Comparison of Grain Size and Delta Morphology East and West of the Elwha River Mouth

Abstract

For almost 100 years sediment has been building up behind two dams on the Elwha River in Washington State (Warrick et al., 2009). It was determined that in order to restore the watershed, the most effective process would be the removal of the dams (Gelfenbaum et al., 2009). This study looks at the effects of the increased sediment budget on comparative grain-size and topographical and bathymetry profiles of two transect lines, east and west of the Elwha River mouth. We found a higher accumulation of sediment on the East transect versus the West transect which is likely due to the oblique wave angle on the delta and net alongshore transport to the east. The morphology of the East transect line after dam removal changed to a steeper slope which could be caused from the increased sediment being deposited faster than it can be eroded. Lastly, we found that the beach substrate within the troughs on the subaerial delta was finer while the berms consisted of coarser material, which is likely due to the relative exposure to wave energy.