Establishing a Reference Site on a Reach of the Middle Fork of San Pedro Creek in Pacifica, California

By Gina Lee, Jon Niemczyk, and Lou Sian

Geography 642 San Francisco State University

December 18, 2006

Table of Contents

I. Introduction

II. Physical and Cultural Setting

a. Watershed Geology

b. Geomorphology

c. Land Use

d. Salmon Habitat

e. Middle Fork Human Setting

f. Channel Studies and Culvert Removal Project 2001

g. Field Observations

III. Methodology

a. Leveler Work

b. Cross Section Survey

c. Compass Traverse

IV. Results

a. Maps and Drawings

V. Discussion

VI. Conclusion

VII. Bibliography

VIII. Appendices

a. Steelhead Trout, Oncorhynchus mykiss

b. Pictures of the Middle Fork Reach

c. Scanned Field Notes

List of Figures

Figure

1. Map of San Pedro Creek Subwatersheds

2. Map of San Pedro Creek Watershed Geology

3. Map of Landslides Triggered by 1982 Storm Event

4. Map of San Pedro Creek Geomorphology

5. San Pedro Creek Longitudinal Profile

6. San Pedro Creek Project Plan View Map

7. Cross Section #1

8. Cross Section #2

9. San Pedro Creek Longitudinal Profile with 2000 data

10. San Pedro Creek Longitudinal Profile with 2000 data (adjusted)

11. Cross Section #2 with 2000 data

12. Cross Section #2 with 2000 data adjusted

13. Stream Restoration Project Plan View

List of Tables

Table

1. Raw Data for Longitudinal Profile 2. Raw Data for Cross Section 1 3. Raw Data for Cross Section 2 4. Raw Data from J. Bradt’s 2000 Longitudinal Profile Study

Establishing a Reference Site on a Reach of the Middle Fork of San Pedro Creek in Pacifica, California

By Gina Lee, Jon Niemczyk, and Lou Sian

December 18, 2006

I. Introduction

A stream found in a landscape is a product of the inextricable force of gravity and water

flowing over surfaces. From areas of high elevation to low elevation, water flows through an infinite

number of pour points in a stream. Depending on the pour point selected, a watershed and its

various influences may be delineated on maps and studied over time. In other words, the pour

point is the lowest point of a delineated watershed (Fig. 1). (Davis, 2006).

A watershed contributes water and sediments to a stream which is in a continuous state of

adjustment. This adjustment balances the forces of aggradation (net deposition), transport, and

degradation (net erosion). Natural and anthropogenic (development, farming, trails, roads)

occurrences within a watershed significantly affect the transport of water and sediment in a stream.

Over time and constant influence of gravity and water, a stream alters its channel longitudinally in

ways that can be measured. [Harrelson et. al., 1994].

We revisited a hydrology and channel design study of a reach of the Middle Fork of San

Pedro Creek in Pacifica. Middle Fork is one of five major tributaries of San Pedro Creek (SPC) and

much of its watershed is located in San Pedro Valley State Park. (Fig. 1). The study was done in

the fall of 2000 before a 24 ft. concrete culvert was removed and replaced by a bridge (Appendix

2). Cross section and longitudinal surveys were done upstream and downstream of the culvert to

analyze existing conditions as a basis for a channel design under the proposed bridge. The culvert

was removed in September 2001 to restore fish passage. In place of a dirt road above the culvert,

a highway-rated steel bridge was installed to preserve park access on Weiler Ranch Road in San

Pedro Valley State Park. (Bradt, 2000).

Initially, the intent of our study was to locate and use the benchmark and monuments

established for the 2000 study and measure hydrological changes in the reach since the culvert

removal project. However, the 2000 surveys were done relative to the culvert that was removed,

and the monuments along the banks of the stream could not be located by the author when he

revisited the site in 2005 (Bradt, 2006). In November 2006, our team found two green steel fencing

posts downstream of the bridge in the approximate locations described by Bradt (2000). Our team

established a cross sectional survey 58 feet downstream of the eastern side of the bridge.

