Case Study: Ecological corridors in ecological networks
This case study is part of the IUCN Guidelines for conserving connectivity through ecological networks and corridors illustrating initiatives around the world that are working toward maintaining, enhancing, and restoring ecological connectivity. See all case studies here.
Pacific salmon watersheds: Restoring lost connections
By Lauren Law and Jonathan Moore, Simon Fraser University
Key lesson
Even in an otherwise protected watershed, dams impair connectivity of the headwaters to the ocean; dam removal can restore biotic and abiotic processes, as demonstrated in the Elwa River in the USA.
Context and challenge
Coastal watersheds that drain into the northern Pacific Ocean support populations of culturally and economically important migratory salmon. Pacific salmon are born and initially develop in freshwater environments and then migrate to the open ocean, where they forage and grow before returning to natal freshwaters to spawn. Across North America and Asia, 8% of high-value catchments that drain into the northern Pacific Ocean are at least partially protected, predominantly in areas that are higher in elevation and distant from the ocean (Pinsky et al., 2009). However, even if portions of catchments are protected, dams have fragmented many salmon systems. Dams, such as for hydroelectric production, may block or hinder salmon migration, alter hydrological regimes and modify downstream river habitat. As a result of the imperilled or extirpated status of many salmon populations (Gustafson et al., 2007), there have been substantial investments in conservation and recovery.
Chinook salmon (Oncorhynchus tshawytscha) © Adobe Stock
Approach
Over the last several decades, there has been increasing dam removal and mitigation to benefit salmon and other migratory fishes. Across the USA, more than 1,200 dams had been removed by 2017 (Bellmore et al., 2017). Dam removal generally occurs through a decentralised decision-making process involving numerous stakeholder groups, including federal agencies, state agencies, and private dam owners. Although some dam removals have been voluntary, many have been the result of legal proceedings that fall under the regulatory powers of the Federal Energy Regulatory Commission. Initial removal efforts were focused on older dam structures, which were too costly to maintain and no longer in compliance with modern safety standards. However, in recent years there has been a greater focus on dam removal for environmental protection and habitat restoration. In the USA, the Wild and Scenic River Act (1968) is a legal mandate to preserve rivers having exceptional natural, cultural and recreational values in a free-flowing state.
Figure 1. Elwha River watershed within Olympic National Park, Washington, USA. The removal of the Elwha Dam and Glines Canyon Dam restored connectivity between the upper and lower portions of the watershed. © Jonathan Moore
Examples of ecological corridors
In the USA, one of the largest dam removals that has restored connectivity in a protected salmon watershed was on the Elwha River. The vast majority of the 72-km-long river is within Olympic National Park in the state of Washington. Historically one of the most productive salmon rivers in the Pacific Northwest, the Elwha was disturbed in the early 1900s when two dams were constructed on it, disconnecting the protected upper portion of the watershed from the seascape that migratory salmon rely on. Migration of salmon was blocked, and the movement of sediment and woody debris was disrupted. The building of these large-scale dams led to a 90% reduction in fish populations, a loss of habitat connectivity and decline in habitat complexity (Pess et al., 2008).
In 1992, the Elwha River Ecosystem and Fisheries Restoration Act authorised the removal of the dams to restore the river ecosystem. The US National Park Service removed the dams in phases, starting with the removal of the smaller dam beginning in 2011 and eventually completing the removal of the larger dam in 2014.
Results
The removal of the Elwha River dams led to renewed riverine fluxes of sediments and large woody debris downstream that had been trapped in the dam reservoirs for nearly a century. Approximately 30 million tons of sediment were released, causing some 60 ha of river delta growth (Ritchie et al., 2018). The supply of sediment and large wood to the fluvial system restored channel morphology to its former complexity and resulted in increased river braiding, sediment-bar growth and pool filling.
Renewed connectivity of upstream protected habitat with the seascape in the Elwha River watershed is fostering the return of several salmon species (Chinook, Oncorhynchus tshawytscha; coho, Oncorhynchus kisutch; chum, Oncorhynchus keta; sockeye, Oncorhynchus nerka; and pink, Oncorhynchus gorbuscha) as well as anadromous trout (e.g. steelhead, Oncorhynchus mykiss; and bull, Salvelinus confluentus). Scientists have already observed record numbers of Chinook salmon returning to the Elwha, with high returns anticipated to follow for other species. About 30,000 Chinook and coho salmon and 270,000 pink salmon are expected to return annually. Salmon returns will eventually sustain local and regional fisheries.
The Elwha is one of many coastal catchments that has protected salmon habitat in its headwaters but whose connectivity to the seascape was severed. As illustrated by the Elwha project, dam removal and restoration of the free-flowing status of rivers can effectively connect protected headwaters with the seascapes on which migratory fishes such as salmon depend.
References
Bellmore, J.R., Duda, J.J., Craig, L.S., Greene, S.L., Torgersen, C.E., Collins, M.J., and Vittum, K. (2017). ‘Status and trends of dam removal research in the United States’. Wiley Interdisciplinary Reviews: Water 4:e1164. https://doi.org/10.1002/wat2.1164.
Gustafson, R.G., Waples, R.S., Myers, J.M., Weitkamp, L.A., Bryant, G.J., Johnson, O.W., and Hard, J.J. (2007). Pacific salmon extinctions: Quantifying lost and remaining diversity’. Conservation Biology 21:1009–1020. https://doi.org/10.1111/j.1523-1739.2007.00693.x.
Pess, G.R., McHenry, M.L., Beechie, T.J., and Davies, J. (2008). ‘Biological impacts of the Elwha River dams and potential salmonid responses to dam removal’. Northwest Science 82:72–91.
Pinsky, M.L., Springmeyer, D.B., Goslin, M.N. and Augerot, X. (2009). Range-wide selection of catchments for Pacific salmon conservation. Conservation Biology 23:680–691.https://doi.org/10.1111/j.1523-1739.2008.01156.x.