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Missoula Valley Water Quality District
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Missoula Valley Water Quality District

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Photograph showing the Milltown Dam Power House.

 

Milltown Dam: Background

The Milltown dam and reservoir were located in Milltown, Montana, about 4 miles upstream of downtown Missoula. The United States Environmental Protection Agency (EPA) listed the area on the federal Superfund site list in 1983 based on high levels of arsenic detected in area drinking water wells.

The dam was built in 1906-7. A large flood in 1908 filled the reservoir with sediments containing wastes from upstream mining operations in Butte and Anaconda. The dam was 21 feet high, with eight foot wooden flashboards on its crest. It had been repeatedly damaged by floods and ice over the years, and re-fortified with concrete – however the original timber crib structure remained in place until its removal in 2008.

The reservoir held about 6.6 million cubic yards of sediments, about 3 million yards of which were heavily contaminated with metals, including 2,100 tons of arsenic, 13,100 tons of copper, 19,000 tons of zinc, 143,900 tons of iron, and 9,200 tons of manganese. The depth of contaminated sediments ranged from 1 to more than 25 feet. Water depth in the reservoir averaged about four to eight feet.

The dam's powerhouse was removed in March 2008, and spillway removal was completed in March 2009. Contaminated sediment removal and shipment by rail was complete in September 2009. Additional sediment removal for restoration of the site was done and will be completed in 2010.  River channel construction and restoration began in 2009 and will continue through 2011.  The Clark Fork River will be diverted into its new channel through the remedial project area sometime in late 2010 or early 2011.  Design of  a new State Park at the former reservoir site will be completed in 2010 and 2011, with construction beginning in 2011.

  Left-facing arrow indicating a hyperlink to a previous page.Back to Milltown Dam Page

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