San Pedro Creek Capistrano Fish Passage Restoration ... Pedro Creek Capistrano Fish Passage Restoration Project Longitudinal Study Brian Crowley Adam Edgell GEOG 642 Fall 2006 12/18/2006

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  • San Pedro Creek Capistrano Fish Passage Restoration Project

    Longitudinal Study

    Brian Crowley Adam Edgell GEOG 642 Fall 2006

    12/18/2006

  • Introduction In the fall of 2005, the City of Pacifica restored approximately 1300 linear feet of

    San Pedro Creek downstream of the Capistrano Bridge. The main objective of the

    project was the removal of a significant obstacle to fish passage while improving habitat

    conditions for migrating steelhead trout. Over the years, severe downcutting by the

    creek, due to the rapid urbanization of the area, had left the bottom entrance to the bridge

    perched approximately nine feet above the downstream channel invert, rendering the

    1960s Denil fish ladder underneath the bridge completely ineffective. This resulted in a

    severe migration barrier to spawning steelhead trout trying to work their way upstream.

    For this project, the City of Pacifica removed the failing fish ladder from beneath

    the bridge, and brought in 12,000 cubic yards of fill to raise and stabilize the streambed at

    its 1950s level and gradient, thus improving fish passage underneath the bridge. A series

    of nineteen rock and log weirs were then placed along the channel, creating a riffle-pool

    and step-pool system that gradually rises in elevation from the downstream gradient up to

    the Capistrano Bridge. This restoration will allow juvenile salmonids to move up the

    channel in a variety of stream flow conditions, while minimizing the height of the jumps

    that they will have to negotiate in order to make it upstream. In addition to enhanced fish

    passage, significant bank reconstruction was done as well. Newly regraded slopes, along

    with the removal of exotic plant species and replacement with natives found throughout

    the watershed, will help to reduce the sediment contribution from bank erosion, to

    increase canopy cover, and to maintain or lower water temperatures. It will also help to

  • create a stable, slightly laid-back bank, with the added benefit of reducing stream

    velocities through the reach (Temple 2006, SPCWC website).

    Our group was given the task of performing a longitudinal and cross-sectional

    study of the reach of San Pedro Creek that was restored in the Capistrano Fish Passage

    Restoration Project. We collected streambed elevation data along the thalweg of the

    creek, beginning at the Capistrano Bridge and working downstream. From this data, we

    were able to construct a longitudinal profile of the stream, from which we could derive

    gradient changes resulting from deposition and/or erosion. We also collected cross-

    sectional elevation data at a number of points along the stream corridor. This data

    allowed us to note any major changes in bank shape and stability, as well as the

    opportunity to document erosional movement along the stream of the originally-placed

    weirs. The main purpose of this study is to compare our data and profile with data

    collected and documented in the as-built plans for the project in November 2005. In this

    way, we hope to provide some initial insight into the ongoing effectiveness of the project

    in preserving and protecting salmonid fish habitat.

    Background on San Pedro Creek Watershed

    San Pedro Creek is a perennial stream that flows northwesterly through San Pedro

    Valley in the city of Pacifica in San Mateo County, California, and empties into the

    Pacific Ocean. It drains a 5,257 acre (8.2 square mile) basin containing the mainstem of

    the creek and five major tributaries (North Fork, Middle Fork, South Fork, Sanchez Fork,

    and one unnamed tributary), and composed of seven major subwatersheds (North Fork,

    Middle Fork, South Fork, Sanchez Fork, Shamrock, Pedro Point, and Hinton) and a

    number of minor subwatersheds. The North, Middle, and South Forks all converge near

  • the eastern end of the San Pedro Valley, and are met downstream to the northwest by the

    Sanchez Fork before reaching the Pacific Ocean. The upper reaches of the creek have

    healthy riparian areas and substantial winter flows that support migrating steelhead trout,

    making it the only creek within 30 miles of San Francisco that offers this type of habitat

    (Collins 2001, McDonald 2004, SPCWC website).

    The area of our study is a roughly 1150-foot reach along the mainstem of the

    creek, extending from the Capistrano Bridge downstream to a bend in the creek near

    where it runs behind the Sanchez Art Center.

