Heather M. Bragg

Initial Fluvial Response to the Removal of Oregon's Marmot Dam

A temporary, 14-meter-high earthen cofferdam standing in place of Marmot Dam was breached on 19 October 2007, allowing the 80-kilometer-long Sandy River to flow freely from Mount Hood, Oregon, to the Columbia River for the first time in nearly 100 years. Marmot Dam is one of the largest dams in the Western United States (in terms of height and volume of stored sediment) to have been removed in the past 40 years, and its removal exposed approximately 730,000 cubic meters of stored sand and gravel to erosion and transport by the newly energetic mountain river. At the time, its breach represented the greatest release of sediment from any U.S. dam removal. (The subsequent March 2008 breaching of Montanas Milltown Dam exposed about 5 to 10 times as much sediment to potential erosion.) Ongoing, intensive monitoring of erosion, transport, and deposition of that sediment is providing the first detailed data from such a voluminous dam-removal sediment release, which will provide a basis for evaluating physical and numerical modeling of the effects of future dam removals from mountain rivers.