USGS Science to Support the Elwha River Restoration Project
Restored supplies of sediment following dam removal are expected to slow or reverse coastal erosion and modify coastal and marine habitats near the Elwha River mouth. A major focus of the ongoing USGS research is to understand the legacy of the dams on coastal processes and the restorative effects of dam removal on coastal features and habitats. An integrated science approach is used with the goal of developing new tools for understanding how coastal ecosystems respond to large perturbations such as dam removal.
Damming of the Elwha River in 1912 dramatically altered the flow of sediment through this river and to the shoreline. This reduction of sediment discharge to the coast coincided with rapid erosion of the shoreline of the Elwha River delta.
To better quantify the erosion of the shoreline and the restorative effects of dam removal, the USGS has been conducting highly accurate GPS surveys of the beach and the seafloor immediately offshore of the beach. Combined with analyses of historical aerial photographs, this work suggests that the beach eroded on average 38 m between 1938 and 2006, and that the rate of erosion has increased with time.
For more information about these results, see the article by Warrick and others (2009), which is published in the journal Geomorphology and available at doi:10.1016/j.geomorph.2009.04.012.
Survey data and supporting information are also available in USGS Data Series Report 288 by Warrick and others (2010), which can be found at http://pubs.usgs.gov/ds/288/.
The removal of Glines Canyon and Elwha Dams during 2012-2015 will release millions of cubic meters of sand and silt to the nearshore waters of the Strait of Juan de Fuca. To better assess the restorative impacts of these renewed supplies of sediment on coastal ecosystems and habitats, the USGS is conducting inventories using both scuba surveys and sonar technologies.
The scuba surveys are used to chronicle the narshore biological communities of the shallow, subtidal regions around the Elwha River delta. During scuba surveys, research divers count and measure all observed kelp, invertebrate and fish taxa, and measure seafloor substrate, relief and depth. Baseline surveys conducted before the dam removal have shown that the biological communities were partly controlled by substrate type, seafloor relief and water depth, and the highest kelp densities and taxa richness have been found on bedrock and boulder reefs. A synthesis of these baseline information can be found in the Rubin and others (2011), which is chapter 6 of the USGS Scientific Investigations Report 2011-5120, found at http://pubs.usgs.gov/sir/2011/5120/. Further information about the scuba surveys can be obtained from Stephen Rubin.
Above: The turbid river plume of the Elwha River during high flow, Dec. 2007, Photo Credit: U.S. Coast Guard.
The dispersal of Elwha River sediment in the coastal waters of the Strait of Juan de Fuca will be increased following dam removal. This additional sediment will cause increased turbidity and sedimentation in the coastal zone. Fine sediment (silt and clay) will be transported offshore in suspended plumes and may plunge to the deeper portions of the Strait of Juan de Fuca in sediment gravity flows. The USGS is investigating the pathways and rates of sediment transport through oceanographic instrumentation placed in the nearshore and detailed surveys of the turbid plumes. Some of this research will be conducted in collaboration with researchers at the University of Washington.
In preparation for the dam removal project, the USGS conducted numerous measurements of the currents and waves offshore of the Elwha River delta and made detailed surveys of the river plume that extends from the river mouth into the Strait of Juan de Fuca. Results of these measurements can be found in an article by Warrick and Stevens (2011), which is available at doi:10.1016/j.csr.2010.11.007.
Fine sediment is also tracked with long-term deployments of benthic tripods with current meters and acoustic and optical turbidity sensor. For more information about these and other river plume survey, please contact Dr. Jonathan Warrick.
The beach of the Elwha River delta is primarily sand and gravel, and the erosion of this beach suggests that the movement of this sediment is rapid. Measurements of the rates of littoral sediment movement have been made with beach cobble tagged with radio frequency identifier (RFID) technology in collaboration with University of California, Santa Cruz graduate student, Ian Miller. This work has shown that beach sediment is transported toward the east along most of the Elwha River delta, and that the rate of transport is proportional to wave height.
For more information about the measurements of beach sediment transport with RFID technology, see the article by Miller and others (2011) in the journal Marine Geology, which is available at doi:10.1016/j.margeo.2011.02.012.