Reservoir drawdown planned for dam work


September 7, 2023

On October 6, NorthWestern Energy will begin drawing down the reservoir at the Thompson Falls Hydro Development about 16 feet from the current full-pool level to replace wooden flash boards along the bottom 8.5 feet on the face of the main dam.

“The wooden flash boards on the main dam have been in place for about 50 years,” said NorthWestern Energy Hydro Foreman Noel Jacobson. “The boards are leaking water which requires a high level of maintenance and frequent repairs. Replacing the boards will reduce maintenance and make the dam perform more efficiently.”

A 250-cfs (cubic feet per second) water leak through the dam, about 1,870 gallons per second, equals almost a megawatt of power, enough energy for about 650 homes a year.

The flashboards, located on the lower 8.5 feet of the main dam’s upstream face in front of steel I-beams, are used to seal water in the dam’s reservoir. Eight-foot drop panels on the upper 8 feet of the main dam control the release of the water through and across the dam to create conditions for maximum energy generation.

The reservoir water drawdown will begin October 6 and work will begin October 11. The construction project will involve removing the eight-foot drop panels to access the old boards and then replacing them with new ones. The reservoir will be back to full pool by October 26.

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