California State Waters Map Series — Offshore of San Francisco, California

Abstract

In 2007, the California Ocean Protection Council initiated the California Seafloor Mapping Program (CSMP), designed to create a comprehensive seafloor map of high-resolution bathymetry, marine benthic habitats, and geology within California’s State Waters. The CSMP approach is to create highly detailed seafloor maps through collection, integration, interpretation, and visualization of swath sonar data, acoustic backscatter, seafloor video, seafloor photography, high-resolution seismic-reflection profiles, and bottom-sediment sampling data. The map products display seafloor morphology and character, identify potential marine benthic habitats, and illustrate both the surficial seafloor geology and shallow (to about 100 m) subsurface geology. The Offshore of San Francisco map area is centered on the City of San Francisco and the Golden Gate channel, a waterway that connects the Pacific Ocean to the San Francisco Bay between the Marin Headlands and San Francisco Peninsula. The San Francisco Bay Area is the second-largest urban area on the U.S. West Coast with a combined population of over seven million. The bay supports several major cargo ports and the Port of San Francisco’s Fisherman’s Wharf is a major center for Northern California’s commercial and sport fishing fleets. The coastal part of the map area predominantly consists of high bluffs and vertical sea cliffs shaped by uplift and erosion of the Marin Headlands and San Francisco Peninsula east of the San Andreas and San Gregorio Fault Zones. The seafloor in the map area extends from the shoreline and western end of the Golden Gate channel to water depths of about 30 to 50 m, except for the San Andreas graben area, where water depths reach 75 m. Sea-level rise, tidal currents, and tectonics have shaped bathymetry in the map area. During the Last Glacial Maximum, Sea level was about 125 m lower than present day and the shoreline was more than 45 km west of San Francisco near the Farallon Islands. At that time, the map area was part of a large alluvial plain connected to a drainage basin that included much of California’s Central Valley. A river system flowed westward through the narrows of the Golden Gate channel and an alluvial valley bounded to the north and south by bedrock highlands, including the present-day Pacifica-Pescadero and Bolinas shelves. Rising seas entered the Golden Gate about 11,000 to 10,000 years ago and subsequent marine flooding led to progressive growth of the San Francisco Bay. Strong tidal currents, accelerating through the relatively narrow Golden Gate, have scoured the bedrock channel to a depth of 113 m. East and west of the channel, tidal currents decelerate and form large fields of sand waves. Offshore of the Marin Headlands, eastward transfer of right-lateral fault slip in a complex of faults northwest of the map area has caused extension and the formation of a sediment basin called the San Andreas graben on the continental shelf. The accommodation space created by extension on the shelf and the proximity to sediment transported to the ocean through San Francisco Bay results in a sand-dominated offshore shelf environment. Seafloor habitats in the Offshore of San Francisco map area comprise significant sand-dominated sediment habitat with sand wave and ripple bedforms indicative of high wave and current energy. North of the Golden Gate, biological productivity resulting from coastal upwelling supports populations of Sooty Shearwater, Western Gull, Common Murre, Cassin’s Auklet, and many other less populous bird species. In addition, an observable recovery of Humpback and Blue Whales has occurred in the area; both species are dependent on coastal upwelling to provide nutrients. For the first time in 65 years, Pacific Harbor Porpoise returned to San Francisco Bay in 2009. On the coast north of the Golden Gate, the large extent of exposed inner shelf bedrock supports large forests of “bull kelp,” which is well adapted for high wave-energy environments. Common fish species found in the kelp beds and rocky reefs include painted greenling, kelp greenling, lingcod, and several varieties of rockfish. Circulation over the continental shelf in the Offshore of San Francisco map area is dominated by the southward-flowing California Current, an eastern limb of the North Pacific Gyre that flows from Oregon to Baja California. At its midpoint offshore of central California, the California Current transports subarctic surface waters southeastward, about 150 to 1,300 km from shore. Seasonal northwesterly winds that are, in part, responsible for the California Current, generate coastal upwelling. Ocean temperatures offshore of central California have increased over the past 50 years, driving an ecosystem shift from the productive subarctic regime towards a depopulated subtropical environment.