2001 74.6 73.6 247.8 50.1 40.2 55.1 68.8 240.0 26.9 16.9 53.7 66.2 204.7 24.0 15.3 51.6 67.0 173.1 22.5 14.2 41.8 54.9 59.1 16.7 12.8 2002 51.2 79.8 257.0 168.6 125.8 28.0 58.8 238.3 148.4 106.4 26.5 55.6 183.2 133.9 93.5 24.6 52.2 140.3 125.3 81.8 19.5 40.7 60.0 72.5 57.8 2003 40.2 108.1 79.5 66.6 30.7 17.1 91.5 55.1 42.1 7.9 16.9 89.4 52.7 41.1 7.9 16.4 86.2 49.3 39.8 7.7 13.6 67.0 38.9 38.7 7.7 2004 42.1 563.2 277.8 307.6 68.7 19.0 556.9 267.4 305.2 51.1 18.6 473.6 239.6 295.1 46.0 17.6 399.8 220.5 299.7 44.7 15.2 120.7 114.3 165.1 77.5

Blue Earth - Rapidan

2005 79.4 80.2 57.6 688.8 279.2 58.4 73.4 40.0 689.6 272.6 57.0 67.6 39.1 574.7 218.9 55.5 65.9 38.8 464.6 186.0 56.0 79.3 32.5 159.8 54.5 2001 210.1 160.3 469.3 68.0 52.5 204.0 159.5 467.8 55.4 39.2 202.0 157.7 405.0 52.6 38.0 195.2 151.4 337.1 56.2 33.0 162.5 140.5 152.4 45.9 29.7 2002 65.4 143.4 602.5 337.2 244.2 61.5 138.4 598.3 329.6 242.6 59.1 126.7 500.5 312.6 232.4 54.8 112.3 415.2 298.9 214.0 45.9 93.0 212.4 197.5 171.4 2003 39.2 244.6 104.8 104.9 24.9 34.3 241.5 95.7 84.0 17.7 34.6 240.9 92.5 84.4 18.3 34.9 236.7 90.9 102.5 21.7 30.4 197.9 74.4 100.9 21.6 2004 44.2 1197.2 519.2 536.0 108.4 39.1 1,196.7 521.4 537.9 107.3 39.0 1,042.8 497.0 512.8 99.3 40.1 901.7 479.9 493.7 93.5 35.8 358.0 322.6 352.3 145.2

Blue Earth - Good

Thunder

2005 161.1 171.5 89.7 1492.4 508.9 158.0 170.8 85.4 1,505.4 510.4 155.0 163.7 84.6 1,273.7 422.2 150.5 152.9 84.4 1,040.8 347.7 127.1 178.9 77.3 429.6 137.3 2001 31.6 117.3 29.1 90.8 48.5 36.3 130.8 28.6 89.1 47.3 34.4 121.4 31.8 107.7 62.9 28.6 116.7 27.9 89.5 48.3 22.5 90.3 36.4 97.5 51.8 2002 44.2 28.3 91.5 86.8 90.8 43.6 29.4 91.9 87.3 90.7 47.6 27.0 94.3 91.0 92.7 41.6 20.2 86.4 82.2 87.6 31.9 14.2 66.7 70.6 75.2 2003 71.4 51.8 44.8 93.5 23.2 71.6 57.5 55.1 93.6 23.2 72.4 49.5 47.2 95.2 25.4 69.5 41.5 35.1 92.5 22.4 54.6 30.8 24.0 86.7 22.6 2004 51.6 104.0 63.5 83.3 98.3 51.1 103.5 73.3 91.9 110.9 54.8 107.5 62.5 96.8 107.5 49.6 96.7 44.4 78.7 105.2 38.3 73.6 26.1 68.3 73.5

Chippewa - Montevideo

2005 104.8 35.7 37.1 76.5 48.1 105.4 45.7 42.5 109.2 63.4 103.5 39.2 37.6 82.4 46.5 100.0 30.9 29.6 65.4 42.3 80.1 17.3 21.5 34.8 26.5 2001 89.4 172.8 117.9 91.1 113.5 88.1 172.6 116.4 88.6 112.6 86.5 163.8 114.4 80.2 98.5 71.3 217.3 97.7 65.2 76.8 60.5 146.3 74.2 45.0 43.9 2002 64.7 81.3 105.7 97.1 49.3 63.9 80.5 103.6 94.0 48.5 63.1 85.8 101.7 90.8 47.7 58.5 76.1 85.8 63.1 38.4 38.8 67.5 63.5 72.9 30.8 2003 42.1 115.0 67.4 116.4 36.1 41.0 114.0 64.3 108.7 33.9 40.3 108.3 65.4 105.9 33.9 30.7 93.8 53.0 63.6 24.7 23.2 73.7 55.6 95.5 23.4 2004 45.5 1782.9 722.6 87.5 41.5 43.7 1789.6 723.1 83.8 40.6 43.0 1394.3 563.3 77.5 40.4 30.0 1079.4 470.0 55.8 35.1 28.0 156.6 130.9 61.9 28.6

Cottonwood - New Ulm

2005 50.0 113.4 65.0 1534.1 66.8 49.2 112.0 63.1 1537.6 66.5 46.1 126.0 74.5 1090.3 55.6 35.2 129.7 64.8 722.8 88.0 27.5 105.4 69.1 117.1 68.2 2001 46.6 244.3 54.7 54.7 34.5 48.9 247.2 55.9 52.3 32.3 48.2 223.4 53.9 50.4 32.0 47.8 216.4 53.2 46.7 30.2 38.6 111.5 41.8 45.1 28.2 2002 27.0 63.0 119.4 254.9 44.0 27.7 63.0 123.4 264.7 46.3 27.2 61.1 109.0 240.4 43.2 26.9 60.8 107.0 233.0 42.7 22.5 48.2 59.3 113.1 34.4 2003 9.7 79.3 132.7 35.5 17.7 9.8 79.1 136.3 37.4 16.2 10.0 74.8 123.3 36.0 16.1 9.8 74.3 121.3 33.7 15.2 8.9 54.8 66.9 32.9 14.6 2004 16.6 211.8 136.3 56.5 196.2 16.2 211.0 139.9 52.7 198.5 16.2 189.2 124.7 51.1 170.4 15.6 183.0 123.3 48.0 166.2 14.1 77.1 70.0 44.9 60.5

Hawk - Sacred Heart

2005 26.3 91.0 74.8 116.5 180.6 27.3 91.5 76.9 116.9 183.7 27.4 86.4 72.4 106.5 159.8 27.3 86.0 71.6 104.9 155.8 23.6 60.7 51.9 72.2 63.9 2001 175.2 197.3 739.0 64.5 57.0 177.5 196.0 739.3 73.2 58.7 177.1 200.4 662.6 80.5 64.2 169.8 204.7 536.8 57.5 43.6 140.3 175.5 230.0 53.0 39.1 2002 137.7 204.8 307.3 204.0 339.1 138.3 206.1 309.2 207.0 340.1 134.4 197.3 279.6 189.5 302.5 128.3 190.6 256.9 171.8 275.7 103.9 154.5 171.7 114.6 193.1 2003 97.3 236.0 73.2 80.9 29.9 99.1 236.9 76.0 84.1 31.2 100.7 230.8 81.8 95.1 34.7 94.2 227.8 66.1 70.4 26.5 72.3 192.5 62.3 68.0 26.4 2004 92.0 424.0 621.7 911.6 103.6 93.5 424.1 621.5 907.8 104.0 88.7 399.0 588.1 864.5 98.9 79.6 363.2 576.8 749.7 115.9 65.7 211.5 394.4 400.7 108.2

Le Sueur - Rapidan

2005 160.0 310.3 127.0 1769.0 198.9 161.0 310.0 128.9 1765.8 193.3 160.6 294.3 132.6 1587.7 202.6 158.4 290.5 120.6 1305.1 197.3 127.5 254.9 130.0 499.5 163.6


20

Baseline Calibration Scenario 1 Scenario 2 Scenario 3 Scenario 4

Season Season Season Season Season

Location Crop Year

Dec-Mar

Apr-May

Jun-Jul

Aug-Sep

Oct-Nov

Dec-Mar

Apr-May

Jun-Jul

Aug-Sep

Oct-Nov

Dec-Mar

Apr-May

Jun-Jul

Aug-Sep

Oct-Nov

Dec-Mar

Apr-May

Jun-Jul

Aug-Sep

Oct-Nov

Dec-Mar

Apr-May

Jun-Jul

Aug-Sep

Oct-Nov

2001 95.6 150.6 193.1 84.8 56.0 93.9 146.1 193.1 77.5 52.9 93.2 140.6 179.8 73.0 49.9 91.1 141.1 167.5 67.8 42.3 61.3 96.2 98.7 50.8 30.6 2002 87.6 150.7 179.1 158.0 134.1 87.7 150.9 176.9 158.1 133.9 84.4 150.2 164.5 147.4 125.3 79.1 151.2 157.4 138.1 117.3 50.7 110.1 103.1 91.3 79.3 2003 55.2 149.8 107.8 65.2 20.3 56.0 148.2 108.7 67.2 20.7 55.7 146.2 104.2 64.8 20.6 53.7 145.2 98.4 61.1 19.1 31.8 104.2 70.1 50.5 15.1 2004 52.6 144.0 332.8 291.4 117.1 53.8 141.0 296.4 291.7 118.8 53.3 132.9 255.7 283.7 110.8 51.1 125.5 230.9 266.0 110.3 29.1 73.1 105.9 143.1 84.4

Minnesota River - Jordan

2005 93.2 123.6 138.9 531.7 183.8 93.2 121.6 138.5 454.2 183.4 90.8 119.2 136.7 379.8 162.9 88.0 122.3 136.0 301.9 153.4 62.0 91.5 100.0 117.6 73.4 2001 130.3 390.9 301.2 129.0 100.8 120.9 382.0 299.5 123.1 89.5 120.1 380.5 276.4 119.7 87.2 118.8 397.6 255.8 113.5 79.7 82.4 313.5 143.7 91.2 59.1 2002 116.8 141.2 237.1 192.7 187.4 114.6 141.2 235.2 195.4 186.2 113.5 140.1 212.6 185.1 173.1 111.4 139.0 200.4 178.1 165.6 78.9 108.9 121.6 122.1 112.9 2003 96.1 178.5 151.8 126.2 49.5 96.5 173.8 152.8 131.0 49.7 96.5 172.5 148.3 123.1 49.5 94.7 175.1 144.7 117.6 47.3 64.9 131.8 107.0 101.6 29.6 2004 81.5 507.2 620.2 507.7 132.2 82.3 446.8 542.5 506.4 137.3 81.4 390.4 472.9 486.2 135.0 78.8 345.5 428.8 461.0 139.1 49.7 152.5 208.6 258.7 105.6

Minnesota River -

Mankato

2005 121.3 188.3 132.9 1023.2 225.4 121.5 187.9 133.2 852.4 224.6 120.5 184.4 135.1 710.4 201.1 119.1 193.4 135.6 570.5 200.8 85.1 152.5 106.8 220.3 112.5 2001 79.4 191.8 182.3 82.7 69.0 76.5 185.7 181.2 76.7 62.3 76.2 181.8 165.9 73.8 61.2 74.8 187.5 150.6 69.6 55.9 44.5 133.6 75.9 52.4 37.6 2002 78.8 83.9 154.8 134.5 119.9 79.3 84.7 152.4 135.8 120.2 78.7 84.2 136.6 127.5 112.0 77.3 84.1 128.6 122.1 106.6 48.0 56.7 69.4 79.0 65.3 2003 66.0 108.5 95.7 77.6 39.6 67.1 106.5 97.4 79.8 40.2 66.8 105.4 94.6 76.8 40.3 65.1 106.1 91.3 72.8 38.5 38.6 70.5 62.5 60.2 24.5 2004 64.1 263.3 387.4 322.1 90.7 65.4 241.3 339.6 322.2 94.6 64.8 213.8 290.3 311.1 90.9 62.5 189.6 260.1 291.2 93.4 35.2 82.4 108.2 151.2 68.2

Minnesota River -

St. Peter

2005 85.3 112.1 84.5 641.4 158.7 86.4 110.7 85.2 543.1 157.9 85.6 106.8 85.2 450.6 137.3 84.6 109.3 85.0 357.1 130.5 57.2 80.0 59.1 127.6 59.1 2001 80.9 113.8 108.6 93.0 94.0 84.0 110.4 86.6 77.5 88.8 75.1 115.2 72.3 59.2 67.0 69.4 120.0 70.8 54.3 66.0 49.1 108.2 57.5 46.4 54.3 2002 60.1 94.7 93.8 117.1 90.8 55.7 91.7 87.2 110.8 60.0 56.0 92.6 71.8 79.2 55.3 55.3 93.4 71.0 72.2 53.4 43.4 81.5 50.2 59.3 41.2 2003 57.9 130.3 60.0 147.9 27.0 59.5 126.4 60.3 140.2 26.5 58.4 126.5 49.5 110.1 21.4 57.2 130.0 46.4 103.2 20.1 42.4 124.4 36.7 85.3 12.9 2004 58.2 1017.8 169.1 110.4 39.3 58.9 1158.0 199.4 122.3 38.0 56.3 1064.4 175.5 85.7 29.5 54.7 1010.3 169.6 77.1 26.6 40.6 333.9 105.9 66.4 18.8