Uncertain as this may seem, information about this reach (portion) of the stream channel before

the culvert removal project may turn up in the future. It is our hope that our establishment of a

bench mark and monuments, and our longitudinal and cross sectional surveys of the stream

channel will facilitate future comparisons of the reach beneath the bridge over time.

II. Physical and Cultural Setting

A stream’s course is to flow downhill making continuous adjustments to inputs from the

watershed by altering local conditions whenever possible. Leopold et. al., (1964) identified eight

major variables that influence a stream’s morphology: flow resistance (roughness of bed and bank

materials), sediment size, sediment load, slope, discharge, width, depth, and velocity. As the

stream transports water and materials downstream, a change in one or more of these variables will

result in a change in the other variables toward an ideal state of equilibrium.

Watershed Geology The area for this study is located on the Middle Fork of San Pedro Creek in Pacifica,

California (Figure 1). The focus for this study is a reach spanned by a bridge on Weiler Ranch

Road in San Pedro Valley Park. The Middle Fork watershed is 2.39 square miles (6.19 km²) and

includes the Middle Fork and South Fork of San Pedro Creek. Together, they comprise the

southeastern most drainage of the San Pedro Creek Watershed. (Amato, 2003). The bridge is

located east of the confluence of Middle Fork and South Fork where Weiler Ranch Road crosses

Middle Fork.

Middle Fork and the main stem of San Pedro Creek closely align with Pilarcitos Fault

which is a northwest to southeast trace of San Andreas Fault. The two faults delineate the San

Pedro Creek watershed into the northeastern Pilarcitos Block and the southern La Honda Block.

The Pilarcitos Block contains some of the oldest sedimentary and igneous rocks in the Bay Area.

The fault is characterized by Franciscan Complex dating back to the Jurassic and Cretaceous

periods. It consists of sandstone and pyroclastic greenstone (graywacke), limestone, serpentine,

conglomerate, chert, and glaucophane schist which in this location has been weathered and

crushed by tectonics. The slopes in the northern portion of the watershed are steep and consist of

unconsolidated soils, slopewash, ravine fill and colluvium that are highly unstable. (Sims, 2004).

South and nearly parallel to the Pilarcitos Fault is the La Honda Block of sandstone shale

and conglomerates formed in the Tertiary period and rocks formed in the Quaternary period. The

southern portion of the La Honda Block abuts a highly fragmented span of granitics demarcated by

the San Pedro Mountain Fault running approximately parallel and south of Pilarcitos Fault. Soils

of the La Honda Block are highly susceptible to slope failure when over-steepened through faulting

or over-saturation with rain. (Sims, 2004)(Figure 2).

The thin soils of this area tend to move down hill and accumulate in hollows, at the base of

hills, and in valleys. In some instances of severe storms, soils become saturated and move rapidly

down slopes. In 1982, hundreds of mudslides were observed in Pacifica. (Davis, 2006). On

January 4, 1982, 150 – 200 mm (5.9 – 7.9 in.) of rainfall fell in less than 30 hours, resulting in

approximately 475 landslides throughout Pacifica. (Figure 3). Rainfall records collected by park

personnel since 1978 show an average annual rainfall in San Pedro Valley Park of approximately

970 mm (38.2 in.) with much of the precipitation occurring in November through March (Sims,

2004).

Geomorphology

The San Pedro Creek Watershed is dominated by Montara Mountain (elevation 1,898 feet,

578.5 m) which is a source of groundwater year-round. Together with the lower elevations of San

Pedro Mountain and Whiting Ridge, they form the southern drainage of the watershed. To the east

is Sweeney Ridge in the Golden Gate National Recreation Area which is the northern extent of the

Santa Cruz Mountains. The total channel length of San Pedro Creek is approximately 4 miles (6.4

km). Sweeney Ridge (elevation 1,340 ft., 408.4 m) is approximately 19 miles (5.8 km) from the

Pacific Ocean. (Sims, 2004). (Figure 5).

Land Use

Drainage in the watershed is compounded by a history of land use in the watershed.

Unless otherwise stated, all of the land use information in this section is from Amato (2003).

Beginning with the Ohlone Native Americans who occupied the valley before the arrival of Spanish

explorer Captain Gaspar de Portola in 1769, the valley was probably impacted to a lesser degree.

