The feasibility study began in June 2023 and is slated to conclude in February, 2025.

The Similkameen River is located approximately four (4) miles northwest of Oroville, Washington. The headwaters of the Similkameen River begin in Canada and drain the west side of the Canadian Cascade Mountains and the interior Thompson Plateau. Major tributaries from both Canada and the U.S. flow into the Similkameen upstream from its confluence with the Okanogan River just outside of Oroville, WA.

Okanogan PUD ceased operating the dam for power generation in 1958 and in 2018 passed a resolution to cease to pursue electrification on the dam because of complexity, risk, and economically unviable. In 2019, FERC terminated the hydropower license. In 2022, the PUD voted to support a process (feasibility study) that objectively evaluates the removal of Enloe Dam.

The feasibility study is currently being funded by NOAA, Resources Legacy Fund and Chelan PUD Habitat Conservation Plan Tributary Funds. The Washington State Legislature previously funded a 2022 report which provided background reports on community engagement processes for engaging the public in the feasibility study, summarized the current status of data related to sediment behind the dam, a dam removal roadmap and laid out options for selecting an entity to remove the dam. One of the components of the feasibility study is to provide the feasibility of a funding package for a potential dam removal project.

The range of dam removal alternatives is one of the components being studied as part of the feasibility study and assessment. Alternatives will range from a no-action alternative to actual dam removal. Additional alternatives will be modelled as information is developed and studied.

The cost for removing Enloe Dam is currently unknown. Once the preferred alternative is determined, cost estimates will be generated for additional design, permitting and project implementation.

Fill out the Contact Form so we can contact you with updates. Public meetings are being scheduled at key milestones in the development of the feasibility study and your participation is welcome.

In the Okanogan River Basin, steelhead are federally listed as “threatened” and spring Chinook salmon are extirpated or locally extinct. The recovery goal for steelhead in the Okanogan River is 500 natural-origin adults. Over the past 5 years the number of adult returns were 50% of the recovery goal or less.

Because the Okanogan River exceeds the lethal temperature for steelhead, steelhead are relegated to complete their freshwater life history in the tributaries. Most of the 114 kilometers of habitat within the tributaries exist at low elevations (< 1,500 ft.). Temperature models, considering climate change, indicate much of the existing habitat will contain water temperature near or exceeding the thermal tolerance for steelhead by 2040.

In 2021-2022, NOAA’s Northwest Fisheries Science Center, LSIB, USIB, Colville Tribe and TU conducted a geomorphic analysis of potential Chinook salmon and steelhead habitat upstream from Enloe Dam. The specific objectives of this work was to (1) estimate rearing capacity for the first summer parr life stage for both Chinook salmon and steelhead, and (2) estimate the spawning habitat capacity for Chinook salmon and steelhead. The habitat assessment resulted in an estimated 1520 miles of habitat available above Enloe Dam .

Over 90% of the Similkameen River Basin is above 1,500 feet elevation. Air temperatures will be cooler at higher elevations and consequently, most of the 2,500 km (about 1553.43 mi) of habitat will be thermally favorable for steelhead and spring Chinook salmon to complete their freshwater life history even amid climate change.

Removing Enloe Dam will create economic opportunity by providing local construction jobs and restored salmon and steelhead runs would bring tourism and recreation dollars into the local community.

Enloe Dam was constructed to supply power to local mining camps, consequently there is concern regarding contaminants contained within the sediment accumulated behind the dam.

In 2019, United States Geological Survey (USGS) conducted a survey of 41 trace elements including lead, mercury and arsenic, in the bed sediment accumulated behind Enloe Dam. Seventy-six subsurface samples from six sediment cores were collected from the lower 1/3 of the impoundment (closer to the dam). In addition, 36 surface samples were collected from 27 locations along 10 transects within the impoundment.

Sediment cores were subsampled at a range of 4.3 to 10.9 m from the sediment-water interface (SWI). Sediment samples were collected from different depths and analyzed as un-sieved or sieved (< 63 microns, silt/clay size fraction). Sieved samples contained the highest concentration of trace elements, because fine sediments have a larger surface area by volume, surface charge, organic content, and sorption capacity. On average, the fine grain particles made up 10.2 percent of the sediment, by mass, in core samples. Trace elements detected in sediment samples were compared at threshold effects concentration (TEC), a level where effects on survival or growth of aquatic organisms are expected to occur only rarely and a probable effect concentration (PEC), where if the element concentration exceeds this level harmful effects on sediment-dwelling organisms are expected to occur frequently. Arsenic, chromium and copper were measured exceeding PEC levels at depth at one core location where small grain size (< 63 microns) were 47% of the total mass of the sample. Arsenic exceeded PEC levels at three other core locations, however chromium and copper did not.

Surface grab samples were below TEC level for all trace elements with the exception of arsenic and no trace element concentrations exceeded PEC.

During the spring of 2020, Washington Department of Ecology (DOE) augmented the effort by USGS to achieve sediment characterization behind Enloe Dam. Sediment core samples were collected at 22 locations within the impoundment. Twenty surface grab samples were collected downstream of Enloe Dam. Washington Department of Ecology incorporates two thresholds, a sediment cleanup objective (SCO) and cleanup screening level (CSL). The SCO establishes the threshold for no adverse effects, above this threshold adverse effects are more likely to occur. Results exceeding the CSL are more likely to exhibit severe adverse biological effects or a greater number of effects. No analyzed materials exceeded the CSL, but arsenic and sulfide exceeded the sediment cleanup objective (SCO) threshold in 20 and 1 samples, respectively.

Thirty-seven reservoir core sediment samples were collected from 22 locations in 2022. The samples were analyzed for grain size, total organic carbon (TOC), ammonia, sulfide, metals, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and pesticides. Sulfide and arsenic were the only constituents detected at concentrations greater than their respective SCOs and CSLs. Six (20 percent) of the locations contained arsenic at concentrations greater than the CSL, with concentrations an order of magnitude greater than the CSL at 3 locations. Similar to the results from the USGS sediment survey, arsenic does not follow a uniform pattern in exceedances throughout the reservoir but exists at elevated concentrations in pockets within depositional areas, primarily in the lower 1/3 of the reservoir.

Finally, leaching tests indicated that arsenic leachate concentrations indicated a low potential for arsenic to leach at concentrations that would influence groundwater quality in the event that reservoir sediments were removed and placed in an upland environment (e.g. containment cell).