Redwood - Redwood

Falls

2005 27.4 138.7 75.1 163.4 53.1 28.7 144.1 72.7 183.6 55.3 25.4 133.5 53.0 136.1 47.6 22.8 126.2 50.2 127.5 44.3 13.8 102.0 42.0 77.3 35.1 2001 22.9 121.5 91.4 44.4 35.0 22.3 121.1 90.3 42.3 33.5 22.1 115.3 87.0 39.8 32.4 21.7 113.2 85.6 38.0 31.7 19.5 85.8 69.6 32.0 29.9 2002 17.9 53.6 54.0 110.7 17.4 17.3 52.5 51.4 107.3 16.9 16.9 50.7 51.4 103.2 16.6 16.2 48.7 50.2 101.6 16.1 15.0 41.6 47.6 91.7 14.7 2003 28.3 64.4 69.1 154.0 30.2 26.8 63.5 65.8 146.2 28.1 26.9 61.7 65.4 141.9 28.2 26.1 59.8 63.7 136.6 27.1 24.9 50.4 60.8 135.8 27.0 2004 29.5 57.4 152.5 86.8 38.5 28.0 55.4 151.7 85.7 38.2 28.1 53.7 127.4 80.1 36.6 27.3 51.9 117.8 77.0 35.3 26.0 46.8 67.5 60.1 30.2

Watonwan - Garden City

2005 28.4 104.0 66.2 289.8 298.2 27.9 103.4 64.1 289.4 298.3 27.4 96.5 63.4 256.4 248.6 26.8 93.7 61.7 229.8 226.3 23.4 69.9 55.4 105.3 89.1 2001 29.7 193.0 50.6 35.9 100.1 29.6 193.0 50.1 34.6 100.4 29.3 190.3 50.6 34.3 93.0 28.8 188.8 50.8 32.6 82.9 23.8 146.5 44.9 31.8 68.2 2002 54.7 179.3 229.7 43.8 61.0 54.8 179.4 229.5 43.5 61.7 52.3 177.7 205.7 41.1 53.7 49.0 175.7 184.2 40.2 47.5 41.4 140.0 88.0 36.5 40.1 2003 39.6 168.6 32.5 57.0 10.0 39.8 168.7 31.7 56.3 9.7 37.9 163.6 33.7 55.3 9.7 36.6 158.3 33.9 54.6 9.5 30.8 121.1 31.6 51.4 9.0 2004 18.5 327.3 231.4 59.7 50.5 18.5 326.8 225.9 58.7 50.5 17.9 315.1 238.6 55.8 47.0 16.9 309.2 234.4 54.9 47.3 14.4 160.7 151.3 49.9 39.9

Yellow Medicine -

Granite Falls

2005 17.1 243.5 96.4 108.9 135.8 17.1 243.1 94.4 108.4 135.1 16.6 235.1 145.5 99.7 135.2 16.0 232.1 168.0 86.0 156.7 13.5 145.1 113.1 69.8 111.0


21

Chippewa at Montevideo

0

20

40

60

80

100

120

140

Dec-M

ar 01

Apr-May

01

Jun-J

ul 01

Aug-S

ep 01

Oct-Nov

01

Dec-M

ar 02

Apr-May

02

Jun-J

ul 02

Aug-S

ep 02

Oct-Nov

02

Dec-M

ar 03

Apr-May

03

Jun-J

ul 03

Aug-S

ep 03

Oct-Nov

03

Dec-M

ar 04

Apr-May

04

Jun-J

ul 04

Aug-S

ep 04

Oct-Nov

04

Dec-M

ar 05

Apr-May

05

Jun-J

ul 05

Aug-S

ep 05

Oct-Nov

05

Seaonal Period

Ave

rage

Sim

ulat

ed S

imul

ated

TSS

(mg/

l) Fl

ow W

eigh

ted

Mea

n C

once

ntra

tion Calibration

Scenario 1Scenario 2Scenario 3Scenario 4

Figure 2-2. Seasonal Flow-Weighted Average TSS, Chippewa River at Montevideo

Yellow Medicine at Granite Falls

0

50

100

150

200

250

300

Dec-M

ar 01

Apr-May

01

Jun-J

ul 01

Aug-S

ep 01

Oct-Nov

01

Dec-M

ar 02

Apr-May

02

Jun-J

ul 02

Aug-S

ep 02

Oct-Nov

02

Dec-M

ar 03

Apr-May

03

Jun-J

ul 03

Aug-S

ep 03

Oct-Nov

03

Dec-M

ar 04

Apr-May

04

Jun-J

ul 04

Aug-S

ep 04

Oct-Nov

04

Dec-M

ar 05

Apr-May

05

Jun-J

ul 05

Aug-S

ep 05

Oct-Nov

05

Seasonal Period

Ave

rage

Sea

onal

Sim

uate

d TS

S Fl

ow W

eigh

ted

Mea

n C

once

ntra

tion

(mg/

)

CalibrationScenario 1Scenario 2Scenario 3Scenario 4

Figure 2-3. Seasonal Flow-Weighted Average TSS, Yellow Medicine River at Granite Falls


22

Hawk Creek at Sacred Heart

0

200

400

600

800

1000

1200

1400

Dec-M

ar 01

Apr-May

01

Jun-J

ul 01

Aug-S

ep 01

Oct-Nov

01

Dec-M

ar 02

Apr-May

02

Jun-J

ul 02

Aug-S

ep 02

Oct-Nov

02

Dec-M

ar 03

Apr-May

03

Jun-J

ul 03

Aug-S

ep 03

Oct-Nov

03

Dec-M

ar 04

Apr-May

04

Jun-J

ul 04

Aug-S

ep 04

Oct-Nov

04

Dec-M

ar 05

Apr-May

05

Jun-J

ul 05

Aug-S

ep 05

Oct-Nov

05

Seasonal Period

Ave

rage

Sea

sona

l Sim

ulat

ed T

SS F

low

Wei

ghte

d M

ean

Con

cent

ratio

ns (m

g/l)


Figure 2-4. Seasonal Flow-Weighted Average TSS, Hawk Creek at Sacred Heart

Redwood at Redwood Falls

0

200

400

600

800

1000

1200

1400

Dec-M

ar 01

Apr-May

01

Jun-J

ul 01

Aug-S

ep 01

Oct-Nov

01

Dec-M

ar 02

Apr-May

02

Jun-J

ul 02

Aug-S

ep 02

Oct-Nov

02

Dec-M

ar 03

Apr-May

03

Jun-J

ul 03

Aug-S

ep 03

Oct-Nov

03

Dec-M

ar 04

Apr-May

04

Jun-J

ul 04

Aug-S

ep 04

Oct-Nov

04

Dec-M

ar 05

Apr-May

05

Jun-J

ul 05

Aug-S

ep 05

Oct-Nov

05

Seasonal Period

Seas

onal

Sim

ulat

ed T

SS F

low

Wei

gtht

ed M

ean

Con

cent

ratio

n (m

g/l)


Figure 2-5. Seasonal Flow-Weighted Average TSS, Redwood River at Redwood Falls


23

Cottonwood River at New Ulm

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Dec-M

ar 01

Apr-May

01

Jun-J

ul 01

Aug-S

ep 01

Oct-Nov

01

Dec-M

ar 02

Apr-May

02

Jun-J

ul 02

Aug-S

ep 02

Oct-Nov

02

Dec-M

ar 03

Apr-May

03

Jun-J

ul 03

Aug-S

ep 03

Oct-Nov

03

Dec-M

ar 04

Apr-May

04

Jun-J

ul 04

Aug-S

ep 04

Oct-Nov

04

Dec-M

ar 05

Apr-May

05

Jun-J

ul 05

Aug-S

ep 05

Oct-Nov

05

Seasonal Period

Ave

rage

Sea

sona

l Sim

ulat

ed T

SS F

low

Wei

ghte

d M

ean

Con

cent

ratio

n (m

g/l)


Figure 2-6. Seasonal Flow-Weighted Average TSS, Cottonwood River at New Ulm

Watonwan at Garden City

0

200

400

600

800

1000

1200

1400

Dec-M

ar 01

Apr-May

01

Jun-J

ul 01

Aug-S

ep 01

Oct-Nov

01

Dec-M

ar 02

Apr-May

02

Jun-J

ul 02

Aug-S

ep 02

Oct-Nov

02

Dec-M

ar 03

Apr-May

03

Jun-J

ul 03

Aug-S

ep 03

Oct-Nov

03

Dec-M

ar 04

Apr-May

04

Jun-J

ul 04

Aug-S

ep 04

Oct-Nov

04

Dec-M

ar 05

Apr-May

05

Jun-J

ul 05

Aug-S

ep 05

Oct-Nov

05

Seaonal Period

Ave

rage

Sea

sona

l Sim

ulat

ed T

SS F

low

Wei

ghte

d M

ean

Con

cent

ratio

n (m

g/l)


Figure 2-7. Seasonal Flow-Weighted Average TSS, Watonwan River at Garden City


24

Blue Earth at Good Thunder

0

200

400

600

800

1000

1200

1400

1600

Dec-M

ar 01

Apr-May

01

Jun-J

ul 01

Aug-S

ep 01

Oct-Nov

01

Dec-M

ar 02

Apr-May

02

Jun-J

ul 02

Aug-S

ep 02

Oct-Nov

02

Dec-M

ar 03

Apr-May

03

Jun-J

ul 03

Aug-S

ep 03

Oct-Nov

03

Dec-M

ar 04

Apr-May

04

Jun-J

ul 04

Aug-S

ep 04

Oct-Nov

04

Dec-M

ar 05

Apr-May

05

Jun-J

ul 05

Aug-S

ep 05

Oct-Nov

05

Seasonal Period

Seas

onal

Sim

ulat

ed T

SS F

low

Wei

ghte

d M

ean

Con

cent

ratio

n


Figure 2-8. Seasonal Flow-Weighted Average TSS, Blue Earth River at Good Thunder

Blue Earth at Rapidan

0

200

400

600

800

1000

1200

1400

Dec-M

ar 01

Apr-May

01

Jun-J

ul 01

Aug-S

ep 01

Oct-Nov

01

Dec-M

ar 02

Apr-May

02

Jun-J

ul 02

Aug-S

ep 02

Oct-Nov

02

Dec-M

ar 03

Apr-May

03

Jun-J

ul 03

Aug-S

ep 03

Oct-Nov

03

Dec-M

ar 04

Apr-May

04

Jun-J

ul 04

Aug-S

ep 04

Oct-Nov

04

Dec-M

ar 05

Apr-May

05

Jun-J

ul 05

Aug-S

ep 05

Oct-Nov

05

Seasonal Periods

Sim

ulat

ed S

eaon

al A

vera

ge T

SS (m

g/l)

Flow

Wei

ghed

Mea

n C

once

ntra

tion Calibiration

Scenario 1Scenario 2Scenario 3Scenario 4

Figure 2-9. Seasonal Flow-Weighted Average TSS, Blue Earth River at Rapidan


25

Blue Earth at Mankato

0

200

400

600

800

1000

1200

Dec-M

ar 01

Apr-May

01

Jun-J

ul 01

Aug-S

ep 01

Oct-Nov

01

Dec-M

ar 02

Apr-May

02

Jun-J

ul 02

Aug-S

ep 02

Oct-Nov

02

Dec-M

ar 03

Apr-May

03

Jun-J

ul 03

Aug-S

ep 03

Oct-Nov

03

Dec-M

ar 04

Apr-May

04

Jun-J

ul 04

Aug-S

ep 04

Oct-Nov

04

Dec-M

ar 05

Apr-May

05

Jun-J

ul 05

Aug-S

ep 05

Oct-Nov

05

Seasonal Period

Seas

onal

Sim

ulat

ed T

SS F

low

Wei

ghte

d M

ean

Con

cent

ratio

n (m

g/l)


Figure 2-10. Seasonal Flow-Weighted Average TSS, Blue Earth River near Mankato

Le Sueur at Rapidan

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Dec-M

ar 01

Apr-May

01

Jun-J

ul 01

Aug-S

ep 01

Oct-Nov

01

Dec-M

ar 02

Apr-May

02

Jun-J

ul 02

Aug-S

ep 02

Oct-Nov

02

Dec-M

ar 03

Apr-May

03

Jun-J

ul 03

Aug-S

ep 03

Oct-Nov

03

Dec-M

ar 04

Apr-May

04

Jun-J

ul 04

Aug-S

ep 04

Oct-Nov

04

Dec-M

ar 05

Apr-May

05

Jun-J

ul 05

Aug-S

ep 05

Oct-Nov

05

Seasonal Periods

Ave

rage

Sea

sona

l Sim

ulat

ed T

SS F

low

Wei

ghte

d M

ean

Con

cent

ratio

n (m

g/l)


Figure 2-11. Seasonal Flow-Weighted Average TSS, Le Sueur River at Rapidan


26

Minnesota at Mankato

0

200

400

600

800

1000

1200

1400

Dec-M

ar 01

Apr-May

01

Jun-J

ul 01

Aug-S

ep 01

Oct-Nov

01

Dec-M

ar 02

Apr-May

02

Jun-J

ul 02

Aug-S

ep 02

Oct-Nov

02

Dec-M

ar 03

Apr-May

03

Jun-J

ul 03

Aug-S

ep 03

Oct-Nov

03

Dec-M

ar 04

Apr-May

04

Jun-J

ul 04

Aug-S

ep 04

Oct-Nov

04

Dec-M

ar 05

Apr-May

05

Jun-J

ul 05

Aug-S

ep 05

Oct-Nov

05

Seaonal Period

Ave

rage

Sea

sona

l TSS

Flo

w W

eigh

ted

Mea

n C

once

ntra

tion

(mg/

l)


Figure 2-12. Seasonal Flow-Weighted Average TSS, Minnesota River at Mankato

Minnesota at St. Peter

0

200

400

600

800

1000

1200

1400

Dec-M

ar 01

Apr-May

01

Jun-J

ul 01

Aug-S

ep 01

Oct-Nov

01

Dec-M

ar 02

Apr-May

02

Jun-J

ul 02

Aug-S

ep 02

Oct-Nov

02

Dec-M

ar 03

Apr-May

03

Jun-J

ul 03

Aug-S

ep 03

Oct-Nov

03

Dec-M

ar 04

Apr-May

04

Jun-J

ul 04

Aug-S

ep 04

Oct-Nov

04

Dec-M

ar 05

Apr-May

05

Jun-J

ul 05

Aug-S

ep 05

Oct-Nov

05

Seasonal Period

Seas

onal

Sim

ulat

ed T

SS F

low

Wei

ghte

d M

ean

Con

cent

ratio

n (m

g/l)


Figure 2-13. Seasonal Flow-Weighted Average TSS, Minnesota River at St. Peter


27

Minnesota at Jordan

0

200

400

600

800

1000

1200

1400

Dec-M

ar 01

Apr-May

01

Jun-J

ul 01

Aug-S

ep 01

Oct-Nov

01

Dec-M

ar 02

Apr-May

02

Jun-J

ul 02

Aug-S

ep 02

Oct-Nov

02

Dec-M

ar 03

Apr-May

03

Jun-J

ul 03

Aug-S

ep 03

Oct-Nov

03

Dec-M

ar 04

Apr-May

04

Jun-J

ul 04

Aug-S

ep 04

Oct-Nov

04

Dec-M

ar 05

Apr-May

05

Jun-J

ul 05

Aug-S

ep 05

Oct-Nov

05

Seasons

TSS

FWM

C CalibrationScenario 1Scenario 2Scenario 3Scenario 4

Figure 2-14. Seasonal Flow-Weighted Average TSS, Minnesota River at Jordan

While Scenario 4 achieves load reductions, the objective of the TMDL is to meet turbidity criteria. As discussed further in Section 3.1, MPCA has derived TSS objectives that are consistent with meeting the turbidity criteria, ranging from 50 mg/L in the upper watershed to 100 mg/L in the lower mainstem Minnesota River. The performance of Scenario 4 relative to these TSS objectives is summarized in Table 2-7 and Figure 2-15 through Table 2-16 and Figure 2-24. The surrogate objectives are frequently exceeded at all analysis points, often by a large amount, and are exceeded 100 percent of the time in some individual seasons. It thus appears that the aggressive reductions of Scenario 4 are not sufficient to achieve water quality standards.

