The Milltown Reservoir Sediments Superfund Site is a major Superfund site in Missoula County, Montana, seven miles 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 three years after its construction in 1905, which washed millions of tons of mine waste into the Clark Fork River, ultimately ending up behind the Milltown Dam.