What could happen to Outlet Bay?
Outlet Bay is the start of Priest River, not Priest Lake as most probably think. There is a natural bar at the entrance to Outlet Bay which would have limited flow before the Priest River Dam was constructed in 1950. An Idaho State statute now requires the lake level to be maintained at a minimum of 3.0 feet throughout the recreational period. The bay is approximately 2/3 of a mile long. Lamb Creek flows into the bay just before the dam.
In the summer of 2021 Priest Lake experienced a public health advisory for HAB (hazardous algae bloom). The advisory was a first for Priest Lake, and it occurred in the exact area that would be impacted by the siphon which will reduce the water volume and flow rates by 75%. Historically, warm surface water continually flows out the dam. With the bypass, the water in the Outlet Bay/channel (being the only outlet for the entire Priest Lake drainage) will become more stagnant, become warmer, and further exacerbate the occurrences of HAB.
Factors contributing to HAB are complex, but warm, slow-moving, stagnant water certainly contributes to its development. HAB can cause poisoning and death to animals and humans as well.
In discussing this issue with IDFG, IDFG relied on a study in 2020 that concluded that an unidentified source of cooler water in the outlet may prevent HAB. Obviously, this conclusion was erroneous based upon the 2021 incident.