Table 2-7. Blue Earth-River near Mankato, Percentage of Time the Surrogate TSS Standard of 90 mg/L is Exceeded under Scenario 4

Crop Year Dec-Mar Apr-May Jun-Jul Aug-Sep Oct-Nov

2001 7.3% 100.0% 67.9% 5.0% 0.0%

2002 7.7% 63.8% 40.3% 42.6% 25.4%

2003 2.3% 85.2% 7.4% 22.5% 0.0%

2004 1.0% 19.5% 100.0% 71.0% 35.4%

2005 31.8% 100.0% 41.9% 31.9% 34.9%


28

Blue Earth- Mankato

0

200

400

600

800

1000

10/01/00 04/19/01 11/05/01 05/24/02 12/10/02 06/28/03 01/14/04 08/01/04 02/17/05 09/05/05

TSS

(mg/

L)

SimulatedTSS Surrogate

Figure 2-15. Blue Earth River near Mankato, Scenario 4 Simulated TSS

Table 2-8. Blue Earth River near Rapidan, Percentage of Time the Surrogate TSS Standard of 90 mg/L is Exceeded under Scenario 4


2001 3.5% 23.0% 11.0% 0.0% 0.0%

2002 0.0% 0.0% 8.1% 21.5% 6.9%

2003 0.0% 13.3% 0.0% 0.0% 0.0%

2004 0.0% 9.0% 48.3% 26.6% 19.7%

2005 0.0% 27.4% 0.0% 9.1% 0.0%

Blue Earth- Rapidan

0

200

400

600

800

1000

10/01/00 04/19/01 11/05/01 05/24/02 12/10/02 06/28/03 01/14/04 08/01/04 02/17/05 09/05/05

TSS

(mg/

L)


Figure 2-16. Blue Earth River at Rapidan, Scenario 4 Simulated TSS


29

Table 2-9. Blue Earth at Good Thunder, Percentage of Time the Surrogate TSS Standard of 90 mg/L is Exceeded under Scenario 4


2001 8.3% 47.5% 36.4% 11.4% 0.0%

2002 2.8% 27.1% 45.3% 47.0% 25.2%

2003 0.0% 78.9% 15.8% 49.8% 0.0%

2004 0.0% 15.4% 100.0% 74.6% 48.5%

2005 30.9% 88.3% 23.8% 26.4% 63.8%

Blue Earth- Good Thunder

0

200

400

600

800

1000

10/01/00 04/19/01 11/05/01 05/24/02 12/10/02 06/28/03 01/14/04 08/01/04 02/17/05 09/05/05

TSS

(mg/

L)


Figure 2-17. Blue Earth River at Good Thunder, Scenario 4 Simulated TSS

Table 2-10. Chippewa River at Montevideo, Percentage of Time the Surrogate TSS Standard of 50 mg/L is Exceeded under Scenario 4


2001 2.5% 44.4% 40.5% 89.2% 42.6%

2002 8.9% 1.6% 66.3% 76.1% 86.4%

2003 40.3% 26.6% 9.9% 74.0% 0.0%

2004 9.9% 76.8% 34.5% 78.8% 90.4%

2005 91.2% 5.7% 4.9% 41.7% 36.7%


30

Chippewa- Montevideo

0

100

200

300

400

500

10/01/00 04/19/01 11/05/01 05/24/02 12/10/02 06/28/03 01/14/04 08/01/04 02/17/05 09/05/05

TSS

(mg/

L)


Figure 2-18. Chippewa River at Montevideo, Scenario 4 Simulated TSS

Table 2-11. Cottonwood River at New Ulm, Percentage of Time the Surrogate TSS Standard of 70 mg/L is Exceeded under Scenario 4


2001 9.2% 90.8% 16.2% 33.3% 0.0%

2002 3.9% 34.3% 23.6% 35.6% 4.2%

2003 0.0% 37.8% 12.2% 31.2% 0.0%

2004 0.1% 27.9% 64.2% 43.1% 0.0%

2005 0.0% 68.4% 37.5% 93.9% 22.1%

Cottonwood- New Ulm

0

100

200

300

400

500

10/01/00 04/19/01 11/05/01 05/24/02 12/10/02 06/28/03 01/14/04 08/01/04 02/17/05 09/05/05

TSS

(mg/

L)


Figure 2-19. Cottonwood River at New Ulm, Scenario 4 Simulated TSS


31

Table 2-12. Hawk Creek at Sacred Heart, Percentage of Time the Surrogate TSS Standard of 50 mg/L is Exceeded under Scenario 4


2001 6.5% 84.2% 12.5% 20.3% 5.6%

2002 0.0% 28.0% 25.1% 35.5% 6.7%

2003 0.0% 41.2% 32.2% 5.6% 0.0%

2004 0.0% 20.6% 44.9% 25.0% 17.6%

2005 0.0% 47.1% 22.6% 36.4% 33.7%

Hawk- Sacred Heart

0

100

200

300

400

500

10/01/00 04/19/01 11/05/01 05/24/02 12/10/02 06/28/03 01/14/04 08/01/04 02/17/05 09/05/05

TSS

(mg/

L)


Figure 2-20. Hawk Creek at Sacred Heart, Scenario 4 Simulated TSS

Table 2-13. Le Sueur River at Rapidan, Percentage of Time the Surrogate TSS Standard of 90 mg/L is Exceeded under Scenario 4


2001 7.7% 67.1% 41.3% 15.4% 0.0%

2002 8.9% 75.1% 47.4% 35.0% 26.2%

2003 2.9% 83.7% 7.7% 23.5% 0.0%

2004 2.4% 23.1% 100.0% 76.4% 26.6%

2005 32.6% 100.0% 70.0% 45.8% 33.9%


32

Lesueur- Rapidan

0

200

400

600

800

1000

10/01/00 04/19/01 11/05/01 05/24/02 12/10/02 06/28/03 01/14/04 08/01/04 02/17/05 09/05/05

TSS

(mg/

L)


Figure 2-21. Le Sueur River at Rapidan, Scenario 4 Simulated TSS

Table 2-14. Redwood River at Redwood Falls, Percentage of Time the Surrogate TSS Standard of 70 mg/L is Exceeded under Scenario 4


2001 7.8% 34.2% 17.3% 10.2% 8.8%

2002 1.4% 15.5% 10.8% 21.9% 10.4%

2003 0.0% 29.2% 0.0% 11.7% 0.0%

2004 0.0% 13.7% 8.7% 24.4% 0.0%

2005 0.0% 33.5% 9.5% 56.5% 0.0%

Redwood- Redwood Falls

0

100

200

300

400

500

10/01/00 04/19/01 11/05/01 05/24/02 12/10/02 06/28/03 01/14/04 08/01/04 02/17/05 09/05/05

TSS

(mg/

L)


Figure 2-22. Redwood River at Redwood Falls, Scenario 4 Simulated TSS


33

Table 2-15. Yellow Medicine River at Granite Falls, Percentage of Time the Surrogate TSS Standard of 50 mg/L is Exceeded under Scenario 4


2001 0.0% 87.9% 25.0% 25.8% 10.5%

2002 16.6% 100.0% 39.5% 20.0% 11.9%

2003 0.0% 66.7% 0.0% 10.8% 0.0%

2004 0.0% 32.0% 59.6% 27.1% 23.4%

2005 0.0% 68.8% 54.5% 69.4% 26.1%

Yellow- Granite

0

100

200

300

400

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10/01/00 04/19/01 11/05/01 05/24/02 12/10/02 06/28/03 01/14/04 08/01/04 02/17/05 09/05/05

TSS

(mg/

L)


Figure 2-23. Yellow Medicine River at Granite Falls, Scenario 4 Simulated TSS

Table 2-16. Watonwan River at Garden City, Percentage of Time the Surrogate TSS Standard of 90 mg/L is Exceeded under Scenario 4


2001 0.0% 25.6% 13.5% 15.5% 0.0%

2002 0.0% 0.0% 9.2% 31.7% 0.0%

2003 0.0% 0.0% 22.3% 46.9% 0.0%

2004 0.0% 8.0% 10.8% 17.1% 0.0%

2005 0.0% 4.3% 13.7% 41.1% 12.7%


34

Watonwan- Garden City

0

200

400

600

800

1000

10/01/00 04/19/01 11/05/01 05/24/02 12/10/02 06/28/03 01/14/04 08/01/04 02/17/05 09/05/05

TSS

(mg/

L)


Figure 2-24. Watonwan River at Garden City, Scenario 4 Simulated TSS

2.2.2 Analysis of Loading Sources Tetra Tech next conducted detailed analyses of the sources of the excess TSS load that remains under Scenario 4. Graphical comparisons were developed for each major watershed (two per watershed) that examine the change in total delivered load between existing baseline and Scenario 4 conditions and the difference in loading rates. These are shown in Figure 2-25 through Figure 2-40.

A detailed examination of the first pair, for Blue Earth River, will serve to explain the remainder. The first graph (Figure 2-25) shows, in bar form, the source loading attributable to the major categories of sheet and rill erosion, ravines, and bed, bank, and bluff loading. The fourth bar shows the total sediment load delivered at the downstream exit of the watershed. The delivered load is not exactly equal to the source loads due to the presence of other minor sources, such as point source loads. The magenta bars show baseline conditions, while the violet bars show Scenario 4 predictions. In general, there are significant reductions in upland loads, particularly ravine loads, but not in the bed, bank, and bluff loads. The latter are derived primarily from remobilization of bluff sediment in the lower reaches that pass through the bluff area, on which only limited controls are placed under Scenario 4. As this load source, on average, constitutes around one third of the total load, failure to further control this source limits the ability of Scenario 4 to reduce total loads. In some cases, as in Blue Earth, the bed, bank, and bluff load actually increases slightly under Scenario 4. This occurs because the reduction in upland sediment load leaves unmet capacity of the streamflow to mobilize additional non-cohesive coarse sediment from the bed. The inset pie chart shows how the total change in delivered load is apportioned to the input source categories.

The second graph (Figure 2-26) shows the upland loading rates (tons per acre per year) for urban and agricultural land uses. This includes loads generated by both sheet/rill and ravine incision processes; however, ravine formation processes are only simulated on cropland. After the Scenario 4 reductions, “urban” land (which, in the model, represents all developed land, including towns and cities as well as rural residential parcels and roads) generally has the highest remaining delivered loading rate in terms of tons per acre. This occurs because the Scenario 4 management measures for developed areas apply only to those urban lands that fall within regulatory MS4 boundaries. Further, urban stormwater is typically conveyed to streams by surface drainage, increasing delivery, whereas much of the runoff from agricultural land is conveyed by subsurface tile drains. While a basin-wide average reduction in sediment


35

loads of 88.5 percent is simulated from urban lands within MS4 boundaries under Scenario 4, only 15 percent of the total urban land in the watershed lies within MS4 boundaries. Much of the remainder of the load in this category comes from rural roads.

Under Scenario 4, large reductions in sediment loading occur for both conventional and conservation tillage. This is due primarily to the reduction in ravine incision and the water retention requirement, which reduces transport capacity for detached sediment, rather than to improvements in tillage. Because this scenario involves a large shift in land use from conventional tillage to CRP, with the shift focused on the most erodible lands, the average loading rate for the remaining conventional tillage is reduced to levels similar to the average for agriculture in conservation tillage across the watershed.

