CalFed Science Conference, October 2000
David H. Schoellhamer and Randal L. Dinehart
U.S. Geological Survey, Sacramento, California
The U.S. Geological Survey measures sediment transport in the Delta primarily to describe the movement of sediment affecting habitats and the availability of sediment needed for habitat restoration and levee construction.
The Sacramento River is the primary supplier of suspended sediment to the Delta. From 1961 to 1998, the suspended-sediment discharge of the Sacramento River was seven times greater than that of the other main Delta tributary, the San Joaquin River. During water year 1999, 45 percent of the annual suspended-sediment discharge at the gaging station on the Sacramento River at Freeport occurred during the winter months.
During a typical flow pulse past Freeport, two peaks in suspended-sediment concentration are observed: an immediate rise to peak in response to local resuspension, and a smaller, broader peak 4 to 5 days later. As flow increases, resuspension decreases the supply of erodible sediment on the bed, therefore, the first peak begins to diminish in 1 to 2 days. Flow pulses of similar size later in the season resuspend less sediment. The second smaller peak is most likely finer sediment derived from watershed sources far from Freeport.
Both peaks move downstream to Rio Vista in less than a day. Unlike Freeport, pronounced resuspension did not occur at Rio Vista during water year 1999 because bi-directional tides dominated the flow pulse. The finer sediment of the second peak settles more slowly and thus the second peak was greater than the first peak at Rio Vista.
Sediment is deposited between Freeport and Rio Vista because flow in the River changes from unidirectional to bi-directional and velocity decreases as the channel widens. During winter of water year 1999, suspended-sediment discharge at Freeport exceeded the tidally averaged discharge at Rio Vista by 2.9´108 kg, even though discharge at Rio Vista included discharge from the Yolo Bypass.