Recommendations are due in October
Removal of Enloe Dam is feasible, although formal recommendations about the future of the dam on the Similkameen River near Oroville hinge on considerable details and planning that are still underway.
The Okanogan County Public Utility District (PUD), which owns the dam, received an update and administrative draft of the feasibility study at the end of June from Trout Unlimited and the Confederated Tribes of the Colville Reservation (TU/CTCR), who are co-managing the study and reviewing data. But the process requires substantial research and review before the TU/CTCR team is ready to release a final version of the study, TU North Central Washington Project Manager Michael Ward said.
The PUD has received regular updates, and this draft contained appreciably more data, particularly regarding sediment collected behind the dam, PUD Commissioner Lauren McCloy said.
Advanced technology found significantly less sediment behind the dam than previously thought. New research also provided specifics about what’s in the sediment and how much there is, along with an enhanced picture of the riverbed, McCloy said. This was primarily an update for PUD board and staff and doesn’t include adequate information for a decision on the dam — nor was it intended to, she said.
Among the greatest concerns surrounding dam removal is the presence of arsenic in the sediment. The new research provides additional information about the arsenic concentration, McCloy said.
Because the sediment is essentially held in place by the dam, researchers have been evaluating various approaches, including letting it all flow downriver, removing all of it to an off-site location, or removing only the most hazardous materials and allowing the rest to be carried downstream, according to the presentation TU/CTCR made to the PUD board.
“The feasibility study is ‘complete’ only in the sense that is has been assembled, edited, and it has all the relevant content, but it is not ‘final’ at this point,” Ward told the Methow Valley News. In July, the consulting team was incorporating comments from the PUD and other reviewers to prepare a technical draft for review by the technical advisory committee in August. The technical feedback will be included in the feasibility study, which is expected in early October, Ward said.
Sediment findings
The preliminary study recommendation is to release all the sediment into the Similkameen River.
The latest research has reduced the total amount of sediment from between 2.5 and 3 million cubic yards to 560,000 cubic yards, according to TU/CTCR. Scientists also analyzed the size of the sediment particles (silt, sand and gravel) and evaluated toxicity levels. The vast majority (90%) of sediment is sand.
Among the greatest concerns for human and ecological health is the presence of arsenic in the sediment. Most of the arsenic is adhered to silt, which constitutes just 3% of the sediment and is concentrated in the reservoir behind the dam. The latest research found less arsenic than previous studies, according to the presentation.
The arsenic doesn’t present significant toxicity to river organisms that fish eat, and much of the arsenic is stable and not available to humans, according to preliminary findings. The most toxic form of arsenic is buried and would transform to a less toxic form when exposed to oxygen after dam removal, according to the study.
The study evaluates impacts from deposition of sediment over time, including to irrigation systems, looks at potential flooding, and sets out mitigations.
Recommendations to come
The final study will include a recommended alternative and three other alternatives, including “no action” (leaving the dam in place). One of the primary distinctions between the alternatives is how much sediment to release to the river or to remove.
The study will include an initial detailed design at the 30% level, with later modifications to produce 60% and final designs, Ward said. The 30% design is expected to be completed early next year.
Actual removal of the dam and associated structures would be similar in all action alternatives. Costs of alternatives are expected to vary widely based on the amount of sediment to be taken off-site, with estimates ranging from $4.6 million to $38.6 million.
Advocates for dam removal say restoration of the Similkameen River could allow threatened and endangered steelhead and other native salmonids to reach additional habitat, much of it in British Columbia. Recent research found more than 1,520 miles in additional habitat could be opened up by dam removal, Ward said.
The PUD is evaluating multiple scenarios, including how fish would pass through the area if the dam were removed and the suitability of habitat for fish above the dam. The Canadian government would need to allow new fish populations to cross the border, the PUD said.
The PUD acquired Enloe Dam by eminent domain in 1942. The district stopped generating hydropower at the dam in 1958 and later spent several decades developing plans to re-energize it. The district abandoned that goal in 2018 when electrification proved financially unfeasible. The PUD has continued to conduct regular safety inspections and maintenance.
The PUD board passed a resolution in 2022 supporting evaluation of the potential for dam removal. TU and CTCR received more than $7.2 million in federal and other grants to conduct the feasibility study.
If dam removal were to proceed, the parties would need to find an entity other than the PUD to do the removal and guarantee complete indemnity for the PUD.
People can learn more about the study at https://www.enloefeasibilityassessment.com and https://www.okanoganpud.org/about-your-pud/environmental-generation/enloe-dam.
