PREDICTING SEDIMENT ROUTING ON THE SANDY RIVER, OREGON FOLLOWING THE REMOVAL OF THE MARMOT DAM

The 14-meter-tall Marmot Dam was removed from the Sandy River, OR in October 2007. Its removal exposed nearly 730,000 m 3 of impounded sand and gravel to the river for erosion and transport downstream. Prior to the dam removal a two-fraction 1-D morphodynamic model was run to understand the probable river response to the dam removal and the predicted changes in morphology associated with the dam removal were assessed to be acceptable. As part of a large monitoring effort to capture the morphologic changes in the Sandy River following the dam removal, we measured the evolution of the bed downstream of the dam over a multi-year time period. The predicted reach-averaged values from the model corresponded well to our field measurements. We observed two reaches with bed elevation changes less than that predicted by the model. These are attributed in one case to uncertainty in the transport and storage capacity of a difficult-to-access bedrock gorge 2.5 km below the damsite, and in another case to a large logjam-induced rapid causing extreme local relief between a bar and the channel. We find the performance of the 1-D model robust, but urge users to be familiar with the objectives and limitations of any sediment routing and morphodynamic model when interpreting the results.