0

50000

100000

150000

200000

250000

300000

350000

400000

Sheet & Rill Ravines Bed, Bank, & Bluff Delivered

Sedi

men

t (to

ns/y

r)

BaseS4

Change: -51.4%

Bed, Bank, & Bluff 3.99%

Ravines, -43.40%

Sheet & Rill, -12.01%

Figure 2-25. Sediment Load Sources - Blue Earth River above Rapidan River

0.0

0.1

0.2

0.3

0.4

0.5

Conservation Tillage Conventional Tillage Manured Cropland Pasture/CRP Urban

tons

/ac/

yr

BaselineScenario 4

Figure 2-26. Sediment Loading Rates for Blue Earth above Rapidan River


36

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000


Sedi

men

t (to

ns/y

r)

BaseS4

Change: -35.0%

Bed, Bank, & Bluff, -1.40%

Sheet & Rill, -17.07%Ravines,

-16.56%

Figure 2-27. Sediment Load Sources - Chippewa River at Montevideo

0.0

0.1

0.2

0.3


tons

/ac/

yr

BaselineScenario 4

Figure 2-28. Sediment Loading Rates for Chippewa River at Montevideo


37

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

200000


Sedi

men

t (to

ns/y

r)

BaseS4

Change: -59.7%

Ravines, -29.73%


Bed, Bank, & Bluff,

-2.81%

Figure 2-29. Sediment Load Sources - Cottonwood River at New Ulm

0.0

0.1

0.2

0.3


tons

/ac/

yr

BaselineScenario 4

Figure 2-30. Sediment Loading Rates for Cottonwood River at New Ulm


38

0

5000

10000

15000

20000

25000

30000

35000


Sedi

men

t (to

ns/y

r)

BaseS4

Change: -60.0%

Ravines, -29.47%


Bed, Bank, & Bluff,

-7.68%

Figure 2-31. Sediment Load Sources - Hawk Creek at Sacred Heart

0.0

0.1

0.2

0.3


tons

/ac/

yr

BaselineScenario 4

Figure 2-32. Sediment Loading Rates for Hawk Creek at Sacred Heart


39

0

50000

100000

150000

200000

250000

300000

350000

Sheet & Rill Ravines Bed, Bank, & Bluff (net) Delivered

Sedi

men

t (to

ns/y

r)

BaseS4

Change: -47.2%

Bed, Bank, and Bluff,

3.28%

Ravines, -40.69%


Figure 2-33. Sediment Load Sources - Le Sueur River at Rapidan

0.0

0.1

0.2

0.3

0.4


tons

/ac/

yr

BaselineScenario 4

Figure 2-34. Sediment Loading Rates for Le Sueur River at Rapidan


40

0

10000

20000

30000

40000

50000

60000


Sedi

men

t (to

ns/y

r)

BaseS4

Change: -42.7%

0.55%

Ravines, -26.68%


Figure 2-35. Sediment Load Sources - Redwood River at Redwood Falls

0.0

0.1

0.2

0.3


tons

/ac/

yr

BaselineScenario 4

Figure 2-36. Sediment Loading Rates for Redwood River at Redwood Falls


41

0

10000

20000

30000

40000

50000

60000

70000

80000

90000


Sedi

men

t (to

ns/y

r)

BaseS4

Change: -58.9%

Ravines, -31.73%


Bed, Bank, & Bluff,

-4.28%

Figure 2-37. Sediment Load Sources - Watonwan River at Garden City

0.0

0.1

0.2

0.3


tons

/ac/

yr

BaselineScenario 4

Figure 2-38. Sediment Loading Rates for Watonwan River at Garden City


42

0

10000

20000

30000

40000

50000

60000

Sheet & Rill Ravines Bed, Bank, & Bluff (net) Delivered

Sedi

men

t (to

ns/y

r)

BaseS4

Change: -50.0%

Bed, Bank, & Bluff (net),

-0.80%Sheet & Rill,

-19.35%

Ravines, -29.82%

Figure 2-39. Sediment Load Sources - Yellow Medicine River at Granite Falls

0.0

0.1

0.2

0.3


tons

/ac/

yr

BaselineScenario 4

Figure 2-40. Sediment Loading Rates for Yellow Medicine River at Granite Falls

To elucidate the path toward achieving compliance, more detailed analyses were undertaken for the Le Sueur River, which is one of the areas where achieving the TMDL targets appears most challenging.

Going from Baseline to Scenario 4 conditions reduces average concentrations (Figure 2-41) and the percent of concentrations above the surrogate of 90 mg/L (Figure 2-42) by about half upstream of the bluff area. Reduction is much less within the bluff area – mostly because the bluff loads are not significantly addressed in Scenario 4. The 95th percentile concentrations suggest that upstream of the bluffs Scenario 4 just meets the 90 mg/L surrogate (Figure 2-43).


43

Average Concentration

0

30

60

90

120

150

R600 R620 R640 R645 R642 R622

TSS

(mg/

L)

Baseline

Scen 4

Bluff Area

Model Reach Figure 2-41. TSS Average Concentrations for Le Sueur River Segments, 1993-2006

Percentage Greater than 90 mg/L TSS Surrogate

0%

10%

20%

30%

40%

50%

60%

R600 R620 R640 R645 R642 R622

Perc

enta

ge

Baseline

Scen 4

Bluff Area

Model Reach Figure 2-42. Percentage of TSS Observations Greater than 90 mg/L for Le Sueur River, 1993-2006


44

95%le Concentration

0

50

100

150

200

250

300

350

400

450

500

R600 R620 R640 R645 R642 R622

TSS

(mg/

L)

Baseline

Scen 4

Bluff Area

Model Reach Figure 2-43. 95th Percentile TSS Concentrations for Le Sueur River, 1993-2006

Scenario 4 includes a variety of practices (including crop changes, reduction in ravine loads, and retention of 1” of runoff near the source, and other practices) that drastically reduce the sediment loading rate from agricultural lands (tillage and pasture). Reductions are also imposed on “urban” or developed lands, but only for those lying within MS4 boundaries. Loading rates by land use, shown above in Figure 2-34, show that the net decrease in loading from urban land is much less than the reduction obtained from cropland in Scenario 4.

Agriculture is the dominant land use in the watershed, and the decline in agricultural loading rates provides a large reduction in the total upland sediment yield. Yet, the change in total suspended solids at the mouth of the Le Sueur is relatively modest, and many observations (about 40 percent) remain above the surrogate (for turbidity) target TSS level of 90 mg/L.

Overall, Scenario 4 provides a 53.7 percent reduction in delivered sediment loads at the mouth of the Le Sueur, despite much larger reductions in loads from agricultural lands. Sediment in the river derives from three types of processes: sheet and rill erosion, ravines, and channel processes (in which are included bed, bank, and bluff sources). An examination of the total load at the mouth of the Le Sueur by these categories (Figure 2-33, above) shows that sheet and rill erosion declined by 50 percent and ravine erosion declined by 89 percent - but the net contribution of channel processes actually increased by 11 percent. Fully 70 percent of the remaining sediment load at the mouth of the Le Sueur under Scenario 4 is due to channel erosion processes.

A series of detailed diagnostics on the Le Sueur model were run to further understand the remaining sources of excess sediment load under Scenario 4 One informative test is to examine the ratio of the load generated from channel processes to load generated from the uplands for a 3-day window up to and including days on which the simulated TSS concentration is greater than the surrogate (a three-day windows is used due to long travel times in part of the basin, such that concentrations at the mouth may reflect preceding day loads from higher in the basin). When this ratio is greater than 1, channel processes are the main cause of the excursion; when it is much less than 1, upland sources dominate.


45

Results of this experiment are shown in Figure 2-44. The highest predicted concentrations are still upland dominated (ratio less than 1) and are associated with loads from agricultural land during extreme rain events, as was determined by single land use sensitivity analyses. These high loading conditions occur, however, less than 1 percent of the time.

In Figure 2-44, the group at the lower left represents relatively small flows during which neither significant channel scour nor cropland erosion occurs. These excursions (approximately 10 percent) occur at moderate flows arising from smaller precipitation events and have important contributions from “urban” land, which contains impervious areas that provide runoff during moderate rain events. As noted above, the so-called “urban” category in the model includes all pervious and impervious surfaces associated with human development, including any isolated farmsteads and roads that are resolved by the NLCD land use coverage. The scenario imposes controls on only the small fraction of urban land use areas that are within designated MS4 boundaries.

0.00

0.01

0.10

1.00

10.00

100.00

1000.00

10000.00

100000.00

1000000.00

10000000.00

0 200 400 600 800 1000 1200 1400

TSS (mg/L)

Ratio

of 3

-d S

cour

to U

plan

d Lo

ad

Dominated by Agricultural LoadsDominated by "Urban" Loads

Dominated by Channel Processes

Figure 2-44. Index of Importance of Channel versus Upland Processes in Le Sueur River

Sediment Load (at Mouth) under Scenario 4

By far the majority of the TSS concentrations above the surrogate target are driven by channel processes (ratio much greater than 1), which includes the mobilization of any contributions from bluff collapse. In the Le Sueur there are three reaches that receive bluff loads. The channel processes within these reaches contribute about 88 percent of the total channel/bank/bluff load from the entire Le Sueur. This suggests that significant further progress toward achieving the TMDL targets can only be attained by reducing the channel loads in the bluff areas, not from controlling upland loads alone.

The loads from the bluff areas could be reduced by reducing the sediment supply in these areas, reducing the erosion potential in these reaches, or by controlling the occurrence of high flows, which provide the erosive power. In theory, clipping the flood peaks generated by the uplands should provide a reduction in the channel loads. But, channel loads actually go up in Scenario 4 relative to the baseline simulation. To understand this result, it is important to note that the critical shear stresses (the points at which motion of silt and clay commence) are set rather low relative to other reaches, so that scour begins at moderate


46

flows. This was found to be needed to achieve calibration, and it was reasoned that bluff contributions of unconsolidated sediment, which may choke channels, would be easily eroded and indeed the flow would need to perform this work to maintain a channel. Reducing the magnitude of flow peaks would reduce the rate of mobilization of this material, but only incrementally, as erosion is simulated as occurring well below peak flow levels. As a result, the retention of flow peaks on the uplands has only a limited effect on predicted channel erosion in the bluff reaches.

Within the model, loads from the bluff areas could be reduced by (1) reducing the supply of load from the bluffs that replenishes the bed sediment available for scour in these reaches, (2) increasing the critical shear stress at which erosion commences, or (3) reducing the maximum potential rate of scour (the model scales up actual scour rates from zero at the critical shear stress value to the user-specified maximum rate).

The model simulates bluffs as contributing sediment to the channel that is then available for scour when flow exceeds the critical shear stress. (This is necessary due to the one-dimensional nature of the model, which does not allow exact representation of the detailed real-world three dimensional processes of channel evolution.) When the sediment available for scour is depleted (i.e., the channel scours down to a more resistant substrate), channel erosion stops until the supply is replenished. Simulating a reduction in the rate of bluff contribution would thus result in more frequent periods of zero scour (because the supply is depleted), but at other times would still provide significant loads at moderate flows, leading to non-achievement of the targets.

The most promising approach to reducing sediment loads in the bluff areas (in the model) is thus to increase the critical shear stress values. In the real world this might be expected to occur if the channel and floodplain were controlled and configured such that (1) collapsing bluffs tended not to fall directly into the channel causing choking, and (2) grade controls prevent the upstream migration of headcuts.

For reaches outside of the bluff areas, critical shear stresses are assigned such that motion of silt commences at around the 95th percentile of the flow distribution and motion of clay commences at about the 85th percentile of the flow distribution. As noted above, the critical shear stresses in the bluff reaches are set lower in the baseline model.

A test simulation for the Le Sueur (using Scenario 4 conditions) was therefore conducted with critical shear stress (tau) values for the three bluff reaches increased to these default values. Results of this experiment are summarized in a histogram in Figure 2-45, which compares the frequency of TSS ranges under these assumptions to those obtained in the baseline and Scenario 4 simulations. As can be seen in the figure, the change from the baseline to Scenario 4 results in only rather small leftward shifts in the frequency bins. In contrast, increasing the critical shear stress values to defaults results in a very substantial shift in frequency to lower concentrations, with most concentrations less than 90 mg/L.


47

0

200

400

600

800

1000

1200

1400

1600

0 20 40 60 80 100

150

200

250

300

350

400

450

500

600

800

1000

1200

1400

TSS Bin (mg/L)

Freq

uenc

y BaselineScenario 4Increased Tau

Figure 2-45. Histogram Showing Results of Increased Critical Shear Stress Values in the Bluff

Reaches of the Le Sueur

Table 2-17 provides the statistics for the three simulations. Scenario 4 yields a significant reduction in the maximum sediment concentration (because the highest loads from agricultural uplands are reduced), but only a small reduction in the median concentration and the frequency greater than the target value of 90 mg/L. In contrast, increasing the critical shear stress provides a much lower median and reduces the percentage of days greater than 90 mg/L to just over 8 percent.

Table 2-17. Statistics for Daily Average TSS Concentrations at the Mouth of the Le Sueur under Baseline, Scenario 4, and Scenario 4 with Increased Critical Shear Stress

Statistic Baseline Scenario 4

Scenario 4 with Increased Critical

Shear Stress

Percentage greater than 90 mg/L 48.20% 42.00% 8.10%

Average (mg/L) 127.0 99.4 36.5

Median (mg/L) 84.8 73.0 19.7

90th %le (mg/L) 288.0 243.1 76.8

95th %le (mg/L) 395.0 304.8 115.3

Maximum (mg/L) 4,240.0 1,180.3 1,046.4

The test with increased critical shear stress still results in 8.1 percent of observations above 90 mg/L. It will be recalled, however, that Scenario 4 still contains relatively high loads from “urban” land uses, due to loading from developed land outside MS4 boundaries, which appear to be responsible for a number of excursions of the target under moderate flows (Figure 2-44). Therefore, additional reductions in the frequency of excursions can likely be obtained by imposing a reduction in the sediment yield rate from developed land outside of MS4s.

Under Scenario 4, only that small fraction of developed land that falls within MS4 boundaries was subject to any controls. A general 50 percent reduction in sediment loading rates from those dispersed developed


48

lands (pervious and impervious) that lie outside of MS4 boundaries results in about a 20 percent reduction in average TSS concentration, but the percentage above the surrogate declines only to 6.2 percent (Table 2-17).

