New Boards for Thompson Falls

New flashboards will allow Thompson Falls Dam to operate more efficiently.

Until recently, the water in the Thompson Falls Reservoir was held back by boards that were about a half-century old. Overtime, those boards became worn from the water.

“The wooden flashboards on the main dam had been in place for about 50 years,” said NorthWestern Energy Hydro Foreman Noel Jacobson. “The boards were leaking water, which requires a high level of maintenance and frequent repairs.”

In October, our crew at Thompson Falls removed the flashboards and replaced them with all new boards. The project was no small task. There are about 175 boards along the face of the main dam, all 24 feet in length.

Crews remove old flashboards at Thompson Falls Dam.

Many of the boards our crew removed had small holes in them. Others were worn in the spots where they sit against the steel I-beams of the dam. Those small leaks can add up to a lot of lost energy generation. 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 old flashboards at Thompson Falls Dam had become worn and cracked after about 50 years of use.

In order to replace the boards, we had to lower the reservoir elevation by about 16 feet.

“In my 25 years here, I’ve never seen the reservoir this low,” Noel said.

The low water did uncover some rarely seen artifacts in Thompson Falls Reservoir. The most noticeable was an old set of train tracks dating back to 1915 that were used when the dam was under construction. The tracks run along the bank before falling off into what is now the reservoir. There are also two piers that can be seen rising above the low water. Those piers allowed the tracks to reach the dam.

“It’s surprising they didn’t at least remove the tracks,” Noel said.

Crews also found some other items in the river, such as chains, pipes and an old rock foundation.

There were about 175 boards along the face of the main dam that all needed to be replaced.

While the reservoir was drained, NorthWestern Energy conducted a LiDAR flight. LiDAR, which stands for Light Detection and Ranging, will allow us to create a detailed map of the bottom of the reservoir.

“The LiDAR provides a high-resolution snapshot of the entire reservoir shoreline and exposed reservoir bed paired with aerial photography,” said Andy Welch, Manager of Hydro License Compliance at NorthWestern. “This data will enable NorthWestern to evaluate such things as habitat complexity, location of shoreline infrastructure, bank stability, and reservoir bed slope and provides something to compare changes to those characteristics with in the future.”

Once the boards were all replaced, Thompson Falls reservoir returned to its normal level. The new boards will likely be in place for another 50 years.

“Replacing the boards will reduce maintenance and make the dam perform more efficiently,” Noel said.