Priest River Coldwater Bypass Concept - FAQs

Frequently Asked Questions about the Priest River coldwater bypass concept.

Below is a list of common questions about the coldwater bypass concept.

Q: What is the coldwater bypass concept and how would it work?

A: The warmest water in Priest River is immediately downstream of the Outlet Dam. This concept involves replacing a portion of the surface outflow with water drawn from the much colder depths of Priest Lake. This would reduce water temperatures in the river and create more suitable habitat for coldwater fish, such as westslope cutthroat trout and bull trout.

Q: How far downstream would it cool the Priest River?

A: Experts at Portland State University modeled flows and demonstrated that replacing 50-75% of the surface outflow with cold water would result in suitable water temperatures for trout in roughly 46 miles of river from Outlet Dam to the mouth of the Priest River. This would result in a tremendous amount of enhanced river habitat.

Q: Has something like this been done before?

A: Yes. Nearby examples include Sullivan Lake, WA and Payette Lake, ID. This concept has been successfully implemented in those waters without harming the ecology of either lake. A variation of this concept is commonly used at dams throughout the country to create what are known as tailwater fisheries by pulling water from depth in an upstream reservoir to regulate river temperatures downstream.

Q: Would a coldwater bypass change the lake level or flows in the river?

A: No. This would simply replace surface flow with colder water from depth, resulting in the same net outflow from Priest Lake. Thus, the lake level in Priest Lake would be unaffected and streamflow would be identical. A project that alters the Priest Lake water level cannot be implemented because of a statutory water right on the lake.

Q: Would a coldwater bypass change the aesthetics of Priest Lake?

A: No. The bypass pipe would be buried through the shallow outlet of the lake and then rest on the lakebed in deeper water. It would not be visible at any lake level, nor would it create noise.

Q: Would water quality in Priest Lake, including the outlet, be negatively affected?

A: No. A limnology and water quality study completed in 2021 concluded that a coldwater bypass would not negatively affect water quality or limnology of the lake. This included an assessment of conditions contributing to blue-green algae blooms in the outlet, which would not be worsened by a coldwater bypass. In fact, the study suggested that conditions may actually be less favorable for blue-green algae with a coldwater bypass in operation.

Q: I commonly hear this referred to as a siphon, so is that how this would be engineered?

A: No. An engineering study evaluated multiple methods for moving cold water from depth in Priest Lake to just below the Outlet Dam. A siphon was evaluated, but the preferred alternative was a gravity-feed system that relies on head pressure from the lake to move the water through a pipe to just below the dam.

Q: How much would this cost to construct and how would it be funded?

A: An engineered design and cost estimate have not been completed. However, it is expected that construction costs could exceed $10 million based on a preliminary feasibility study. Funding has not been pursued since a project has yet to be proposed, but mitigation funds and grants would be the likely source for construction and operation. While expensive to build, the relative cost is lower than most habitat restoration and enhancement projects because about 46 river miles would be improved. It is rare for these types of projects to affect habitat at such a large scale.

Q: What if a coldwater bypass was constructed and caused an unexpected problem?

A: The preferred design would be engineered with the added safety of a shutoff valve. Additionally, the entire system would not be operable at winter lake levels. The bypass would only be operated when colder water is needed downstream. When the bypass is not operating, all outflow would come from the lake’s surface like it does now.

Q: What are the expected benefits of a coldwater bypass?

A: Native westslope cutthroat trout, native bull trout (ESA listed as “Threatened”) and other salmonids would be expected to substantially increase in abundance, thus providing a conservation benefit. Increased trout abundance would support a far more popular recreational fishery, attracting anglers from the region and beyond. This additional angler effort would provide a benefit to the local economy.

Q: Are there plans to construct a coldwater bypass?

A: Not at this time. Idaho Fish and Game and the Kalispel Tribe have been evaluating the feasibility of the coldwater bypass for nearly a decade. Studies indicate that the concept would provide the intended benefits without causing harm to Priest Lake. However, public sentiment towards the idea has been mixed. As a result, no project has been proposed. The current focus for Idaho Fish and Game is on public outreach and engagement, including participation in the Priest River Watershed Group that is being formed by Trout Unlimited.