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NorthWestern plans August repair of Hebgen Dam

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PHOTO FROM ADOBE STOCK.

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.

By Amanda Eggert MONTANA FREE PRESS

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 minutes. Representatives 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 Butcher, who oversees operation and maintenance of dams operated by the company.

“‘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.

Divers, fabricators and engineers will be on hand to complete repairs to the dam in August. Normal dam operations will be affected by the repair, but NorthWestern said it anticipates it will maintain its ability to release water from Hebgen Reservoir into the Madison River, which helps cool the river during the hottest weeks of the year.

Several community members at the presentation questioned the timing of the scheduled repairs, given that August tends to be ecologically challenging for the Madison, a cold-water fishery popular with trout anglers from all over the country. Flows on the Madison are generally quite low by the end of summer, and water temperatures frequently climb into the mid- or high-60s, which can put trout in a state of physiological stress.

The repair won’t preclude NorthWestern from releasing water through the spillway as stream and weather conditions dictate, said Andy Welch, who’s tasked with ensuring that NorthWestern remains in compliance with the terms of its FERC license. Those water releases support NorthWestern’s ability to issue cooling “pulse flows” of water from Madison Dam, which is further downstream in the system.

Asked if dam operators need a certain amount of water in Hebgen Reservoir to make spillway release a viable option, NorthWestern said the spillways become ineffective when the reservoir level drops 9 feet below full pool, but the company’s models indicate that won’t be an issue this year. 

The 2021 dam malfunction rendered NorthWestern Energy out of compliance with a condition in its FERC license that requires the company to maintain flows of at least 600 cubic feet per second at a U.S. Geological Survey gauge located downstream of Earthquake Lake. The license also includes provisions pertaining to maximum flows at gauges along the Madison, Hebgen Reservoir levels, and how much the company can increase or decrease flows in a 24-hour period. 

NorthWestern said it submitted its plan to mitigate the dam malfunction’s impact to FERC in late March, the agency has yet to formally approve or disapprove of the plan, and there are no requirements that the agency respond to the proposal within a specified period of time. FERC has not responded to calls and emails from Montana Free Press seeking an update on the agency’s response to the dam malfunction.

The mitigation proposal includes plans to ramp up fishery monitoring — including NorthWestern’s work with Montana Fish, Wildlife and Parks to conduct an electrofishing survey of the Madison below Hebgen Reservoir — and projects to mitigate streambank erosion and support gravel recruitment, both of which are expected to benefit the river’s fish.

Jon Hanson, NorthWestern’s fisheries biologist, said the company and partner agencies including FWP, the U.S. Fish and Wildlife Service, the Bureau of Land Management and the U.S. Forest Service agree that “from a fisheries standpoint, the event was probably not a catastrophic event.”

“We didn’t lose an entire year-class of fish,” he said, adding that not all of the Madison’s brown trout redds — nests made by spawning fish — were compromised by the dewatering.

He said FWP completed one electrofishing survey between Hebgen Reservoir and Ennis Lake in June, but is still waiting on data from that survey regarding fish sizes and year classes. Other monitoring programs NorthWestern is planning include a fall redd count, juvenile salmonid sampling, and a continuation of long-term population estimates along the Pine Butte section of the Madison.

MTFP’s email to an FWP spokesperson seeking details about the June electrofishing survey was not returned by press time Wednesday afternoon.

Other mitigation measures that NorthWestern has proposed include a fencing project to exclude cattle from streambanks on tributaries of the Madison, which is expected to reduce sedimentation and erosion, and an analysis of other measures aimed at improving spawning habitat and embryo survival on the main stem of the Gallatin River.

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