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Rivers around the world provide valuable ecosystem services like food, drinking water, and recreational opportunity, as well as important infrastructure for human activities such as transportation and electricity generation. Rivers are also home to aquatic organisms, providing essential habitats for fish spawning, rearing, and migration.

Former Lake Mills five years after dam removal
The Elwha River flows through the footprint of the former reservoir Lake Mills. The wide-angle image also shows revegetated uplands dominated by alder trees meeting coniferous forest along the former shoreline of the reservoir. Photo taken from the retained spillway of the former Glines Canyon Dam five years after completion of the historic dam removal project that saw two large dams removed for anadromous fish and ecosystem restoration. Image taken by Jeff Duda, USGS.

Given the many people and aquatic animals that rely on rivers, the sometimes-conflicting demands can alter the structure and function of these ecosystems. Understanding how river ecosystems work and how they respond to alterations, including the removal of dams, is critical for supporting these resources.

With the removal of two large dams on the Elwha River, WA, in 2011-2014, scientists from USGS have used innovative ways to document the outcomes of dam removal, including how fish return to previously blocked upstream waters. Jeff Duda at USGS Western Fisheries Research Center and Christian Torgersen at the USGS Forest and Rangeland Ecosystem Science Center have teamed up with multiple partners to ask questions about how to collect data in rivers ecosystems to visualize relevant patterns and how fish species return following removal of dams. Our research plays an important role in understanding the ecological outcomes of dam removal including a suite of research studies on the Elwha River, home to the largest dam removal to date and a significant salmon restoration project.

How can we visualize and understand an ecosystem when it’s completely underwater?

You’ve probably heard of landscapes, but how about “riverscapes?” In recent decades, scientists have increasingly adopted a landscape perspective to study rivers. Using this view alongside technological advancements allowing high-resolution data collection across broad spatial scales, mapping the entire spatial scene of the river environment gives new understanding about how river processes, hierarchies, and connectivity interact to create scale-dependent patterns of aquatic organisms and their habitat. In a recent publication of Biological Reviews, USGS scientists, along with federal, university, and international partners describe riverscape approaches and how they have been used for research and management. Recent technological advancements in remote sensing and modelling are discussed that allow riverscape approaches to be used in practice across a wide array of river systems. This state-of-the-art review paper establishes an authoritative benchmark and holistic framework for using "riverscape approaches" to study design, data collection and analysis.

Innovative riverscape approaches are informed by high-resolution data collected over broad spatial scales to better study, visualize and communicate spatiotemporal complexity of freshwater systems to tackle complex problems and advance scientific understanding of river systems.

If we remove it, will they come? 

The removal of two large dams on the Elwha River opened over 110 km of pristine river habitat in Olympic National Park to anadromous migratory fish populations, including Pacific salmon.  Historically, there were 8 anadromous species (Chinook, Coho, Sockeye, Pink, Chum, Steelhead, Pacific Lamprey, and Bull Trout) in the Elwha River, but after the dams were constructed, numbers of returning adults in these populations had steadily declined to low levels or experienced complete extirpation. But, once a dam is removed, do these fish return? And if they do, when and how will they distribute and reestablish? In a new publication of Frontiers in Ecology and Evolution, USGS scientists and their partners put riverscape approaches into action, as they describe spatial and temporal patterns of fish before and after dam removal in the Elwha River. They used spatially continuous snorkel surveys in consecutive years before and after dam removal during low summer flow to assess changes in fish distribution and density over 65 km of mainstem river (compared to the 7.9 km that was available prior to removing the dams). After dam removal, there was rapid passage into areas upstream of Elwha Dam, with 8 anadromous species observed above the lower dam site within 2.5 years. By 5 years, all these species, except for Chum salmon, were observed upstream of the previous Glines Canyon Dam (the upper dam). The spatial extent of adult Chinook salmon and Summer Steelhead increased upstream of the dams and densities increased. The spatial extent of resident 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. 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, setting the stage for a return to pre-dam species diversity and population levels.

Want to learn more? Check out these recent publications!

Duda, J.J., and J.R. Bellmore. 2021. Dam removal and River Restoration. Encyclopedia of Inland Waters 2nd Edition, Vol. 2 (T. Mehner and K. Tockner, editors), pp. 576-585.

Duda, J.J., C.E. Torgersen, S.J. Brenkman, R.J. Peters, K.T. Sutton, H. Connor, P. Kennedy, S.C. Corbett, E.Z Welty, A. Geffre, J. Geffre, P. Crain, D. Shreffler, J.R. McMillan, M.L. McHenry, and G.R. Pess. 2021. Reconnecting the Elwha River spatial and temporal patterns of fish response to dam removal: Frontiers in Ecology and Evolution, 9:765488.

Duda, J.J., M.S. Hoy, D.M. Chase, G.R. Pess, S.J. Brenkman, M.M. McHenry, and C.O. Ostberg. 2020. Environmental DNA is an effective tool to track recolonizing migratory fish following large-scale dam removal. Environmental DNA. 3: 121–141.

Torgersen, C.E., C. Le Pichon, A.H. Fullerton, S.J. Dugdale, J.J. Duda, F. Giovannini, É. Tales, J. Belliard, P. Branco, N. Bergeron, M.L. Roy, D. Tonolla, N. Lamouroux, H. Capra, and C.V. Baxter. 2021. Riverscape approaches in practice: perspectives and applications: Biological Reviews 97 (2): 481-504.

Our science is collaborative! Partners who helped make these publications happen:

Lower Elwha Klallam Tribe, Olympic National Park, U.S. Fish and Wildlife Service, Washington Department of Fish and Wildlife, National Oceanic and Atmospheric Administration, University of Zurich, Shreffler Environmental, Trout Unlimited, University of Paris-Saclay, University of Nottingham, INRAE, University of Lisbon, Institut national de la recherche scientifique, Environment and Climate Change Canada, Zurich University of Applied Sciences, and Idaho State University.

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