To help decide on the future of Enloe Dam, technicians are doing a comprehensive analysis of sediment and the riverbed as part of a feasability study.
Updated information could affect prospects for dam’s removal
New methodology to analyze the Similkameen River bed suggests the amount of sediment behind Enloe Dam could be just a fraction of what’s previously been estimated.
The quantity of sediment — and extent of any contamination — are crucial elements in an assessment of the feasibility of removing the dormant hydropower dam owned by the Okanogan County Public Utility District (PUD).
What’s in the sediment — and how to deal with it safely — is an essential aspect of the decision about Enloe’s future. Allowing sediment to flow downstream could be harmful if the sediment contains contaminants from historic mining activity, or if the quantity is so great that it would affect aquatic life in the Okanogan basin, according to biologists who’ve studied the river. The primary concern about sediment is arsenic contamination.
PUD Director of Regulatory Affairs Jeri Timm provided an update on a feasibility study about the future of Enloe dam, including the potential for dam removal, to the PUD board of commissioners at their meeting last week. The commissioners asked Timm about the findings that dramatically reduced the amount of sediment.
Preliminary research by Stantec Consulting Services and their technical team suggests that the amount of sediment is 560,000 cubic yards — not 2.92 million, as estimated in a previous study by the U.S. Geological Survey (USGS), Trout Unlimited National Restoration Director Warren Colyer told the Methow Valley News last week.
That vast difference depends on whether the Similkameen River flows directly on bedrock or whether natural alluvial deposits such as sand, cobble and rock have accumulated on top of the bedrock, Colyer said.
That layer of sand and cobble had previously been believed to be sediment that collected after the dam was constructed, but the new methodology suggests it could be natural alluvial deposits. If the dam is removed, the goal would be to return the Similkameen to its pre-Enloe state, Colyer said.
The feasibility study will evaluate at least three alternatives — leaving the dam in place, letting all sediment flow down the Similkameen River, or removing the sediment for safe disposal at an off-site facility or on the banks of the river, Colyer said. The study may also consider a hybrid alternative, where a portion of the sediment could be allowed to flow downstream. The quantity and composition of sediment would have a major impact on the cost of dam removal.
Trout Unlimited and the Confederated Tribes of the Colville Reservation received a $2.31-million grant for the study from the National Oceanic and Atmospheric Administration (NOAA) last year and are jointly managing the analysis.
Enloe Dam hasn’t produced power in 65 years. Restoration of the Similkameen River could allow access to more than 300 miles of habitat for threatened and endangered steelhead and other native salmonids, according to biologists who’ve researched the watershed.
Risk assessment
Trout Unlimited and the Colville Tribes have agreed to a wider role for Richard Roos-Collins, a principal with the Water and Power Law Group PC who produced a detailed proposal for the PUD in 2022 that led to the commissioners’ support for a feasibility study. Roos-Collins is an attorney with considerable experience in dam removal, including of the Klamath River Dam in Oregon.
Roos-Collins is working on a comprehensive liability-management plan, and it’s critical he be involved long-term in planning for the dam, Timm told the commissioners. Although he has been involved in evaluating risk and liability for the PUD, the PUD also wanted him to review the technical assessment from a risk perspective, Colyer said.
One goal of the feasibility study is to identify an entity that could assume responsibility for dam removal if that’s the study recommendation, Timm said. The most likely scenario would be a group of nongovernmental organizations rather than a state or governmental agency, she said.
The feasibility study includes regular consultation with the Traditional Ecological Knowledge group, which includes representatives from the Colville Tribes and the Upper and Lower Similkameen Indian bands to incorporate traditional ecological knowledge and indigenous perspectives in the decision-making process.
‘Aggressive timeline’
The feasibility study is on an aggressive timeline to comply with terms of the NOAA grant. They aim to complete all technical assessments this fall so consultants can recommend a preferred alternative for the future of the dam — and then draft the first 30% of a corresponding design plan by February, Colyer said. Although it’s an ambitious timeline and “everything has to go just right,” the project is on pace and on budget, he said.
But PUD staff believe that schedule is probably unrealistic, primarily because of the unresolved sediment issues, Timm said. The outcome of the feasibility study hinges on completion of the sediment analysis and will remain uncertain until all the sediment issues are addressed, she said.
Neither the PUD nor a prospective dam-removal entity would agree to removal without complete information, Timm told the commissioners.
The technical advisors and all federal and state stakeholder agencies will meet in April, with a follow-up meeting at the dam in May, Colyer said.
More funding
The study team has received an additional $400,000 grant for hydraulic monitoring from the Habitat Conservation Plan Tributary Committees’ General Salmon Habitat Program of the Wells, Rocky Reach and Rock Island dams in the Columbia Basin. The grant will allow them to map the bottom of the reservoir behind the dam to understand the flow of the river before the dam was built so that the river can be restored to its natural condition, Colyer said.
The team has also applied for a $4.8-million grant from the U.S. Fish & Wildlife Service. That grant would allow them to do additional studies as part of the feasibility assessment and additional design work if the study leads to a recommendation for dam removal. A decision on the grant is expected in April.
The fate of Enloe has been debated for decades. The PUD had been pursuing re-energization, but in 2018 the commissioners determined that producing electricity at Enloe wasn’t economically viable. The PUD commissioners adopted a resolution supporting a comprehensive feasibility study in 2022 after reviewing the Water and Power Law Group memo. Their requirements include finding an agency to handle dam removal that would release the PUD from all liability, as well as a source of funding to cover all costs of removal.
Detailed information about the dam and the feasibility assessment are at www.enloefeasibilityassessment.com. The PUD has considerable history and information about Enloe on its website at www.okanoganpud.org/environmental/enloe-dam-project.
