The water level at Mission Dam Reservoir will be restricted this season due to deficiencies in the dam’s spillway, according to a recent press release.

The Confederated Salish and Kootenai Tribes (CSKT) Safety of Dams Program, in collaboration with the Bureau of Indian Affairs Branch of Safety of Dams and the Flathead Indian Irrigation Project (FIIP), have implemented a precautionary management measure at the reservoir to ensure continued public safety and responsible water operations.

“There is no immediate danger to the public,” said Trevor Bras, CSKT Safety of Dams Program Manager. “This is a proactive step to ensure the long-term safety and reliability of the facility while continuing to meet water needs for the community and surrounding resources.”

Following a thorough engineering review, dam safety professionals recommended a temporary fill restriction due to deficiencies they identified in the dam’s spillway. The restriction is designed to maintain reservoir levels below the point where the spillway would be activated, while still allowing for effective water storage and use throughout the year.

Under the updated guidance:

• During the spring runoff period (April 1 through June 20), reservoir levels will be maintained at around 3.2 feet below the crest of the spillway, with an additional restriction of around 6.2 feet in wetter years.

• During the remainder of the year, reservoir levels will be allowed to rise closer to full, while still maintaining a safety buffer.

Bras said the changes may impact irrigation operations from May-June since FIIP personnel won’t be able to store as much water as they typically would. They also disperse water from the reservoir during those months for irrigation deliveries, to meet in-stream flows, and help fill other reservoirs in the irrigation system.

Mission Reservoir typically discharges around 37,000 acre-feet of water annually, feeding Mission Creek, Post Creek, Crow Creek and Pablo Reservoir, among other outputs.

Prior to the fill restriction, Mission Reservoir could contain 7,072 acre-feet at full pool. Now, the level would be 6,188 acre-feet during dry-to-normal years and 5,390 acre-feet in wet years.

Still, Bras said, “the expectation is that this fill restriction will not impact irrigation releases in a significant way,” since the fill restriction will be significantly reduced to 1.2 feet when inflows from snowmelt begin to decrease, typically by June 21.

Bras said the restriction will be evaluated every season, or on an as-needed basis by a working group consisting of the CSKT Safety of Dams Program, BIA Branch of Safety of Dams, and FIIP. He noted that this approach reflects standard, risk-informed dam safety practices and allows operators to adjust for changing conditions such as snowpack, temperature and precipitation forecasts.

As to when the restriction might be lifted, Bras said it is apt to remain in place until a “rehabilitation and betterment project” takes place on the spillway that sufficiently reduces the risk. No timeline was available for that project.

According to the press release, CSKT will continue to monitor conditions closely and make adjustments as needed, and share updates as more information becomes available.