Trends in the sediment yield of the Sacramento River, 1957-2001.
Scott A. Wright and David H. Schoellhamer

Successful wetland restoration projects require an adequate sediment supply and many wetland restoration projects are being planned for the Sacramento-San Joaquin River Delta and San Francisco Bay. The primary source of sediment to the Delta and Bay is the Sacramento River. In this paper, daily water and sediment discharge in the lower Sacramento River were analyzed and show that the watershed sediment yield has decreased by about one-half during the period 1957 to 2001. The annual sediment yield record was separated into two discharge ranges (above and below the median annual water discharge) to compare the sediment yield through time for similar water discharge. Two nonparametric correlation coefficients were used to test for a monotonic trend and a very high probability (>99%) of a decreasing trend in annual sediment yield within each discharge range was shown. These tests also indicated no trend in annual water yield or flow variability. Individual measurements of suspended-sediment concentration, separated into ten discharge ranges, also have a very high probability (>99%) of a decreasing trend. Peak concentrations during large floods also have decreased during the period of record. Many factors may contribute to the decreasing sediment yield. Depletion of erodible sediment from hydraulic mining in the late 1800s may contribute to the trend. Trapping of sediment in reservoirs also is a factor; deposition in two large reservoirs in the watershed, Oroville and Folsom Lakes, is on the same order as the decrease in sediment yield of the lower Sacramento River. Riverbank protection is another watershed alteration that may decrease sediment yield, while levees and altered land uses may cause an increase. The relative balance among these competing effects will determine future sediment yield.