Influence of Dams on Size-Specific Sediment Transport and Storage on the Elwha River, Washington

Abstract:

The Elwha River recently underwent the largest dam removal project in history with the deconstruction of the Elwha and Glines Canyon Dams. According to recent USGS and USBR estimates, the project released 21±3 million m³ of sediment, approximately 420,000 m³ of which was gravel and cobble. Much of the coarse sediment released from the reservoir deposits has been stored in the channel bed and floodplain. Our project focuses on the gravel and cobble sediment budget for the middle and lower Elwha Rivers for pre- and post-removal periods. Prior to removal, the reduction in sediment load caused by the dams likely led to coarsening and incision despite regular lateral channel change, with the floodplain representing an important source of bed material. Air photo analysis (1939–2015) and creation of a map of relative floodplain elevation (topographic surface minus elevation of nearby vegetation line) helped test the hypothesis that post-dam (but pre-removal) floodplain deposits were built to a lower elevation than pre-dam surfaces. Preliminary results indicate that pre-removal but post-dam banks are, on average, lower than older banks, suggesting that floodplain built during the period when dams were in place did not completely replace sediment eroded from nearby banks. Bank erosion thus almost certainly represented a net source of sediment for the channel, and differences in the size distributions of eroded and deposited material could have had important geomorphic implications. Facies mapping and surface and sub-surface sampling on recent bars and along cut banks allow us to compare the coarseness of pre- and post-dam bulk deposits. We note that the coarsest fraction in eroding banks may be correlated to riffle location. In addition, bulk sampling in recently exposed reservoir deposits allows us to estimate the gravel and cobble fractions of the pulse of sediment released to the downstream river after the final portion of Glines Canyon Dam was removed in August 2014.