SCOUR AND DEPOSITION IN LOWER GRANITE RESERVOIR, SNAKE AND CLEARWATER RIVERS NEAR LEWISTON, IDAHO, U.S.A.

Abstract

Calculation of the final levee grades for Lower Granite Reservoir at Lewiston, Idaho, required adjustment to the streambed to account for sediment deposits during the 50-year life of the project. Since the backwater reach of the reservoir behaves like a sediment trap during low flow and re-entrains material during high flow, both deposition and scour influence the final bed profile with the coarser material playing a dominant role. The simulation technique used to distribute sediment deposits (computer program known as “Scour and Deposition in Rivers and Reservoirs”) required data on inflowing sediment load, gradation of the load, gradation of material on the streambed, water-discharge hydrographs, and operating policies for the reservoir. Sediment data were lacking, so a collection program was initiated to satisfy two objectives: (1) to provide sediment-inflow data for the study, and (2) to calibrate the simulation technique. The field data, including bedload-transport rates, provide one of the most comp...