Dam Removal Produces Largest Release of Sediment in History
During the past decade, the number and size of dams removed on rivers across the United States has been increasing. Dam removal typically involves the release of at least some of the sediment stored in the reservoir behind the former dam. When released sediment moves downstream, it has the potential to dramatically change the form and behavior of the downstream channel.
Nowhere has this been more closely studied than on the Sandy River, outside Portland, OR, after the removal of Marmot Dam in 2007. At the time, its removal produced the largest intentional release of sediment from any dam removal in history.
A new report describes how the Sandy River responded to the release of sediment during the next 2 years. Key findings include: (1) an energetic river can rapidly incise and remove large volumes of unconsolidated stored sediment, even under very modest flows; (2) channel change is initially quite rapid but diminishes over time as sediment sources diminish; (3) allowing rivers to naturally process stored sediment rather than manually removing it before dam removal may be a tractable option for coarse, clean sediment in cases where sediment deposition will not create a flood risk downstream.
Forest Service Partners