HYDROLOGIC SURVEY
Return to Hydrologic Survey Table of Contents

HYDROGEOLOGIC SETTING

Geography and Land Use

Stinson Beach is situated where the westernmost flanks of Mount Tamalpais reach the Pacific Ocean. As a result the community is confined by topography to the lower slopes of Bolinas Ridge, which forms the community's backdrop, to small alluvial fan areas at the base of the ridge, and to an elongated line of sand dunes that parallel the coast. These dunes form a long, east-west trending sandspit that separates Bolinas Lagoon from the ocean except for a narrow channel at the foot of Bolinas Mesa to the northwest. Topography along the sandspit and sand dunes generally is within 20 feet of mean sea level (MSL). The topography steepens abruptly at the foot of Bolinas Ridge, rising to 400 feet MSL within the study area and to more than 1,900 feet at the crest of the ridge.

The community is relatively isolated from the greater San Francisco Bay metropolitan area, but attracts numerous visitors and temporary or seasonal residents as a result of its recreational attractions. These attractions include Stinson Beach Park and other portions of the Golden Gate National Recreation Area (GGNRA), which completely surrounds the community. Stinson Beach includes a resident population and small commercial area, but is best known for its numerous weekend and summer residences. The population is approximately 1,500 persons year-round, but can swell to ten times that number on summer weekends, with an added influx of numerous day visitors. Such seasonal peak populations place considerable demands on both water supply and wastewater disposal capacity.

The community has existed as a recreational destination since the 1870's. Resort hotels and vacation homes were built around Old Town before the 1920's, followed by development of the Calles and Patios in the 1940's. The period after 1945 was marked by development of the Highlands subdivision and by development of Seadrift beginning in the 1960's. In 1972, establishment of the GGNRA effectively encircled the community, limiting any future growth to infilling. Nonetheless, the Stinson Beach water supply and onsite wastewater disposal systems could experience increased demand and loading, respectively, as a result not only of increased residential density, but also increased year-round residence.

Climate and Surface Water

Stinson Beach enjoys a Mediterranean climate characterized by cool, rainy winters and dry, foggy summers. Precipitation generally ranges from about 25 inches near the coast to over 50 inches at higher elevations. District rainfall records indicate an average annual rainfall of 28 inches between 1978 and 1997; however, recent years have been characterized by higher rainfall, averaging 43 inches between 1994 and 1997.

Surface water drainage in Stinson Beach is generally towards Bolinas Lagoon. Bolinas Lagoon is an estuary encompassing about 1,100 acres with a watershed of 16.7 square miles Wetlands Research Assoc., August 1995). The Lagoon is publicly owned and managed as open space and is a valuable natural habitat for marine invertebrates, fish, birds, and mammals. The major management issue is continuing sediment accumulation and loss of estuarine habitats within Bolinas Lagoon.

Seadrift Lagoon was created during development of the Seadrift Subdivision, when the sandspit was widened on the Dipsea side and dredged in the center to create an artificial lagoon. Water supply to the lagoon is provided by means of an intake near the northwestern end (see Figure 1), which typically is opened during high tide to allow inflow from Bolinas Lagoon. In addition, an outfall to Bolinas Lagoon exists on the southeastern end. The intake and outfall facilities are operated to maintain a stable elevation in the lagoon and to allow regular flushing of water through Seadrift Lagoon.

The major stream in the study area is Easkoot Creek, which originates above Stinson Beach on the flank of Bolinas Ridge and flows through Old Town. The historic stream likely flowed from its Old Town channel directly toward the ocean in the vicinity of the Beach Park, but now turns northwestward into Bolinas Lagoon. The next major stream to the northwest is Stinson Gulch, which also originates on Bolinas Ridge and flows to Bolinas Lagoon. Stinson Gulch and its associated groundwater are the major source of water supply to Stinson Beach.