They practiced slash and burn agriculture to encourage native grasses and gray pines (Pinus

sabiniana) for nuts and seeds while controlling native scrub. Perhaps the practice also attracted

deer, rabbit and other game to the area. The Ohlone were eventually decimated by disease and

displaced by Spanish settlers. In 1782, an assistencia or support farm was established for Mission

San Francisco de Asis, (Mission Dolores) and the presidio in San Francisco.

Subsistence farming gave way to the Mexican Land Grant system and for a time, ranching

in the valley prospered. Cattle were allowed to forage freely on riparian and hillside vegetation.

Over time, formerly pervious soils were compacted by livestock and road building. Runoff

increased and ditches were dug by hand to divert water. In turn, ranch lands were divided and sold

for small scale farming which supplied markets in San Francisco.

A low-lying area formerly known as Lake Mathilda near the mouth of the creek was drained

to create additional arable land. The channel was moved south to its present location and

straightened to connect with the ocean. This added .8 miles to the creek and significantly lowered

the base level of the stream. As a result, the creek incised its bed and eroded headward. In

addition, farmers along the creek removed water for irrigation lowering the water table. Base flow

supplied by groundwater in the summer dry months declined. Amato surmises that more than two

hundred years of agricultural use in the San Pedro Creek watershed had perhaps lowered the

channel in San Pedro Valley Park to its current location of approximately 18 feet below the

ancestral flood plain.

After World War II, housing development boomed in San Pedro Valley. By the time USGS

printed its 1968 map, the North Fork was no longer on the map because it flowed in culverts

beneath housing developments. The hardscapes of urban development (roof tops, roads) and

culverting increased hydraulic power in the North Fork during storm events. Banks were

destabilized and flooding occurred in the lower reaches of San Pedro Creek. Residents along the

creek and in low-lying areas responded with home-spun revetments. (Davis, 2006). Government

grading and construction projects further destabilized banks in other areas of the profile. The

sediment supply increased in the creek, thus reducing its capacity. In 1962, 1972, and 1982,

Pacifica sustained major flooding in the low-lying areas of Linda Mar that is most likely due to

urban development. (Amato, 2003).

Salmon Habitat

San Pedro Creek boasts a population of steelhead trout, Oncorhynchus mykiss, which

return to the upper reaches of the watershed from the ocean during winter flows. It is the only

habitat to contain steelhead trout within 30 miles of San Francisco. However, a series of bridges

hinder fish migration to prime spawning areas in the upper reaches of the watershed. Nonetheless,

an extensive habitat assessment and snorkel survey in the fall of 2004 found the threatened

species in each of the four tributaries studied, implying that the obstacles were overcome by at

least some of the adults. Steelhead trout were found in greater numbers in the Middle Fork than in

any other tributary (Johnson, 2005).

Johnson found that steelhead trout are spatially distributed according to stage of life.

Adults (unless migrating upstream to spawn) and resident fish were found in the lower reaches of

SPC where deep pools, long flat water, and few riffles occur. In the upper reaches of SPC,

especially in Middle Fork, populations of the young of year (YOY) were high and gaining

throughout the main stem, yet almost absent in the lower reaches. Age 1 were distributed

throughout the main stem and into the upper reaches, while the age 2 fish were in the lower

reaches of the mainstem.

Small fry were found mainly in riffles, while larger fish were in pools, undercut banks, and

large woody debris. The smaller fish also occurred in reaches shaded by riparian vegetation that

kept waters cool and insulated from extreme temperature. The canopy provided cover from

piscivorous (fish-eating) animals and insects for food. Johnson found no correlation for larger fish

in shaded areas.

Community members of the San Pedro Creek Watershed Coalition, the City of Pacifica,

the San Mateo County Parks Department and others have restored portions of the creek to

increase its capacity, remove obstacles to fish passage, and restore habitat. In the summer of

2000, the City of Pacifica began construction of the San Pedro Creek Flood Control and

Ecosystem Restoration Project. Wetlands were restored and a new channel at the lower reach of

the creek was created to add sinuosity. Depressions were added in the flood plain for red-legged

frog habitat, thousands of native riparian plants were planted, and additional pumps were added to

increase pumping capacity. In 2005, the city removed a dilapidated fish passage at the Capistrano

Avenue bridge that scoured a plunge pool too deep for juvenile fish to negotiate. Community

groups removed non-native invasive species and replanted the banks with native riparian species

that will shade the reach and stabilize the banks. (Davis, 2006).

