A substantial number of fish died in the Clark Fork River when ice jams and flooding in January forced a drawdown of Milltown Reservoir, a fisheries manager said Friday.
The river was so full of silt, debris and ice that the dead fish washed away unnoticed, said Dennis Workman of the Montana Department of Fish, Wildlife and Parks in Missoula.
The Milltown Reservoir area is contaminated with metals from upstream mining years ago.
“There was literally tons and tons of sediment carrying the metals down the river” last month, Workman said. There will be more fish deaths this spring and summer as insects pick up traces of the metals and fish eat the insects, he said.
Workman said he was alarmed by high concentrations of arsenic, copper and zinc in river samples taken Feb. 9 and 10.
Samples downriver from the reservoir included copper levels of up to 770 parts per billion. The standard for support of aquatic life is 18 parts per billion.
“Just based on past experience, back when the dam used to be drawn down every spring and we would get surges of muddy water for a few days, we know it doesn’t take concentrations as high as we saw in the ice-jam event to kill fish,” Workman said.
In previous years, biologists caged fish during seasonal drawdowns and observed the effects of metal poisoning. The fish died.
“The question during the drawdown last month was whether or not the fish were able to find a way to escape when the sediments came down the river,” Workman said. “The only tributaries, though, were Rattlesnake Creek and the Bitterroot. They didn’t have many options.”
“I have to conclude that we had a substantial fish kill.”
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