Since the days of hydraulic gold mining (29 KB), sedimentation in San Francisco Bay has changed drastically. From the 1850's until at least the late 1800's, debris from hydraulic mining in the Sierra Nevada filled the Bay. As hydraulic mining practices ceased, the amount of sediment deposited decreased. In the latter half of the 20th century an increase in the implementation of flood control and water distribution projects in the Central Valley caused the Bay to be erosional due to the reduction of the frequency and duration of peak flow conditions, which in turn decreased sediment supply to the Bay.
Suisun Bay (10 KB) makes up the northeastern section of the San Francisco Bay watershed and is the area of focus in this study. Because the study area includes not only Suisun Bay proper, but also Carquinez Straits, Grizzly Bay, Honker Bay, and the entrance of the Sacramento and San Joaquin Rivers, for the purpose of this study the term Suisun Bay will refer collectively to these five areas.
From 1867 (the year of the first detailed hydrographic survey of the area) until at least 1887, Suisun Bay was depositional and filled with hydraulic mining debris. After 1887, Suisun Bay was erosional and the volume of material eroded increased up to the end of the study period. These web pages detail the change in sedimentation in Suisun Bay from 1867 to 1990 by using Geographic Information Systems for analyses. These changes are based on historical hydrographic surveys made by the U.S. Coast and Geodetic Survey (USCGS) and the National Ocean Survey (NOS). The Suisun Bay survey timeline (9 KB) shows the occurence of these hydrographic surveys in relation to activites such as hydraulic mining and flood control projects in central California.
A future step will be to apply this information as well as the data from a similar study in San Pablo Bay to gain a better understanding of change in the San Francisco Bay ecosystem.
Sedimentation change is important because sedimentation is a basic control on many processes that affect the ecosystem including:
- the transport of sediment to wetlands, which affects wetland health
- erosion of wetlands
- the redistribution of sediment, which may bury or expose trace metals and toxic sediments, thus affecting their bioavailability.
Additionally, if sedimentation sufficiently alters bathymetry, particularly in channels, changes can occur in flow patterns and tidal exchange, which in turn affects habitat distribution.