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Western Coastal and Marine Geology
U.S. Geological Survey
Open-File Report 2006-1262
Version 1.0

A History of Intertidal Flat Area in South San Francisco Bay, California; 1858 to 2005

By Bruce Jaffe and Amy Foxgrover


Preliminary map of intertidal flat area (region shaded brown), bathymetry, and tidal marsh in South San Francisco Bay from 1858 to 2005


A key question in salt pond restoration in South San Francisco Bay is whether sediment sinks created by opening ponds will result in the loss of intertidal flats. Analyses of a series of bathymetric surveys of South San Francisco Bay made from 1858 to 2005 reveal changes in intertidal flat area in both space and time that can be used to better understand the pre-restoration system. This analysis also documents baseline conditions of intertidal flats that may be altered by restoration efforts. From 1858 to 2005, intertidal flat area decreased by about 25% from 69.2 +6.4/-7.6 km2 to 51.2 +4.8/-5.8 km2. Intertidal flats in the north tended to decrease in area during the period of this study whereas those south of Dumbarton Bridge were either stable or increased in area. From 1983 to 2005, intertidal flats south of Dumbarton Bridge increased from 17.6 +1.7/-2.5 km2 to 24.2 +1.0/-1.8 km2. Intertidal flats along the east shore of the bay tended to be more erosional and decreased in area while those along the west shore of the bay did not significantly change in area. Loss of intertidal flats occurred intermittently along the eastern shore of the bay north of the Dumbarton Bridge. There was little or no loss from 1931 to 1956 and from 1983 to 2005. Predictions of future change in intertidal flat area that do not account for this spatial and temporal variability are not likely to be accurate. The causes of the spatial and temporal variability in intertidal flat area in South San Francisco Bay are not fully understood, but appear related to energy available to erode sediments, sediment redistribution from north to south in the bay, and sediment available to deposit on the flats. Improved understanding of sediment input to South San Francisco Bay, especially from Central Bay, how it is likely to change in the future, the redistribution of sediment within the bay, and ultimately its effect on intertidal flat area would aid in the management of restoration of South San Francisco Bay salt ponds.

Download Open-File Report 2006-1262 as a 32-page PDF file (6.2 MB)

For questions about the content of this report, contact Bruce Jaffe

Also of interest:

USGS Open-File Report 2006-1287, Sediment Deposition and Erosion in South San Francisco Bay, California from 1956 to 2005, By Bruce Jaffe and Amy Foxgrover

USGS Open-File Report 2004-1192, Deposition, Erosion, and Bathymetric Change in South San Francisco Bay: 1858-1983, By Amy C. Foxgrover, Shawn A. Higgins, Melissa K. Ingraca, Bruce E. Jaffe, and Richard E. Smith

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Maintained by: Michael Diggles
Created: September 1, 2006
Last modified: October 16, 2006 (mfd)