Geology topics

Oscar A. Wilkerson

Publications and source records attributed to Oscar A. Wilkerson.

2 recordsLinked to original sources

USGS2026

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 (Mm 3 ) compared to 1.37 Mm 3 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.

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USGS2025

Divergent trends in fluvial suspended-sediment concentrations following improved land-use practices, southwest Washington State

Improvements in logging practices since the mid-20th century are widely presumed to have reduced suspended sediment loads in streams across the Pacific Northwest. However, there have been few opportunities to directly assess this, particularly in larger rivers. We compare modern (2019–22) and historical (1960s) suspended sediment monitoring in three large, actively managed watersheds in western Washington with similar land-use histories. In the two watersheds draining the southern Olympic Mountains (Satsop and Wynoochee Rivers), modern sediment yields were around 300 t/km 2 /yr, two to three times lower than historical conditions. Most suspended sediment exiting these watersheds came from rolling terrain mantled by glacial deposits in the lower watersheds, not the steep headwaters. Modern sediment yields in the Chehalis River, draining the low-relief Willapa Hills, were lower (70 t/km 2 /yr), though this represented a 50 % increase relative to historical conditions. SSC-discharge relations in the Chehalis River were steady from 1961 to 1994, indicating this increase happened sometime after 1994. The Chehalis River headwaters were uniquely impacted by landsliding during a 2007 storm, though there is some evidence against that storm as the cause of the recent increase. Ultimately, improved land-use practices appear to have reduced suspended sediment loads in large rivers of the southern Olympic Mountains several-fold, consistent with prior findings in the western Olympic Mountains, primarily due to reduced sediment delivery from the lower watersheds. Countervailing SSC-discharge trends and lower yields in the Chehalis River underscore that background sediment delivery rates and sensitivity to land-use disturbance may vary substantially within a region.

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