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Milltown Dam: Water to Flow into Clark Fork River Bypass Channel

Release Date: 03/14/2008
Contact Information: Russ Forba, EPA (406) 457-5042; Diana Hammer, EPA (406) 457-5040; Keith Large, DEQ, (406) 841-5039

(Helena, Mont. -- March 14, 2008) The Milltown Dam cleanup near Missoula, Mont., is approaching another milestone. After a year of construction, the Clark Fork River bypass channel is complete and ready for water. On Tuesday, March 18, the plan is to open the outlet of the Clark Fork River bypass channel to fill the channel with water from the Blackfoot River. It should take approximately 14 hours and 44 million gallons of water to fill the bypass channel.

EPA, in consultation with the State of Montana, has approved Envirocon’s plan to fill the bypass channel slowly enough so that plenty of water remains in the Clark Fork River below the Dam. Maintaining an adequate and steady river flow is critical for the fishery. Earlier this week, Envirocon removed the sheet piling at the bypass channel outlet, leaving only an earthen berm between the Blackfoot River and the bypass channel. On Tuesday, March 18, this berm will be breached and water from the Blackfoot River will flow upstream into the bypass channel. A rock-lined 15 foot wide opening in the outlet structure will allow about 2 feet of water to flow into the bypass channel. This will result in a temporary and slight drop in the Clark Fork River level below the Milltown Dam.

Late next week, the inlet of the bypass channel will also be opened, allowing the Clark Fork River to be diverted into the bypass channel. 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. 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 percent. It also means that no sediments with high metals concentrations from the project area will be sent downstream. Most of the scour will come from the clean materials in the Blackfoot and the earthen coffer dam that is washed away as the Milltown Dam is breached in late March 2008.

Breaching of the Milltown Dam in late March 2008 is an important step in reaching the goals of restoring the local drinking water supply, protecting the fishery, and restoring the Clark Fork and Blackfoot Rivers to a natural and free-flowing state.

The Milltown Dam was built in 1906-1907 to supply power to the local lumber mill, which in turn, 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.

REMINDER: 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:

A web camera view of the bypass channel breach as it happenscan be viewed at: