Beach response to the removal of dams on the Elwha River

Conceptual models of sediment distribution from the mouth of the Elwha River developed prior to dam removal suggest two responses on the intertidal beach; 1) the cessation or reversal of chronic erosion, especially east of the river mouth, due to the addition of new sediment volume to the beach and 2) a reduction in the average bed grain size of the beach due to the composition of the expected sediment load from the reservoirs. Furthermore, the documentation of strong alongshore transport directed to the east along this shoreline suggests that the two responses described above should vary in space and time, with a response occurring first near the river mouth and progressing to the east over time. To track the intertidal response and test this conceptual model, profile and grain size measurements have been collected at four cross-shore oriented transects on the Elwha River delta, one to the west of the river mouth and three to the east. Measurements were made intermittently between 2008 and 2011, and approximately bi-weekly since February 2011. These high frequency measurements supplement bi-annual large scale topographic and bathymetric surveys conducted by the US Geological Survey. At the measurement site immediately to the east of the river mouth patterns of profile accretion and a reduction in the mean grain size of the beach suggests that, by July 2012, sediment associated with the dam removal was recruiting to the foreshore and nourishing what has been a coarse eroding beach. In November 2012 a large pulse of sediment arrived at the river mouth which built portions of the shoreline seaward >200 m and initiated a transition of some portions of the formerly intertidal and sub-tidal shoreline to back-shore habitat. The profile to the west of the river mouth has also added sediment volume since the dam removal started, and our data also suggest a reduction in the mean grain size of the beach there. By contrast, the two profiles furthest to the east on the delta have continued to erode, though possibly at a declining rate. There is little evidence for a reduction in grain size at these sites, though, suggesting that a reduction in the erosion rate may not be due to the alongshore transport of sediment from the river mouth and the addition of sediment volume. Rather, we hypothesize here that the changing morphology of the river mouth and/or the sea-floor may be altering the pattern of wave energy delivery to the beach and reducing alongshore energy flux.