Deposition, Erosion, and Bathymetric Change in South San Francisco Bay: 1858-1983By Amy C. Foxgrover, Shawn A. Higgins, Melissa K. Ingraca, Bruce E. Jaffe, and Richard E. Smith |
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Abstract of USGS Open-File Report 2004-1192
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. South San Francisco Bay has lost approximately 90 x 106 m3 of sediment from 1858 to 1983;
however within this timeframe there have been periods of both deposition and erosion.
During the most recent period, from 1956 to 1983, sediment loss approached 3 x 106 m3/yr.
One of the most striking changes that occurred from 1858 to 1983 was the conversion of more
than 80% of the tidal marsh to salt ponds, agricultural, and urban areas. In addition,
there has been a decline of approximately 40% in intertidal mud flat area. Restoration
of these features will require a detailed understanding of the morphology and sediment
sources of this complex system. Learn More
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