Pescadero Creek

Problem Statement / Impairment Assessment Report, June 2003

Introduction

The Pescadero Creek watershed is listed as impaired by excessive sedimentation on the 303(d) list of impaired waterbodies in the San Francisco Bay Region.  The listing is based upon the consensus opinion of scientists and resource professionals, and is primarily driven by concern over the degradation of aquatic habitat and declines in the populations of rare and endangered species, including coho salmon (Oncorhynchus kisutch), steelhead trout (Oncorhynchus mykiss), tidewater goby (Eucyclogobius newberryi), red-legged frog (Rana aurora draytonii), and San Francisco gartner snake (Thamnophis sirtalis tetrataenia).  Beneficial uses of Pescadero Creek and its tributaries are believed to be impaired by accelerated rates of erosion and sedimentation resulting from natural geologic and climatic processes augmented by human land use practices.  In addition to excessive sedimentation, scientists have identified other factors that may be limiting populations of sensitive species, such as recruitment and retention of woody debris, water withdrawal during critical low-flow periods, lagoon circulation and management, and water quality. This report summarizes the current state of knowledge of the Pescadero Creek Watershed regarding sediment-related issues and the attainment of beneficial uses.

Watershed Background

The Pescadero Creek watershed, located in western San Mateo County, has a drainage area of approximately 82 square miles (Figure 1).  Pescadero Creek and its tributaries flow from the western slope of the Santa Cruz Mountains, a tectonically active mountain range bordered on the east by the San Andreas Fault system, to the Pacific Ocean.  Major tributaries of Pescadero Creek include Butano Creek (23 square miles), Honsinger Creek (3 sq. mi.), McCormick Creek (2 sq. mi.), Jones Gulch (2 sq. mi.), Tarwater Creek (2 sq. mi.), Peters Creek (10 sq. mi.), Slate Creek (3 sq. mi.), Oil Creek (5 sq. mi.), and Waterman Creek (2 sq. mi.).  Pescadero Marsh, a 320 acre brackish and freshwater wetland at the confluence of Pescadero Creek and Butano Creek, is one of the most significant coastal wetlands on the central California coast (Curry et al, 1985).   It is composed of an estuary/seasonal freshwater lagoon, fresh and brackish water marshes, brackish water ponds, and riparian areas along stream channels (Smith and Reis, 1997).

Geology

The evolution and current condition of the Pescadero/Butano watershed is tremendously influenced by regional and global geologic processes and controls including local bedrock, rising sea level, and faulting along the boundary between the Pacific and North American plates.  The watershed shows signs of recent uplift common to the north coast of California, including steep hillslopes in the upper watershed and creeks downcutting into steep v-shaped canyons (Curry et al, 1985).  Bedrock in the watershed is primarily composed of thick sequences of Tertiary marine sandstones, mudstones, and shales.  Many of these sedimentary rock units are mechanically weak and highly susceptible to landsliding, debris flows, and gullying.  The watershed is bisected by two important fault zones: the San Gregorio-Hosgri Fault system is a north trending right-lateral fault zone, roughly parallel to and two miles inland from the Pacific coast; the Butano Fault system connects the San Andreas and San Gregorio-Hosgri fault zones, following Pescadero Creek from its headwaters to Memorial Park (Figure 1).  Right-lateral offset on the San Gregorio-Hosgri fault, on the order of 1-2 meters per century, has offset the portions of Pescadero, Butano, and Little Butano Creeks west of the fault by roughly 6 km over the last few hundred thousand years (Curry et al, 1985).  The effect of this stretching and lengthening of Butano Valley is to slowly decrease the already low gradient of Butano Creek west of the fault, reducing transport capacity and causing severe aggradation.

Pescadero Marsh is an ephemeral landform over geologic time, subject to the rise and fall of sea level and infilling with sediment from the upstream watershed.  The marsh has only existed in its present form for several thousand years, as sea levels rose following the last glacial maximum 14,000 years ago and sediments filled in the deep canyon of Pescadero Creek (Williams, 1990).  Accelerated rates of watershed sedimentation effectively reduce the lifespan of the Marsh, filling in valuable aquatic habitat and reducing flood carrying capacity.  Although sea levels have been relatively stable over the past 5000 years, anthropogenic induced global warming may result in increases in sea level as high as several feet over the next few hundred years, further confounding the balance of sedimentation and sea level.

