Plans had called for dropping the reservoir by 10 feet this summer as a first step in the eventual removal of Milltown Dam and restoration of the confluence of the Clark Fork and Blackfoot rivers. The drawdown began June 1.
The release was stopped, however, when the reservoir reached 9 1/4 feet - and after all the rainbow trout fingerlings died in cages in the Clark Fork River near the Deer Creek Bridge and below the confluence with the Bitterroot River, said Russ Forba, EPA's Milltown project manager.
“The small fingerling rainbow trout are very susceptible to any sort of change,” Forba said. “They're meant to be the canary. We're not exactly sure what killed them. It could have been some sort of viral or bacterial infection.”
The drawdown has been closely monitored, and so far there haven't been any spikes in dissolved metals, Forba said.
The levels of metals and suspended solids were actually higher earlier in the process, but the water was also much cooler, he said. Water temperatures bumped up to the 70-degree mark over the past few days.
Stopping the drawdown will likely reduce the fine particles of sediment coming out of the reservoir, which may be part of the reason the fish died, Forba said.
The drawdown will probably start again sometime near the end of August when water temperatures drop.
“We're doing what we can to protect the resource,” Forba said. “Our monitoring system has worked just as we expected it would. These fingerlings couldn't escape to cooler or deeper water like other fish can.”
Montana Fish, Wildlife and Parks will perform a necropsy on the trout in an attempt to discover what killed them.
About a third of the larger trout fitted earlier with radio transmitters have also died.
“It's hard to know what that means,” Forba said. “We don't know what the impact has been on resident fish. We won't know that until we do population census work next June.”
Forba said the amount of sediment that scoured out from the reservoir was quite a bit less than what had been anticipated.
“We didn't have the high flows that we expected,” he said. “When we began the drawdown on June 1, the river was at 8,400 cfs. Our models expected almost double that.”
If the levels are higher than 8,400 cubic feet per second next spring, Forba said there will likely be additional sediment scoured from the reservoir.
The Milltown Reservoir Superfund cleanup is on schedule, with work on the bridge over the Blackfoot River planned for this fall and removal of Milltown Dam planned for spring 2008.
Reporter Perry Backus can be reached at 523-5259 or at pbackus@missoulian.com
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