The Remedial Investigation/Feasibility Study conducted by the U.S. Environmental Protection Agency within the Spokane River Basin of northern Idaho and eastern Washington included extensive data-collection activities to determine the nature and extent of trace-element contamination within the basin. As part of the investigation, the U.S. Geological Survey designed and implemented a sampling program to assess sediment transport in the Coeur d’Alene River Basin. Suspended and bedload sediments were sampled at four stations at or near base flow and at eight stations during low, moderate, and high discharge conditions between February 1999 and April 2000.
The concentrations and loads of suspended and bedload sediment at all stations were directly related to stream discharge. To quantify these relationships, a power function was used to develop sediment-transport curves at all stations. Although the transport curves for most of the stations indicate a good log-log relationship between stream discharge and suspended- and bedload-sediment discharge, there was a fair amount of scatter about the best-fit regression at most stations. For suspended-sediment discharge, the scatter can be primarily attributed to a hysteresis effect in the concentration of suspended sediment as stream discharge rises and falls. The effects of hysteresis on bedload-sediment discharge were difficult to assess because of a paucity of samples collected over the stream hydrograph.
At most of the stations, and at the stream discharges sampled, the transport characteristics for fine- and sand-sized suspended sediment were similar. However, at the two main-stem Coeur d’Alene River stations, Rose Lake and Harrison, the suspended-sediment load was primarily composed of fine-grained sediment at stream discharges of less than 15,000 cubic feet per second. These two stations are characterized by relatively slow water velocities, which appear to be insufficient to transport sand-sized sediment at lower stream discharge.
At most of the stations, and at the stream discharges sampled, the bedload was primarily composed of material greater than 2 millimeters in diameter, the break between sand and gravel. A predominance of sand-sized bedload was noted at only a few stations, and generally only during low stream discharge. The particle-size distribution of bedload sediment at most stations became proportionately coarser as stream discharge increased. During the peak of snowmelt runoff for water years 1999 and 2000, gravel-sized material between 2 and 64 millimeters in diameter comprised more than 70 percent of the bedload transport at most stations. However, at the station on the Coeur d’Alene River at Rose Lake, the bedload was predominantly composed of fine-grained material of less than 1 millimeter in diameter for all measured stream discharges. The slow water velocities at Rose Lake accounted for the predominance of fine-grained sediment transport.