Assessment of Sediment Quality and Volume behind Enloe Dam

Science Center Objects

The Issue:

The Enloe Dam, a 100-yr old structure on the Similkameen River in Washington State, has not produced hydropower since 1958 and options are being considered for potential removal of the dam. The concentrations of sediment contaminants (primarily trace elements from a long history of mining and smelting operations in the US and Canada) trapped by Enloe Dam are unknown and may be high enough to require special handling of the sediment if the dam were removed. In 1972, the volume of sediment trapped behind the dam was estimated at 1.79 million cubic yards but the present-day volume of sediments is unknown.

How USGS will help:

The USGS will provide unbiased and up-to-date information on the concentrations of contaminant trace elements such as arsenic and mercury and precious metals such as gold and silver in bed sediments stored in the 1.6-mile long impoundment behind Enloe Dam on the Similkameen River in Okanogan County, WA. USGS will also generate an updated estimate of the volume of sediments stored behind the dam.

Enloe sediment boat

Tripod mounted on pontoons used for collecting sediment cores

(Public domain.)

The Problem:

A variety of options are being considered by resource managers and stakeholders to determine the future of the Enloe Dam on the Similkameen River in Washington State. In 1972, USGS originally investigated the probable effects on the Similkameen and Okanogan Rivers of the evacuation, transport, and deposition of sediment now deposited behind Enloe Dam if the dam were removed. Critical unanswered questions are whether or not contaminant concentrations (primarily trace elements from past mining and smelting operations) of the sediment trapped by Enloe Dam are high enough to trigger special handling of the sediment, and what is the total volume of sediment that may need handling. More information on the chemistry—trace element and precious metal concentrations—and volume of sediment trapped behind the dam is needed to evaluate possible removal strategies with consideration of risk to downstream water and sediment quality, and channel changes and flooding in the Similkameen and Okanogan Rivers

Objectives:

The objectives of this study are  to provide additional, recent data on trace element and precious metal concentrations in sediments, at the river-bed surface and at depth, stored in the 1.6-mile long impoundment behind Enloe Dam on the Similkameen River in Okanogan County, WA, and to provide an updated estimate of the volume of sediments stored behind the dam.

Relevance and Benefits:

This study will directly provide the Priest Rapids Coordinating Committee Habitat Subcommittee and other stakeholders with current data on chemical concentrations and volume of bed sediments stored in the 1.6-mile long impoundment behind the dam. Science to support safe and effective dam removal and habitat restoration is highlighted in the USGS Washington Water Science Center Strategic Science Plan for 2018-2023. This study also addresses several USGS Mission Area science priorities including those for Ecosystems (habitat restoration for threatened and endangered species), Environmental Health (contaminant sources and environmental transport) and Water Resources (understanding of human interactions with water availability). The study directly addresses Goal 1 in the DOI Strategic Plan For Fiscal Years 2018 – 2022 “Utilize science in land, water, species and habitat management supporting decisions and activities.”

Enloe sediment scientist

Lakebed surface sediment collected with a ponar grab sampler

(Public domain.)

Approach:

The proposed approach for characterizing sediment chemistry is to sample and analyze sediments at an intensity that will refine estimates of the magnitude and distribution of trace-element contaminant levels in bed sediments in the Enloe dam impoundment. This will include collecting sediment cores along the length of the reservoir to characterize the vertical distribution in trace-element concentrations throughout the entire thickness of post-dam sediments, and collecting surface-sediment samples along the length of the reservoir to characterize the small and large-scale spatial variation in trace-element concentrations in surface sediments.

The proposed approach for estimating sediment volume is to survey the reservoir bathymetry (elevation of the top of the bed sediments) and the pre-dam sediment interface (elevation of the bottom of the bed sediments) and generate digital-elevation models (DEMs) of each surface. The bathymetry survey will use an acoustic Doppler current profiler (ADCP) or echosounder and the sub-bottom profile survey will use geophysical methods (candidate methods include Continuous Seismic-Reflection Profiling (CSP), Ground Penetrating Radar (GPR), or  Continuous Resistivity Profiling) to measure depth to pre-dam sediment and/ or bedrock at selected river cross sections and/or profiles. Differencing of DEMs in ArcGIS will provide the basis for determining the sediment volume in the reservoir behind Enloe Dam.