Suspended Sediment & Trace Elements Loads in the Clark Fork Basin, Before and After Removal of Milltown Dam

Abstract

About 6.6 million cubic yards of sediments enriched in trace elements from historical mining operations were deposited in Milltown Reservoir after construction of Milltown Dam in 1907. Milltown Dam was removed in March 2008, as part of remedial activities. In preparation for the removal of Milltown Dam, permanent drawdown of Milltown Reservoir began in June 2006. The annual mass balance (that is, the inflow load minus the outflow load) of suspended sediment and trace element loads within the project area (the historical Clark Fork channel and floodplain affected by impoundment of water behind Milltown Dam) were estimated for water years 2004–2009. During water years 2004–2005, inflow loads of suspended sediment and most trace elements exceeded outflow loads indicating net gain in the project area. Net loss of suspended sediment and trace elements from the project area began after the start of the permanent drawdown in June 2006 and continued through water year 2009. In water year 2008, there was net loss of about 391,000 tons of suspended sediment and about 157 tons of unfiltered-recoverable copper with most of the transport occurring after the removal of Milltown Dam in March. Substantially smaller net loss of suspended sediment and copper occurred during water year 2009 (provisional estimates of about 76,000 and 31 tons, respectively) than during water year 2008 even though streamflow conditions were similar. The estimated cumulative net loss of suspended sediment from the project area from June 2006 through water year 2009 is about 623,000 tons