Lower Granite Reservoir Bed Sediment Coring and Analysis

Science Center Objects

Some groups are concerned that dredging the reservoir and the confluence of the Snake and Clearwater Rivers to remove excess sediment may mobilize contaminants in sediment that could negatively affect endangered species such as steelhead and salmon.

A specially trained and equipped crew from the USGS Texas Water Science Center extracted 69 bed-sediment core samples from Lower Granite Reservoir, the Snake River, and the Clearwater River. From these 69 cores, 105 subsamples were analyzed for sediment grain size. Fifty of the subsamples were also analyzed for major and trace elements.

Surficial-sediment grain-size analyses revealed that:

  • Samples collected from the lower end of the reservoir near the dam, where stream velocities are lower, contained more than 80 percent silt and clay
  • Surfical-sediment subsamples collected near midchannel at the confluence generally had more silt and clay than most surficial-sediment subsamples collected from sites on the Snake and Clearwater Rivers
  • All samples collected from the Snake River contained less than 20 percent silt and clay, and most of the samples collected from the Clearwater River contained less than 40 percent silt and clay
  • Cores collected from the Clearwater River and from the confluence were extracted from a thick sediment layer that may be associated with Clearwater River floods in 1996 and 1997

Major and trace element analyses revealed that:

  • Concentrations of major and trace elements were low in most cases
  • Major and trace element concentrations were lower in samples collected from the Snake River than in those collected from other sites
  • In general, coarser sediment material contained lower concentrations of major and trace elements, whereas finer sediments contained higher concentrations