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Dam removal has gained traction as a powerful tool for restoring aquatic habitats and eliminating high-risk infrastructure. While most previous studies have concentrated on river and watershed responses to dam removal, the dam removals on the Elwha River, a short river within Olympic National Park that drains to the coast, offered an unprecedented chance to investigate the impact of dam removal on coastal ecosystems. 

A key finding of the study is the profound effect of sediment deposition on nearshore communities. Where sediment deposits persisted, mostly near (within two kilometers of) the river mouth, sites exhibited wholesale changes in their biological community composition, resulting in a shift that so far has not reverted to its pre-dam removal state. Following the sediment influx, these sites saw colonization by new species that were largely absent prior to the dam removal. Sediment deposition created habitat for ecologically important species such as sand lance—a forage fish—and shellfish such as geoducks and cockles. 

By contrast, seafloor sites without sediment deposition demonstrated a different response. High turbidity—sediment clouding the water column—was common during the dam-removal phase.  These sites saw a dramatic reduction in the abundance of kelp and other algae during the dam-removal process, followed by a rapid recovery as sediment flux to the nearshore environment decreased, allowing coastal waters to clear. Abundances of invertebrates like crab and polychaete worms fluctuated depending on their reliance on these algae, but in most cases returned to pre-disturbance levels. This suggests that while disturbances like increased turbidity can temporarily affect coastal ecosystems, they may quickly recover once the cloudiness diminishes. 

These phenomena underscore the importance of understanding the role that sediment deposits play in shaping the habitats of coastal ecosystems and highlights the need for careful management strategies during dam removal projects to mitigate, or in some cases take advantage of, their effects. 

The implications of this study may extend beyond the Elwha River project. As dam removals become more common and similar sediment perturbation events are anticipated, the study provides a crucial foundation for predicting and managing the response of marine ecosystems to these changes. 

The study, “Nearshore subtidal community response during and after sediment disturbance associated with dam removal”, is authored by USGS (Western Fisheries Research Center and Pacific Coastal and Marine Science Center), San Francisco Estuary Institute, Washington Sea Grant, Washington Department of Natural Resources, and the Lower Elwha Klallam Tribe.

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