Fish in Montana's Flathead River below Hungry Horse Dam have had the best conditions possible in recent years as the dam has been operated to mimic natural flows, according to a comprehensive study recently published in a prestigious science journal.
And conversely, previous dam operations that caused erratic river flows were harmful to threatened bull trout and other species, concludes the study published in the River Research and Applications Journal.
“Our results suggest that past flow management policies that created sporadic streamflow fluctuations were likely detrimental to resident salmonids and that natural flow management strategies will likely improve the chances” of protecting bull trout, cutthroat trout and other species,” the study states.
That conclusion reinforces the state of Montana’s long-held resistance to demands for late-summer “flow augmentation” releases that were intended to benefit salmon in the lower Columbia River system.
Clint Muhlfeld, a U.S. Geological Survey researcher and the study’s lead author, said the published work is a culmination of a series of research projects.
“This is a comprehensive evaluation of the system over time, and it represents a ton of work,” said Muhlfeld, who is based at Glacier National Park.
A similar study on the effects of Libby Dam on the Kootenai River will be published this year, and “what we are finding are similar patterns between the two basins,” Muhlfeld said.
Brian Marotz, a Montana Fish, Wildlife and Parks biologist and one of the study’s six co-authors, has worked for about 20 years on improving dam operations for Montana fish and says the new study backs up much of that work.
“Before we didn’t have as much detail to rely on,” he said, noting that it was long suspected that higher summer flows were not normal and potentially harmful. “Now we have proof of how it is damaging.”
What’s new about the study is its use of a three-dimensional in-stream flow model that is coordinated with GPS measurements on the river and data from bull trout that were tracked during snorkel surveys and with radio telemetry devices over a period of about six years.
“Now we have an actual, empirically based model of the river showing how the fish habitat and the fish locations relate to flow patterns,” Marotz said.
The fish monitoring efforts were carried out during all seasons and at all hours, revealing that bull trout make use of the deep main channels during the day and move into shallow, slow moving waters near shorelines to feed at night.
“We integrated the biology with the physical and hydrological characteristics” of the river, Muhlfeld said. “Our data basically quantifies the impacts on the environment” when releases from the dam are increased and decreased.
The model showed that shallow shoreline areas and riffle habitat are most impacted by sporadic flow increases.
“Small changes in the flow regime result in substantial changes in those critical habitats,” Muhlfeld said.
Another major component of the study is that historic river data were applied to the in-stream flow model, showing the impacts of different dam operations over the years.
The study examined seven periods of changing operations since the dam became operational in 1952. The first period, through 1968, was erratic with dam discharges being adjusted experimentally. During this period, flows changed by more than 20 percent from one day to the next for 40 to 113 days per year.
From 1969 to 1985, the dam was operated “with sporadic and extreme hourly, daily and weekday flow peaking events for power generation and flood control,” the study states, adding that flow changes exceeded 20 percent 58 to 160 days per year.
In 1995, selective withdrawal technology was installed at the dam to allow for release temperatures to be regulated, a significant improvement for fish.
That same year, however, late-summer flow augmentation to benefit lower Columbia salmon was initiated, creating “double peak” runoff conditions on the river that were met with stiff resistance, including a lawsuit, from the state of Montana.
That resistance led to significant changes to end the double-peak releases in 2000, and a gradual move over the last decade for releases to more closely simulate natural runoff conditions.
This strategy, referred to as the Main Stem Amendments, wasn’t fully implemented until 2009. While more water than usual still is being released for downstream salmon, those releases are now being stretched into the fall.
“In an attempt to reduce adverse impact to resident fish, the most current objective of the summer operation strategy is to mimic the natural spring run-off event, within flood constraints, gradually reducing dam discharge toward stable flows for the biologically productive summer and fall periods,” the study states.
“It’s the best operation we’ve ever achieved in the history of the dam,” Marotz said.
While there continues to be pressure from downstream states for increasing flows in the summer, the new study “definitely” demonstrates impacts on the Flathead River, Marotz said.
“We want to be good neighbors” to downstream states, Marotz said. “But we want everyone to realize that it’s not harmless. There are actual impacts associated with this” on Montana rivers.