• Helena Dore Bozeman Daily Chronicle
  • 1 hr ago
Volunteers search a channel of the upper Madison River for fish stranded by drastically lowered water levels below Hebgen Dam on Wednesday, Dec. 1, 2021. A malfunction at Hebgen Dam caused flows in the upper Madison River to drop suddenly, leaving parts of the river dry and prompting state officials to order a full fishing closure for the upper river.Samuel Wilson, Bozeman Chronicle/Report for America

Fish experts said at a meeting on Wednesday that juvenile fish likely died at the highest rates, compared with adult fish and fish eggs, after a malfunction at Hebgen Dam triggered an abrupt drop in flows along the upper Madison River in the fall of 2021.

Based on the initial results from electrofishing surveys and a literature review, biologists inferred that because the dewatering of the upper Madison River on Nov. 30, 2021 was a one-time event, it likely didn’t do catastrophic damage to the fishery.

Water leaves Hebgen Dam and flows out of a culvert and into the upper Madison River on Dec. 15, 2022. In late November 2021, a malfunction at the dam caused flows to drop suddenly, leaving parts of the river dry.Rachel Leathe, Bozeman Chronicle

However, they believe juvenile fish probably suffered the greatest impacts, since they are smaller, less mobile and tend to congregate in the side channels that dried out fastest, according to Montana Fish, Wildlife and Parks Fisheries Technician Jenna Dukovcic.

Biologists presented their initial conclusions about how the Hebgen Dam malfunction likely impacted fish survival in the upper Madison River during a public meeting hosted by NorthWestern Energy near Ennis on Wednesday evening.

“When the dam gate failed, it was kind of everybody’s ‘Oh crap’ moment. It was kind of the perfect storm,” said FWP Conservation Technician Travis Lohrenz. “We had brown trout eggs in the gravel. We had stranding of juvenile fish because the dewatering was so precipitous.”

It will take years of monitoring to determine the extent of the event’s impacts on the fishery, but so far, surveys in 2022 showed that about 92% of brown trout redds were located within side channel habitats, according to Lorhenz.

The mountains and hills surrounding the upper Madison River were covered in snow on Dec. 15.Rachel Leathe, Bozeman Chronicle

That could have been a problem, but the surveys of the side channels also showed that young-of-year brown trout were still present, which suggests that the dam malfunction didn’t result in a complete, catastrophic loss of the cohort — the group of fish that share the same age, he said.

“As you move further down the age continuum to these juvenile fish, the big result was that these fish were still here, occupying these habitats that were dewatered,” Lorhenz said. “They were pushed down, then they moved back in.”

Dukovcic pointed out that the dam malfunction occurred in late November, at the end of the spawning season for brown trout and mountain whitefish. At that time of the year, fish eggs have just been laid in the gravel, and they’re on track to incubate throughout the winter.

Based on a review of the literature, biologists learned that salmonid eggs “tend to be fairly tolerant to dewatering, especially when they are early in their development,” Dukovcic said. At that early stage, the eggs can still diffuse oxygen through the air and moisture in the gravel.

Volunteers and employees of Montana, Fish Wildlife and Parks use electrofishing tools and nets to collect trout and other fish stranded in a channel of the upper Madison River below Hebgen Dam on Dec. 1, 2021.Samuel Wilson, Bozeman Chronicle/Report for America

Because the redds were in the early stages of their development, because the gravel stayed moist and because temperatures didn’t drop below freezing overnight, they suspect the impact of the dam malfunction on fish eggs was relatively low.

The experts also concluded that the impacts of the event on adult fish are also relatively low, since adult fish typically occupy deeper habitats and are more mobile, meaning they can better respond to a sudden drop in flows.

Lohrenz emphasized that while the first year of electrofishing surveys gave biologists a sense of the relative abundance of fish between Ennis and Hebgen lakes, they still don’t know what the numbers were prior to the dam failure, and “we can’t hang our hat on one year of data.”

That’s why they’re planning to conduct long-term monitoring through 2025, where they’ll track the cohorts that were affected by the dam malfunction as they age. They’ll compare the numbers they have now with numbers they mark down in subsequent years.

Two years ago in late November, a bronze coupler that was attached to a gate stem on Hebgen Dam broke overnight, causing the dam’s gate to fall. That limited flows in the upper Madison River. The effects were more abrupt further upstream.

Brown trout redds — the gravel where female fish lay their eggs — were exposed to the elements. Locals rushed to save fish that were stranded within side channels.

NorthWestern Energy controls flows from Hebgen Dam, and the Federal Energy Regulation Commission (FERC) later found that the utility violated two requirements from its operating license when the gate stem broke.

The utility has since agreed to fund monitoring of fish populations in the river, and it is compiling a report to identify mitigation projects that will improve fish embryo survival, gravel recruitment and spawning habitat.

Helena Dore can be reached at 406-582-2628 or hdore@dailychronicle.com.