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 complex stretches from the Warm Spring Ponds area near Butte, MT to the Milltown Dam site east of Missoula, MT. 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.