Milltown Reservoir Superfund Site
The Milltown Reservoir Sediments Superfund Site is part of the largest Superfund complex in the western United States. Situated in the Clark Fork River Basin, the Milltown Reservoir Sediments/Clark Fork River site stretches from the Warm Spring Ponds area near Butte to the Milltown Dam site east of Missoula. It was added to the National Priorities List in 1983 when arsenic groundwater contamination was found in the Milltown area. The contamination resulted from a massive flood in 1908 which washed millions of tons of mine waste into the Clark Fork River, ultimately ending up behind the Milltown Dam. The additional designation of the Clark Fork River between Milltown and Warm Springs Ponds in the Deerlodge Valley came in 1992.
The Milltown Dam (46°52′18″N 113°53′33″W / 46.87167°N 113.89250°W) was an earth-fill gravity-type hydroelectric dam on the Clark Fork river in Missoula County, in the western part of Montana. The dam was located about seven miles east of Missoula, Montana, at the confluence of the Blackfoot River with the Clark Fork. Built in 1908 by copper mining tycoon William A. Clark, it was meant to supply hydroelectricity to his sawmills in nearby Bonner, Montana. Clark's sawmills supplied the giant timbers used to shore up the walls of the mine shafts in Butte. Since the 1870s, the Anaconda and Butte areas had been mined as one of the richest deposits of copper sulfate ever found in North America. With the dam just months old, however, a record flood on the Clark Fork washed tons of toxic mining sediment downstream, where it settled at the base of the dam to remain until remediation began, with a cumulative total of around 6.6 million cubic yards of sediment contaminated with arsenic, lead, zinc, copper, and other metals in the former reservoir bed.
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