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Breaching the Milltown Dam

Release Date: 03/31/2008
Contact Information: Diana Hammer, EPA, (406) 457-5040.

(Helena, Mont. -- March 27, 2008) The Milltown Dam was completed in 1908 and on Friday, March 28, 2008, a century later, the Milltown Dam will be officially breached. This is an historical moment both in the cleanup of the Milltown Reservoir Sediments Superfund Site and in the rich cultural and natural history of the Bonner-Milltown area. Late Friday morning, a crowd is expected to gather on the bluff overlooking the Dam while project personnel and elected officials gather below; everyone watching for the first cracks and seeps through the earthen coffer dam that currently holds back the Clark Fork and Blackfoot Rivers. The breach is expected to begin sometime after 11:30 a.m.

Russ Forba, EPA Project Manager, has been overseeing this work for the past 11 years and sees this as yet another step in completing the cleanup at Milltown. “Every day we move a little bit closer to cleaning up the drinking water supply and restoring the rivers. Today, however, is a giant step forward.” This day has been 20 years in the making and this week Envirocon has been making the final preparations for opening up the Milltown Dam. Wednesday workers finished removing the sheet piling between the earthen coffer dam and the rivers. Thursday and Friday, workers are excavating fill from the coffer dam and creating a pilot channel to direct the rivers toward the area to be breached. Workers are also creating a 15 foot wide lip that will serve as the initial spillway in the coffer dam.

Up until now, the rivers have been flowing over radial gate of the Dam. Tomorrow morning, the radial gate will be raised, causing the river levels to rise behind the Dam. As the river levels rise, they will begin to slowly overtop the coffer dam. As more water flows through the ever-increasing opening in the coffer dam, the Milltown Dam will be officially breached. An era will come to an end.

As the coffer dam erodes and the water level drops, project managers estimate that approximately 300,000 tons of clean material will wash downstream from the eroding coffer dam and scoured sediments from the Blackfoot River. Breaching the Milltown Dam will lower the reservoir level by another 12-14 feet and formally end Stage 2 of the Milltown cleanup plan. The Clark Fork River will also temporarily rise following the breach. Project managers estimate that the river will be 4 feet higher immediately below the dam, 3 feet higher at Pine Grover, 2 feet higher at Deer Creek Bridge, and 12”-18” higher in Missoula. At the Bitterroot, the Clark Fork is only expected to rise about 6 inches. As the waters recede, some mud will remain on the rocks below the breach high water line. This mud is low in metals and poses no risk to public health or the environment. The muddy coating should wash away with spring flows.

According to Matt Fein, Envirocon Project Director, “This is a significant achievement and represents a beginning and an end. It is the culmination of over a year's worth of work on the bypass channel and the end of the Milltown Dam. Looking ahead, Fein also said, “It also represents the beginning of the process to reclaim the floodplain of the Clark Fork River.”

Opening the Milltown Dam, we are all one 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.

The public is asked to stay off the Clark Fork River on Friday, March 28, following the breach of the Milltown Dam. The river level will temporarily rise and potentially dangerous debris (e.g., logs and ice) will be in the river. Warning signs have been posted at access sites and wardens will be patrolling the River. 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