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By Helena Dore, The Bozeman Daily Chronicle

Hebgen Dam

NorthWestern Energy plans to study the impacts of last year’s dam malfunction along the upper Madison River on fish populations and enhance spawning habitat along the river and its tributaries.

NorthWestern Energy wrote about the plans in a March 23 letter submitted to Federal Regulatory Energy Commission Secretary Kimberly Bose. FERC has 60 days to file a response. The Billings Gazette first reported on the letter.

According to the letter, an advisory group of biologists and staff from NorthWestern Energy discussed a monitoring plan for the upper river.

In the early morning hours of Nov. 30, a broken metal coupler on a gate stem at Hebgen Dam, controlled by NorthWestern Energy, caused flows below it to drop from 648 to 278 cubic feet per second in 15 minutes.

The sudden loss of flows stranded fish in side channels along the Upper Madison River, and it exposed brown trout redds — the spawning beds where the fish lay their eggs — to the elements.

Volunteers rushed to move fish back into the river, and the company was ultimately able to fix the broken component and restore flows on Dec. 1. Biologists didn’t know exactly how the event would impact the fishery.

According to the letter filed with FERC on March 23, resource biologists discussed several conclusions they reached at the Jan. 20 advisory group meeting.

The biologists determined that the dewatering event did have an effect on the fishery and the aquatic health of the Madison River, and those effects were “likely largest for embryos and young-of-the-year salmonids.”

Such impacts “may be evident for the next four years but not fully realized until 2025,” they concluded, and “traditional sampling is inadequate to assess or quantify” them. The effects likely aren’t “catastrophic, such as losing an entire year class” of fish, the letter says.

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