Scripps Institution of Oceanography

We studied the streamflow and sediment flux characteristics of the 20 largest streams entering the Pacific Ocean along the central and southern California coast, extending for 750 km from Monterey Bay to just south of the U.S./Mexico border. Drainage basins ranged in area from 120 to 10,800 km2, with headwater elevations ranging from 460 to 3770 m. Annual streamflow ranged from 0 to a maximum of 1 × 109 m3/yr for the Santa Clara River in 1969, with an associated suspended sediment flux of 46 × 106 ton. Trend analyses confirm that El Niño/Southern Oscillationinduced climate changes recur on a multidecadal time scale in general agreement with the Pacific/North American climate pattern: a dry climate extending from 1944 to about 1968 and a wet climate extending from about 1969 to the present. The dry period is characterized by consistently low annual river sediment flux. The wet period has a mean annual suspended sediment flux about five times greater, caused by strong El Niño events that produce floods with an average recurrence of ca. 5 yr. The sediment flux of the rivers during the three major flood years averages 27 times greater than the annual flux during the previous dry climate. The effects of climate change are superimposed on erodibility associated with basin geology. The sediment yield of the faulted, overturned Cenozoic sediments of the Transverse Ranges is many times greater than that of the Coast Ranges and Peninsular Ranges. Thus, the abrupt transition from dry climate to wet climate in 1969 brought a suspended sediment flux of 100 million tons to the ocean edge of the Santa Barbara Channel from the rivers of the Transverse Range, an amount greater than their total flux during the preceding 25-yr dry period. These alternating dry to wet decadal scale changes in climate are natural cycles that have profound effects on fluvial morphology, engineering structures, and the supply of sediment and associated agricultural chemicals to the ocean.