Middle Fork Human Setting

Middle Fork is a pristine habitat ideal for steelhead trout. It has few anthropogenic

structures except trails and roads and a few buildings owned by the San Mateo County Parks

Department and the North Coast County Water District. In the 1950s, a trout farm was located in

two places in the park: near the confluence of Middle Fork and South Fork and by the Visitors

Center. It was destroyed by severe storm conditions in 1962 that flooded homes in Pacifica and

caused mudslides throughout the park. Remnants of the operations can be seen in the park and

memorabilia of the trout farm is exhibited in the Visitors Center. (Heisinger, 2006).

Near the terminus of Weiler Ranch Road are two culverts that extend approximately south

to north beneath the road. The easternmost culvert is dry year-round, even during wet winters.

(Heisinger, 2006). The culvert closest to the bridge has flowing water, though fish are not likely to

pass through it. The opening is very small and approximately 1 foot above its channel. The

watershed is bisected by many trails that are well maintained and well used, including the Hazelnut

trail which wends its way over the ridge to the Visitors Center.

Channel Studies and Culvert Removal Project 2001

In the spring of 2000, a hydrological study of the stream at the Weiler Ranch Road

crossing was done to assess current conditions in the creek and design a stable channel bed that

would allow fish passage. The new span would allow emergency and park vehicles to traverse

Middle Fork. Field observations, a longitudinal profile, and cross sections were done to collect

baseline data that would guide the design of the channel. (Bradt, 2000).

Bradt found that though the culvert had an impact in the immediate areas of the creek,

Middle Fork appeared to be in excellent condition with stable banks, good sinuosity, and well

established willows and dogwoods. Cobbles were medium to large in size in the riffles and gravels

were seen in deeper, outside portions of meanders. He observed good pool and riffle spacing,

ample amounts of large woody debris for fish habitat, and little evidence of aggradation.

However, immediately upstream of the 24 ft. culvert, large bed material from eroded banks

widened the streambed against the soft, exposed soil of the left bank where young, herbaceous

plants grew on vertical slopes. In contrast, woody mature plants grew on the gently sloping

opposite bank.

Downstream of the culvert, a 6 ft. drop in elevation – the impetus for the restoration project

– destabilized the right bank which had a vertical slope of 15 feet. The left bank was steep and

had remnants of concrete terraces, apparently abutments of the culvert.

In September 2001, park personnel began the culvert removal project while discharge was

low in the creek. Middle Fork was impounded upstream of the culvert and the water was diverted

through a pipe that snaked around the construction site. The dirt road was dug out and the culvert

removed. They completed the site and foundation work for a new channel and steel bridge that

was delivered on a flatbed truck almost too wide for Weiler Ranch Road (Appendix 2). Rocks for

the step pools and dirt were brought in to transition the 6 ft. drop in elevation downstream based on

Bradt’s design analysis of longitudinal and cross sectional surveys. Appropriate sinuosity and run-

rise spacing were incorporated in the step pool design. Eight rows of rock, two abreast, were

placed across the channel (Figure 7). Live willow stakes were pounded into coir-covered banks for

stability (Heisinger, 2006).

Field Observations

During the winter of 2001, Heisinger photographed stormflow under the bridge (Appendix

2). Photographs show the channel aggraded and the rock steps partially buried in mud. Though the

rocks steps remained in place, the left bank showed undercutting. Approximately five years later,

the creek has migrated dramatically to the south, and the abandoned stream bed is high and dry.

Also, large portions of the left and right banks no longer exist.

In spite of the changes, the bridge is apparently sound and the area appears to be

naturalizing. Willows have grown thick below the bridge and alders have succeeded in gaining a

foothold along the banks. Left bank failures continue immediately upstream of the bridge,

contributing ponderous amounts of large woody debris from banks of soft conglomerates. Medium

to large cobbles along the profile in this area were likely entrained during past storm events.

