Role of Channel Banks in the Adjustment of the River Bed along the Elwha River

The Elwha River has now undergone several years of adjustment since Elwha and Glines Canyon Dams were removed over the 2011-2014 period. The removal released sediment that had accumulated in the reservoirs for nearly 100 years. Much of the released sediment consisted of relatively fine-grained (sand and finer) material that has not been stored permanently on the relatively coarse-grained (cobble and boulder) river bed. Nevertheless, the channel has experienced significant geomorphic change since the dams were removed. Bank erosion and channel avulsions have mobilized coarse-grained sediment that had been stored in floodplain deposits adjacent to the channel. To characterize this material and new sediment deposits now present in the channel, we collected size distributions from cut banks and new point bars using traditional Wolman counts, bulk subsurface samples, and physical and photograph-based samples of surface sediment. Results show that material deposited in new point bars is similar in texture to material in cut banks and imply that floodplain deposits could be the major source of much of the new bed material now present in the channel. It is not clear, however, how far the system remains from the original, pre-dam condition. To address this question, we have used a size-specific sediment transport model, MAST-1D, to simulate bed coarsening and incision during the entire ~100-year period when the dams were present. The model allows the channel to exchange sediment with the floodplain in a way that stabilizes the channel and that is consistent with the large amount of bed material that has been released from banks since the dam removal occurred. The inclusion of bank sediment in the bed material budget is a stabilizing feature of the model. Results illustrate the important role played by streambanks in both storing and sequestering sediment in geomorphically active gravel bed rivers