Abstract

The Santa Clara River is the largest source of detrital sediment supplying the coast of the California Borderland. During the last 50 years, the average annual sediment supply was about 3.5 × 10⁶ metric tons. However, in the semiarid climate of southern California, about half of the total supply occurred during 1941 and 1969, the two years of high rainfall. Due to the direct approach of waves from the west, and the prevailing longshore drift toward the south, the average supply of 1 × 10⁶ metric tons of sand and gravel is transported away from the river mouth within one year. During 1941 and 1969, the higher rates of sediment supply resulted in the formation of a subaereal delta on the shelf, adjacent to the Santa Clara River mouth. For example, the maximum rate of supply for one 24 h period in 1969 exceeded 22 × 10⁶ metric tons. A repetitive series of bathymetric surveys permits delineation of the rates of erosion and transport of delta sediments following the 1941 and 1969 events. In the area of maximum deposition, sediments deposited at a depth of less than 12 m following a major flood are removed within several years. However, remnants of the delta were present at a depth of 30 m for more than 20 years after the 1941 influx of material. Bottom current speeds of 15 to 30 cm/s on the inner shelf are significantly influenced by tidal currents. Transport of eroded deltaic sediments occurs in an offshore as well as an alongshore direction. Some of the sediment transported to the south on the inner shelf enter Hueneme Canyon, which serves as a conduit for transport of coarse-grained material into deeper water. Additional information from beach profiles, surficial sediment samples and analysis of vibracores in the vicinity of the delta and adjacent shelf provide detailed documentation of the pathways of transport for sediments deposited from episodic floods and subsequently reworked by wave and current action.