Upstream and beyond the extent of our study area, entrained cobbles were found in

remnants of concrete channels in contrast to the weathered bank of clay and silt that was exposed

behind a shell of concrete. Other possible signs of bankfull include clear demarcation of lichen and

exposed soil, a u-shaped, flat-bottomed glide, and large cobbles piled at the base of an exposed

bank of clay and silt.

Downstream of the bridge, ancient flood plains are well above a wide area of lower

elevation terraces. Undercut banks beneath woody roots of mature plants contain medium to large

cobbles. An exposed toe of bedrock seeps groundwater; and the meanders hug bars of medium to

large cobbles. The channel bed widens downstream and is overlain with many cobbles. Walking

upright is easy as the vegetation has transitioned into large trees or woody shrubs on terraces well

away from the creek bed. At least within the 200 feet distance of the downstream reach, the creek

displays some sinuosity, though down-cutting in the channel bed continues as evidenced in the

downstream cross-sectional survey.

III. Methods

The methods employed to gather data were consistent with the USDA Forest Service’s

procedures (Harrelson, 1994). The procedures followed the steps of site selection based upon the

existing body of knowledge, creating a site map (a hand-drawn map on-site), and conducting

survey work to create a longitudinal profile and two cross sectional profiles of the chosen reach.

Median sediment size was recorded when conducting the cross-sectional profiles, rater than from a

single point in the stream.

Site selection took place based upon the absence of data regarding the middle fork of San

Pedro Creek in the five years since a box culvert was removed and replaced with a less intrusive

bridge by the San Pedro Creek Watershed Coalition. The reach chosen was to be centered under

the bridge, with survey work to be conducted upstream and downstream from the control point

(eastern edge of the bridge). Also factoring in to the decision was the previous study conducted

(including a longitudinal profile and cross-sectional data) by Joshua Brandt to study the feasibility

of removal of said box culvert in 2001. This would prove a benchmark to which a comparative

analysis of our readings could me made.

The survey work to create a longitudinal profile began on November 5, 2006. This involved

surveying the reach downstream of the bridge. The southeast corner bridge footing was used as a

control point, and assumed to be 100 feet above mean sea level. This would provide the elevation

benchmark needed to make the survey readings useable. The elevation was assumed to be 100

feet because previous studies failed to provide an elevation reading for any landmark this far

upstream. A rock on a pool wall within the streambed was chosen as the control point. Survey work

was then conducted downstream for 200 feet following the thalweg, taking readings at applicable

points of interest: runs, riffles, pools, bars, terraces and depositional features. Water depth was

also measured as part of the profile, as was terrace level at randomly selected points along the

reach. Field notes are provided as Appendix C.

The second phase of surveying took place on November 18, 2006. This involved

surveying upstream for 145 feet to create a longitudinal profile, using the same zero point and

incorporating all the same elements used to create the downstream profile. Survey work was also

conducted to create two cross-sectional profiles. One was conducted underneath the bridge mid

span, and another 58 feet south of the bridge to maintain geographic consistency with Brandt’s

previous study. When creating the cross-sectional profile, elevation was sighted to the original zero

point to determine elevation. Foresight readings were taken every 6 inches along the profile’s

horizontal distance to maintain consistency in detail. The profile was created to show the

streambed and twice-bankfull width on either side of the channel. Also included as part of the

cross-sectional profile were ocular estimations of median sediment size at each point on the

transect.

Throughout all survey work, feet were used as unit of measurement to remain consistent

with Brandt’s previous study. The materials used were a level, tripod, stadia rod (with feet as

measurement standard), and measuring tape. For the longitudinal profile, Gina Lee, with Lou Sian

holding the stadia rod, took all foresight readings to maintain consistency in data collection. Jon

Niemczyk conducted all foresight readings in the cross-section surveying, with Gina Lee holding

the stadia rod, for the same reason. displays the elevation readings and sediment size

observations.