Table 2-18. Statistics for Daily Average TSS Concentrations at the Mouth of the Le Sueur

Statistic Baseline Scenario 4

Scenario 4 with Increased Critical

Shear Stress

With 50% Reduction from “Urban” Land Outside MS4s

With 50% Reduction in

Maximum Scour Rates in Bluff

Reaches

Percentage greater than 90 mg/L 48.20% 42.00% 8.10% 6.20% 5.10%

Average (mg/L) 127.0 99.4 36.5 30.6 29.2

Median (mg/L) 84.8 73.0 19.7 16.7 16.7

90th %le (mg/L) 288.0 243.1 76.8 64.2 59.9

95th %le (mg/L) 395.0 304.8 115.3 102.1 91.0

Maximum (mg/L) 4,240.0 1,180.3 1,046.4 1,005.4 986.9

Following this experiment, the index of the importance of channel versus upland processes was recalculated for the new results (Figure 2-46). As in the previous analysis, there are still a small fraction of events that are dominated by upland loading from agricultural land during extreme rain events, as well as some residual influence from urban loads. The events dominated by channel processes, although reduced, still constitute the bulk of the events in excess of 90 mg/L. Therefore, achieving a lower frequency of excursions of the surrogate will require additional attention to this load source.

0.00

0.01

0.10

1.00

10.00

100.00

1000.00

10000.00

100000.00

1000000.00

10000000.00

0 200 400 600 800 1000 1200 1400

TSS (mg/L)

Ratio

of 3

-d S

cour

to U

plan

d Lo

ad

Dominated by Agricultural LoadsDominated by "Urban" Loads

Dominated by Channel Processes

Figure 2-46. Index of Importance of Channel versus Upland Processes in Le Sueur River

Sediment Load (at Mouth) after Implementing Increases in Critical Shear Stress in Channel Reaches and Reductions in Load from Developed Land


49

An investigation of the sensitivity of results to the resupply rate from bluff collapse showed only a small effect. Specifically, a 25 percent reduction in bluff supply dropped the percentage of observations greater than the surrogate from 6.2 to 6.1 percent. Only a small effect on concentration occurs because it is the hydraulic resuspension of load derived from the bluffs that determines instream concentrations, rather than the rate of supply of material. Reductions in bluff resupply thus reduce the total sediment mass exported, but have little effect on the frequency of high-concentration events.

The previous experiments increased the critical shear stress for cohesive sediment movement in the channel, but did not alter the erodibility coefficient (M, lb/ft2/d). The predicted rate of erosion is a function of M times the ratio of shear stress (τ) to critical shear stress (τC). Load generated from the channel thus scales directly with M.

The M values for the bluff reaches are set higher than for upstream reaches, reflecting a characterization of these reaches as actively processing large amounts of unconsolidated sediment derived from the bluffs. If the channel is rehabilitated, with inclusion of grade control to prevent headcuts, and kept away from the bluff foot where possible, it is likely that the value of the erodibility coefficient will also decline.

A reduction of 50 percent in the M values for silt and clay (which still leaves the erodibility coefficients for these reaches greater than for reaches upstream of the bluff area) results in a reduction to 5.1 percent in the percentage of daily average TSS predictions greater than 90 mg/L (see Table 2-18 above), while the 95th percentile concentration is just above the surrogate at 91 mg/L.

This suggests a strategy for achieving targets in Scenario 5: A scenario that is to achieve the TSS surrogate targets would need to combine the upland land management practices incorporated in Scenario 4 with a representation of decreased loading from channel processes in the bluff areas.

2.2.3 Phosphorus and Sediment Export at Jordan Total loads exported from the Minnesota River at Jordan are also of interest for their downstream effects. The Lake Pepin TMDL is expected to call for reductions in both phosphorus and sediment load from the Minnesota River. The export of phosphorus from the Minnesota River under the different scenarios is compared to Baseline results in Table 2-19 and Figure 2-47. This indicates that management measures to reduce sediment load also achieve a substantial reduction in phosphorus load, as most phosphorus is transported in associated with sediment. An alternative version of Scenario 4 was also run with all phosphorus loading from wastewater treatment plants eliminated. As these loads had already been reduced for Scenario 4, full elimination reduced the delivered load by only about 11 tons per year.

Table 2-19. Phosphorus Export (lbs/yr), Minnesota River at Jordan, by Water Year

Water Year Baseline Scenario 1 Scenario 2 Scenario 3 Scenario 4

2001 2,795,501 2,732,350 2,476,655 2,368,248 2,229,519

2002 1,341,317 1,269,866 1,189,869 1,155,355 1,073,832

2003 935,214 963,131 885,210 773,083 738,597

2004 1,673,772 1,307,146 1,061,308 1,193,600 980,095

2005 1,946,685 1,855,693 1,708,003 1,594,932 1,396,321

2006 2,198,274 2,178,752 1,988,533 1,925,747 1,691,036

Total 10,890,762 10,306,937 9,309,578 9,010,965 8,109,399

Average 1,815,127 1,717,823 1,551,596 1,501,828 1,351,567


50


Average (tons/yr) 908 859 776 751 676

Reduction Relative to Baseline -5.36% -14.52% -17.26% -25.54%

Total P Export at Jordan

0

100

200

300

400

500

600

700

800

900

1000

Baseline Scenario 1 Scenario 2 Scenario 3 Scenario 4

Tota

l P (t

ons/

yr)

Figure 2-47. Phosphorus Export, Minnesota River at Jordan, Water Years 2001-2006

The total export of sediment at Jordan is summarized in Table 2-20 and Figure 2-48.

Table 2-20. Sediment Export, Minnesota River at Jordan, by Water Year (tons)


2001 1,344,108 1,313,686 1,240,249 1,193,557 779,240

2002 592,342 590,103 551,765 517,220 352,724

2003 364,710 364,206 346,515 324,904 228,586

2004 1,177,525 1,091,248 948,346 824,943 391,145

2005 1,035,719 970,232 875,574 791,911 502,140

2006 904,593 887,081 823,461 944,968 531,320

Total 5,418,998 5,216,555 4,785,909 4,597,503 2,785,156

Average (tons/yr) 903,166 869,426 797,652 766,250 464,193

Reduction Relative to Baseline -3.7% -11.7% -15.2% -48.6%


51

Total Suspended Sediment Export at Jordan

0

100,000

200,000

300,000

400,000

500,000

600,000

700,000

800,000

900,000

1,000,000

Baseline Scenario 1 Scenario 2 Scenario 3 Scenario 4

Tota

l Sed

imen

t (to

ns/y

r)

Figure 2-48. Sediment Export, Minnesota River at Jordan, Water Years 2001-2006


52



53

3 TMDL Targets Scenario (Scenario 5)

3.1 TMDL TARGETS Scenario 5 represents “paradigm change” – the level of changes to the management of both the landscape and the stream network that is needed to achieve compliance with the total suspended solids surrogate objectives associated with the turbidity standard. The TMDL TSS loading objectives were set by the MPCA using a load-duration curve approach. This approach focuses on flows up to the 95th percentile (i.e., the mid-point of the high flow zone on the load duration curves). In addition, a 10 percent explicit margin of safety will be built in, so the objective will be set at 90 percent of the TSS surrogate value. For example, in the Le Sueur River near Rapidan, the scenario should achieve a daily average suspended sediment concentration of 81 mg/L (90 percent of the 90 mg/L target) on days when the flow is less than or equal to the 95th percentile flow of 2,870 cfs. Attainment of compliance is assessed, as previously, over the period from December 2000 to November 2005 (“crop years” 2001-2005). Compliance is evaluated at model reaches corresponding to all 18 segments listed for turbidity impairment, using the 2008 303(d) list. (The 2008 list adds 07020004-515, Minnesota River from Minnesota Falls Dam to Hazel Cr., and deletes 07020007-508, Minnesota River from Eight Mile Cr. to Cottonwood River.)

3.1.1 TSS Surrogates MPCA has conducted detailed analyses of the relationship between monitored turbidity and TSS at stations throughout the Minnesota River system (Campbell, 2009). The relationship between TSS and turbidity changes across the basin. As a result, lower TSS surrogate values are required in the western watersheds and higher values are required in the eastern watersheds to achieve the turbidity criterion. These are shown in Table 3-1.

Table 3-1. TSS Surrogate Concentrations for Minnesota River

Stream Name Description MPCA

Assessment Unit Drainage Area (mi2) TSS Surrogate

Minnesota River Rush R to High Island Cr 07020012-503 15,994 100 mg/l

Minnesota River Shanaska Cr to Rogers Cr 07020007-501 15,105 100 mg/l

Minnesota River Blue Earth R to Shanaska Cr 07020007-502 15,023 100 mg/l

Minnesota River Swan Lk outlet to Minneopa Cr 07020007-505 11,337 100 mg/l

Minnesota River Cottonwood R to Little Cottonwood R 07020007-503 10,876 85 mg/l

Minnesota River Beaver Cr to Birch Coulee 07020007-514 9,026 85 mg/l

Minnesota River Timms Cr to Redwood R 07020004-509 8,200 75 mg/l

Minnesota River Minnesota Falls Dam to Hazel Cr 07020004-515 6,439 75 mg/l

Minnesota River Chippewa R to Stony Run Cr 07020004-501 6,221 75 mg/l

Blue Earth River Le Sueur R to Minnesota R 07020009-501 3,540 90 mg/l


54

Stream Name Description MPCA

Assessment Unit Drainage Area (mi2) TSS Surrogate

Blue Earth River Rapidan Dam to Le Sueur R 07020009-509 2,427 90 mg/l

Le Sueur River Maple R to Blue Earth R 07020011-501 1,112 90 mg/l

Watonwan River Perch Cr to Blue Earth R 07020010-501 877 90 mg/l

Cottonwood River JD 30 to Minnesota R 07020008-501 1,313 70 mg/l

Redwood River Ramsey Cr to Minnesota R 07020006-501 705 70 mg/l

Yellow Medicine River Spring Cr to Minnesota R 07020004-502 674 50 mg/l

Hawk Creek Spring Cr to Minnesota R 07020004-587 506 50 mg/l

Chippewa River Watson Sag to Minnesota R 07020005-501 2,083 50 mg/l

3.1.2 Margin of Safety An explicit Margin of Safety will be incorporated into the TMDL by reducing the TSS surrogate objectives to 90 percent of the values shown in Table 3-1. The development of the compliance scenario endeavored to meet the TSS surrogate objectives 100 percent of the time. The reduced values of 90 percent of the surrogate objectives were met all the time when possible, but were allowed to be exceeded up to 5 percent of the time when necessary.

3.1.3 Flow Duration Analysis Interpretation of the scenario depends on the 95th percentile determined in the flow duration analysis. Flow percentiles were derived by MPCA for those sites with streamflow. Due to concerns over changes in hydrology over time, these percentiles are based on the 1977-2006 record, rather than the complete period of gaging. Relevant estimates of the 95th percentile flow provided by MPCA are as follows:

Table 3-2. 95th Percentile Flows (cfs) at USGS Gages Determined by MPCA

Minnesota River nr Montevideo 5,430 Blue Earth River nr Rapidan 5,552

Yellow Medicine at Granite Falls 870 Blue Earth River at Mouth 8,322

Redwood River at Redwood Falls 1,020 Le Sueur River nr Rapidan 2,870

Cottonwood River nr New Ulm 2,180 Minnesota River at Mankato 20,000

Watonwan River nr Garden City 1,730 Minnesota River at Jordan 22,300

Blue Earth River nr Good Thunder 3,875

Flow percentiles were also needed for a number of sites not gaged by USGS. This was accomplished as follows:

• For the Chippewa River, the listed segment is below the Watson Sag Diversion, which shunts high flows into Lac qui Parle. The Chippewa below this point is not gaged by USGS. However, the US Army Corps of Engineers does measure flow leaving the Watson Sag dam and proceeding into the lower Chippewa. Tetra Tech developed an analysis of this time series, including the


55

filling of gaps, as part of the characterization of the Chippewa Diversion during model development. For 1977-2006, the 95th percentile flow is 938 cfs, which is used in the analysis.

• For Hawk Creek, the flow duration curve analysis for the Yellow Medicine River was used to determine the appropriate translation between the 95th percentile flow in the 30-year record and the percentile point of that flow within the model simulation period. The corresponding cutoff point in the simulated flows for Hawk Creek is 939 cfs.

• For ungaged segments on the Minnesota River upstream of Mankato and the confluence with the Blue Earth River, the flow duration curve already developed for the Minnesota River at Montevideo was scaled up by the increase in drainage area to estimate the 95th percentile flow.

• For ungaged segments on the Minnesota River downstream of Mankato, a similar process was first used based on the flow duration curve analysis at the Mankato gage. However, some further refinement was needed to account for the influence of the Blue Earth River. Under certain uncommon conditions, the Blue Earth River is at flood stage while flows from the upper Minnesota River are low. During the flow transition, the Blue Earth River may be above the 95th percentile flow while the Minnesota River at Mankato is still below the 95th percentile flow. Compliance with the TSS surrogate objective is not required on the Blue Earth (because above the 95th percentile), but essentially the same flow and sediment concentration is present in the mainstem below the Blue Earth. Therefore, two transition days were also exempted from meeting the TSS surrogate when the Blue Earth River was above the 95th percentile flow and the mainstem was transitioning to flow greater than the 95th percentile on the following day.

It should be noted that the analysis of compliance is made relative to the flow percentiles for existing conditions. That is, the flow duration analysis is not revised to reflect any change in flows that may be caused by the management scenario. This is a conservative approach as the 95th percentile flows decline due to use of increased water storage measures. The hydrologic changes that result from implementation do have important impacts on pollutant loading, which is accounted for in the analysis.

3.2 TMDL TARGETS SCENARIO SETUP The TMDL targets scenario (Scenario 5) was developed iteratively. First, a set of baseline measures was applied to all watersheds (Section 3.2.1). Then, additional measures were applied on an individual watershed basis, as needed, to achieve targets (Section 3.2.2).

