Sediment Deposition on the Elwha Submarine Delta: Changes in Substrate Composition and Conditions for Fine-Grain Deposition

Abstract

The coarse Elwha River Delta is now exposed to an influx of fine-grained sediment from the removal of two dams. The coarse substrate of the delta and energetic oceanographic processes will result in complicated sediment deposition patterns. This study used grain-size distribution and water velocity data compared with prior studies to conclude where change in the delta substrate has occurred and under what conditions deposition is likely to occur. Grab samples and video footage were used to map the substrate and a shipboard acoustic Doppler current profiler was used to measure water velocity and subsequently to estimate bed shear velocity. Currents west of the river were on average lower then on the east suggesting deposition is likely to occur on the west side. Deposition of fine sediment was found west of the river while the seabed east of the river has remained coarse. Deposition of sand is likely to occur during periods of low shear velocities during ebb and slack tides. This study provided background work for future research on the oceanographic processes that allow deposition on the west side of the river.