Using Be and textual characteristics to trace the source of sediment in the Elwha

Abstract

The removal of the two dams on the Elwha River provides a large-scale backdrop for sediment transport research. The demolition will stimulate sediment-laden flows downstream, directly impacting the Elwha delta and the marine environment. This study uses two different tracers to track sediment movement along the Elwha River and in the Strait of Juan de Fuca: beryllium-7 (7Be) and grain texture and mineralogy. The objective of this study is to draw conclusions about the source of sediment reaching the mouth of the river. Using these tracing methods, we found that (1) 7Be functions as a useful tool when researching sediment transport during dam removals, (2) the grains examined at the mouth of the Elwha River are a mixture of both sediment from upriver and from the glacially deposited bluffs along the shoreline, and (3) there is evidence of sediment from the former reservoirs making its way to the delta. The source of sediment reaching the delta is most likely a mixture of normal depositional processes due to fluctuating river discharge and reworking of the river bed sediments due to dam removal as the former reservoirs, Lake Mills and Lake Aldwell, erode into their pre-dammed form.