DOWNSTREAM RESPONSE OF DEPOSITIONAL CHANNEL-FORMS TO DAM REMOVAL, CLARK FORK RIVER, MONTANA

The March 2008 breaching of Milltown Dam, at the confluence of the Clark Fork River and Blackfoot River, MT, exposed some 3 million m³of predominantly fine (< 2 mm) reservoir sediment deposits to erosion. Some of these sediments, particularly on the Clark Fork arm of the former Milltown reservoir, contained high levels of arsenic (As), copper (Cu), and other mining-derived metals. We documented textural, volumetric, and geochemical changes to depositional features on the Clark Fork River downstream of Milltown Dam in response to dam removal and river erosion of hundreds of thousands of cubic meters of sediment out of Milltown reservoir. Pre-dam removal topographic surveys and bed-material samples of mid-channel and lateral sand and gravel bars were compared with repeat, post-dam removal measurements in 2008 and 2009. Downstream depositional features have experienced a net volumetric increase of sand sized material. Composition in these features has tended towards a net decrease in grain size. Additionally, geochemical data showed increased levels of As and Cu within texturally equivalent deposits at various locations within the Clark Fork River corridor in 2008; 2009 results are pending. Field measurements were complemented by image analysis techniques to discern the location of sediment deposits and to develop volumetric deposition estimates from aerial photographs over broader spatial scales.