Deposition, Erosion, and Bathymetric Change in South San Francisco Bay: 1858-1983

Since the California Gold Rush of 1849, sediment deposition, erosion, and the
bathymetry of South San Francisco Bay have been altered by both natural
processes and human activities. Historical hydrographic surveys can be used to
assess how this system has evolved over the past 150 years. The National Ocean
Service (NOS) (formerly the United States Coast and Geodetic Survey (USCGS),
collected five hydrographic surveys of South San Francisco Bay from 1858 to
1983. Analysis of these surveys enables us to reconstruct the surface of the
bay floor for each time period and quantify spatial and temporal changes in
deposition, erosion, and bathymetry.

The creation of accurate bathymetric models involves many steps. Sounding data
was obtained from the original USCGS and NOS hydrographic sheets and were
supplemented with hand drawn depth contours. Shorelines and marsh areas were
obtained from topographic sheets. The digitized soundings and shorelines were
entered into a Geographic Information System (GIS), and georeferenced to a
common horizontal datum. Using surface modeling software, bathymetric grids
with a horizontal resolution of 50 m were developed for each of the five
hydrographic surveys. Prior to conducting analyses of sediment deposition and
erosion, we converted all of the grids to a common vertical datum and made
adjustments to correct for land subsidence that occurred from 1934 to 1967.
Deposition and erosion that occurred during consecutive periods was then
computed by differencing the corrected grids. From these maps of deposition and
erosion, we calculated volumes and rates of net sediment change in the bay.

[Summary provided by the USGS.]