IV. Results

Longitudinal Profile

The longitudinal profile of the project area was entered into an Excel worksheet in order to evaluate

the change in gradient over the project area, as well as the morphological aspects of the thalweg

river bed. Mean grain size in the creek thalweg were observed to be predominately pebble to

cobble sized in the riffle and step run reaches of the stream, and predominately sandy to gravel

sized in the pool areas of the stream. Raw data is provided in Table 1, a figure depicting the

longitudinal profile is provided as Figure 5. Due to the particularities of Excel, distances were

plotted as relative to our farthest point upstream. Thus the eastern edge of the bridge, which is our

“zero point” in the field is represented as 145 feet downstream, and the downstream end of our

study is depicted as 345 feet downstream. It is important to note that the elevations given are

relative to an artificial benchmark created in the field – i.e. the concrete foundation bench located

on the southeastern corner of the bridge, for reasons discussed previously.

Our data indicates that over a span of 345 feet, the change of gradient was a total of 9.12 feet.

Given the calculation for slope:

Slope = �y/�x = Rise (change in gradient)/Run (longitudinal distance)

The slope of our stream can be calculated to be:

Slope = 9.12 feet/345 feet = 0.026

Two Dimensional Compass Traverse

A two dimensional compass traverse was conducted in order to plot the horizontal river

morphology of the study area, and for the purpose of sinuosity. The data collected in the compass

traverse is depicted in Figure 6. Absolute compass bearings and distances from “zero” – i.e. the

eastern edge of the bridge, are potted on the plan view map as well. Also provided on the map are

the relative positions of various aspects of the creek study area, including terraces, pools and

riffles, large woody debris deposits, and locations of various control points and benchmarks. The

compass traverse however, was conducted prior to the longitudinal profile was completed

upstream. It was decided later in the field to continue further upstream, thus the compass traverse

terminates at 120 feet upstream, instead of 145 feet upstream measured in the longitudinal profile.

Given the plan view map of the stream, valley gradient can be determined from measurements

depicted in Figure 6. The total valley distance was determined by a straight line from the two

endpoints of the creek reach (Figure 6). This line was then measured, and found to be a total of

241.6 feet long. Using data from the longitudinal profile (Table 1), 120 feet upstream on Figure 6

correlates to 25 feet downstream in Figure 6 and Table 1, thus the change in gradient over this

distance was determined to be 8.86. Thus the valley gradient over this portion of the reach area

was determined to be:

Valley Slope = 8.86 feet/ 241.6 feet = 0.0367

Cross Sectional Profiles

Two cross sectional profiles were conducted along our reach area for the purpose of correlation of

data to previous studies and tocreate a baseline study to understand channel morphology. This

data can also be used to determine the Rosgen classification for the stream. Cross Section #1 is

located mid-span of the bridge at Wieler Creek Road. Cross Section #2 is located 58 feet

downstream of the eastern side of the bridge, or 203 feet downstream from the furthest point

upstream. This location was chosen because monuments from the 2000 study by J. Bradt were

observed in the field, thus was done for a comparison study. Locations of the cross sections are

provided in Figures 7 and 8. Raw data from our cross sections are provided in Table 2 and 3. This

data was then plotted using Excel and are provided as Figures 5 and 6.

V. Discussion

Longitudinal Profile

The longitudinal profile plotted by our data indicates that this portion of the San Pedro Valley Creek

is dominated by a step run morphology, with pools occurring mainly where obstacles in channel

bed occur. As observed in Figures 5 and 6, a total of 6 visible large pools occur during this reach

of the creek. The two located upstream of the creek are due to a sharp channel bend and an

increase of large woody debris (due to hill slope failure from undercutting) creating a pool. The two

pools located under the bridge are due to the stream restoration project conducted approximately 5

years ago. Finally, the two down stream pools are due to a large alder tree forcing a change in

river geometry and creating a bend, and another forced bend due to the creek encountering a well

consolidated conglomerate bedrock. The area 50 feet upstream from the bridge appears to have a

greater gradient compared to the rest of the stream profile. This is most likely due to effects of the

creek restoration project, where a forced gradient change with rock weirs was installed to

compensate for soil loss due to the previously existing culvert (Bradt, 2000). The two uppermost

weirs have been eroded away, and the effect of this gradient change appears to be propagating

upstream as a result.

