Missoula Dam Letting Toxic Sediments Through Tons Of Sludge Polluting Clark Fork River, Say Managers

The Milltown Dam is letting thousands of tons of toxic mine sediments through to the Clark Fork River downstream, managers conceded Thursday.

Some 68,000 tons of toxic sediment flow into the reservoir behind Milltown Dam each year, and the dam stops only about half, said Russ Forba, the Environmental Protection Agency’s cleanup project manager at Milltown.

The reservoir now holds 6.6 million cubic yards of contaminated sediment, he added. “There’s not a lot of storage capacity left.”

His comments, at a meeting to review progress on planning a Milltown cleanup, surprised Missoula Mayor Mike Kadas.

“I was always under the impression that the dam was a catchall,” Kadas said. “That is not the case. It all flushes down.”

“What occurs is about eight years of catching sediments and one year with huge, elevated levels of metals coming over the dam,” replied Forba.

Forba said the EPA is considering excavating up to 500,000 cubic yards of sediment to make room for new arrivals, with further excavation as cleanup work progresses upstream in Butte, Anaconda and along the Clark Fork from Deer Loge to Milltown.

Only the upper 2 to 4 feet of sediments are affected by changes in the reservoir’s water level; deeper sediments are stable.

Cleanup costs are the responsibility of the Atlantic Richfield Co., which acquired the Anaconda Copper Co., whose mines and smelters produced the toxic wastes. But the dam is operated by the Montana Power Co., and the meeting raised the question of whether MPC will continue its ownership after its current dam license expires in 2004.