Sedimentation and Sedimentation Rates in San Pablo Bay

In the twenty-one years after the start of hydraulic mining of gold in the Sierra foothills massive quantities of sediment were washed down from the mountains to the Delta and San Francisco Bay. More than 250 million cubic meters of this sediment was deposited in San Pablo Bay.

To give you a sense of how much material this is, it would take 10 quarry trucks a second dumping their loads for 1 year to equal this volume. Sedimentation decreased in the following years. From 1951 to 1983, San Pablo Bay lost sediment. This was the first period we studied where San Pablo Bay lost sediment. One possible reason for the change from sediment accumulating in the Bay to eroding from the Bay is a decrease in sediment supply. The decrease in sediment supply is likely the result of upstream flood control and water distribution projects that have reduced peak flows (conditions when most sediment is transported). This possibility will be explored further in future work.

Sedimentation, normalized by the time between surveys, gives the rate that San Pablo Bay is filling or loosing sediment. The peak in sedimentation rate corresponds to the peak in hydraulic mining debris entering the Bay. By the early 1900's, sedimentation rates level off to about 2 million cubic meters per year. Sedimentation continues at the same rate until the 1950's. From 1951 to 1983, San Pablo Bay lost about one-quarter of a million meters of sediment a year.