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Milltown Dam: Diversion of Clark Fork River set for tomorrow

Release Date: 03/27/2008
Contact Information: Diana Hammer, EPA, (406) 457-5040

(Helena, Mont. -- March 20, 2008) The Milltown Dam cleanup is approaching yet another milestone. On Friday, March 21, the inlet of the bypass channel at the Milltown Reservoir Superfund Site will be opened and the Clark Fork River will be diverted into its new, temporary channel. The diversion will not cause any change in the Clark Fork River level but project managers expect to the river to be murkier for several days following the diversion. Earlier this week, the outlet of the bypass channel was opened and the bypass channel filled with water from the Blackfoot River. EPA required construction of the bypass channel to isolate the Clark Fork River from the contaminated sediments in the Milltown Reservoir, thereby reducing sediment scouring during the next reservoir draw down and impacts to downstream ecology and river users.

When the Milltown Dam is breached on March 28 (expected date), the reservoir level will drop another 12-14 feet (Stage 2 draw down) and approximately 300,000 tons of clean material will wash downstream.

The bypass channel represents a major effort to minimize downstream impacts of dam removal. When the dam is breached, having the Clark Fork River flowing through the bypass channel will reduce the amount of material scoured from the reservoir area by 60%. It also means that no sediments with high metals concentrations will be sent downstream. Most of the material that is scoured will come from the Blackfoot River and the from the earthen coffer dam that is washed away as the Milltown Dam is breached.

With the diversion of the Clark Fork River into the bypass channel, the project is another giant step closer to our goals of restoring the local drinking water supply, protecting the fishery, and restoring the Clark Fork River and Blackfoot Rivers to a more natural and free-flowing state.

The Milltown Dam was built in 1905-1908 to supply power to the local lumber mill and the surrounding area. The mill supplied lumber for the mining operations upstream in Butte. In 1908 an enormous flood washed mine waste downstream from the Butte Mining District, depositing the wastes behind the newly constructed Milltown Dam.

Over time, the arsenic in the reservoir sediments made its way into the Milltown aquifer, polluting the local drinking water supply. Copper in the sediments would periodically scour from the reservoir and kill fish downstream of the dam. Due to the public health risks, the site was listed as a federal Superfund Site in 1983.

In 2004, EPA and the Montana Department of Environmental Quality announced the decision to remove the Milltown Dam and contaminated sediments. Since then, EPA, DEQ and the site trustees have been working with Atlantic Richfield Company, NorthWestern Energy, and Envirocon to carry out the cleanup plan.


For safety reasons, the project area is closed to the public. This includes the area around the Dam, Powerhouse and portions of the Clark Fork (CFR is closed from Turah Bridge Fishing Access Site downstream to the Dam and railroad bridge below the Dam) and Blackfoot Rivers (Closed from Weigh Station downstream to the Dam) near the Superfund Site.

Non-authorized personnel are not allowed on site.

For more information about viewing site activities, please contact Diana Hammer, EPA, (406) 457-5040.


EPA’s Milltown Reservoir Sediments Superfund Site website:

Clark Fork River Technical Assistance Committee’s Milltown website:

View events as they happen via the Milltown “Dam cam” web cam: or