Erosion of reservoir sediment following the removal of the Marmot Dam, Sandy River,
Oregon

Johannah Withrow-Robinson

Whitman College, Walla Walla WA 99362

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The Marmot Dam, a 13-meter high concrete diversion dam located on the northwestern Oregon, was removed by controlled blasting in July, 2007. A coffer dam upstream diverted water around the Marmot Dam until excavation of the concrete structure was complete. On October 19, 2007, the gravel coffer dam was breached in a moderate flow event; at approximately 5:45 pm, a knickpoint was initiated with the aid of backhoes. This knickpoint retreated rapidly upstream through the unconsolidated reservoir sediments at an initial rate of approximately 3 meters per minute. By the morning of October 20, the knickpoint had migrated hundreds of meters upstream, removing approximately 750,000 cubic meters of sediment, and the rate of upstream retreat had decreased. To calculate the rate of knickpoint migration, we analyzed still and time-lapse photos taken at the removal site to determine the location of the knickpoint in relation to fixed points. Documenting knickpoint retreat and sediment transport at the Marmot Dam site is relevantto the planning and management of dam removals on mountain streams across the western United States.