Climate

The Pescadero Creek watershed has a Mediterranean climate typical of the Central California Coast, with a mild, wet winter season (November-April) and a warm, very dry summer season (May-October).  The watershed averages approximately 40 inches of precipitation annually, with nearly 100% falling during the 6-month wet season.  Anadromous fish have adapted to the strong seasonal hydrology of the region, migrating and spawning during the wet season and rearing over the summer in cold, deep pools.

Land Use History

Human settlement and land use in the Pescadero watershed is inextricably linked to the natural resources of the area.  Early settlers began farming potatoes and other crops in the fertile alluvial soils of the valleys and marshlands around Pescadero in the mid- to late- 19th century.  At the same time, loggers began harvesting the old growth redwood forests covering a huge portion of the upper Pescadero and Butano watersheds.  As a result of its location beyond the zone of commuters to the larger cities of the Bay Area, land use in the watershed has changed remarkably little in the past century, and relatively little urbanization has occurred (Curry et al, 1985).  Table 1 details the important periods of land use history in the Pescadero watershed.

Table 1: Land use history in the Pescadero Creek watershed

Problem Statement

The 303(d) listing of the Pescadero Creek Watershed is prompted by the loss of suitable habitat and the decline in populations of sensitive aquatic species, especially salmonids, as well as threatened and endangered aquatic species found in Pescadero Marsh.  As described below, Pescadero Creek is not meeting narrative water quality objectives which require that beneficial uses are not to be adversely affected by excessive sediment (Table 2).

Table 2: Beneficial uses and water quality objectives related to sediment (Basin Plan, 1995).

Note: COLD=Cold Freshwater Habitat, MIGR=Fish Migration, RARE=Preservation of Rare and Endangered Species, SPWN=Fish Spawning, WILD=Wildlife Habitat

Anadromous Fish

As identified in the Basin Plan (1995), the Pescadero Creek watershed provides the following beneficial uses relating to anadromous fish: cold freshwater habitat, fish migration, preservation of rare and endangered species, and fish spawning (Table 2).  Historically, Pescadero Creek and many of its tributaries supported substantial runs of coho salmon.  Although there is little definitive data on historic coho salmon abundance, the Department of Fish and Game has estimated that the San Mateo County coastal streams (Pescadero, San Gregorio, and Gazos Creeks) supported an annual run of approximately 1,000 adult coho during the early 1960’s (Baker, 1998).  Historic (pre-1850) abundance was undoubtedly much greater.  In 1995, coho salmon south of San Francisco Bay were designated as a state ‘Endangered’ species.  At the federal level, the

Central California coast evolutionary significant unit (ESU) was listed as a ‘Threatened’ species under the Endangered Species Act in 1996.  Presently, the Pescadero Creek watershed is not believed to support a viable self-sustained run of coho salmon, although a small remnant population of one brood year (1999…2002) may still exist (J. Smith, personal communication).  Recent sightings of adult coho may be strays from nearby Scott Creek (J. Nelson, personal communication).  Extinction of the two (or three) brood years is believed to have occurred sometime in the 1970s (Baker, 1998).  Significant steelhead trout populations exist in both the Pescadero Creek and Butano Creek watersheds. The highest densities of steelhead are in Butano Creek; lower than expected numbers relative to other similar watersheds are observed in Pescadero Creek (J. Nelson, personal communication).  Scientists working in the watershed believe that excessive sedimentation resulting from a combination of natural and human induced factors is the primary cause for the decline of the anadromous fish populations in Pescadero Creek (R. Curry, personal communication; J. Nelson, personal communication; B. Hecht, personal communication).