3.2.1 Scenario 5 Baseline Setup Analysis of the results of Scenario 4 demonstrated that large reductions were achieved in loads from upland sources – including both sheet/rill and ravine loading. Most of the remaining load is derived from channel and bank/bluff erosion, particularly in the bluff reaches. Reduction in these sources is needed to achieve standards. However, it was also determined that some excursions appear to be caused by runoff from developed (“urban”) land outside of MS4 boundaries, for which no enhanced management has been proposed in earlier scenarios.

Based on the scoping analyses presented in Section 2.2.2, Scenario 5 was set up with the following baseline components:

• All upland management practices simulated in Scenario 4 are carried forward to Scenario 5.

• An additional reduction in sediment load from developed land outside of MS4 boundaries was imposed: An 85 percent reduction is applied to load from impervious surfaces and a 50 percent reduction is applied to loads from pervious surfaces. These are applied as direct reductions in sediment load because it is assumed that this improvement will be achieved by a variety of


56

dispersed management practices, including source reduction, rather than explicitly through retention of runoff. As a result, this reduction in load is represented without a change in hydrology.

• The scenario increased critical shear stress values for silt and clay within the bluff reaches to default values for the model (95th percentile of the instantaneous flow distribution for silt and 85th percentile for clay). The critical shear stresses in the bluff reaches are set lower than the default in the baseline model, representing a situation in which the channel is clogged with unconsolidated bluff sediment that must be periodically moved to maintain the flow pathway. This improvement is assumed to be an approximation of the effects of channel stabilization and rehabilitation in the bluff reaches.

• Rates of sediment supply from the bluffs were reduced by 25 percent. This has only a minor direct effect on the frequency of elevated instream suspended sediment concentrations, as the bluffs serve to replenish the channel supply, which is subsequently scoured by high flow events. Nonetheless, some improvement can be expected through a combination of management of ravines that intersect the bluffs and orientation of the stream channel away from bluff faces. This is implemented in the SPECIAL ACTIONS section that addresses bluff load.

• The scenario reduced the erodibility factors (M) for silt and clay in the bluff reaches. The M values for the bluff reaches are set higher than for upstream reaches, reflecting a characterization of these reaches as actively processing large amounts of unconsolidated sediment derived from the bluffs. If the channel is rehabilitated, with inclusion of grade control to prevent headcuts, and kept away from the bluff foot where possible, it is likely that the value of the erodibility coefficient (an index to the maximum erosion rate) will also decline. We have previously seen that a reduction of the M values by 50 percent (which still leaves the coefficients for these reaches greater than for reaches upstream of the bluff area) approaches the target for Le Sueur. The Scenario 5 baseline includes a 50 percent reduction in M.

• Sand transport capacity in the bluff reaches was similarly reduced to default rates in those cases where the calibration had established a higher capacity.

• Similar modifications to match default rates were made to the channel erodibility and transport factors in the mainstem Minnesota. Although most bluff loading does not directly enter the mainstem, similar transport characteristics are assumed to apply due to the proximity of bluff loading on the tributaries. The mainstem sediment transport is expected to respond to reductions in excess loading from the bluff areas of the tributaries; however, achieving this may also require direct management of the mainstem channel.

• Added a practice (P) factor of 0.5 to the grass/CRP land use. Under baseline conditions, the grass land use category is conceptualized as including areas of pasture where erosion may be enhanced by grazing, trampling, and presence of eroded pathways. Under scenarios 4 and 5, this land use category primarily represents CRP. The P factor accounts for the presence of better cover for CRP lands that will reduce the rate of detached sediment generation available for sheet and rill erosion. In the Minnesota River watershed, sediment load delivered to streams appears to be, on average, more limited by overland flow transport capacity than by sediment availability, so the effect of this change is small.

3.2.2 Additional Management Measures The baseline components of Scenario 5 reduced rates of excursion of the TSS surrogate objectives to around 5 – 10 percent, but did not fully achieve the targets. Therefore, additional measures were applied on a watershed by watershed basis to meet the targets. In many cases, these additional measures were


57

needed to address a few isolated excursions of the objectives. These additional measures are summarized below.

• The transport capacity for sheet and rill erosion from the pasture/grass/CRP land use was reduced in all basins. Such a reduction appears reasonable, as there is a shift from grazed pasture to CRP within this land use class. Intact grass lands with good cover should provide low erosion rates, but significant erosion can occur in areas that are overgrazed, from tracks and paths, and in areas where cattle have access to streams. A 50 percent reduction in transport capacity was required for the Chippewa, Hawk Creek, Yellow Medicine, and Watonwan watersheds; an 80 percent reduction was required in the Cottonwood, Redwood, Blue Earth, Le Sueur, Middle Minnesota, and Lower Minnesota watersheds.

• Sediment load derived from the pervious portion of developed lands outside of MS4 boundaries was further reduced in several watersheds, with a 75 percent reduction in the Cottonwood, Blue Earth, and Middle Minnesota watersheds, and an 85 percent reduction in the Le Sueur watershed.

• In the Hawk Creek, Yellow Medicine, Redwood, Cottonwood, Blue Earth, Le Sueur, and Middle Minnesota watersheds, model subwatersheds that had less than 75 percent of cropland in conservation tillage under Scenario 4 were increased to a minimum of 75 percent conservation tillage.

• Erosion from ravines prevented attainment in the Blue Earth watershed and was further reduced to about half of the Scenario 4 rates.

• Additional reductions to stream transport capacity in the bluff reaches, beyond those specified in the baseline, were applied in the Blue Earth and Redwood watersheds to address model simulation of elevated concentrations during peak flow events that were primarily driven by channel erosion.

The final configuration of Scenario 5 represents an extremely aggressive suite of management measures. The full set of measures is not necessarily practicably achievable, but does demonstrate what might be required to achieve TMDL targets with the turbidity standard as currently defined. Different combinations of management measures that achieve similar levels of reduction might also achieve targets, but it is clear that any successful strategy would need to address both channel and upland loads, and include measures that reduce loading during both large events (where upland loading is dominated by runoff from cropland) and smaller events (where significant contributions come from impervious surfaces associated with developed land).

3.3 TMDL TARGETS SCENARIO RESULTS

3.3.1 TMDL Targets Scenario Results − Tributary Watersheds TMDL Targets Scenario (Scenario 5) results for the tributary watersheds are shown graphically in Figure 3-1 through Figure 3-10. In these figures, the TSS concentrations associated with flows above the 95th percentile are marked in magenta. Tabular summaries of the frequency of excursions of the TSS surrogate objectives (Table 3-3 through Table 3-12) show that there are no remaining excursions of the surrogate objectives, and only a few excursions of the MOS level of 90 percent of the objective.


58

Blue Earth- Mankato

0

100

200

300

400

Oct-00 Oct-01 Oct-02 Oct-03 Oct-04 Oct-05

Date

TSS

(mg/

L)

Above 95th Percentile

TSS Surrogate

Simulated TSS

Figure 3-1. Scenario 5 Results for Blue Earth River near Mankato

Blue Earth- Rapidan

0

50

100

150

200


Date

TSS

(mg/

L)


TSS Surrogate

Simulated TSS

Figure 3-2. Scenario 5 Results for Blue Earth River at Rapidan


59

Blue Earth- Good Thunder

0

50

100

150

200

250


Date

TSS

(mg/

L)


TSS Surrogate

Simulated TSS

Figure 3-3. Scenario 5 Results for Blue Earth River at Good Thunder

Chippewa- Montevideo

0

30

60

90

120

150


Date

TSS

(mg/

L)


TSS Surrogate

Simulated TSS

Figure 3-4. Scenario 5 Results for Chippewa River at Montevideo


60

Cottonwood- New Ulm

0

50

100

150

200


Date

TSS

(mg/

L)


TSS Surrogate

Simulated TSS

Figure 3-5. Scenario 5 Results for Cottonwood River at New Ulm

Hawk- Sacred Heart

0

50

100

150

200


Date

TSS

(mg/

L)


TSS Surrogate

Simulated TSS

Figure 3-6. Scenario 5 Results for Hawk Creek at Sacred Heart


61

Lesueur- Rapidan

0

100

200

300

400

500


Date

TSS

(mg/

L)


TSS Surrogate

Simulated TSS

Figure 3-7. Scenario 5 Results for Le Sueur River at Rapidan

Redwood- Redwood Falls

0

150

300

450

600


Date

TSS

(mg/

L)


TSS Surrogate

Simulated TSS

Figure 3-8. Scenario 5 Results for Redwood River at Redwood Falls


62

Watonwan - Garden City

0

30

60

90

120

150


Date

TSS

(mg/

L)Above 95th Percentile

TSS Surrogate

Simulated TSS

Figure 3-9. Scenario 5 Results for Watonwan River at Garden City

Yellow- Granite

0

30

60

90

120

150


Date

TSS

(mg/

L)


TSS Surrogate

Simulated TSS

Figure 3-10. Scenario 5 Results for Yellow Medicine River at Granite Falls


63

Table 3-3. Scenario 5 Results for Blue Earth River near Mankato

Fraction greater than surrogate TSS of 90 Fraction greater than surrogate TSS of 81

Crop Year Dec-Mar Apr-May Jun-Jul Aug-Sep Oct-Nov Crop Year Dec-Mar Apr-May Jun-Jul Aug-Sep Oct-Nov

2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.0% 0.0%

Table 3-4. Scenario 5 Results for Blue Earth River at Rapidan



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.1% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.0% 0.0%


64

Table 3-5. Scenario 5 Results for Blue Earth River at Good Thunder



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.3% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.0% 0.0%

Table 3-6. Scenario 5 Results for Chippewa River at Montevideo



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.0% 0.0%


65

Table 3-7. Scenario 5 Results for Cottonwood River at New Ulm



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 1.1% 0.0%

Table 3-8. Scenario 5 Results for Hawk Creek at Sacred Heart



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.0% 0.0%


66

Table 3-9. Scenario 5 Results for Le Sueur River at Rapidan

Fraction greater than surrogate TSS of 90 Fraction greater than surrogate TSS (with MOS) of 81


2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 1.3% 0.0% 0.0% 0.0%

Table 3-10. Scenario 5 Results for Redwood River at Redwood Falls



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.0% 0.0%


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Table 3-11. Scenario 5 Results for Watonwan River at Garden City



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.0% 0.0%

Table 3-12. Scenario 5 Results for Yellow Medicine River at Granite Falls



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.0% 0.0%


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The drastic reductions in instream loads needed to meet the TSS surrogates result in a corresponding reduction in flow-weighted mean concentrations. Results for the Blue Earth River at Mankato are compared to existing conditions and to Scenario 4 in Figure 3-11.

Blue Earth at Mankato

0

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Dec-M

ar 01

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onal

Sim

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ed T

SS F

low

Wei

ghte

d M

ean

Con

cent

ratio

n (m

g/l)

CalibrationScenario 4Scenario 5

Figure 3-11. Seasonal Flow-Weighted Average TSS for Scenario 5, Blue Earth River at Mankato

3.3.2 TMDL Targets Scenario Results – Minnesota River Mainstem Scenario 5 results for the impaired segments on the Minnesota River mainstem are shown graphically in Figure 3-12 through Figure 3-21. Results are also provided for the gage locations at Mankato and Jordan. As with the figures for the tributary watersheds, the TSS concentrations associated with flows above the 95th percentile are marked in magenta. Tabular summaries of the frequency of excursions of the TSS surrogate objectives (Table 3-13 through 22) show that there are no remaining excursions of the surrogate objectives, and only a few excursions of the MOS level of 90 percent of the objective.


69

Chippewa River to Stony Run Creek

0

50

100

150

200


Date

TSS

(mg/

L)

Above 95th Percentile TSS Surrogate Simulated TSS

Figure 3-12. Scenario 5 Results for Minnesota River from Chippewa River to Stony Run Creek

Minnesota Falls Dam to Hazel Creek

0

100

200

300

400


Date

TSS

(mg/

L)


Figure 3-13. Scenario 5 Results for Minnesota River from Minnesota Falls Dam to Hazel Creek


70

Timms Creek to Redwood Creek

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100

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400


Date

TSS

(mg/

L)


Figure 3-14. Scenario 5 Results for Minnesota River from Timms Creek to Redwood Creek

Beaver Creek to Birch Coulee

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100

200

300

400

500


Date

TSS

(mg/

L)


Figure 3-15. Scenario 5 Results for Minnesota River from Beaver Creek to Birch Coulee


71

Cottonwood River to Little Cottonwood

0

150

300

450

600


Date

TSS

(mg/

L)


Figure 3-16. Scenario 5 Results for Minnesota River from Cottonwood River to Little Cottonwood

Swan Lake Output to Minneopa Creek

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600


Date

TSS

(mg/

L)


Figure 3-17. Scenario 5 Results for Minnesota River from Swan Lake Output to Minneopa Creek


72

Mankato

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450

600


Date

TSS

(mg/

L)


Figure 3-18. Scenario 5 Results for Minnesota River at Mankato

Shanaska Creek to Rogers Creek

0

75

150

225

300


Date

TSS

(mg/

L)


TSS Surrogate

Simulated TSS

Figure 3-19. Scenario 5 Results for Minnesota River from Shanaska Creek to Rogers Creek


73

Rush River to High Island Creek

0

100

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400


Date

TSS

(mg/

L)


Figure 3-20. Scenario 5 Results for Minnesota River from Rush River to High Island Creek

Minnesota River at Jordan

0

75

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225

300


Date

TSS

(mg/

L)


.