Geology and Soils

The major geologic feature in the Stinson Beach area is the San Andreas fault zone, which trends in a northwesterly direction through Bolinas Lagoon. The fault separates Franciscan bedrock complex on the eastern side of the fault and Bolinas Lagoon from younger sedimentary rocks, such as Merced Formation and Monterey Shale, on the western side of Bolinas Lagoon in the vicinity of Bolinas. The youngest geologic units in the area are alluvium and dune sand. Given its position largely on the eastern side of the fault, the Highlands and Old Town are underlain by Franciscan Complex, an assemblage of sandstones, shales, cherts, limestones, conglomerates, and basalts that have been deformed intensively (Galloway, 1977; Boudreau, July 1979). Locally, the Franciscan includes highly fractured and deeply weathered sandstones and shale along the lower slopes of Bolinas Ridge that is marked by landsliding. The higher and steeper portions of Bolinas Ridge are underlain by a harder and less weathered sandstone (Boudreau, July 1979; Harding-Lawson, December 1980).

Alluvium occurs in alluvial fans at the mouth of Stinson Gulch and along Easkoot Creek in the Old Town area. The alluvium consists of a wedge of unconsolidated sand, gravel, and clay deposits that thicken toward the stream and in a downstream direction. In the vicinity of Alder Grove on Stinson Gulch, the alluvium reaches a thickness of 67 feet (Boudreau, July 1979) and is characterized by a low-permeability clay layer overlying sands and gravels.

Dune sands parallel the coast, underlying the Stinson Beach Park area, Calles, Patios, and Seadrift. The dune sands consist generally of loose and well-sorted sand. Seadrift is largely a natural sandspit, but has been altered, particularly on the Dipsea side, by addition of finer-grained artificial fill from dredging of Seadrift Lagoon. The bedrock underlying Seadrift is unknown but is assumed to be Franciscan.

Figure 5 shows the locations of cross sections in the study area, while Figure 6 and Figure 7 are cross sections illustrating the geology underlying the Seadrift and Calles areas, respectively. Figure 6 crosses Seadrift through the new monitoring wells; in this area, Seadrift Lagoon is about 500 feet wide. As shown, Seadrift is underlain by about 20 to 30 feet of sand that thickens toward the ocean. Underlying this sand on the Dipsea side is a clay zone overlying gravel. These clay and gravel zones likely correspond to the clay layer and gravel and sand layer encountered in alluvium at the lower end of Stinson Gulch. As shown, the clay layer extends to an unknown extent beneath Seadrift Lagoon and is absent on the Seadrift side of the sandspit. The underlying gravel layer is known to extend under the Dipsea side and Seadrift side in the vicinity of MW-2 and MW-5, respectively.

Figure 7, extending along Calle del Resaca (see Figure 5), shows that the surficial geology consists of a wedge of dune sand that thickens toward the ocean. Bedrock was encountered in both MW-6 and MW-7. The material in MW-6 consisted of weathered limestone and sandstone in a clay matrix. The geologic materials found below about ten feet depth in MW-7 also were clay, and may have included alluvial clay (like that encountered at Dipsea) overlying bedrock. Bedrock was not encountered in MW-8 and MW-9 at depths of 25 and 35 feet, respectively.

Soils in the Stinson Beach area have been mapped as part of the overall Marin County soil survey (USDA Soil Conservation Service, 1985). Two major soil types are present, including the beach and sand dune soils along the coast and Seadrift sandspit, and the Cronkhite-Barnabe soils complex on the lower slopes of Bolinas Ridge. Additional soil types occur higher on the ridge. The beach and sand dune soils, which underlie the Seadrift, Patios, Calles and Beach Park areas, are deposits of loose sand with rapid permeability. The Cronkhite-Barnabe complex include the deep, slow-permeability Cronkhite loams; the shallow, moderate-permeability very gravelly Barnabe loams; and other related soils and rock outcrops.




SBCWD HomeContactCurrent WeatherFAQRates and ChargesMeter ReadingConservationDo's and Don'ts
Homeowner's GuidePermit ProcessDisaster/EmergencyBoard of DirectorsBoard and Committee MeetingsMinutes
BudgetWest Nile VirusPesticide PolicyWater QualityHydrologic SurveyTitle 3Title 4Title 5GlossaryLinks
SBCWD Home

Contact

Current Weather

FAQ
Rates and Charges
Meter Reading
Conservation
Do's and Don'ts
Homeowner's Guide
Permit Process
Disaster/Emergency

Board of Directors
Board and Committee
Meetings

Minutes
Budget

West Nile Virus
Pesticide Policy
Water Quality
Hydrologic Survey
Title 3
Title 4
Title 5
Glossary
Links