Sediment Fluxes from California Coastal Rivers: The Influences of Climate, Geology, and Topography

The influences of geologic and climatic factors on erosion and sedimentation processes in rivers draining the western flank of the California Coast Range are assessed. Annual suspended, bedload, and total sediment fluxes were determined for 16 river basins that have hydrologic records covering all or most of the period from 1950 to 2006 and have been relatively unaffected by flow storage, regulation, and depletion, which alter the downstream movement of water and sediment. The occurrence of relatively large annual sediment fluxes are strongly influenced by the El Nino-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). The frequency of relatively large annual sediment fluxes decreases from north to south during La Nina phases and increases from north to south during El Nino phases. The influence of ENSO is modulated over a period of decades by the PDO, such that relatively large annual sediment fluxes are more frequent during a La Nina phase in conjunction with a cool PDO and during an El Nino phase in conjunction with a warm PDO. Values of mean annual sediment flux, (S) over bar (f), were regressed against basin and climatic characteristics. Basin area, bedrock crodibility, basin relief, and precipitation explain 87% of the variation in (S) over bar (f) from the 16 river basins. Bedrock erodibility is the most significant characteristic influencing (S) over bar (f). Basin relief is a superior predictor of (S) over bar (f) compared with basin slope. (S) over bar (f) is nearly proportional to basin area and increases with increasing precipitation. For a given percentage change, basin relief has a 2.3-fold greater effect on (S) over bar (f) than a similar change in precipitation. The estimated natural (S) over bar (f) from all California coastal rivers for the period 1950-2006 would have been approximately 85 million tons without flow storage, regulation, and depletion; the actual (S) over bar (f) has been approximately 50 million tons, because of the effects of flow storage, regulation, and depletion.