Potential transport of sediment from Enloe Reservoir by the Similkameen and Okanogan rivers, Washington

Abstract

This study was undertaken to determine the probable effects on the Similkameen and Okanogan Rivers of the removal, transport, and deposition of sediment now deposited behind Enloe Dam on the Similkameen River, if the dam were removed. Under existing conditions of sediment transport, the average annual suspended-sediment discharges at three streamflow-measuring sites are calculated to be 134,000 tons for the Similkameen River near Nighthawk, 175,000 tons for the Okanogan River near Tonasket, and 175,000 tons for the Okanogan River at Malott. The volume of sediment in Enloe Reservoir was computed to be 1.79 million cubic yards (about 2.4 million tons). The sediment is largely sand. If the dam were removed the maximum amount of reservoir sand transportable (in addition to normal sediment discharge), under stated conditions of velocity and depth during any average 10-year period, would range from about 17,000 to 580,000 cubic yards per year, with about 320,000 cubic yards being transported during a year of average water discharge. These volumes represent a range of 32 to less than 1 percent of the volume of sediment now deposited in the reservoir. The reservoir deposits not carried to the mouth of the Okanogan River (at Columbia River) would be deposited largely in a 17-mile reach of the Okanogon River immediately downstream from its confluence with the Similkameen River. Deposition of the reservoir sediment would tend to reduce the capacity of the channel of the Okanogan River in this reach; also there possibly would be some long-tern adjustments of the stream course to the temporary increase in the sediment load. Locally, the reduction in the channel cross section probably would increase the depth of the water and area of inundation during floods. Deposition of sediment in the Okanogan River channel and accompanying undesirable effects would be less severe if the dam were removed in segments over a period of several years. If the dam is removed in segments the amount of sediment transported by the river could be monitored to evaluate (1) the timing of the removal of the remaining segments of the dam, based on actual sediment-transport data, (2) the amount of sediment rremaining in the reservoit, and (3) the impact of sediment deposition on channel configuration and flooding.