Sediment Sampling in the Snake and Clearwater River Basins

Science Center Objects

Are there ways to manage sediment before it accumulates in Lower Granite Reservoir? If so, resource managers must know exactly how much sediment is being transported in the lower Snake and Clearwater River basins, the grain-size distribution of the sediment that is being transported, which subbasins are contributing the most sediment, and how the sediment is being deposited once it is delivered to Lower Granite Reservoir.

Our scientists, working in cooperation with the U.S. Army Corps of Engineers, started a comprehensive monitoring program for suspended and bedload sediment transport in the lower Snake and Clearwater River basins. This was the first effort to quantify sediment transport in these basins since the 1970s, when the USGS and the Corps established a monitoring program before and after the impoundment of Lower Granite Reservoir. This most recent study also evaluated the use of surrogate technologies for estimating suspended sediment concentrations.

  • During water years (October-September) 2009-11, Lower Granite Reservoir received about 10 million tons of sediment from the combined Snake and Clearwater River basins.
  • The Snake River accounted for about 89 percent of total suspended sediment, 90 percent of suspended sand, and 87 percent of suspended silt and clay.
  • The Salmon River alone accounted for 51 percent of suspended sediment, about 56 percent of suspended sand, and about 44 percent of suspended silt and clay.
  • Comparing data from this study to data collected during 1972 through 1979 showed a significant increase in suspended sediment, represented mostly by suspended sand. The increase in suspended sand concentrations is probably attributable to the numerous severe forest fires that have burned large areas of central Idaho.