Abstract Title

Session S-07F: Elwah River Restoration: Evolution of Habitats and Ecosystems During a Dam Removal Project

Keywords

Restoration

Start Date

1-5-2014 3:30 PM

End Date

1-5-2014 5:00 PM

Description

Similar to other natural and anthropogenic influxes of sediment to coastal rivers, the removal of two dams on the Elwha River is expected to cause a wide range of impacts to nearby coastal environments. For example, fine-grained sediment may be advected seaward by currents and waves and deposited beyond shallow nearshore zones. The timing, location, and character of such deposits, which have the potential to alter the existing benthic habitat, depend on the available coastal energy. In this study, we characterize and explain the formation mechanisms of patchy sediment deposits that have developed in 10-60 m water depth offshore of the river mouth associated with dam removal. Prior to removal, much of the submarine Elwha Delta and adjacent Freshwater Bay substrate was coarse sands, gravels, and boulders. During two years of dam removal, ship-based surveys and nearbed instrument data show that fine-grained sediment pulses delivered via the buoyant surface plumes have generated new deposits near the river mouth and in Freshwater Bay. In contrast, the remaining three-quarters of the study area have received no measurable new sediment accumulation. The largest of the new deposits near the river mouth is > 30 cm thick and contains up to 90% mud. It has formed in a zone of slow currents and regional-scale flow eddies. In contrast, habitats in zones of strong current flow (up to 100 cm/s) north of the headland and across most of the outer delta have retained their gravelly character. Ephemeral deposition may occur on tidal time scales, but generally, fine-grained sediment appears to bypass these areas. These results expand our understanding of how dam removal has affected the Elwha Delta ecosystem, namely in the form of increased sediment flux through the surface plume and tidal partitioning of sediment to patchy seabed deposits. In a broader context, this study shows how high-energy coastal environments might efficiently process anthropogenic or natural sediment pulses.

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May 1st, 3:30 PM May 1st, 5:00 PM

Formation and character of fine-grained sediment deposits offshore of the Elwha River during two years of dam removal

Room 602-603

Similar to other natural and anthropogenic influxes of sediment to coastal rivers, the removal of two dams on the Elwha River is expected to cause a wide range of impacts to nearby coastal environments. For example, fine-grained sediment may be advected seaward by currents and waves and deposited beyond shallow nearshore zones. The timing, location, and character of such deposits, which have the potential to alter the existing benthic habitat, depend on the available coastal energy. In this study, we characterize and explain the formation mechanisms of patchy sediment deposits that have developed in 10-60 m water depth offshore of the river mouth associated with dam removal. Prior to removal, much of the submarine Elwha Delta and adjacent Freshwater Bay substrate was coarse sands, gravels, and boulders. During two years of dam removal, ship-based surveys and nearbed instrument data show that fine-grained sediment pulses delivered via the buoyant surface plumes have generated new deposits near the river mouth and in Freshwater Bay. In contrast, the remaining three-quarters of the study area have received no measurable new sediment accumulation. The largest of the new deposits near the river mouth is > 30 cm thick and contains up to 90% mud. It has formed in a zone of slow currents and regional-scale flow eddies. In contrast, habitats in zones of strong current flow (up to 100 cm/s) north of the headland and across most of the outer delta have retained their gravelly character. Ephemeral deposition may occur on tidal time scales, but generally, fine-grained sediment appears to bypass these areas. These results expand our understanding of how dam removal has affected the Elwha Delta ecosystem, namely in the form of increased sediment flux through the surface plume and tidal partitioning of sediment to patchy seabed deposits. In a broader context, this study shows how high-energy coastal environments might efficiently process anthropogenic or natural sediment pulses.