Milltown Dam Removal Marks Milestone, Water Rerouted

By Peter Metcalf, 3-19-08

	Caption: The Milltown Dam removal project as seen from the bluff overlooking the site. On Tuesday, March 18, the temporary dam was removed and the Blackfoot river filled the bypass channel. Photos by Emily Haas

The removal of Milltown Dam marked another milestone Tuesday when the Blackfoot River poured into the bypass channel dug along Interstate 90.

Project managers chose to backfill the channel from the Blackfoot River first to protect equipment and better manage downstream flows, said Ben Johnson, an Envirocon project engineer.

On Friday, Envirocon workers will remove the steel and earthen coffer dam at the channel’s inlet to reroute the Clark Fork River and Deer Creek into the bypass channel in anticipation of the next big event, the breach of the Milltown dam itself.

If all goes according to plan, the coffer dam at the site of the old powerhouse will be breached next Friday, allowing the both the Clark Fork and the Blackfoot rivers to flow free for the first time in a century.

	An excavator removes the remnants of the earthen berm holding back water from the Blackfoot river.

“Initially there will be a bit of a surge. It’s not like a wave. It’s like a tide,” Johnson said.

Engineers anticipate downstream river levels will rise about four feet immediately below the dam, and about a foot to a foot and half near Missoula. Over the next couple weeks, the free flowing rivers will scour out clean sediments deposited over the century by the Blackfoot River, slowly dropping the river channel on the upstream side of the old dam.

After the dam is breached, engineers will turn their attention to removing the dam’s spillway over the next six to eight months.

The $100 million cleanup and restoration of the old Milltown Reservoir is part of the nation’s largest Superfund cleanup site. The whole 120-mile stretch of the Clark Fork River upstream from Milltown is contaminated with toxic and other heavy metals, including arsenic, lead and mercury from the mines and smelters in the Butte and Anaconda area.

	Sediments from the Milltown Dam reservoir are loaded onto train cars to be deposited up river near Opportunity.

Over the last century, large amounts of contaminated sediments built up behind the dam, contaminating ground water and threatening the health of fish and other wildlife. The EPA decided to remove the dam in 2004 in response to citizen concerns and the listing of Bull Trout under the Endangered Species Act. The cleanup and restoration of the Milltown site is expected to last at least through 2010.

The bypass channel took 18 months to dig is designed to hold a 100 year flood event and provides resting space for fish It will serve as a temporary channel for the Clark Fork while the contaminated sediments of the old channel and reservoir are cleaned up.

Eventually the Clark Fork will flow through an entirely new channel that meanders across its historic flood plain, now buried under as much as 20 feet of sediment. Designs for the new channel have not been finalized, Johnson said.
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