California rivers have historically delivered between 70-85% of the sand naturally supplied to the coastline. This delivery has been greatly reduced by dams, which prevent the sand from getting to the coast and nourishing the beaches naturally; this in turn has resulted in narrowing beaches. Currently, more than 500 dams control over 42,000 km 2, or 38%, of California’s watersheds. This study determined cumulative volumes of sand-sized material trapped behind the major coastal dams of California. The study focused on sand-sized material, since it would typically be large enough to remain on beaches.

Twenty-one coastal streams were included in the study, from the Klamath River in the north to the Tijuana River in the south. Sand reduction effects of 66 dams were analyzed on these 21 streams. During ‘natural’ conditions (i.e., with no dams present), the 21 streams formerly delivered about 10,000,000 m 3/yr of sand to the coast. With dams in place, the river discharge has been reduced to about 7,700,000 m 3/yr of sand. Therefore, dams have reduced the annual sand flux to the California coast by 2,300,000 m 3/yr or 23%. The natural annual sand flux for northern California rivers has only been reduced by about 5%, while the annual central and southern California sand flux has been reduced by 31% and 50%, respectively. Overall, the 66 dams analyzed in this study have impounded a cumulative volume of about 125,000,000 m 3 of sand. This volume will continue to increase unless some dams are removed or some form of sand bypassing strategy is established.

The reduction in global modern sediment flux has been calculated to be about 10%. The trends in northern California agree well with this global 10% reduction, but central and southern California have experienced far greater reductions than the global average.

The CSMW and the University of California at Santa Cruz, have developed this report as an educational tool as part of their public outreach effort associated with the Sediment Master Plan. The document was prepared with significant input from CSMW personnel, but does not necessarily represent the official position of member agencies. The impounded material represents a significant potential source of sediment for mitigation of narrowing beaches, and will be investigated for use as part of Regional Sediment Management in California.