Estimation of impounded sediment volume in the Similkameen River upstream of Enloe Dam, Okanogan County, Washington

The Enloe Dam was built in 1920 for hydropower generation and impounds a steep-banked, narrow reach of the Similkameen River in north-central Washington. During the subsequent century, sediment from the Similkameen River watershed, which includes historical mining operations, accumulated within Enloe Dam’s impoundment. Enloe Dam ceased hydropower production in 1958 and is currently under consideration for removal, but there are concerns that the remobilization of sediments may have harmful impacts on downstream water quality. To complement previously published analyses of heavy metal concentrations within sediments and assess the total volume that may be transported following dam removal, this report presents estimates of the volume of sediment trapped behind Enloe Dam which was measured in 2020. The volume of sediment was estimated by comparing a bathymetric survey, collected using an acoustic Doppler continuous profiler, and a survey of the bedrock-sediment interface, collected using a continuous resistivity profiler.

The study area spanned a 2.6-kilometer reach of the Similkameen River upstream from Enloe Dam. The volume of impounded sediment was calculated in 2020 by subtracting the elevation of the bedrock-sediment interface measured using the continuous resistivity profiler from a bathymetric surface measured by the acoustic Doppler current profiler. In 2020, the estimated volume of sediment impounded by Enloe Dam was 2.17±0.04 million cubic meters (Mm³) compared to 1.37 Mm³ measured in 1971. This equates to a deposition rate of approximately 16,300 cubic meters per year from 1971 to 2020. Continuous resistivity profiles revealed that bedrock within Enloe Dam’s impoundment was deepest (about 20 m) in the southern, downstream end of the profiles and shallowest (less than 5 m) in the northern upstream end of the profiles.