Sedimentary deposits and processes in the lower Elwha River, Washington, USA, during dam removal

Removal of two dams on the Elwha River, Washington, USA, began in September 2011, constituting the largest dam removal and controlled sediment release in history and providing a valuable opportunity to evaluate physical and biological response to restoration of natural upstream sediment supply. Topographic and grain-size data collected regularly over the 5 years before dam removal allow a detailed comparison with changes that have occurred since the start of dam removal. High suspended-sediment concentrations and lack of large floods over winter 2011-2012 promoted widespread deposition in the Elwha River downstream of the dam-removal sites. New sediment deposits were evident throughout the lower river in spring 2012 and were composed of a much finer grain size (very fine sand, silt, and clay) than occurred on the riverbed before dam removal began. Although bed aggradation had been measured several times in previous years, 2012 marked the first instance of deposition of muddy bed material. No similar new deposition of fine sediment occurred in a reference reach upstream of the dam sites, indicating that the newly deposited sediment is derived entirely from former reservoir material. New sediment deposits in the lower river were typically 1-20 cm thick, with a maximum measured thickness of approximately 50 cm. Although this new deposition was insufficient to raise the local water surface (flood stage) appreciably, the common occurrence of new fine sediment and organic matter in interstitial spaces between gravel and cobble grains could have substantial ecosystem effects.;