Last year’s dam malfunction was “probably not a catastrophic event” for the fishery, the company said at a meeting where it outlined its plan to mitigate impacts to the Madison River.

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WEST YELLOWSTONE —  At a Tuesday event, NorthWestern Energy outlined its plan to prevent future Hebgen Dam malfunctions like the one that dewatered the Madison River late last year, cutting flow to one of the state’s flagship trout streams in half in a matter of minutesRepresentatives from the company, which operates the 108-year-old dam under a hydropower license administered by the Federal Energy Regulatory Commission, also presented a plan to monitor the Nov. 30 event’s impact on the Madison River’s fishery, which is a cornerstone of Madison County’s economy.

Approximately 20 people attended the presentation, which followed a format similar to an informational meeting NorthWestern hosted in Ennis in April.

In addition to reviewing the event that led to the dam malfunction — the failure of a coupling on the dam’s gate stem that broke, freezing the outflow gate in a nearly closed position and restricting the amount of water exiting Hebgen Reservoir for a 46-hour period — the state’s largest energy utility discussed its plan to replace other couplings that perform a similar function in August.

A third-party analysis of the failed coupling found that the alloy used to fabricate the part reacted poorly with the “chemical composition of the environment” it was located in, according to NorthWestern’s hydro superintendent Jeremy Butcherwho oversees operation and maintenance of dams operated by the company.

This NorthWestern Energy diagram demonstrates the relationship between Hebgen Reservoir, Hebgen Dam, the Madison River and downstream lakes like Earthquake Lake and Ennis Lake.

“‘Stress corrosion cracking’ was the term that was used,” Butcher said, adding that all four such couplings that allow dam operators to raise and lower Hebgen Dam’s outflow gate are slated for replacement during a roughly three-week period in August. 

In addition to the new coupling fabricated by an Anaconda company in the hours after the dam malfunction, which is working as anticipated, operators have a temporary back-up installed. Cables threaded through two 20-ton chain hoists allow operators to raise and lower the outflow gate in the event of another coupling failure, he said.

Butcher also said the company has installed an alarm system that will alert operators of sudden decreases in flow registered by stream gauges on the Madison River.

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