THE BREN SCHOOL OF ENVIRONMENTAL SCIENCE AND MANAGEMENT
at the University of California, Santa Barbara

Presents

"The Very Hungry River: Spectacular Geomorphic Response

of the Sandy River to Removal of Marmot Dam"

Hosted by Bren Assistant Professor Christina Tague

Partial Abstract

We provide an initial report on the impressively rapid fluvial response to the first dam removal involving significant release of sediment. Marmot Dam, a 15-meter concrete diversion dam located on the Sandy River east of Portland, Ore., was removed in the summer of 2007. Removal of the concrete structure required construction of a temporary cofferdam composed of sediment upstream of the main structure. Here we report on the sequence of geomorphic events accompanying breaching of this cofferdam in October 2007, and accompanying erosion and transport of a significant fraction of the roughly 750,000 cubic meters of sand and gravel stored behind it. The breach occurred during a modest rainstorm and rising hydrograph, and was initiated by removal of groundwater pumps that had kept phreatic surfaces within the dam low, and excavation of a small notch at the crest of the cofferdam. Notch position was chosen based on physical models that indicated where an initial breach would lead to maximum erosion of stored sediments. 

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