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San Francisco Bay Floor Explored in Public Lecture
A public lecture on "Shifting Shoals and Shattered Rocks—How Man Has Changed the Floor of San Francisco Bay" drew more than 100 people to the U.S. Geological Survey (USGS) campus in Menlo Park, Calif., on the evening of November 17, 2005. The audience sat spellbound as geologists John Chin and Florence Wong of the USGS Western Coastal and Marine Geology Team (WCMG) related historical events that have affected the bay and described mapping techniques that let us view its floor in unprecedented ways. John began the lecture with a short history of human influences on the bay. He showed striking photographs of hydraulic mining washing away whole hillsides in the foothills of the Sierra Nevada during the 19th-century California Gold Rush; before this practice was banned in 1884, it sent enormous volumes of sediment down the rivers and into San Francisco Bay, reducing the water depth over large areas. He identified bedrock knobs on the west-central bay floor that have been repeatedly blasted to accommodate increasingly larger vessels, and showed photographs of blasting in the early 1900s. He explained how San Francisco's Marina District, once sand dunes, was transformed into the site of the 1915 Panama-Pacific Exposition (only the Palace of Fine Arts remains from this era), and how Treasure Island was created from dredged sand for the 1939 Golden Gate Exposition.
John also described multibeam swath-sonar mapping, a fairly recent technology that has enabled scientists to study the San Francisco Bay floor in more detail than ever before. In contrast to single-point soundings that were the norm through the early 1900s, and lines of soundings that were collected from World War II into the 1990s, multibeam mapping allows scientists to map virtually 100 percent of the sea-floor surface, producing photograph-like views of the sea floor. In her section of the lecture, Florence Wong explained how geographic-information-system (GIS) techniques allow scientists to reconfigure multibeam and other mapping data for numerous purposes—from detecting and quantifying past changes, such as alteration of land by urbanization, to making models and predictions about the future, such as the likely intensities of earthquake shaking in different areas. She showed how multibeam data collected by the USGS in San Francisco Bay had been used to calculate how much rock must be removed from a bedrock knob to eliminate its threat to deep-draft vessels, and to reconstruct how disposal of dredged material transformed a deep depression off Alcatraz Island into a large mound (contradicting expectations that tidal currents would sweep away the disposed material).
Florence closed the lecture with a computer-generated "flythrough" of the central bay floor, created from multibeam-mapping data by Peter Dartnell, also of WCMG. This virtual tour, in which the viewer skims past Alcatraz Island, around Angel Island, over large sandwaves, deep depressions, and bedrock knobs, and out to the brink of the Golden Gate, drew so many oohs and ahhs from the crowd that it was played twice. The lecture was based on a USGS Circular that John and Florence coauthored with Paul Carlson, now a WCMG emeritus scientist. Circular 1259 is posted online at URL http://pubs.usgs.gov/circ/2004/c1259/.
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in this issue:
special feature: Offshore Impacts of Hurricane Katrina Sediment-Toxicity Studies in Western Long Island Sound Sea-Floor Geology Off Massachusetts Coast Alvin Dives to Deep-Water Coral Habitats
Briefing on Coastal Research in Hawai'i USGS Research on the Kona Coast, Hawai'i
Native-Plant Landscaping in Florida
Dec. 2005 / Jan. 2006 Publications List  |
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U. S. Department of the Interior | U.S. Geological Survey
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