Pescadero Marsh

Pescadero Marsh and environs provide estuarine habitat, wildlife habitat, and the preservation of rare and endangered species, including the San Francisco garter snake, tidewater goby, and California red-legged frog.  The availability of suitable habitat in the marsh for sensitive species is affected by variable natural factors, the legacy of land use changes in the marsh and watershed, and current management practices and restoration efforts.  Tidewater gobies, which tolerate fresh or saltwater but avoid strongly tidal areas, are found in the lagoon following formation of the sandbar, as well as in non-tidal marsh habitats of Pescadero Marsh (Smith and Reis, 1997).  In recent years the sandbar has been forming much later in the summer (August-October) than usual, delaying the timing of suitable habitat for tidewater gobies.  The change in timing of sandbar formation is most likely a result of changes in the supply of sediment to the marsh and the transport capacity of flows.  The restoration of tidal action to North Pond and North Marsh in 1995 reduced available habitat for tidewater gobies in areas where they were once common (op cit).  California red-legged frogs use areas of the marsh that remain fresh through spring and mid-summer, such as East Butano marsh, the trout ponds, and portions of North marsh; they are limited by the drying and/or salinization of pond and marsh habitats during this period.  The restoration of tidal action to North and Middle Butano marshes and North Pond effectively limits the value of these habitats to red-legged frogs.  San Francisco gartner snakes rely on red-legged frogs for food, and also require freshwater habitat.  San Francisco gartner snakes are only observed in areas where red-legged frogs are abundant, such as the freshwater portions of North Marsh and the boundary of Middle and East Butano marshes (op cit).  Diking, channelization, reclamation, and excessive sedimentation have dramatically altered the size and character of Pescadero Marsh over the past 150 years.  Between 1900 and 1960 the size of the delta/open water area of the marsh decreased by over 50%, primarily due to reclamation of marshland for agriculture (Violis, 1979).  Human alteration to geomorphic and hydrologic processes in Pescadero Marsh has dramatically reduced the availability of suitable habitat for sensitive species (Smith and Reis, 1997; J. Smith, personal communication).

While not designated as a beneficial use, flooding is an important issue for local residents that should be addressed when approaching other sedimentation issues.  The main access route to the town of Pescadero, as well as surrounding farm and residential land, is routinely flooded during heavy winter storms.  The magnitude and frequency of flooding has increased dramatically over the past few decades, as a direct result of the decreased channel capacity of Butano Creek and the marsh system (Cook, 2002).

Relevant Data

Hydrology

Current problems in the Pescadero Creek Watershed are directly related to the movement of water and sediment.  Very large flood events are capable of tremendous sediment transport, and can have dramatic effects on aquatic habitat.  The largest flood events for the period of record at the Pescadero USGS gage (1951-present) are shown in Table 3.  The largest flood of record occurred in February 1998, with a peak flow of 10,600 cubic feet per second (cfs) at the Pescadero Creek gage.  Large “critical” flow events capable of substantial sediment transport occur at above 500 cfs (Curry et al, 1985).  The 1982 and 1998 water years had the greatest number of days of critical flow in recent history (Table 4).

Table 3: Largest peak flow flood events on record (1952-2001) at the Pescadero Creek USGS gage.  2002 data is preliminary and is not included in the calculation of the recurrence interval.

Table 4: Water years with the greatest number of days of “critical flows” capable of significant sediment transport.  Critical flows in Pescadero Creek are mean daily flows greater than 500 cfs as measured at the USGS gage.  The flow record for the period 1937-1951 was modeled using the mutually overlapping records of 1951 –1960 from the San Lorenzo River, Saratoga, and Pescadero Creek USGS gages by Curry et al (1985).

Turbidity

Regional Board staff performed a limited turbidity monitoring study in winter 2002-2003 to assess how long turbidity remained elevated following storm events.  Turbidity as low as 25 NTU can prevent salmonids from feeding, and can eventually slow growth rates in juveniles (Sigler et al, 1984).  Following the December 16, 2002 storm, a large event with approximately a six-year recurrence interval, turbidity in the mainstem of Pescadero Creek was 268 NTU in the town of Pescadero and 180 NTU at the USGS gage 30 hours after peak flood flow (Figure 2).  Seventy-six hours after peak flow turbidity had decreased to 55 NTU and 40 NTU, respectively.  Due to the number and severity of the

December storms, salmonid feeding was probably inhibited for well over one week.  Approximately two days (52-54 hours) following peak flows in the smaller storm of March 15, 2003, turbidity was much lower (Table 5), remaining elevated above 25 NTU only in small tributaries such as McCormick Creek and Tarwater Creek.  Background

Table 5: Turbidity approximately 2 days (52-54 hours) following the March 15, 2003 storm event.  The storm had a recurrence interval of approximately 1.05 years.

