Effects of the Elwha River Dam Removal Project on Suspended-Sediment Concentrations in the River and Nearshore Environment
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Suspended-sediment concentrations (SSCs) in the Elwha River and nearshore environment off the river mouth were observed in order to determine the effects of the Elwha dam removal project on sediment supply to the river and marine environment. River gauge data over a 4-month period of dam removal from November 2011 to March 2012 was analyzed to identify deconstruction-caused SSC increases. Nearshore SSC was measured over the same period and was compared to oceanic and river data to determine controls on nearshore SSC levels. The 2011-2012 data was compared to data from a 2008 study to see what effects dam removal has had on SSC levels. The river had a background SSC level of about 100 mg L-1 with a maximum observed concentration of 2610 mg L-1. Nearshore SSC had a background level of 0-20 mg L-1 during the study with periodic events reaching levels of 50 mg L-1 and a maximum of 121 mg L-1 on March 30. The highest recorded river SSC levels corresponded with major deconstruction events, while the highest recorded nearshore SSC levels corresponded with strong sediment input from the river. Comparison with the 2008 data was made difficult by the large difference in river discharge between the two study periods. Dam removal resulted in pulses of sediment to the river that were measurable off the mouth of the river, indicating that deconstruction had a direct on effect on SSCs in both the river and nearshore environment.