Research Article: Increased sediment load during a large-scale dam removal changes nearshore subtidal communities

Date Published: December 8, 2017

Publisher: Public Library of Science

Author(s): Stephen P. Rubin, Ian M. Miller, Melissa M. Foley, Helen D. Berry, Jeffrey J. Duda, Benjamin Hudson, Nancy E. Elder, Matthew M. Beirne, Jonathan A. Warrick, Michael L. McHenry, Andrew W. Stevens, Emily F. Eidam, Andrea S. Ogston, Guy Gelfenbaum, Rob Pedersen, James P. Meador.


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.

Partial Text

Benthic marine habitats are often defined by their substrate characteristics, yet sediment dynamics such as erosion, transport, and deposition are also important structuring mechanisms in marine ecological communities [1–3]. Human landscape alterations have changed the flux of sediment to the global coastal ocean [4], patterns of deposition and erosion in the marine environment [5], and sediment linkages between catchments and marine environments [6]. For example, activities such as deforestation [7], mining [8], and urbanization [9] have increased the delivery of sediment to coastal habitats, and consequently driven community change in coral reefs [10], seagrass meadows [11], estuaries [12], and rocky reefs [2, 13]. Increased sediment delivery can directly affect habitats and organisms and lead to community-scale changes by burying substrates [14–16], preventing propagules from settling [17, 18], or reducing growth and survival [19, 20]. Sedimentation can also indirectly affect community composition by altering rates of competition and predation, for example, through creating refugia for deposition tolerant species [21, 22].

The Elwha River (Fig 1) originates in a mountainous landscape and descends from an elevation of approximately 1100 m to sea level over 72 km. The Elwha River has a mean annual discharge of 43 m3·s-1, with a bimodal hydrograph shaped primarily by rain in the winter months and snowmelt in the spring. The discharge magnitude for 2-year recurrence flood is 400 m3·s-1 [51].

We assessed the effects of dam removal on nearshore subtidal communities using a Before-After-Control-Impact (BACI) approach [65]. Ecological data were collected along gradients of expected impact associated with turbidity and sediment deposition east and west of the river mouth, at 3–17 m depths referenced to mean lower low water (MLLW). We also collected data in two control areas where effects of dam removal were expected to be minimal: Green Point, 21 km east of the Elwha River (“Control” and “Dungeness Bluffs”; Fig 1a); and Crescent Bay, 15 km west of the Elwha River (Fig 1a). Our study was conducted on lands managed by the Washington Department of Natural Resources but lacking any other protection status (i.e., they were not in preserves). No specific permissions were required for our study. We did not observe any endangered or protected species during our study.

The degree to which sediment deposition drives changes in benthic marine habitats and biological communities is an important scientific and management issue, particularly in regards to anthropogenic sources like those derived from a large-scale dam removal [1, 4, 89]. During the simultaneous removal of two dams on the Elwha River that was phased over three years, decades worth of annual sediment load was released that changed nearshore subtidal communities. Sediment entering the coastal system increased suspended sediment concentrations and formed persistent deposits on the seafloor in some areas. Community change differed depending on whether elevated suspended sediment was accompanied by sediment deposition or not. Spatial and temporal patterns of suspended and deposited sediments determined where and when each type of community change occurred.

The large amount (> 10 Mt) of sediment released into marine waters during removal of the Elwha River dams altered the benthic community. The characteristics of community response differed depending on the context of sedimentation. The two main types of sediment effects were increased turbidity and sediment deposition. Turbidity increases had a greater spatial footprint than formation of sediment deposits, allowing us to separate how these two physical changes affected the community. This might have relevance to future dam removal projects occurring in coastal areas, but might also be more broadly applicable to any scenario of increased coastal sedimentation effects on marine benthic communities.