Data from the 2000 study by Bradt was then plotted against our data. This was done by using the

“Reference Cross Section” Bradt conducted “43 feet downstream of the end of the culvert” (Bradt,

2000). It is our belief that this reference cross section was the monumented cross section (in the

form of two metal rods located on either side of the creek) observed 58 feet downstream of the

bridge. Thus this point was used to match Bradt’s longitudinal data to ours. Data is provided in

Table 4, and the comparison is provided in Figure 9. As discussed previously, elevation

benchmarks could not be found for this study, thus the elevation data is incompatible. In order to

force a comparison, an additional 2 feet was added to Bradts data, and is depicted in Figure 10. In

Bradt’s profile, the effects of the culvert are obvious, and the change since the restoration project

was installed in gradient is apparent. According to Bradt’s report in 2000, restoration plans called

for filling of the downstream plunge pool and rising of the grade downstream with fill material. No

quantitative data exists for post restoration longitudinal profiles. Thus, an absolute measurement

of change cannot be made since the restoration project. The difference in elevation data also

makes pre-restorations comparisons difficult. It is hoped that future studies will be able to find true

elevation data by GPS or other means to accurately compare the two studies.

Channel slope was calculated previously in Section III and was found to be 0.026 for a length of

345 feet. Bradt’s study found the slope to be much steeper, with a slope of 0.046 for a length of

202 feet of the creek, with the former culvert in place. For a true analysis to be made, a calculation

for the approximate same reach of the creek was made. Thus the slope for the approximately

same reach as the previous study by Bradt is:

Bradt’s Channel Current Slope = 5.79 feet/202 feet = 0.0286

Difference in Slopes = 0.046-0.0286 = 0.0174

Thus the effect of the culvert removal and restoration (including fill to raise the gradient) has

resulted in a decrease of 0.0174 feet per foot of gradient change.

Plan View of the Creek

The effects of the geology and other physical characteristics of the stream can be seen in the river

geometry (Figure 6). As stated previously, the beds of the river mid and downstream were due to

manmade and natural interruptions in stream geometry. The cause of the sharp bend upstream of

the bridge however, is unknown, because no change in bedrock or other outside forces were

observed. The creek in this part of the creek is deeply incised, with a large terrace plateau located

on the left bank of the river. Thus the change in river geometry at this point must have been

caused some time ago, with the terraces above indicating the former river geometry. This change

may have been due to a previous flood event.

The much smaller pool just upstream of the creek is due to what we believe from large woody

debris in the creek caused by slope failure on the left bank of the creek. According to various

historical data, a large flooding event occurred at the end of 2001. This created a marked change

in giver geometry at the site. Figure 6 depicts the previously estimated river geometry, based on

historical data. As a result of the change in river morphology, deep undercutting of the left stream

bank is occurring, creating further stream bank instability in this portion of the creek.

The area upstream of the restoration project appears to be far more dynamic of a system in

comparison to the downstream portion of the creek. The downstream portion of the creek appears

to be relatively stable, with a broader stream valley area to provide migration of the creek. The

upstream portion however, shows indications of movement, with deeply incised areas and

undercutting. This rapid migration and undercutting may be due to the creek system responding to

forced changes in gradient from the restoration project.

Cross Sections

As state previously, two cross sections were done in order to create a baseline study for future

groups to study the creeks morphological changes. The first cross section (Cross Section #1) was

done mid span of the bridge. The results of this study (Figure 7) indicates this part of the creek is

characterized by a Rosgen G type channel due to its gentle gradient, terraced walls, and slightly

entrenched channel. Based on the median grain size of the channel (pebble to cobble sized) it can

be assumed that this portion of the creek is typified by a Rosgen G3 system. No comparison could

be made to the 2000 study by Bradt, since no cross section was conducted at this point of the

stream.

The second cross section (Cross Section #2) was conducted 58 feet downstream of the eastern

edge of the bridge (Figure 8). This cross section indicates that the stream is also a Rosgen G3

type stream. Bradt’s 2000 data from this cross section is given in Figure 9. The elevations were

then adjusted similar to the longitudinal profile, where 2 feet was added, however this is a crude

comparison at best (Figure 10). Yet, it can be inferred that the general morphology of the river has

not changed drastically from the previous study, it can be seen that the thalweg of the creek has

shifted somewhat to the left bank. As said previously, since absolute elevations were not possible

for this study, a quantitative analysis of change could not be accurately made.

