The USGS-CALFED Program to Measure Sedimentation in the Delta
The Sacramento-San Joaquin River Delta was formed by deposition of sediment transported through the Central Valley of California. We understand the basic movement of sand and fine sediment into the Delta, but we do not have data to specify the availability, quantity, and efficient use of sediment. This activity supports the CALFED effort to assess, monitor and restore aquatic habitats throughout the Delta. Our primary objective is to describe the transport of sediment needed for habitat restoration (see complete Project Description).
Sediment transport in the Delta was last studied comprehensively more than 40 years ago, before the Central Valley watershed and Delta were altered by the State Water Project. Distant sources of sands in the mountains have been disconnected from the Delta by construction of dams along major tributary rivers. Without direct measurements, we are left to wonder whether sand transport is dominant or nominal in the main tributaries to the Delta.
Sand is transported as bedload along the bottom of large, slow rivers such as the Sacramento and San Joaquin. Where there is relatively little turbulent force acting on the riverbed, sand will tend to deposit. When turbulent forces increase, as during floods, or in steep reaches, sand will tend to erode and be moved downstream. In some rivers, the movement of sand is revealed by the movement of the associated bedforms and by changes in channel topography. Sand moving as bedload grows into sand dunes along the bottom of the channel.
Water and bedload sampling is often used to measure the quantity of sand transported by rivers. This method is usually required in steep mountain streams or at flood stage. Water and bedload sampling are standard methods and are widely used. The fact that sand settles out of the water as rivers slow down near the Delta means that water samples detect less sand. Bedload sampling is designed to detect the sand moving along the bed, but it is expensive because many samples are needed to define transport rates, and it has limited accuracy because transport rates fluctuate so widely.
Where the river is slower and the channel is accessible, the bedforms can be examined directly to measure sand transport. Fortunately, the waters of the lower Sacramento and San Joaquin Rivers are relatively calm. This permits sonar measurements of the bed elevation from moving boats.
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