EFFECTS OF THE MARMOT DAM REMOVAL ON GRAIN SIZE DISTRIBUTION IN THE B REACHES OF THE SANDY RIVER IN OREGON

The Marmot dam, a medium-large sized dam on the Sandy River in Oregon, was removed in October 2007 to restore fish spawning habitat. This decommissioning project provided the opportunity to study fluvial geomorphologic changes as affected by the release of impounded sediment behind a dam. This research project examined change in grain size distribution (GSD) in Reach B in the downstream of the Sandy River, relatively close to the dam site. Data was collected in July 2008 and compared to data collected by others in July 2007, prior to the dam removal. Seven bars in this reach were facies mapped and pebble counted using the Wolman pebble counting technique. This study hypothesized that fine material had already traveled downstream without deposition whereas larger grains had yet to be transported to this reach. Therefore, Reach B should not have changed significantly since the dam removal. Data collected in 2008 suggest that the GSD of some individual bars changed though these changes are not apparent when evaluating the reach as a whole. In Reach B1, one of the three bars had noticeable changes in the size of finer grains though in contrast, the change in fraction of sand for the other two bars was more evident. In Reach B2, where four bars were studied, only one showed a noticeable change in D16. The data suggest that grain size in Reach B2 changed less than in Reach B1. Overall thus far, grain size in Reach B appears to be fairly the same, pre- and post-dam removal.