Climate change and the episodicity of sediment flux of small California rivers. Inman, Douglas L; Jenkins, Scott A.
The Journal of Geology:
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 m3/yr for the Santa Clara River in 1969, with 9 1#10 an associated suspended sediment flux of ton. Trend analyses confirm that El Nino/Southern Oscilla- 6 46#10 tion–induced 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 Nino events that produce floodswith 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.
Reductions in Fluvial sediment discharge by coastal dams in California and implications for beach sustainability. Willis, Cope M; Griggs, Gary B.
The Journal of Geology:
The long-term sustainability of California's beaches depends on periodic deliveries of sand and gravel from coastal rivers and streams. To assess the long-term health of California's beaches, this study characterized the current state of fluvial sediment delivery and quantified, on a littoral cell basis, the cumulative impacts of dams in decreasing annual discharge. Presently, more than 500 dams impound more than 42,000 km² (or 38%) of California's coastal watershed area. Flow modeling suggests that by diminishing flood hydrographs, these dams have reduced the average annual sand and gravel flux to 20 major littoral cells by 2.8 million m³/yr (or 25%). In 70% of the streams considered in this study, suspended sediment loads during equivalent discharge events have declined over the past three decades, which indicates that dams have also significantly reduced downstream sediment supplies. Approximately 23% (or 274 km) of the 1193 km of beaches in California are downcoast from rivers that have had sediment supplies diminished by one-third or more. Moreover, 192 km (or 70%) of these threatened beaches are located in southern California, where most of the state's beach recreation and tourism activities are concentrated. Although past large-scale nourishment activities associated with coastal construction and harbor dredging have offset fluvial sediment supply reductions, particularly in southern California, many of these threatened beaches can be expected to undergo longterm erosion in the future.