(low-flow) turbidity in the mainstem of Pescadero Creek is generally below 2 NTU.  In Butano Creek, low-flow turbidity ranges from 4 to 8 NTU.  It is unclear whether the higher background turbidity in Butano Creek is due to natural or anthropogenic factors.

Salmonid habitat

Department of Fish and Game (DFG) staff completed salmonid habitat surveys in Pescadero and Butano Creeks and tributaries in 1995.  The most suitable habitat for coho salmon spawning is the mainstem of Pescadero Creek in the redwood canyon that runs from above Portola State Park to below Loma Mar (J. Nelson, personal communication; J. Smith, personal communication).  Below the redwood canyon, temperatures are probably too warm for optimal growth of coho salmon.  Potential spawning grounds in Butano creek exist from Cloverdale Road upstream to Butano Falls, a natural migration barrier.  Bed sediment in Pescadero Creek may be finer than ideal spawning gravel

composition (B. Hecht, personal communication).  Much of Pescadero Creek, especially the section upstream of Wurr Road, suffers from a dearth of woody debris in the channel and large riparian trees (J. Nelson, personal communication).  The lack of recruitment and retention of large woody debris (LWD) precludes the formation of deep pools and limits the complexity of spawning and rearing habitat.

Steelhead can utilize a larger area of potential habitat, and have been found in fish surveys throughout the watershed, including the mainstem of Pescadero and Butano Creeks, Little Butano, Peters Creek, Evans Creek, Slate Creek, Oil Creek, and Pescadero Marsh.  The limiting factor for steelhead populations is the overwintering period (J. Smith, personal communication).  Small numbers of steelhead survive over the winter, with yearlings making up less than 10% of the total population (op cit).  A large percentage of the steelhead population may use the Marsh as primary rearing habitat (op cit).  Several salmonid fish kills have been reported in recent years in Pescadero Marsh.  When the sandbar at the mouth of the Marsh opens with a weak storm, anoxic marsh waters can drain into the lagoon, killing steelhead and other aquatic organisms (J. Smith, personal communication; R. Curry, personal communication; M. Gingras, personal communication).

Sediment Sources

Although a sediment budget has not been produced for the Pescadero-Butano watershed, the collective observations of numerous scientists and resource professionals working in the watershed allows for an initial qualitative summary of known sediment sources.  Although bedrock type may affect erosion potential, topography and land use history may be the most important factors in determining sediment yield (B. Hecht, personal communication; R. Curry, personal communication).

In the upper Pescadero watershed, numerous logging related landslides and road failures delivered huge volumes of sediment into the creek in the late 19th century.  As a result of this pulse of sediment, Pescadero Creek near the town of Pescadero aggraded up to 8 feet between the turn of the 20th century and 1981 (Williams, 1990).  Much of this sediment has been carried out of the system by flood flows since the 1980s, and Pescadero Creek is now downcutting at the Old Stage Road Bridge in the town of Pescadero (Williams, 1990; R. Curry, personal communication).  In the Butano Creek watershed, a similar pulse of sediment from logging and road building entered the creeks in the 1950s through 1970s.  Flushed by 1982-83 floods, the sediment pulse is now near the mouth of Butano Creek and is entering Pescadero marsh (R. Curry, personal communication).  An incredible amount of aggradation in Butano Creek near Pescadero Road has occurred in the last few decades, and especially in the last 5 years (R. Curry, personal communication; Williams, 1990; Cook, 2002).

