During the past 150 years, human activities in the Bay watershed have had a large impact on the rate that sediment has accumulated within the Bay (Figure 2). Soon after the California Gold Rush began in 1849, miners began to employ a method called hydraulic mining to search for gold pieces in sediments deposited millions of years ago in the foothills of the Sierra Nevada Range. Miners aimed monster hoses with high-pressure water at hillslopes to wash ancient sediments into sluices and remove gold pieces. Vast quantities of sediment were removed from hillslopes and entered into streams, rivers, the Delta, and finally into San Francisco Bay. Farmers downstream from the mining activities began to complain loudly, as their fields became covered and the streams became clogged with sediment. The hydraulic mining practice was soon outlawed, but the badlands topography that was created by this destructive method can still be seen today along Interstate 80.
Beginning in the early 1900s, state and federal water projects built dams on many rivers in the Sierran foothills, Central Valley, and Coastal Ranges (Figure 2). Large canals were built to transport the stored water to farms in the Central Valley and to urban areas throughout the state. Dams collect sediments as well as fresh water carried by rivers. Whereas the effect of hydraulic mining was to increase the amount of sedimentation in the Bay, the effect of water projects has been to decrease sedimentation in the Bay.
Figure 2. Timeline of activities in the San Francisco Bay watershed. The period of hydraulic mining is shown in light brown. The solid blue area shows the increased capacity of water storage created by damming of rivers in the Bay's watershed. Red vertical lines are times when bathymetric surveys were completed in San Pablo Bay (north of the central part of San Francisco Bay).
Bruce Jaffe and his colleagues at the U.S. Geological Survey used the bathymetric surveys in San Pablo Bay (Figure 2) to estimate the amount of sediment accumulation during the past 150 years. For more information about their methods, see Jaffe's web site. They found that in the latter part of the 1800s, following the hydraulic mining period, about 300 million cubic meters of sediment were deposited in San Pablo Bay, filling it an average of 12 meters (3 feet). In the middle 1900s, San Pablo Bay lost about 7 million cubic meters of sediment, probably because of California water projects.
Changes in San Pablo Bay affected the ecosystem in various ways. For example, more than 9,000 acres of tidal mudflat, rich habitats and sources of sediment to wetlands, were lost from the fringes of San Pablo Bay as hydraulic mining debris deposited in the 1800s was eroded by currents and waves in the 1900s. Sediment input into the system was not great enough to overcome the erosive power of currents and waves.
Figure 3. Change in sedimentation rate (the amount of sediment accumulated) on tidal mudflats during the past 150 years. Tidal mudflats are the upper parts of shallow areas that are exposed above water during low tides (brown color in maps below).
The diagrams below are maps of some of the bathymetric surveys, and maps that show the amount of sediment accreted (gained) or eroded (lost) between the times of the surveys. The upper maps show the areal extent of marsh, tidal mudflat and water at various depths in San Pablo Bay.
1. Summarize the changes in San Pablo Bay between 1856 and 1887; for example, did marshes and tidal mudflats expand or contract, and which areas were accreting or eroding. Explain your observations in light of the information in Figures 1 and 2 and the text above.
2. Summarize the changes in San Pablo Bay between 1951 and 1983; for example, did marshes and tidal mudflats expand or contract, and which areas were accreting or eroding. Explain your observations in light of the information in Figures 1 and 2 and the text above.
Diagrams and information are from a USGS web site: SEDIMENTATION CHANGES IN SAN PABLO BAY, 1856 - 1983, by Bruce Jaffe, Richard E. Smith, and Laura L. Zink.