Sinuosity

Sinuosity can be measured by the following equation:

Sinuosity = Valley Slope/Channel Slope

Valley slope for the channel length of 320 feet of the creek was calculated in Section IV, and was

found to be 0.0367. The channel slope needs to be recalculated to match the same length of the

creek, instead of the whole length measured in the longitudinal profile (as stated previously, the

lengths are different due to decisions made in the field). Thus the adjusted channel slope would

be:

Adjusted Channel Slope = 8.86 feet/ 320 feet = 0.0277

Thus the sinuosity for the creek reach depicted in Figure 6 of 320 feet is:

Sinuosity = 0.0367/0.0277 = 1.325

Bradt’s previous study calculated a sinuosity of 1.27, indicating a 0.055 increase in channel

sinuosity since the culvert was removed and the creek was restored.

Restoration Project

As stated previously, a restoration project was conducted approximately 5 years ago in 2001. The

restoration consisted of the removal of the existing culvert, filling in a deep plunge pool and raising

the gradient downstream of the culvert, and the installation of rock weirs to create step pools for a

rapid decrease in slope. A figure of the proposed restoration project is provided as Figure 11 and

is shaded in grey.

According to the model, 8 rock weirs were to be installed creating two step pools. Field

observations indicated that the evidence of 7 weirs in varying conditions remained and depicted in

Figure 11. Evidence of one of the central weirs was not observed. The two uppermost weirs were

almost completely eroded away, as well as the last downstream weir. Step pools however

appeared to remain in their designed place. Riparian vegetation has re-grown in the area creating

bank stabilization.

Changes in the channel morphology due to a storm event 2001 may be the cause of the erosion of

the two upstream and one downstream weir. The loss of the two upstream weirs providing a step

system for dispersing gradient is creating an upstream propagation of the gradient as observed in

the longitudinal profile.

VI. Conclusions and Recommendations

Limitations to Our Study

An attempt to make a quantitative measurement of the discharge of the stream beneath the bridge

was not possible due to the relatively low flows. Due to the low flows, the flow meter used did not

register the stream velocity.

Thick vegetation prohibited extending the cross section at Cross Section #2 further up the valley

walls. However, the cross section distance was similar to that of the 2000 study by Bradt. Time

constraints also prohibited more cross section measurements to be made.

Deep undercutting of the stream and large woody debris prevented thalweg measurements in

some areas of the creek. Every attempt was made to capture as much data as possible.

Time constraints also prevented measuring exact distances of the terraces relative to the creek.

Thus distances from the creek are estimates only in Figure 6.

No post restoration study was conducted until our study. Thus changes since the restoration

project could not be measured. Changes mentioned in this report are based on eyewitness

accounts and models only.

Evidence of bankfull stage in the creek were not readily observed. Thus Rosgen classifications of

the stream given are based on observed morphology.

Finally, elevation data from the previous study in 2000 by Brandt could not be correlated to our

study, due to the absence of benchmarks. Thus several quantitative measurements of change

since the 2000 study could not be made accurately.

Conclusions and Recommendations

Our geomorphological study of the Middle Fork of the San Pedro Creek at Weiler Ranch Road

Bridge indicates that the restoration project in terms of fish habitat has been successful. Although

several quantitative changes could not be made due to the lack of correlation of data from the 2000

study by Bradt, some changes could be measured that were not elevation dependant.

Our study indicates that the restoration project and culvert removal has created a decrease in

slope of 0.0174. Sinuosity has also increased by 0.055. Step pools created by the project remain

in place, and appear to be self maintaining. However, some of the rock weirs have been eroded

away, creating a upstream propagation of the increase of gradient.

Our group also found that the area directly upstream of the restoration project has experienced

channel migration resulting in deep undercutting and slope failure on the left bank of the stream.

Large woody debris is also seen in the creek, with may prevent upstream migration of fish. Some

sort of bank stabilization should be done to prevent further slope failure.

Finally, we hope that this study provided an accurate benchmark study for future groups for

comparison of the morphological changes of this reach of the creek.

BIBLIOGRAPHY

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