Abstract
The removal of two large dams on the Elwha River was completed in 2014 with a goal of restoring anadromous salmonid populations. Using observations from ongoing field studies, we compiled a timeline of migratory fish passage upstream of each dam. We also used spatially continuous snorkeling surveys in consecutive years before (2007, 2008) and after (2018, 2019) dam removal during summer baseflow to assess changes in fish distribution and density over 65 km of the mainstem Elwha River. Before dam removal, anadromous fishes were limited to the 7.9 km section of river downstream of Elwha Dam, potamodromous species could not migrate throughout the river system, and resident trout were the most abundant species. After dam removal, there was rapid passage into areas upstream of Elwha Dam, with 8 anadromous species (Chinook, Coho, Sockeye, Pink, Chum, Winter Steelhead, Summer Steelhead, Pacific Lamprey, and Bull Trout) observed within 2.5 years. All of these runs except Chum Salmon were also observed in upper Elwha upstream of Glines Canyon Dam within 5 years. The spatial extent of fish passage by adult Chinook Salmon and Summer Steelhead increased by 50 km and 60 km, respectively, after dam removal. Adult Chinook Salmon densities in some previously inaccessible reaches in the middle section of the river exceeded the highest densities observed in the lower section of the river prior to dam removal. The large number (>100) of adult Summer Steelhead in the upper river after dam removal was notable because it was among the rarest anadromous species in the Elwha River prior to dam removal. The spatial extent of trout and Bull Trout remained unchanged after dam removal, but their total abundance increased and their highest densities shifted from the lower 25 km of the river to the upper 40 km. Our results show that reconnecting the Elwha River through dam removal provided fish access to portions of the watershed that had been blocked for nearly a century.
Highlights
Societies around the world are confronting the many interacting—at times conflicting—demands placed on rivers
After the removal of Elwha Dam, anadromous fish species progressively returned to areas upstream: Winter Steelhead were first observed within 2 months, followed by four other anadromous species within the first year (April 2012 to March 2013)
The reconnection of the Elwha River will have far-reaching implications for genetic diversity, life history diversity, and habitat use of these species, which has started to be documented for Steelhead (Fraik et al, 2021), Pacific Lamprey (Hess et al, 2021), Coho Salmon (Liermann et al, 2017), and Bull Trout (Quinn et al, 2017; Lincoln et al, 2018; Brenkman et al, 2019)
Summary
Societies around the world are confronting the many interacting—at times conflicting—demands placed on rivers. Rivers are home to or provide essential habitat for aquatic organisms, including spawning, rearing, and migratory habitat for fish species. Many of the world’s large and medium sized rivers are dammed or otherwise fragmented (Lehner et al, 2011; Grill et al, 2015, 2019), impacting anadromous and potamodromous fish species by eliminating riverine habitat inundated by reservoirs, inhibiting connectivity and within-river migrations, and altering flow, sediment, temperature, and nutrient regimes in downstream areas (Petts, 1984; Poff et al, 1997; Bunn and Arthington, 2002; Poff, 2018). Others have shown that anadromous fish can return to and spawn in upstream areas following dam removal (Burdick and Hightower, 2006; Hogg et al, 2015; Allen et al, 2016; Battle et al, 2016; Liermann et al, 2017)
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