GEOMORPHIC EVOLUTION OF THE CLARK FORK RIVER, MONTANA IN THE FIRST TWO YEARS FOLLOWING BREACHING OF MILLTOWN DAM

The 2008 removal of Milltown Dam resulted in base-level lowering of 9 m at the dam site and introduced a pulse of fine reservoir sediment, some of which carried mining-derived contaminants, into a gravel and cobble-bedded river. The location of Milltown Dam at the downstream terminus of the largest Superfund site in the nation and at the confluence of two regionally significant rivers, the Clark Fork and Blackfoot Rivers, Montana, have created substantial challenges and opportunities for dam removal studies. Documentation of the geomorphic effects of this dam removal has included bedload and bed-material sampling, surveys of topographic change, aerial photograph analysis, development of a sediment budget for Milltown reservoir, and hydraulic modeling. 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 both the Blackfoot and Clark Fork arms of Milltown reservoir and also produced substantial new deposition of coarse sediment from upstream watersheds in the former reservoir, complicating analysis of reservoir erosion volumes. 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 aggradation on the order of one meter has been observed in side-channels of a multi-thread, lower-gradient reach extending from 21 to 25 km downstream of the dam site. Substantial infiltration of fine reservoir sediments into the interstices of the Clark Fork River’s gravel and cobble bed has also been observed. Continued study to assess the duration and spatial extent of geomorphic changes to ecologically valuable side-channel and interstitial habitats is being used to evaluate whether dam removal has produced short-term perturbations or threshold-crossing shifts to new ecogeomorphic conditions.