Home | Session 5 | Features of SF Bay Pg 1, 2, 3
Features of San Francisco Bay: From River Valley to Estuary
Presented
by Lise White
San Francisco State University
B. Climate and sea level change
San Francisco Bay is a drowned river valley and its formation as an estuary is related to rising sea level since the last ice age 20,000 years ago. The position of the shoreline was 120 meters lower than present day sea level (figures 7 and 8, below) and would have extended just past the Farallon Islands and Cordell Bank. The succession of rising sea level has important implications for entrainment of sediments along the continental shelf during sea level highstands. The combination of sea level rise and tectonism has shapes many features along the continental shelf including marine terraces.
Figure 7
Figure 8
Figure 9
As sea level rose, the sediments accumulate in the shallow regions as mudflats and marshes along the Bay's shores (read more at the Estuaries and San Francisco Bay home page).These wetland areas are some of the most modified parts of the estuary where their total area is 80% of what is was in 1850 (figure 9, above).
Figure 10This figure (10, left) shows where the shoreline would be if global warming continues. Sea level is expected to rise about 30 meters and the Central Valley would be an inland sea.
C. Circulation and sedimentation patterns and human activities
San Francisco Bay is a partially mixed estuary where the salinity ranges from 2 parts per thousand in Suisun Bay to 30 parts per thousand near the Golden Gate. In places, the estuary is stratified with saltier, denser waters become entrapped beneath less dense, fresher water.
updated April 1, 2002
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