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The Removal Of The Milltown Dam--Mitigating The Aquatic Impact Of An Engineering Marvel


Wed Apr 30 00:00:00 MDT 2008

The removal of the Milltown Dam has caught the interest of fisheries biologists worldwide, said David Schmetterling, Montana Fish, Wildlife & Parks Region 2 fisheries biologist charged with monitoring the related fisheries for the past 11 years.

"It is an engineering marvel to remove a dam of this size and the millions of tons of toxic sediments from behind it," Schmetterling said. "A project of this scope has the short-term potential to harm the very aquatic environment it will ultimately benefit. Fortunately, we are able to work with the EPA and contractors on ways to minimize the impact on the fishery."

 He said the best part of the job is that the project team is open to adjusting the plan to minimize the short-term effects of the project on fish and aquatic organisms. 

In one example, the EPA and the state, including the Montana Department of Environmental Quality, and the Montana Department of Justice Natural Resource Damage Program, made a $12 million decision that will pay huge dividends to the state's fish and wildlife resources. They constructed an armored bypass channel 4,100 feet long for the Clark Fork River to flow through.  By re-routing the Clark Fork River into this temporary channel, workers can remove more than two million cubic yards of sediments without fear of the sediment entering the river.  An earlier plan would have allowed sediment from the Clark Fork River to be scoured downstream.

In another example, because fish may not be able to migrate through the project area during high flows this year, fish passage structures were installed in the bypass channel and former powerhouse area and the shape of the new channel altered to encourage fish passage. 

The changes should help fish move upstream past the project area and prevent them from being subject to the heavy sediment load that is expected from the work this year.

In order to evaluate the success of fish passage through the bypass channel and powerhouse area, Schmetterling's team will capture fish upstream in both the Blackfoot and Clark Fork rivers, equip them with radio transmitters and release them below the dam to observe how well they make it back upstream. At least one trout is known to have already made the trip upriver successfully.

Schmetterling began studying the effects of Milltown Dam on fish in the Blackfoot and Clark Fork Rivers in 1998. Now he monitors the effects of removing the dam, which involves testing for water quality, fish density, species composition of the rivers, and the physical condition of the fish as the dam and toxic sediment are removed.

FWP monitors the Clark Fork from Turah to Alberton, and the lower Blackfoot and Bitterroot rivers daily. They compare reaches affected by dam and sediment removal to stretches of the Blackfoot and Clark Fork rivers upstream that are not affected by the dam removal. The monitoring includes checking on caged fish in six different locations, up and downstream of the dam, to make certain they are alive. Periodically fish technicians also test the fish for metals in their organs and overall condition.

FWP has also implanted 40-70 free-ranging fish with radio transmitters to track mortalities.

Where the water volume nearly doubles at the confluence of the Clark Fork and the Bitterroot rivers down to Superior the sediment and toxins will be diluted and the impact reduced. From Superior to St. Regis the impacts on the river are expected to be negligible.

"We'll know more about how fish will be affected by late summer when the stress of the dam removal and clean up could be compounded by high water temperatures and low flows," said Schmetterling. "This project’s long-term benefits are far-reaching and unequivocal, a giant environmental step forward. Our job in the short-term is to do all we can to minimize the effect of the project on fish, and to pass what we learn along to others."