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Dam Removal on the Elwha River in Washington—Nearshore Impacts of Released Sediment
The Elwha River drains the rugged Olympic Mountains of Washington, flowing northward to the Strait of Juan de Fuca. Construction of two hydroelectric dams in the early 1900s resulted in the loss of approximately 95 percent of the anadromous salmon spawning habitat on the river. In 1992, the Elwha River Ecosystem and Fisheries Restoration Act was enacted by Congress to authorize removal of the dams in order to restore the once-plentiful salmon runs in the river. Dam removal is currently slated to begin in early 2008. As well as restoring salmon runs, dam removal on the Elwha River will expose more than 14 million m3 of sediment deposited in the deltas within the two reservoirs. The sediment-management strategy is to allow the material to be naturally eroded and transported to the Strait of Juan de Fuca, acknowledging that some sediment will remain in place and be deposited in the river channel and flood plain. Contributions of sediment to the strait may end or even reverse the current trend of coastal erosion near the river mouth. Sediment contributions may also bury or alter nearshore habitats (including kelp beds and geoduck clam burrows) offshore of the river mouth. U.S. Geological Survey (USGS) scientists have begun research to characterize the nearshore impacts of the Elwha River dam removals as part of the USGS Coastal Habitats in Puget Sound project. Project scientists have been working closely with local, tribal, State, and Federal parties to develop coordinated monitoring and modeling plans. Within the USGS, scientists from the Coastal and Marine Geology Program, which is funding the project, are working with hydrologist Chris Konrad, biologist Jeff Duda, and geographer Harvey Case to develop a coordinated Elwha River science plan that links fluvial, ecological, and coastal research. As part of this coordination, USGS geologist Jon Warrick participated in the "Technical Workshop on Nearshore Restoration in the Central Strait of Juan de Fuca" held March 2004 in Port Angeles, WA, where ecologists, fishery scientists, engineers, and geomorphologists agreed on the high-priority need for a conceptual model of sediment transport and deposition off the Elwha River. Since this workshop was held, the USGS has led efforts to develop both a simple conceptual model of sediment transport and a research plan for quantifying sediment-transport rates and pathways.
The USGS Coastal Habitats in Puget Sound Project will employ three major techniques to evaluate dam-removal impacts: mapping, monitoring, and modeling. The mapping work will focus on collecting baseline bathymetric and seabed information from which changes can be tracked through the dam-removal process. Two mapping techniques are being used. In March 2004, Guy Cochrane, Jon Warrick, Jodi Harney, Andy Stevenson, Larry Kooker, Mike Boyle (all of the USGS), and Tina Blewett (Washington Department of Fish and Wildlife) used combined swath-sonar, seabed video, and seabed grain-size sampling techniques from the research vessel Karluk to map the nearshore region off the Elwha River mouth. Preliminary results show that the substrate from the river mouth out to approximately 30-m water depth consists of mixed sand and gravel, with areas of large sand waves (approx 10 m high) and some boulder fields. In September 2004, Guy Gelfenbaum, Peter Ruggiero, Jodi Eshleman (all USGS), and Etienne Kingsley (Washington Department of Ecology) conducted bathymetric and topographic mapping, using a second mapping technique that relies on satellite-based global-positioning-system (GPS) units on land and in watercraft. More than 100 shoreline cross sections were obtained in the September mapping exercises, which will be repeated twice per year through the dam removal. These measurements—along with beach surveys by the Lower Elwha Klallam Tribe at seven sites and aerial photography by the Surfrider Foundation—will be the primary method of tracking changes offshore of the Elwha River mouth.
Because oceanographic information about the strait is very limited, the USGS research project will also monitor and numerically model waves and currents in the region offshore of the river mouth. Oceanographic monitoring will begin during spring 2005 with deployments of acoustic Doppler current profilers (ADCPs) and directional wave gauges, which will be used to develop an understanding of waves, tides, and currents that may affect the sediment released by dam removal. Observations of physical conditions off the river mouth will be used to calibrate and validate a three-dimensional hydrodynamic model of the Elwha River area of the strait currently being developed by Guy Gelfenbaum and Giles Lesser with Delft3D modeling software. Modeling is an important part of the research plan because wave and current conditions along the river-mouth delta are expected to vary widely, owing to the complex bathymetry of the submarine delta offshore of the present river mouth. Preliminary results of the circulation modeling show strong eddies offshore of the river mouth, features that are commonly encountered by local fishermen. The planned mapping, monitoring, and modeling will help characterize the pathways and fate of sediment released from behind the dams of the Elwha River. This information will be crucial in evaluating the impacts of dam removal on the substrate and habitats of the beaches and the nearshore.
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in this issue:
Pulley Ridge Deepest Known US Coral Reef Climate Features Influenced 2004 Hurricane Landfall Count
Oceanographer Joins Western Coastal & Marine Geology Team Netherlands Students Contribute to USGS Studies Florida Integrated Science Center, Gainesville, FL |
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U. S. Department of the Interior | U.S. Geological Survey
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