Geomorphic evolution of the Clark Fork River in response to the removal of Milltown Dam

The 2008 breaching of Milltown Dam resulted in base-level lowering of 9 m at the dam site and introduced a pulse of fine reservoir sediment into the gravel and cobble-bedded Clark Fork River. Studies of the geomorphic effects of the dam removal have evaluated the volume of material eroded from the Blackfoot and Clark Fork arms of Milltown reservoir, downstream transport and deposition of that sediment, and resulting changes in channel morphology. These efforts have incorporated bedload and bed-material sampling, surveys of topographic change, and aerial photograph analysis. In the first two years following dam breaching, successive peak flows with similar magnitudes (3-year recurrence interval) eroded several hundred thousand cubic meters of sediment from Milltown reservoir. Reservoir erosion and downstream geomorphic adjustments were far greater in the first year following removal, when readily mobilizable fines were eroded and transported downstream, than in the second year. Downstream topographic changes (e.g., aggradation) have been limited in study reaches within 4 km of the dam site, but more significant sediment accumulation has been observed in the multi-thread reach extending between the Reserve Street bridge in Missoula and the Bitterroot River confluence (from 21 to 25 km downstream of the dam site). Substantial infiltration of fine reservoir sediments into the interstices of the gravel and cobble bed has also been observed. Studies are ongoing to assess the duration and spatial extent of geomorphic changes resulting from the dam removal