Milltown Reservoir Dam Removal and Sediment Excavation

Abstract

The Milltown Dam is located approximately seven miles east of Missoula, Montana, at the confluence of the Blackfoot and Clark Fork rivers and was originally constructed to provide hydropower-generated electricity for a major sawmill in 1907. In 1908, a major flood resulted in a widely-spread overbank condition that washed tailings from major copper mining operations in the Butte and Anaconda area, approximately 100 mi upstream of Milltown. The backwater condition created by the dam resulted in the deposition of an estimated 7 million cubic yards of sediment behind the reservoir. A portion of these sediments contained elevated concentrations of metals, particularly copper and arsenic. EPA listed the Milltown Reservoir Sediment Operable Unit (MRSOU) on the National Priorities List in 1982 based on arsenic detected in Milltown groundwater wells located adjacent to the reservoir sediments. In late 2004 EPA selected a remedial action for the MRSOU, which included removal of the dam and excavation of 2.2 million yards of contaminated sediments. Initial drawdown and construction for the first phase of the project began in June 2006. This presentation summarized design and construction strategy for removal of the dam, the work conducted to date and measure implemented to minimize impacts to the Clark Fork River fisheries.