Figure 3-21. Scenario 5 Results for Minnesota River at Jordan


74

Table 3-13. Scenario 5 Results for Minnesota River from Chippewa River to Stony Run Creek

Fraction greater than surrogate TSS of 75 Fraction greater than surrogate TSS of 67.5


2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.0% 0.0%

Table 3-14. Scenario 5 Results for Minnesota River from Minnesota Falls Dam to Hazel Creek



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.0% 0.0%


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Table 3-15. Scenario 5 Results for Minnesota River from Timms Creek to Redwood Creek



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.2% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.0% 0.0%

Table 3-16. Scenario 5 Results for Minnesota River from Beaver Creek to Birch Coulee



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.0% 0.0%


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Table 3-17. Scenario 5 Results for Minnesota River from Cottonwood River to Little Cottonwood



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.0% 0.0%

Table 3-18. Scenario 5 Results for Minnesota River from Swan Lake Output to Minneopa Creek



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.5% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.0% 0.0%


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Table 3-19. Scenario 5 Results for Minnesota River at Mankato



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.0% 0.0%

Table 3-20. Scenario 5 Results for Minnesota River from Shanaska Creek to Rogers Creek



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.1% 0.0%


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Table 3-21. Scenario 5 Results for Minnesota River from Rush River to High Island Creek



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.0% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.2% 0.0%

Table 3-22. Scenario 5 Results for Minnesota River at Jordan



2001 0.0% 0.0% 0.0% 0.0% 0.0% 2001 0.0% 0.0% 0.0% 0.0% 0.0%

2002 0.0% 0.0% 0.0% 0.0% 0.0% 2002 0.0% 0.0% 0.0% 0.0% 0.0%

2003 0.0% 0.0% 0.0% 0.0% 0.0% 2003 0.0% 0.0% 0.0% 0.0% 0.0%

2004 0.0% 0.0% 0.0% 0.0% 0.0% 2004 0.0% 0.0% 0.0% 0.3% 0.0%

2005 0.0% 0.0% 0.0% 0.0% 0.0% 2005 0.0% 0.0% 0.0% 0.5% 0.0%


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3.3.3 Total Sediment Loads by Major Watershed Figure 3-22 and Figure 3-23 summarize the total sediment load by major watershed for existing and Scenario 5 conditions, respectively. In these figures “Mainstem” includes the Minnesota River channel itself and smaller tributaries, such as the Little Cottonwood and High Island Creek. Note that the scale differs between the figures; else the contributions under Scenario 5 would not be visible.

0

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3500

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2006

Water Year

Sed

imen

t Loa

d (to

ns/d

)

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5000

6000

7000

8000

9000

10000

Flow

(cfs

)

ChippewaYellow MedicineHawkRedwoodCottonwoodWatonwanLeSueurUpper Blue EarthMainstemFlow at Jordan

Figure 3-22. Total Sediment Load by Major Watershed, Existing Baseline Conditions

0

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2001

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Water Year

Sed

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t Loa

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)

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5000

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7000

8000

9000

10000Fl

ow (c

fs)

ChippewaYellow MedicineHawkRedwoodCottonwoodWatonwanLeSueurUpper Blue EarthMainstemFlow at Jordan

Figure 3-23. Total Sediment Load by Major Watershed, Scenario 5


80

3.3.4 Mass Export from the Minnesota River at Jordan The TMDL targets scenario results in a large reduction in the total sediment export at Jordan. The average annual load (over water years 2001-2006) is reduced to 86,375 tons/yr, or only 10 percent of the baseline load for existing conditions. The mass exported is less than the total loading rate shown in the previous section because a significant amount of sediment trapping is predicted to occur in the Minnesota River mainstem. In addition, a shorter period of analysis (consistent with the TMDL compliance period) is used for the mass export calculations reported in this section.

As should be expected, the reduction in sediment loading also causes a large reduction in phosphorus export. The average phosphorus export at Jordan for water years 2001-2006 is 463 tons/yr, or about half of the baseline export of 908 tons/yr (Figure 3-25).

Total Suspended Sediment Export at Jordan

0

100,000

200,000

300,000

400,000

500,000

600,000

700,000

800,000

900,000

1,000,000

Baseline Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5

Tota

l Sed

imen

t (to

ns/y

r)

Figure 3-24. Comparison of Total Suspended Solids Load Results for All Scenarios (Water Years

2001-2006)


81

Total P Export at Jordan

0

100

200

300

400

500

600

700

800

900

1000

Baseline Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5

Tota

l P (t

ons/

yr)

Figure 3-25. Comparison of Total Phosphorus Load Results for All Scenarios

The net phosphorus loading delivered to the mainstem by major watershed under existing baseline and Scenario 5 conditions is summarized in Figure 3-26 and Figure 3-27.

0

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7000

2001

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Tota

l Pho

spho

rus

Load

(lbs

/d)

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4000

5000

6000

7000

8000

9000

10000

Flow

(cfs

)

ChippewaYellow MedicineHawkRedwoodCottonwoodWatonwanLeSueurUpper Blue EarthFlow at Jordan

Figure 3-26. Total Phosphorus Load by Major Watershed, Existing Baseline Conditions


82

0

500

1000

1500

2000

2500

3000

2001

2002

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2004

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2006

Water Year

Tota

l Pho

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(lbs

/d)

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4000

5000

6000

7000

8000

9000

10000

Flow

(cfs

)

ChippewaYellow MedicineHawkRedwoodCottonwoodWatonwanLeSueurUpper Blue EarthFlow at Jordan

Figure 3-27. Total Phosphorus Load by Major Watershed, Scenario 5 Conditions

3.3.5 TSS Loading by Land Use for TMDL Targets Scenario The TMDL targets scenario represents a combination of implementation strategies that jointly achieve the TMDL objectives. A summary of the average changes in loading rates by land use is provided in Table 3-23. In this table, “Cropland” provides the combined results of land in conventional tillage, conservation tillage, and areas receiving manure application. The MS4 urban loading rates are broken out separately as they are subject to permitting requirements.

3.3.6 Sediment Loading by Land Use for TMDL Targets Scenario The TMDL targets scenario represents a combination of implementation strategies that jointly achieve the TMDL objectives. A summary of the area-weighted average changes in rates of upland sediment delivery to streams, by land use, is also provided in Table 3-23. Note that these are predicted rates of sediment delivery to stream, not sediment loss rates, and delivery is a fraction of the sediment loss at the field scale due to redeposition and retention in small, first-order streams that are not explicitly represented in the model. The delivery rates for sediment from cropland are, on average, low in the Minnesota River basin due to the prevalence of flat topography with subsurface drainage.

In Table 3-23, “Cropland” provides the combined results of land in conventional tillage, conservation tillage, and areas receiving manure application, and includes both sheet-and-rill erosion and ravine scour, as most ravines are assumed to be associated with field runoff and drainage. The largest reductions occur in loading from ravines (see below), leading to a large percentage change in the total cropland loading rate. The MS4 urban/developed land loading rates are broken out separately as they are subject to permitting requirements. Channel, bank, and bluff loads are not shown in this table as they are not assigned on a per-acre basis to the uplands.


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Table 3-23. Summary of Changes in Average Upland Sediment Delivery Rates (tons/ac/yr) by Land Use for TMDL Targets Scenario

Land Use Baseline Loading TMDL Scenario

Loading Change

Forest 0.031 0.027 -13.4 %

Cropland 0.154 0.011 -92.7 %

Grass/Pasture 0.043 0.013 -70.9 %

Developed – MS4 0.216 0.025 -88.5 %

Developed – non-MS4 0.218 0.027 -87.6 %

Note: Based on 1993-2006 simulation.

Evaluation of total load by source takes into account the area in each land use. Cropland occupies the major fraction of the landscape, and so the loading from cropland and from ravines associated with cropland runoff and drainage together account for most of the upland load totals, while bed, bank, and bluff sources contribute slightly less than one-third of the total sediment load across the basin under existing baseline conditions. Total loads by source are summarized in Table 3-24, in which the loading from surface erosion of cropland and loads from ravines are separated.

The net changes in load shown in Table 3-24 represent the combined effects of reduced loading rates and changes in the land use distribution. For instance, loading rates from grass/pasture decrease by 71 percent, but the area in this land use also increases substantially, resulting in a net decline in load from this land use of 18.6 percent. The relative importance of different sediment load sources is summarized in Figure 3-28 and Figure 3-29. Under the TMDL scenario, contributions from cropland and ravines are predicted to decrease from 65 to 32 percent of the total load due to additional management practices and a shift of some cropland to CRP. The relative contribution of the bed, bank, and bluff erosion sources is higher under the TMDL scenario because a smaller percent reduction is achieved for these sources.

Table 3-24. Summary of Changes in Total Sediment Loading (tons/yr) by Source for TMDL Targets Scenario

Land Use Baseline Loading TMDL Scenario

Loading Change

Forest 6,037 5,226 -13.4%

Cropland (surface) 315,063 33,720 -89.3%

Ravines 629,197 26,146 -95.8%

Grass/Pasture 20,212 16,449 -18.6%

Developed (MS4) 16,199 1,822 -88.8%

Developed (non-MS4) 87,265 11,056 -87.3%

Bed, Bank, Bluff 405,712 90,730 -77.6%

Total 1,479,684 185,149 -87.5%

Note: Based on 1993-2006 simulation.


84

Cropland (surface)21%

Ravines44%

Grass/Pasture1%

Developed (MS4)1%

Developed (non-MS4)6%

Bed, Bank, Bluff27%

Forest0.4%

Figure 3-28. Average Annual Sources of Sediment Load to the Minnesota River under Existing

Baseline Conditions

Forest3%

Cropland (surface)18%

Ravines14%

Grass/Pasture9%

Developed (MS4)1%

Bed, Bank, Bluff49%

Developed (non-MS4)6%

Figure 3-29. Average Annual Sources of Sediment Load to the Minnesota River under TMDL

Targets Scenario Conditions

Overall, the TMDL Targets scenario calls for a load reduction of about 87 percent. While this is a large reduction, it is also clear that sediment loads in the system have increased dramatically relative to natural conditions. The best evidence for this comes from studies of dated sediment cores in Lake Pepin, an impoundment downstream on the Mississippi River. Core analysis (Engstrom et al., 2009) indicates that annual sediment accumulation rates in Lake Pepin have increased about eleven-fold from pre-settlement levels, from about 79,000 metric tons/yr before 1840 to 876,000 metric tons/yr in the 1990s. The majority of these increases took place between 1940 and 1970. It is further well known that about 85 percent of the sediment load to Lake Pepin derives from the Minnesota River basin (Kelley and Nater, 2000) and that the changes in load from the Minnesota River basin over time are strongly correlated with row crop acreage (Mulla and Sekely, 2009) – although the mechanisms for increased loading may have as


85

much to do with alterations to hydrology as with land disturbance. Based on Engstrom and Almendinger’s estimates, an overall reduction in sediment load of about 91 percent would be required to achieve the natural sediment mass balance in Lake Pepin. Thus the 87 percent reduction in sediment load required to fully achieve compliance with turbidity objectives in all listed segments of the Minnesota River basin will result in sediment mass loading rates comparable to those generated from the basin under natural conditions.

3.4 SENSITIVITY ANALYSIS The TMDL scenario contains a wide variety of management measures, addressing sediment generation from the land surface, alterations to hydrology, and control of channel degradation (including bluff loads). These different management measures vary widely in their costs and implementability. They also vary widely in the amount of load reduction they achieve.

To provide a basis for discussion of tradeoffs and the development of implementation plans, it is important to develop additional information on cost effectiveness of the measures under consideration. While Tetra Tech cannot develop detailed cost information within the scope of the current work, important information on the effectiveness side can be generated through model sensitivity analyses.

Specifically, Tetra Tech conducted sensitivity analyses that compare results of the full TMDL compliance scenario (Scenario 5) to results with a particular management measure or group of management measures deleted from the mix. The difference between a pair of runs of this type shows how much would be lost by eliminating a specific group of management measures when the other groups of proposed management measures are in place. We believe this is more informative than evaluating the load reduction associated with a single management measure in the absence of other components of the TMDL scenario (the difference between the baseline run and a run with just that management measure) as various different measures may obtain their reductions from the same sources. Comparisons are made in terms of the changes in total sediment load and changes in the number of days above the TSS surrogate target at each of the major watershed outlets and at Jordan.

The sensitivity analysis also provides important information for the evaluation of the practical impacts of uncertainties in model prediction. The modeling framework allows some management measures and their impacts to be represented with a greater degree of assurance than others. In practical terms, the significance of such uncertainty should be evaluated relative to the degree of reduction that is attributed to the management measure.

A large number of different management measures are included in the TMDL compliance scenario. To reduce the number of model reruns to a manageable level, we assessed these in five groups, as follows:

A. Upland – Land use change (added CRP and changes in the characteristics of CRP land, including changes to cover and practice factors)

B. Upland – Soil retention practices (increased conservation tillage, increased crop residue, elimination of surface tile inlets, generalized sediment reductions from non-MS4 urban areas).

C. Upland – Reduction in ravine scour rates (stabilization of ravines through practices such as grade control and installation of drop structures on tile drain outlets, represented through reduction of KGER parameter in SED-PARM3).

D. Drainage/Hydrology Control – Use of controlled drainage (temporary storage or retention of surface flow on agricultural lands through engineered ditches or ponds) and urban infiltration BMPs for the first inch of runoff in MS4 areas.


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E. Channel Restoration – Practices that directly reduce net bank and bluff sediment loads to waterbodies, including control of bluff supply, sediment resuspension in the bluff areas, and baseflow sediment load (channel restoration and stabilization through grade control, riparian vegetation management, energy dissipation, re-routing of flows away from bluff faces, and other techniques).

Evaluations were made over the period 2001-2006, which is the same period analyzed for the TMDL compliance scenario. Results for loading (tons/yr) are summarized in Table 3-25. The table also shows percent reduction, which is the reduction in load between conditions without a given group of management options (with all other groups included) and the load from the full TMDL compliance scenario. Results are shown at the mouth of each major watershed, and for the mainstem at Mankato and at Jordan. The mainstem results represent the cumulative impacts of all upstream areas, and load is lower at Jordan than at Mankato due to substantial deposition in the intervening reaches.

