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1. Southern waters of the Strait of Juan de Fuca near the mouth of the Elwha River, Washington. Five-meter bathymetry contours also are shown.
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Increases in sediment delivery to coastal waters are expected following removal of dams on the Elwha River, Washington, potentially increasing sediment deposition on the seafloor and suspended sediment in the water column. Biological communities inhabiting shallow, subtidal depths (3–18 m) near the mouth of the Elwha River, between the west end of...
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Citations
... During surveys in 2012, one year after dam removal began, it became clear that dam removal effects on macroalgae extended farther from the river mouth than the previously established impact sites. To better capture the spatial extent of dam removal effects, in 2012 we established three additional impact sites (J, K and L; Fig 1b) at locations where macroalgae had been abundant during reconnaissance surveys made in 2008 [60] and were still at least moderately abundant when resurveyed in 2012. All surveys were conducted during summer between 21 July and 12 September (S1 Table). ...
... Biological change at dive sites. We collected benthic community data at each dive site using methods adapted from the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO) kelp forest monitoring program [60,66]. Individual organisms present in a 1-m wide swath along the 30-m transect (Fig 2B) and exceeding a size threshold (! 2.5 cm in any dimension for invertebrates, and > 24 cm long from holdfast to blade tip for kelp [Laminariales] and acid kelp [Desmarestia]) were identified to the lowest practicable taxonomic level and tallied. ...
... We define primary cover as the surface area of substrate occupied by the attachment structures of vegetation or sessile invertebrates. Percent primary cover was estimated using a uniform point contact (UPC) methodology [60]. At points spaced every 0.5 m along each transect (N = 60 points per transect; Fig 2C), we recorded the type of organism attached to the substrate or the substrate type (bare rock or bare sand) if it was unoccupied. ...
The coastal marine ecosystem near the Elwha River was altered by a massive sediment influx—over 10 million tonnes—during the staged three-year removal of two hydropower dams. We used time series of bathymetry, substrate grain size, remotely sensed turbidity, scuba dive surveys, and towed video observations collected before and during dam removal to assess responses of the nearshore subtidal community (3 m to 17 m depth). Biological changes were primarily driven by sediment deposition and elevated suspended sediment concentrations. Macroalgae, predominantly kelp and foliose red algae, were abundant before dam removal with combined cover levels greater than 50%. Where persistent sediment deposits formed, macroalgae decreased greatly or were eliminated. In areas lacking deposition, macroalgae cover decreased inversely to suspended sediment concentration, suggesting impacts from light reduction or scour. Densities of most invertebrate and fish taxa decreased in areas with persistent sediment deposition; however, bivalve densities increased where mud deposited over sand, and flatfish and Pacific sand lance densities increased where sand deposited over gravel. In areas without sediment deposition, most invertebrate and fish taxa were unaffected by increased suspended sediment or the loss of algae cover associated with it; however, densities of tubeworms and flatfish, and primary cover of sessile invertebrates increased suggesting benefits of increased particulate matter or relaxed competition with macroalgae for space. As dam removal neared completion, we saw evidence of macroalgal recovery that likely owed to water column clearing, indicating that long-term recovery from dam removal effects may be starting. Our results are relevant to future dam removal projects in coastal areas and more generally to understanding effects of increased sedimentation on nearshore subtidal benthic communities.
... Boulders of all sizes, which were remnants left from eroding glacial deposits, remained scattered on the seafloor. Rubin et al. (2011) characterized the pre-dam removal delta substrate, based on mapping by Cochrane et al. (2008) and Warrick et al. (2008), as predominately gravel-and cobble-sized sediment, with areas of bedrock, boulders, and some sand. Rubin et al. (2011) classified the subaqueous delta into habitat types based on a series of SCUBA dive surveys to assess benthic habitat and biological communities. ...
