Lessons learned on Milltown Dam drawdown 

The drawdown of the Milltown Reservoir last month which caused the Clark Fork River to run black through town was not a pretty sight, but officials from the state and federal agencies in charge of the project expect to glean a wealth of information once data taken from the drawdown is analyzed.

The reservoir was lowered 10.5 feet, nearly twice the amount of more conventional drawdowns performed to manage predator fish populations at the site. The depth of the drawdown allowed the Clark Fork and Blackfoot rivers to rediscover natural channels they hadn’t used in a long time, and that, says state Department of Environmental Quality remediation specialist Keith Large, was the primary source of increased sediments in the water downstream.

“Once the levels got below six feet, we saw some banks sloughing off into the channels,” says Large. “The drawdown taught us a lot, and we’re looking at ways to try and be a little bit more protective of downstream water quality in the future.”

Perhaps the most alarming development was the loss of one-third of the cutthroat trout fingerlings caged in the water below the dam. Nine of the 27 two-inch fish perished below the dam; only one of the 54 fish held upstream in the river channels died during that time.

“We were a little surprised at the mortality rate,” says Russ Forba, the Environmental Protections Agency’s (EPA) project manager for the site. Forba says that the exact cause or causes of the fish deaths may never be known, but water samples that were taken are being analyzed for metal concentration, suspended solids, and dissolved oxygen levels, among other factors.

Still, Forba says that the drawdown confirmed the EPA’s projection of the long-term effects of sediment removal at the site.

“One thing we are confident about is that the mass loading downstream is a relatively small one,” he says. “We expect the only significant impacts to be felt in the short term, not the long term.”

In addition to conducting water-quality studies, the agencies tested the sediment bed to determine the most feasible combination of dry extraction and wet dredging that would be required to remove the estimated 6.6 million cubic yards of contaminated sediment at the bottom of the reservoir.

Test results are expected in another month or so, and the agencies hope to have a report on the findings ready by November, in time for the decision announcement and subsequent public comment period.

“It was a good exercise,” says Large. “We gathered a ton of information that will help us get construction done at Milltown Dam.”

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