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New Maps Reveal Seafloor off San Francisco Area

Three new sets of maps detail the offshore bathymetry, habitats, geology and submarine environment of the seafloor off the coast of San Francisco, Drakes Bay, and Tomales Point.

SANTA CRUZ, Calif. — Three new sets of maps detail the offshore bathymetry, habitats, geology and submarine environment of the seafloor off the coast of San Francisco, Drakes Bay, and Tomales Point. Critical for resource managers, the maps are part of the California Seafloor and Coastal Mapping Program, a series of maps published by the U.S. Geological Survey with support from the California Ocean Protection Council, NOAA, and 15 other state and federal partners. The maps are designed to be used by a large stakeholder community and the public to manage and understand California’s vast and valuable marine resources.

“OPC is proud to be a partner in this interagency effort,” said California’s Secretary for Natural Resources and OPC Chair John Laird. “These maps are critical to the state’s innovative approach to coastal resource management. USGS’s products form the foundation for assessing the performance of our Marine Protected Area network and preparing for climate change impacts such as sea-level rise.”

“NOAA is pleased to be partnering in this integrated ocean and coastal mapping project. By working with partners from across federal, state, academic, and private sectors, we are able to combine data resources and maximize our efficiency in applying a ‘map once, use many times’ approach that benefits all,” said Rear Admiral Gerd F. Glang, director NOAA’s office of coast survey.

The program was initiated seven years ago with the goal of comprehensively surveying and mapping all of California's state waters. The vision was tremendously ambitious – comparable mapping on this scale has not been attempted anywhere else in the world. Each of the three publications includes 10 map sheets, a pamphlet, and a digital data catalog. The maps and mapping data have a large range of applications. They provide:

  • A foundation for assessing marine protected areas and habitats.
  • Baselines for monitoring coastal change and sea-level-rise impacts.
  • Critical input data for modeling and mitigation of coastal flooding.
  • A framework for understanding coastal erosion and developing regional sediment management plans.
  • Contributions to earthquake and tsunami hazard assessments.
  • More accurate maps for safer navigation.
  • Essential information for planning, siting, or removing offshore infrastructure.

The new “Offshore of San Francisco” maps document the complex submarine environments along the inlet to San Francisco Bay formed by strong tidal currents, including spectacular sand waves, a deep scour pool beneath the Golden Gate, and the dynamic offshore San Francisco mouth bar and “Potato Patch” shoal.  Sediment distribution maps reveal only a thin sediment cover offshore of the Ocean Beach (San Francisco) erosional hotspot (a pattern extending south to San Gregorio), indicating that today's present coastal erosion will be a continuing problem, likely to be exacerbated by continuing sea-level rise. Geologic maps incorporating subsurface data document the location and geometry of the San Andreas, San Gregorio, and Point Reyes fault systems, and show how their interactions led to uplift of Point Reyes and development of a deep sediment-filled basin. The “Drakes Bay and Vicinity,” and “Offshore of Tomales Point” maps reveal the diverse and complex range of seafloor habitats typical of the California coast, ranging from the rugged granitic bedrock along the high-energy west coast of Point Reyes, to smooth sand and mud in the more protected Drakes Bay environment that includes the Point Reyes State Marine Reserve.

Sam Johnson, the USGS project lead, also notes, “There is a ‘WOW!’ factor to the new high-resolution datasets and maps. They're allowing scientists to pose new questions and are having a significant role in stimulating research.  We're also seeing a positive impact on public education and awareness.” 

To date, twelve map sets and catalogs have been published. Ten additional map sets are now being formatted for publication, which will complete coverage in the Santa Barbara Channel (Oxnard to Gaviota) and from Marina northward to beyond the Russian River. The maps are created through the 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 California Seafloor and Coastal Mapping Program is a unique collaborative effort supported by the USGS, the California Ocean Protection Council, NOAA, California State University at Monterey Bay, Moss Landing Marine Laboratories, and many other academic, government, and industry partners.

“Seafloor character” map of the San Francisco Region. This is a type of habitat map that classifies the seafloor based on surface hardness and roughness. Such maps are used in various types of ecosystem assessments and seafloor zoning, such as delineation or monitoring of marine protected areas.
Map of sediment thickness in state waters offshore of San Francisco. About 21,000 years ago, sea level in this area was about 125 m lower and the shelf offshore San Francisco was an emergent land surface. At that time, the Sacramento River drained through the Golden Gate and eroded a valley ("the San Francisco paleovalley”) that was filled with sediment during subsequent sea-level rise. The thickest young sediment in the region occurs in the “San Andreas graben,” a basin that formed by crustal down dropping along the offshore section of the San Andreas fault. There is very little sediment on the shelf offshore of southern Ocean Beach (a pattern that extends south to Pescadero), a factor important for understanding and forecasting coastal erosion in this area.
Bathymetry bounding Tomales Point. Rugged and massive granite outcrops extend offshore from Tomales Point to water depths of as much as 60 meters. Offshore sedimentary rock outcrops (lower left part of image) form distinctive “ribs” on the seafloor and have a notably different appearance. There is minimal sediment on this part of the California shelf because the watersheds draining the west flank of Tomales Point are very small and because Tomales Point and Tomales Bay block sediment transport from the north. Rocky-shelf outcrops and rubble are excellent habitats for rockfish and lingcod, recreationally and commercially important species. Tomales Bay, approximately 20-km long and 1- to 2-km wide, formed along a submerged portion of the San Andreas Fault (very shallow water depths preclude collection of high-resolution bathymetric data at the mouth of Tomales Bay). 
Map of offshore sediment thickness in State Waters between Drakes Bay and Salt Point, north of the Russian River. The thickest sediment in the region occurs offshore of the Russian River, and in a large bar along the south flank of Point Reyes Head. There is a relative lack of offshore sediment between Bodega Head and Point Reyes, where the shelf is characterized by abundant rocky habitat and much of the coastal sediment is trapped in large onshore dune fields.
Perspective view looking to the southeast over entrance to San Francisco Bay. Golden Gate Bridge is to left (east) of this view. The large sand-wave field lies within Golden Gate channel, and formed from sediment transported out of the Bay by strong tidal currents. Profile A–A’ shows that the larger bedforms can reach heights of over 7 m and are asymmetrical with steeper sides towards the open coast. A smaller field of sand waves to south near Baker Beach shows the opposite symmetry (steep sides toward the Bay) indicating that the strongest tidal currents in that local area are directed eastward. 

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