In response to increased snowpack in the Kootenai Bain during the month of March, the U.S. Army Corps of Engineers is discharging full powerhouse capacity, about 25,000 cubic feet per second, at Libby Dam. Corps’ water managers plan to maintain releases at full powerhouse capacity for the remainder of April, or until the start of refill, projected for the first or second week of May.
This action is creating more storage in the Koocanusa Reservoir and will allow the Corps to retain more Kootenai River flow behind Libby Dam during the spring freshet.
Current Koocanusa Reservoir elevation is 2,404 feet and the median projection for the end of month elevation is 2,387 feet, depending upon actual April inflows. The reservoir will continue to be drafted at a rate of roughly one foot per day until the start of reservoir refill, when the dam shifts from emptying the reservoir to create storage space to controlling the refill through the freshet.
The water supply forecast for April through August into Koocanusa Reservoir is 117 percent of normal and snowpack is 115-120 percent of normal in the Kootenai River basin. The water supply forecast increased from 93 percent of normal to 117 percent between the months of March and April. The increase was mainly driven by 250 percent of average precipitation for the month of March that landed mostly as snow in the higher elevations in the Upper Kootenai Basin.
Generally, upper river runoff is regulated by Libby Dam, while lower river runoff is mostly unregulated since it flows into the Kootenai River below the dam. The Corps’ flood risk management operational strategy is to retain as much volume as possible behind Libby Dam while the downstream unregulated tributaries, which are forecasted to be near flood stage, pass as much of their spring snowmelt and rain flows as possible. It is possible that Kootenai River flows may be at or above flood stage from unregulated tributary flows alone.
Current projections show that there is about a 40 percent chance of being over flood stage at Bonners Ferry, elevation 1,764 feet. The potential for being near flood stage is highest late-May through June.
Residents and businesses in the river basin should be prepared for potential flooding once snowpack begins to melt. Should flows approach flood stage, the National Weather Service and downstream communities have plans in place to promptly alert potentially affected people about the situation and what action to take. Citizens are encouraged to contact local emergency managers and work with them to determine the best path to prepare for potential flooding.
The Corps is regulating Libby Dam for flood risk management, closely monitoring weather conditions, snowpack readings and inflows to mitigate flood risk downstream.
A public information meeting for Libby Dam operations is scheduled for May 29 at the Kootenai River Inn, Bonners Ferry, Idaho, at 7 p.m. Public meetings are also being planned for Eureka and Libby during early June.