Recent sediment sources are continuing to supply large amounts of sediment to Pescadero and Butano Creeks and the marsh/lagoon system.  Compressional motion on the San Gregorio-Hosgi fault system is tilting the Butano Valley upward to the west, allowing Butano and Little Butano Creek to incise several meters into loosely consolidated alluvial fill on the west side of the valley (Curry et al, 1985).  One estimate places the amount of eroded alluvium at 500,000 cubic yards in the past 150 years (Williams, 1990).  Bank erosion and downcutting in upper Butano Valley, compounded by the removal of riparian vegetation by streamside property owners, is believed to be one of the major sources of sediment to lower Butano Creek and Pescadero Marsh (R. Curry, personal communication).  Recent large landslides have occurred in the Slate, Oil, and Little Boulder Creek drainages in the upper Pescadero Creek Watershed (J. Nelson, personal communication).  The road network in the upper watershed is also likely still a significant sediment source.  A recent assessment of roads and trails within San Mateo County Park complex in the Pescadero Creek watershed found that stream crossings, road-related landslides, and culverts have the potential to deliver nearly 40,000 cubic yards of sediment in the future (Pacific Watershed Associates, 2003).  The most direct supply of sediment to Pescadero Marsh is from gully erosion, especially on the north side of the marsh.  The gullying, which is common up and down the San Mateo Coast, is believed to be the result of poor historic land use practices (potato farming on steep slopes) combined with a very wet year, which opened up numerous gullies (R. Curry, personal communication).  The gullies operate on cycles of healing and erosion, delivering large amounts of sediment to the marsh every 30-50 years (op cit).

Analytical Strategy

To better understand the sources and transport of sediment in the watershed, a sediment budget will need to be developed.  The sediment budget will help to answer questions about the magnitude, spatial distribution, and controllability of sediment sources in the watershed.  Environmental Science Associates (ESA) is currently performing a geomorphic assessment and sediment transport study as part of the Pescadero-Butano Watershed Assessment, which should facilitate the development of the sediment budget   (D. Sicular, personal communication).  Additional data collection may be needed in order to place sediment issues within the broader context of other watershed issues.

Conclusion

Beneficial uses related to salmonid populations (cold freshwater habitat, fish migration, preservation of rare and endangered species, fish spawning) and other sensitive aquatic species (preservation of rare and endangered species, wildlife habitat) are not being protected in the Pescadero Creek watershed.  The coho salmon population is on the brink of extinction, and several other threatened or endangered species are at risk due to loss of habitat and poor water quality.  Nearly two centuries of intensive human land use, combined with natural geologic and climatic factors, have resulted in a complex history of watershed impacts.  Excessive erosion and sedimentation is believed to be the primary factor impairing these sensitive populations.  Additional factors identified by scientists working in the watershed that may be causing impairment include poor recruitment and retention of woody debris, water withdrawal during critical low-flow periods, lagoon circulation and management, and water quality.  To address these issues, further data collection is needed to define sediment sources and impacts and to place sediment issues within the broader context of other watershed issues.

References

Baker, P. H., 1998.  Restoration plan for the endangered coho salmon south of San Francisco Bay.  Department of Fish and Game. July 1998.

Basin Plan, 1995.  Water quality control plan.  San Francisco Bay Regional Water Quality Control Board.  June 21, 1995.

Cook, B., 2002.  The flooding of Pescadero Road.  Self-published report.  November 2002.

Curry, Robert.  Personal communication, June 23, 2003

Curry, R., Houghton, R., Kidwell, T., and Tang, P., 1985.  Pescadero Marsh management: a plan for persistence and productivity.  January 28, 1985.

Gingras, Marty.  Personal communication, June 12, 2003

Hecht, Barry.  Personal communication, June 24, 2003.

Nelson, Jennifer.  Personal communication, June 13, 2003.

Pacific Watershed Associates, 2003.  Sediment assessment of roads and trails within the Pescadero/Memorial/Sam McDonald County Park Complex, Pescadero Creek Watershed, San Mateo County, California.  Prepared for San Mateo County Parks and Recreation Department and California Department of Fish and Game.  Final report, February 17, 2003.

Sicular, Dan.  Personal communication, June 17, 2003.

Sigler, J.W., Bjornn, T.C., and Everest, F.H., 1984.  Effects of chronic turbidity on density and growth of steelheads and coho salmon.  Transactions of the American Fisheries Society 113: 142-150.

Smith, Jerry.  Personal communication, June 17, 2003.

Smith, J.J., and Reis, D.K., 1997.  Pescadero Marsh Natural Preserve salinity, tidewater goby and red-legged frog monitoring for 1995-1996.  Prepared for the California Department of Parks and Recreation.

Violis, F.S. 1979.  The evolution of Pescadero Marsh.  Master’s thesis, San Francisco State University.

Williams, J., 1990.  Pescadero Marsh Natural Preserve hydrologic enhancement plan.  Prepared for California Department of Parks and Recreation.  Prepared by Phillip Williams and Associates, San Francisco, CA.