Removing dams that are outdated, unsafe, or pose significant economic or environmental costs has emerged in the last 10 years as a major river restoration strategy. The removal of the 45-foot-high Marmot Dam on the Sandy River in 2007 resulted in the biggest sediment release accompanying any dam removal to date. It also provided an unprecedented opportunity for a team of scientists from the Pacific Northwest Research Station and other organizations to predict, monitor, and evaluate the river environment before, during, and after the event.
The dam removal was successful on many levels. The earthen cofferdam that had been temporarily constructed while Marmot Dam was dismantled rapidly eroded on cue, and reservoir sediments were transported downstream on a timetable that surpassed all expectations. This happened with no detrimental effects on fish habitat or increased flooding that might impact downstream properties. Removing Marmot Dam showed that an energetic river can rapidly and efficiently do the work of redistributing huge volumes of unconsolidated sediment, even under very modest flows. Results are likely to guide future dam removals for the next decade or more, and offer dam managers a cost-effective option for sediment disposal in some cases.