... Rubin et al. (2011) characterized the pre-dam removal delta substrate, based on mapping by Cochrane et al. (2008) and Warrick et al. (2008), as predominately gravel-and cobble-sized sediment, with areas of bedrock, boulders, and some sand. Rubin et al. (2011) classified the subaqueous delta into habitat types based on a series of SCUBA dive surveys to assess benthic habitat and biological communities. These substrate types, from sand to bedrock, were favorable for a variety of submerged aquatic vegetation (10 different species of kelp), invertebrates (65 different invertebrate taxa), and certain fish (24 different fish taxa). ...
... The exact causes of the decrease in kelp density are uncertain, but may result from a decrease in light availability during the kelp's growing periods, a decrease in rocky or hard substrate for kelp to attach, scouring of kelp recruits by high suspended sediment loads, or a combination of all three. Even as kelp density is reduced, though, other species may benefit or not be affected at all by the rapid change in substrate (Thrush et al., 2004;Wheatcroft, 2006;Rubin et al., 2011). Continued monitoring of habitat and community response to increased sediment supply following Elwha River dam removal provides a unique opportunity to study the geological and the biological consequences of river restoration in the coastal environment. ...
Two dams on the Elwha River, Washington State, USA trapped over 20 million m3 of mud, sand, and gravel since 1927, reducing downstream sediment fluxes and contributing to erosion of the river's coastal delta. The removal of the Elwha and Glines Canyon dams, initiated in September 2011, induced massive increases in river sediment supply and provided an unprecedented opportunity to examine the geomorphic response of a coastal delta to these increases. Detailed measurements of beach topography and nearshore bathymetry show that ~ 2.5 million m3 of sediment was deposited during the first two years of dam removal, which is ~ 100 times greater than deposition rates measured prior to dam removal. The majority of the deposit was located in the intertidal and shallow subtidal region immediately offshore of the river mouth and was composed of sand and gravel. Additional areas of deposition include a secondary sandy deposit to the east of the river mouth and a muddy deposit west of the mouth. A comparison with fluvial sediment fluxes suggests that ~ 70% of the sand and gravel and ~ 6% of the mud supplied by the river was found in the survey area (within about 2 km of the mouth). A hydrodynamic and sediment transport model, validated with in-situ measurements, shows that tidal currents interacting with the larger relict submarine delta help disperse fine sediment large distances east and west of the river mouth. The model also suggests that waves and currents erode the primary deposit located near the river mouth and transport sandy sediment eastward to form the secondary deposit. Though most of the substrate of the larger relict submarine delta was unchanged during the first two years of dam removal, portions of the seafloor close to the river mouth became finer, modifying habitats for biological communities. These results show that river restoration, like natural changes in river sediment supply, can result in rapid and substantial coastal geomorphological responses.
Characterizing the physical and biological characteristics of the lower Elwha River, its estuary, and adjacent nearshore habitats prior to dam removal is essential to monitor changes to these areas during and following the historic dam-removal project set to begin in September 2011. Based on the size of the two hydroelectric projects and the amount of sediment that will be released, the Elwha River in Washington State will be home to the largest river restoration through dam removal attempted in the United States. Built in 1912 and 1927, respectively, the Elwha and Glines Canyon Dams have altered key physical and biological characteristics of the Elwha River. Once abundant salmon populations, consisting of all five species of Pacific salmon, are restricted to the lower 7.8 river kilometers downstream of Elwha Dam and are currently in low numbers. Dam removal will reopen access to more than 140 km of mainstem, flood plain, and tributary habitat, most of which is protected within Olympic National Park. The high capture rate of river-borne sediments by the two reservoirs has changed the geomorphology of the riverbed downstream of the dams. Mobilization and downstream transport of these accumulated reservoir sediments during and following dam removal will significantly change downstream river reaches, the estuary complex, and the nearshore environment. To introduce the more detailed studies that follow in this report, we summarize many of the key aspects of the Elwha River ecosystem including a regional and historical context for this unprecedented project.
