Mapping the Sea Floor Southwest of Santa Rosa Island, California


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Mapping the Sea Floor Southwest of Santa Rosa Island, California

By Guy Cochrane
November 2007

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Above: Gerry Hatcher (left) and David Finlayson monitor and process sonar data on the research vessel Shearwater. [larger version]

U.S. Geological Survey (USGS) staff used sonar to map approximately 50 km² of the continental shelf southwest of Santa Rosa Island in the northern Channel Islands off California last August. The sonar survey is a continuation of a cooperative mapping project with Channel Islands National Marine Sanctuary (URL http://channelislands.noaa.gov/), designed to map geology and benthic habitat primarily in the sanctuary's jurisdiction, although other areas of mutual interest have also been mapped, including California State Marine Protected Areas.

David Finlayson and Gerry Hatcher participated in the 2-week cruise, from August 6 through 18, 2007, along with project chief Guy Cochrane. Mike Boyle helped install USGS instruments on the Channel Islands National Marine Sanctuary's research vessel Shearwater, used for the survey. The scientists lost 2 days of the cruise to repairs after part of the vessel's port rudder linkage sheared through, but nevertheless accomplished much of the planned mapping.

Data from the interferometric sidescan sonar were processed daily, producing a view of the sea floor surveyed that day. The mapping revealed rocky sea floor off Santa Rosa Island out to depths of 70 m; on the mainland coast, rocky sea floor is largely buried by sediment, and outcrops are limited to shallower water closer to shore. Rock outcrops on the sea floor are of particular interest because they support diverse communities of marine organisms, including rockfish, an important resource for commercial and recreational fisheries.

Above left: California, showing islands mentioned in text and area of Channel Islands National Marine Sanctuary (darker shading). [larger version]

Above right: Shaded-relief bathymetry (gray tones) of area southwest of Santa Rosa Island mapped with sonar in 2007. [larger version]

An unusual circular feature in the northwest section of the survey area is likely a deposit of sand. The scientists hypothesize that sand eroding from sandstone on the southeast side of San Miguel Island is being blown into the ocean and deposited in this area as a result of eddying and attenuation of currents from the northwest. Slumping is visible on the south edge of the sand deposit, where it abruptly rises more than 10 m above the surrounding shelf.

Above: Preliminary images (oblique view) of sea floor southwest of Santa Rosa Island. Top, shaded-relief bathymetry, color coded for depth; bottom, backscatter amplitude (amplitude of sound waves bounced from sea floor back to sonar receiver), with lighter gray tones for higher amplitudes (indicating harder or rougher bottom). Note circular feature at left (northwest), hypothesized to be sand blown off San Miguel Island and deposited by eddying and attenuation of currents from northwest. [larger version]

Recently, a larger cooperative project has been established to map all of the sea floor under the jurisdiction of the State of California, which extends from the shore out 3 mi. In this effort, the USGS is collaborating with California State University, Monterey Bay's Seafloor Mapping Lab; Moss Landing Marine Laboratories' Center for Habitat Studies; Fugro Pelagos, Inc.; and the California Coastal Conservancy. Future cooperative work with Channel Islands National Marine Sanctuary will likely focus on video ground-truthing in State waters, and sonar and video mapping in waters beyond State jurisdiction.

Channel Islands National Marine Sanctuary
National Oceanic and Atmospheric Administration