Extremely high-resolution maps of the seafloors offshore of Tomales Point, Drakes Bay and San Francisco Bay were published late last month by the United States Geological Survey, as part of a multi-million dollar, decade-long partnership with other agencies to pin down baseline conditions of the seafloor. The effort, called the California Seafloor and Coastal Mapping Program, is meant to help state and federal governments and academic institutions study sea-level rise, coastal erosion, earthquake risk and benthic habitat, as well as to help monitor the state’s marine protected areas.
The maps “provide a very high-resolution starting point from which you can monitor change,” said Samuel Johnson, a research geologist with U.S.G.S. who heads the project at the survey. “So, for example, if there are changes offshore due to sea-level rise or due to El Niño storms or due to offshore fault or [underwater] landslides, you can measure the change.” The program is also serving an unforecasted yet critical purpose: maps released earlier of offshore Santa Barbara will be used to evaluate the impacts of the oil spill two weeks ago at Refugio State Beach.
The effort to build the maps began in 2006, as California geared up to demarcate marine protected areas along the coast. The state realized it didn’t have detailed bathymetry maps—essentially a topography of the ocean floor. Bathymetric maps can provide highly detailed information about potential habitat down there; for instance, rocky bottoms are well suited to species like rockfish. The National Oceanographic and Atmospheric Administration had an interest in ocean floor data for nautical maps, and the U.S.G.S. saw that the mapping effort could also provide information about hazards like offshore faults.
In 2007 the California Seafloor Mapping Program—a collaboration between the state’s California Ocean Protection Council, U.S.G.S., NOAA and about 15 or so other partners—began collecting large amounts of data about the seafloor from a suite of sources, including sonar, seafloor video footage, photographs and sediment sampling. The state committed about $15 million, while NOAA invested about $9 million.
The maps for offshore of upper Tomales Bay, Drakes Bay and San Francisco Bay are the most recent to be released, adding to nine previously published map sets. By the end of this year, more seafloor maps will be released, including ones documenting areas off the coast of Bolinas, Bodega Head and the mouth of the Russian River. (The map of the Bodega Head area will show that a portion of the San Andreas Fault lies 800 meters west of where it was previously mapped.)
Each area comes with a set of 10 maps that visualize different kinds of data. For instance, one kind of map for each area focuses on offshore faults; these are created through what Mr. Johnson called “seismic reflection data,” which he described as “CAT scans of what’s below the seafloor.” Understanding where faults connect is important because longer faults can produce more severe earthquakes. Mr. Johnson said the project has found that offshore faults near Diablo Canyon Nuclear Power Plant, in San Luis Obispo, previously thought to be separated by a gap, are in fact connected.
Another map reveals detailed areas of sediment on the seafloor. The Tomales Point map shows “several small…irregular ‘mounds.’” The detailed analysis was fine-tuned enough to show that these are sediment mounds, not rocks, and hypothesized that the curious accumulations could be marine sediment from shipwrecks, or, perhaps, that they sit atop shells.
Tim Doherty, a coastal specialist for NOAA based in Oakland, said the sediment information is also key for sea-level rise predictions. “Sediment is increasingly becoming important for understanding how wetlands or coastal habitats will respond to increases in sea-level rise,” he said. For example, wetlands experiencing sedimentation could be better equipped to handle rises in sea level; rather than being submerged, new wetlands habitat could continue to form.
Yet another map presents potential benthic habitat. For example, the area off Tomales Bay has a “rugged, rocky seafloor,” according to the maps, while Drakes Bay is comprised of softer sediments.
Mr. Doherty noted that in in terms of managing and monitoring the ocean and sealife, maps are crucial. “Think of trying to manage West Marin, but you didn’t have any maps. It’s really hard if you don’t know where certain creeks are, if it has eucalyptus or oaks or grassland. This helps people understand, ‘Well that’s more of an oak area.’…It certainly allows us to get a window into this seafloor arena.”
The data in the maps are enormously useful for the agencies, but the maps themselves are also visually alluring. The map of offshore Tomales Point, color-coded with rosy pinks and mustard yellows to highlight the geology beneath the waters—marine shelf deposits from the Holocene and granitic rock formations dating from the Cretaceous period—is reminiscent of a wood block print. The bathymetric maps use a rainbow of colors overlaid on a relief map to describe the coast’s sinking depths, starting with rusty orange to represent shallower areas near the coast, fading into yellow, green and finally to a dark blue to reflect the deep sea—a sea that’s now a little less mysterious.
To view the maps for the Point Reyes areas, visit http://pubs.usgs.gov/of/2015/1041 and http://pubs.usgs.gov/of/2015/1088.