The results immediately emphasize the importance of channel processes. The change from Scenario 5 without control of channel processes to Scenario 5 with control of channel processes is on the order of 80 percent. In most basins, hydrology controls are second in importance.

It should be reiterated that the results shown here are not additive, as each compares the effect of deleting a single group of management options from Scenario 5 with all other groups of management options present. The absolute magnitude of load reductions associated with upland practices, for instance, is thus downplayed as the analyses of these options already have the large reductions associated with channel practices included, and the upland loads exerted downstream are further reduced by reduced transport capacity in the channel. Instead, the load amounts can be read as the amount of sediment load that would occur if a specified group of management options was omitted from the TMDL scenario.

There are also interactions between components. For instance, the hydrology controls reduce peak runoff rates and thus also contribute to a reduction in channel erosion. This is also why the percentages across management groups add up to more than 100 percent Nonetheless, it is clear from these results that the total load reductions contained in the TMDL scenario cannot be achieved without addressing the channel component – as was previously evident from the results of Scenario 4.


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Table 3-25. Sensitivity Analysis of Sediment Loading (tons/yr) and Percent Reduction Associated with Groups of Management Options, 2001-2006

TMDL Scenario

A: Upland – Land Use Change

B: Upland – Soil Retention C: Upland - Ravines D: Drainage/

Hydrology Control E: Channel Restoration

Location Total Total Percent

Reduction Total Percent Reduction Total Percent

Reduction Total Percent Reduction Total Percent

Reduction

Blue Earth 51,918 52,848 1.76% 59,817 13.20% 62,052 16.33% 83,227 37.62% 191,300 72.86%

Chippewa 3,070 3,812 19.46% 3,903 21.35% 3,122 1.66% 3,107 1.18% 11,233 72.67%

Cottonwood 13,067 14,903 12.32% 17,650 25.97% 13,522 3.36% 27,563 52.59% 45,777 71.46%

Hawk 3,818 4,228 9.70% 5,369 28.88% 3,900 2.09% 5,023 23.99% 10,530 63.74%

Le Sueur 32,912 38,297 14.06% 40,082 17.89% 38,887 15.37% 80,565 59.15% 122,817 73.20%

Redwood 8,183 8,685 5.78% 10,658 23.22% 8,497 3.69% 17,420 53.02% 22,292 63.29%

Watonwan 10,150 10,670 4.87% 12,780 20.58% 10,395 2.36% 11,512 11.83% 22,603 55.10%

Yellow 2,863 3,007 4.77% 3,450 17.00% 3,158 9.34% 3,833 25.30% 17,222 83.37%

at Mankato 98,453 101,385 2.89% 110,868 11.20% 106,920 7.92% 139,488 29.42% 438,992 77.57%

at Jordan 86,265 88,846 2.91% 105,501 18.23% 93,814 8.05% 121,467 28.98% 520,842 83.44%


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Differences in average annual load relative to the TMDL scenario are shown in Table 3-26. As above, these represent the changes in load that are predicted to occur if a specific group of management options was omitted from the TMDL scenario. The results of changes in land use are generally quite small, but the types of land use changes imposed vary significantly between basins. Ravine control is important in the Blue Earth and Le Sueur watersheds, but has a relatively small impact in other watersheds (Figure 3-30).

Table 3-26. Differences in Average Annual Sediment Load (tons/yr) Due to Omission of a Group of Management Options, 2001-2006

A: Upland – Land Use Change

B: Upland – Soil Retention

C: Upland - Ravines

D: Drainage/ Hydrology

Control E: Channel Restoration

Blue Earth 930 7,898 10,133 31,308 139,382

Chippewa 742 833 52 37 8,163

Cottonwood 1,837 4,583 455 14,497 32,710

Hawk 410 1,551 82 1,205 6,712

Le Sueur 5,385 7,170 5,975 47,653 89,905

Redwood 502 2,475 313 9,237 14,108

Watonwan 520 2,630 245 1,362 12,453

Yellow 143 587 295 970 14,358

at Mankato 2,932 12,415 8,467 41,035 340,538

at Jordan 2,581 19,236 7,549 35,202 434,577

0

50,000

100,000

150,000

200,000

250,000

300,000

350,000

400,000

Land Use Soil Retention Ravines Drainage Channel

Management Category

Load

Gai

n (to

ns/y

r)

At MankatoLeSueur RiverCottonwood River

Figure 3-30. Load Gain Associated with Omitting Specific Management Groups, 2001-2006


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Changes in total annual load can be somewhat misleading because the TMDL is based on attaining specified surrogate concentrations (with an exception for concentrations that occur in conjunction with flows above the 95th percentile). A comparison of the average number of days per year greater than the surrogate TSS concentrations is provided in Table 3-27, while Table 3-28 shows the change in the average number of days per year greater than the surrogate.

These tables again emphasize the key role of channel restoration in achieving the TMDL targets. The upland soil retention options (Group B) are also important in many of the watersheds. In contrast, the upland land use changes (Group A) have only a small effect, and indeed result in an adverse effect in some watersheds (the frequency of excursions of the surrogate decreases a little when the option is omitted), despite reductions in total load associated with those options. This occurs due to changes in the hydrograph caused by the land use change which, in some cases, may spread out peak sediment concentrations over a few additional days.

Table 3-27. Average Number of Days per Year Greater than TSS Surrogate Concentration for TMDL Scenario and TMDL Scenario with Specific Groups of Management Options Omitted, 2001-2006

TMDL Scenario

A: Upland – Land Use


C: Upland - Ravines

D: Hydrology Control

E: Channel Restoration

Blue Earth 3.0 2.8 3.2 3.8 5.2 64.8

Chippewa 1.5 1.7 6.0 1.7 2.2 130.3

Cottonwood 0.7 1.3 12.8 1.5 3.5 49.5

Hawk 2.0 2.3 15.0 3.0 3.7 62.8

Le Sueur 3.8 4.5 14.0 4.3 8.2 125.0

Redwood 1.3 1.3 17.8 1.5 2.2 33.5

Watonwan 0.7 0.3 9.3 0.8 1.7 12.5

Yellow 2.2 2.5 20.5 3.5 4.2 84.5

at Mankato 5.3 5.3 6.2 6.0 9.0 163.3

at Jordan 0.3 0.3 0.7 1.0 2.8 86.0


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Table 3-28. Change in Average Number of Days per Year in Excess of TSS Surrogate Concentration Due to Omission of a Group of Management Options, 2001-2006

A: Upland – Land Use


C: Upland - Ravines

D: Hydrology Control

E: Channel Restoration

Blue Earth -0.2 0.2 0.8 2.2 61.8

Chippewa 0.2 4.5 0.2 0.7 128.8

Cottonwood 0.7 12.2 0.8 2.8 48.8

Hawk 0.3 13.0 1.0 1.7 60.8

Le Sueur 0.7 10.2 0.5 4.3 121.2

Redwood 0.0 16.5 0.2 0.8 32.2

Watonwan -0.3 8.7 0.2 1.0 11.8

Yellow 0.3 18.3 1.3 2.0 82.3

at Mankato 0.0 0.8 0.7 3.7 158.0

at Jordan 0.0 0.3 0.7 2.5 85.6

The importance of channel processes is primarily due to erosion in the bluff areas. Upstream of the bluffs this component is of lesser importance, as was shown above in Figure 2-41 and Figure 2-42. These figures suggested that Le Sueur River reaches 600 (mouth) and 645 (Lower Maple River) are generally representative of conditions below and above the bluff areas. Results for the sensitivity analyses (in similar format to Figure 2-41 and Figure 2-42) are shown in Figure 3-31 and Figure 3-32. For both reaches, these figures show results for existing conditions and for the TMDL scenario, followed by the results for the five sensitivity analyses. As with previous figures, each of the sensitivity analysis runs shows the results that would be obtained with a single group of management options omitted from the TMDL scenario.


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Existing

TMDL TMDL

Existing

Land UseLand UseSoil RetentionSoil Retention

RavinesRavines

DrainageDrainage

Channel

Channel

0

20

40

60

80

100

120

140

Reach 600 (Bluff Area) Reach 645 (above Bluffs)

TSS

(mg/

L)

Figure 3-31. TSS Average Concentrations, Le Sueur River Segments 1993-2002, for Existing

Conditions, TMDL Scenario, and Sensitivity Analyses Omitting a Specific Group of Management Options

Existing

TMDL TMDL Channel

Existing

Land Use Land UseSoil Retention Soil Retention

Ravines RavinesDrainage Drainage

Channel

0%

10%

20%

30%

40%

50%

60%

Reach 600 (Bluff Area) Reach 645 (above Bluffs)

Per

cent

age

Figure 3-32. Percentage of TSS Concentrations Greater than 90 mg/L, Le Sueur River Segments

1993-2002, for Existing Conditions, TMDL Scenario, and Sensitivity Analyses Omitting a Specific Group of Management Options

For reach 600, at the bottom of the Le Sueur River bluff area, channel restoration is the main component leading to the reduced concentrations and frequency of excursions of the TSS surrogate in the TMDL scenario. Upland soil retention and drainage (hydrology control) are next in importance in establishing reduced concentrations. Note that ravine control has a larger effect on mass loading than on concentrations because loads associated with ravines tend to occur in a small number of isolated events.


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As a result, eliminating the ravine control component results in only a 1.5 to 1.6 percent increase in the frequency of days with TSS concentrations above 90 mg/L in reaches 600 and 645, even though ravines account for over 50 percent of the total load under existing conditions.

The reach above the bluffs has much lower concentrations, and is predicted to achieve the TSS surrogate most of the time even under existing conditions. In contrast to the area below the bluffs, channel restoration has little effect on concentrations in this reach. Instead, upland soil retention and drainage (hydrology control) are the major factors leading to the TMDL reductions. It will also be noted that, for concentrations, the reduction from existing conditions to the TMDL scenario are more than the sum of the changes predicted by individual management option groups. This reflects a synergistic interaction among the different management options that magnifies the improvement attained by their simultaneous implementation. Given these synergistic effects and the fact that many reaches upstream of the bluffs are also impaired, a combination of practices addressing both the uplands and the channels will be needed to meet the turbidity targets.


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4 Summary MPCA has identified surrogate total suspended solids (TSS) objectives to achieve turbidity standards in impaired segments of the Minnesota River watershed. This report documents the results of a variety of management scenarios designed to reduce TSS loading to the Minnesota River and its tributaries. Efforts currently under way and efforts considered as reasonable by stakeholders (Scenarios 1 through 4) have the potential to provide large reductions in sediment loads – but fall far short of achieving full compliance with the TSS surrogate objectives 100 percent of the time.

One of the major causes of remaining non-compliance after the measures tested in Scenario 4 is channel bank and bluff loading, which constitutes around a third of the load under existing conditions and an even greater proportion as upland loads are reduced. Significant reductions in this source – while technically difficult to achieve – will be essential to meeting water quality standards. Additional upland load reductions, such as control of loading from developed land outside of MS4 boundaries, will also be needed.

Scenario 5 – the TMDL targets scenario – is designed to achieve water quality targets and meet the TMDL, by whatever means necessary. This scenario is not constrained by economic or political feasibility; rather, it shows the level of effort that would be needed to fully achieve standards. This level of effort is large, requiring a reduction in sediment loading from the basin that may be difficult to implement. However, the results are consistent with analyses of historical sediment loading to Lake Pepin. Sediment load reductions to achieve full compliance with the TMDL targets in all listed reaches would approach closer to natural conditions, but still allow somewhat greater average annual sediment load than was transported by the river historically.


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5 References Bicknell, B.R., J.C. Imhoff, J.L. Kittle, Jr., T.H. Jobes, and A.S. Donigian, Jr. 2001. Hydrological Simulation Program – FORTRAN, HSPF Version 12, User’s Manual. National Exposure Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Athens, GA.

Campbell, E. 2009. Development of Total Suspended Solids (TSS) Surrogates for Turbidity in the Minnesota River Basin. Minnesota Pollution Control Agency, St. Paul, MN.

Engstrom, D.R., J.E. Almendinger, and J.A. Wolin. 2009. Historical changes in sediment and phosphorus loading to the upper Mississippi River: mass-balance reconstructions from the sediments of Lake Pepin. Journal of Paleolimnology, 41: 563-588.

Kelley, D.W. and E.A. Nater. 2000. Historical sediment flux from three watersheds in Lake Pepin, Minnesota, USA. Journal of Environmental Quality, 29: 561-568.

Kelley, D.W., S.A. Brachfeld, E.A. Nater, and H.E. Wright, Jr. 2006. Source of sediment in Lake Pepin on the upper Mississippi River in response to Holocene climatic changes. Journal of Paleolimnology, 35:193-206.

MPCA. 2005. Minnesota River Basin TMDL Project for Turbidity. Water Quality/Basins Fact Sheet 3.33. Minnesota Pollution Control Agency, St. Paul, MN.

Mulla, D.J. and A.C. Sekely. 2009. Historical trends affecting accumulation of sediment and phosphorus in Lake Pepin, upper Mississippi River, USA. Journal of Paleolimnology, 41: 589-602.

Mulla, D.J., A.S. Birr, G. Randall, J. Moncrief, M. Schmitt, A. Sekely, and E. Kerre. 2001. Technical Work Paper: Impacts of Animal Agriculture on Water Quality. Prepared for the Minnesota Environmental Quality Board and Citizen Advisory Committee, Generic Environmental Impact Statement on Animal Agriculture.

Tetra Tech. 2008. Minnesota River Basin Turbidity TMDL and Lake Pepin Excessive Nutrient TMDL, Model Calibration and Validation Report. Prepared for Minnesota Pollution Control Agency by Tetra Tech, Inc., Research Triangle Park, NC.

USEPA. 1991. Guidance for Water Quality-Based Decisions: The TMDL Process. EPA 440/4-91-001. Office of Water, U.S. Environmental Protection Agency, Washington, DC.

minnesota river basin turbidity tmdl scenario report · of 90 mg/l is exceeded under scenario 4...

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