After the Elwha Dams: What USGS and Lower Elwha Klallam Tribe Scientists Learned

U.S. Geological Survey Fact Sheet 2018–3025. By Jeffrey J. Duda, Jonathan A. Warrick and Christopher S. Magirl (USGS) and Matt M. Beirne (Lower Elwha Klallam Tribe).

Taking down two dams

After nearly a century of making power, two large hydroelectric dams on the Elwha River in Washington were removed from 2011 to 2014 to restore the river and bring back imperilled salmon. About two-thirds of the 21 million cubic metres of sediment trapped behind them — enough to fill nearly 2 million dump trucks — was released downstream, restarting natural processes in the river, estuary and sea. Scientists from the Lower Elwha Klallam Tribe, universities, non-profit groups, federal and state agencies and the USGS gathered data before, during and after removal.

A long partnership. The Tribe has depended on the Elwha for spiritual, cultural and economic sustenance since time immemorial. Its natural resources staff have worked with USGS scientists for decades, especially on the river and on the coast where the reservation lies — an information legacy that helps the Tribe understand the changes and manage its coast.

Key lessons so far

• The river adjusted to the sediment faster than expected.
• Reconnected, the Elwha now looks more like the Olympic Peninsula's other wild rivers.
• Salmon are returning to the main stem and tributaries above the old dams.
• The estuary and beaches are growing, and plants, aquatic animals and birds are using the new habitat.
• The sea off the river mouth has fundamentally shifted toward a restored state.

The river

The released sediment was comparable to what rivers carry after a moderate volcanic eruption. For the first three years the river was choked: pools filled, and at times the bed turned to sand, unlike the cobble-and-gravel bottom upstream. Once the pulse passed, the bed below the dams began to look as it had before. Monitoring above and below the dams showed very high suspended sediment during removal, falling year by year afterward.

River flow (top) and suspended-sediment concentration by week and by year, measured at USGS station 12046260 from turbidity readings every 15 minutes and samples across a range of flows (photos). USGS, from Magirl and others (2015).

The estuary

• Physical. The river carried more than 90 percent of the sediment eroded from the reservoirs out to the coast. Before removal, the river mouth fluctuated as tides and river flow mixed fresh and salt water; afterward, large deposits cut off the tides and turned most of the "old estuary" into fresh water.
• Biological. New landforms and habitat reversed decades of erosion and loss. Total plant species richness and density held steady, but pioneer vegetation increased on the new ground. Bottom-dwelling invertebrates declined, probably buried by sediment. The fish in the former estuary shifted toward freshwater species while it still served as a nursery for young salmon, and as the delta grew seaward, new estuary habitat drew salmon, shorebirds and their invertebrate prey.

The coast

• Physical. Before removal, beaches near the river mouth — including those of the reservation — were eroding by more than 1 foot a year, and faster over time, costing the Tribe land and coastal wetlands. Removal reversed that, building broad new beaches and a new estuary at the river mouth. Coastal waters stayed murky during removal but have cleared considerably since.
• Biological. For 10 years, divers from the USGS, the Tribe and partners watched the seafloor of the Strait of Juan de Fuca. Near the river mouth, murky water cut the light reaching kelp and algae, which declined sharply. Clams, tubeworms and some fish changed only where sediment actually settled — and some species, such as the Pacific sand lance, increased in the new sand. Most importantly for the Tribe, Dungeness crab returned in abundance along the new delta, reopening a productive fishery for the first time in the memory of many Tribal fishers.

Beach and seafloor mapping of the river mouth before, during and after dam removal, 2011–2017. Image courtesy of Andrew Stevens, USGS.

Sources

• Duda, J.J., Beirne, M.M., Warrick, J.A., and Magirl, C.S., 2018, Science partnership between U.S. Geological Survey and the Lower Elwha Klallam Tribe—Understanding the Elwha River dam removal project: U.S. Geological Survey Fact Sheet 2018–3025. https://doi.org/10.3133/fs20183025 · https://pubs.usgs.gov/publication/fs20183025
• USGS Elwha River science: https://www.usgs.gov/elwha · Lower Elwha Klallam Tribe: https://www.elwha.org
• The two figures come from the fact sheet's PDF. Its photographs of the dams and of the river are by private photographers, and its estuary maps are adapted from journal articles; they are not reproduced here.
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.