    San Pedro Creek Watershed Geology and Geomorphology

    The San Pedro Creek Watershed (SPCW) is located at the northern extent of the

    Santa Cruz Mountains. Montara Mountain, at the southern extent of the watershed, is the

    highest point at 1,989 feet, and, along with San Pedro Mountain and Whiting Ridge,

    forms the southern boundary of the watershed. Sweeney Ridge, peaking at 1,220 feet,

    forms the eastern boundary, and connects to Cattle Hill, which marks the northern extent

    of the watershed. The creek flows towards the northwest where it meets the Pacific

    Ocean.

    The SPCW lies on the western edge of the North American tectonic plate, at its

    junction with the subducting Pacific plate. This leads to significant uplift and faulting in

    the region. The largest fault in the area is the Pilarcitos fault, a strike-slip fault that runs

    through the center of the watershed. The northern side of the fault is called the Pilarcitos

    block, while the southern side is the La Honda block. South of the Pilarcitos fault is the

    smaller San Pedro Mountain fault, which separates the La Honda block from the geologic

    structures to its north and south. This fault moves granitic rocks upward relative to the

  • downward movement of the sedimentary rocks to the north. There are also a number of

    smaller, unnamed faults in the northern half of the valley that run nearly parallel to these

    two faults. The SPCW is characterized by alternating, sheared, northwest-trending beds

    of the Jurassic/Cretaceous Franciscan formation that are faulted against Tertiary

    sedimentary rocks, that are, in turn, faulted against Cretaceous granitic rocks at the

    southernmost ridge top. The bottom of the San Pedro Valley is composed primarily of

    alluvium deposited from runoff from the surrounding highlands. These factors combine

    to make many areas within the SPCW prone to slope failure. (Collins 2001, Sims 2004,

    McDonald 2004).

    SPCW and San Pedro Creek as Steelhead Habitat

    As mentioned earlier, San Pedro Creek is notable in that it is the nearest creek to

    San Francisco that provides habitat to support migrating steelhead trout. In fact, the creek

    even supported a population of Coho salmon up until the 1950s. Although decent habitat

    for spawning is located throughout the mainstem and into the Middle Fork tributary, the

    best spawning habitat appears to be in the upper reaches of the Middle Fork.

    However, the mainstem provides the best conditions for rearing steelhead to smolt size

    (at about two years old) and for stages of development beyond spawning (Davis 2004,

    Hagar Environmental Science 2002, SPCWC website).

    Sediments from upland hillslope sources are a major contributing factor to the

    condition of steelhead habitat. Upland sediments contribute gravels that are important for

    spawning, but also contribute fine sediments that can bury those gravels, thus increasing

    the streams turbidity and disrupting the natural pool and riffle systems that are crucial to

    steelhead spawning and rearing. A growing concern is that, although the San Pedro Creek

  • mainstem is currently supporting steelhead, it is always at risk for elevated

    mobilization of sediment and an increase in fine sediment loads, given the amount of

    development and human activity in the area. Some research suggests that the substrate of

    the mainstem is lower in gravels than what is ideal for supporting steelhead, and that,

    therefore, steelhead using the mainstem are more vulnerable to water quality degradation

    from siltation compared to the Middle Fork tributary. (Hagar Environmental Science,

    2002)

    There are a number of other factors that adversely affect the steelhead

    environment, among them the bridges along the mainstem of San Pedro Creek, which

    serve as obstacles to adult steelhead trying to get to spawning areas, as well as to the free

    movement of the younger steelhead. Also of concern is the destruction of riparian

    corridors due to the regions rapid suburban development, which adversely affects fish

    habitat. This riparian vegetation is vitally important because it provides shade to

    moderate water temperature for the fish, while also providing overhead cover (Hagar

    Environmental Science, 2002).

    Development and Land Use in San Pedro Creek Watershed

    Up until the mid-1800s, what are now the lower reaches of San Pedro Creek were

    comprised of a large seasonal wetland and lagoon. Early maps of the area show a large

    willow thicket in the area east of what is now Linda Mar Shopping Center. West of this

    willow thicket lay a seasonal lagoon. The original stream channel was probably quite

    indistinct through this area, and during the dry season may have dwindled to nothing

    more than a few isolated pools. In all likelihood, the lagoon probably formed during the

    dry season behind a sandbar at the mouth of the stream. The stream would then drain into

  • the lagoon during the wet season when streamflow became high enough to breach the

    sandbar.

    The first known human modifications to the area were by the Ohlone Indians who

    set frequent fires as part of a regular burning regimen, converting much of the coastal

    shrub and forested areas into grassland. As a result, mollisols, which are primarily

    grassland soils, are the dominant soil order in the region. (Collins 2001, Davis 2004,

    Hagar Environmental Science 2002).

