Origin and Recent History of Newport Submarine Canyon, California Continental Borderland

Newport Canyon is the largest of several channels which cut the continental margin off Newport Beach, California. All of the channels were initiated subaerially by the Santa Ana River during the Pleistocene and Holocene. Submarine erosion may have continued until early in the last century. Prior to 1825, the river provided sediment directly to the ocean in the canyon head area. Subsequent construction of a barrier beach by the river caused its diversion into Newport Bay and a change in coastal alignment which altered longshore current patterns. The upper portions of the canyon are now being filled with organic-rich silt and clay at the rate of approximately 1.4 cm/yr. The primary sources of this material are the Orange County outfall and the Santa Ana River. Suspension of fine sediment on the shelf and deposition in the canyon is the normal mode of transport for all modern canyon sediment. Collection of sand and debris in the nearshore head is precluded by the divergence of longshore currents from the head under most wave conditions. The canyon is presently inactive and cannot be the source of modern turbidites in the San Diego Trough.