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The sediment budgets of most beaches in southern California are dominated by sand contributions from coastal streams. Extensive alteration of fluvial systems by the construction of dams and debris basins has reduced substantially the volume of sand reaching the shore. This is one factor in the chronic erosion of these beaches. The purpose of this research is to assess the magnitude of the impacts of coastal dams and debris basins on sand delivery using direct measurements of sediment impoundment rates within eight watersheds. Sediment impoundment data were obtained for 28 dams and more than 150 debris basins in the watersheds of eight rivers: the Santa Ynez, Ventura, Santa Clara, Los Angeles, San Gabriel, Santa Ana, San Dieguito, and San Diego. The cumulative effect of these structures is the impoundment of more than 4,000,000 m3 yr−1 of sediment. This is equivalent to a potential deprivation of beach sand of roughly 3 m3 yr−1 per m of shoreline in the five southern California coastal counties. The sedimentation records for individual watersheds are interpreted in the context of wet and dry climate episodes in the last century. The data are compared to estimates of sediment delivery derived from stream gauge records. In most watersheds, the majority of sedimentation occurs behind one dam, and the contributions of debris basin impoundment are secondary. The net effect is a substantial reduction of potential sand supply to the coast.

Retention of Beach Sands by Dams and Debris Basins in Southern California