    Beginning in the 1850s, intensive agriculture came to the San Pedro Valley.

    Sometime prior to 1928, the willow thicket was entirely removed, and San Pedro Creek

    was confined to an aligned channel that ran through the area, while the original channel

    and lagoon were plowed over and planted with crops. The agriculture practiced in the

    area had a profound effect on the creek by increasing the area of exposed soil, which led

    to increased erosion rates, accelerated channel incision, and heavier sediment loads.

    Suburban development began in the area during the 1950s, leading to a number of

    dramatic changes in the watershed. The most far-reaching of these were the placement of

    most of the North Fork watershed into underground culverts, the development of a series

    of new bridges on the mainstem of the creek, and increasingly dense residential

    development of much of the valley floor and northern hillslopes. This development led,

    in turn, to a drastic increase in the impermeable surface area of the watershed, resulting in

    a decrease in runoff lag time and an increase in the peak amount of runoff. In addition,

    off-road motorcycle use, primarily during the second half of the 20th century, increased

    the impermeable surface area further, especially in the Pedro Point and South Fork

    watersheds. The effects of these changes are evident today in active slopewash, gullies,

  • and landslides throughout the watershed (Collins 2001; Georgeades, et. al. 2005; Hagar

    Environmental Science 2002, McDonald, 2004).

    Today, the San Pedro Creek Watershed exists in a wide variety of land use

    conditions, ranging from dense residential and commercial development along the

    mainstem at the valley floor, extensive culvertization of the North Fork subwatersheds,

    and relatively undisturbed parklands on the Middle and South Fork subwatersheds.

    Estimates of the amount of developed land within the watershed range anywhere from 13

    to 33%, and those numbers continue to increase. Although much of the watersheds

    uplands are designated as county, state, and federal recreation and parklands, and are

    therefore protected from development, residential development continues to increase in

    density both along the valley floor and on the northern and eastern hillslopes. (Collins

    2001, Davis 2006, Hagar Environmental Science 2002). This is especially true in our

    study area, where housing developments run right up to, and sometimes within, the

    riparian corridor.

    Overview of Materials and Methodology

    Our study site was on the San Pedro Creek mainstem channel, starting at the

    Capistrano Bridge and extending 1147 feet downstream. Following basic reference reach

    procedures, we proceeded to perform a longitudinal survey and four cross-sectional

    surveys along this reach. The materials used to perform the surveys included: an optical

    level and tripod, a hand level, a stadia rod (marked in tenths of feet), and a 300-foot field

    tape measure (marked in tenths of feet). In addition, we used rebar and flagging tape to

    monument benchmarks, turning points, and other points of interest; two-way radios to

    enhance communication between team members; and field notebooks to record data and

  • observations.

    We began our survey by setting up the optical level on the tripod on an upslope

    area just downstream of the bridge. By performing a backsight (BS) to a benchmark with

    a known elevation (in this case a drainage grate on the paved surface of the bridge), we

    were able to establish the height of the instrument (HI). By next performing a foresight

    (FS) to an arbitrary turning point, then setting up the instrument at a new location within

    sight of the area under the bridge and performing another backsight to that turning point,

    we were able to establish the height of instrument for the first part of our longitudinal

    survey. Starting at a point in the stream at the northeast corner of the bridge culvert that

    had previously been staked with rebar, we proceeded to pull the field tape measure to its

    full extent along the thalweg of the stream. The stadia rod was then placed at various

    numbered and referenced stations along the thalweg. These points were chosen for their

    significance in portraying the overall shape of the pools and riffles in the stream, and

    include the beginning and ending points of pools and riffles, the deepest point of each

    individual pool, and any notable points of inflection in the streambed. Using the optical

    level, we collected foresight readings from the stadia rod for each station and recorded

    this data in our field notebooks. This data was later used to calculate thalweg bed

    elevations. As we reached the end of the 300-foot tape measure, we placed a monument

    and laid out the tape over the next 300-foot length of thalweg. We performed the

    foresight-backsight operation at various turning points along the way so that the level

    could stay within sightline of the stadia rod as we moved downstream. These turning

    points were recorded in our notebooks, and most were monumented with rebar and/or

    flagging tape as well.

  • On the second day of data collection, we performed cross-section...

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