Anadromous fish historically migrated thousands of kilometers, but dams now delay or halt migrations on nearly every stream in the United States. There are some 2.5 million dams in the country (Pejchar & Warner 2001). Over the past century, fish populations have plummeted and billions of kilograms of biomass have been denied to upstream river reaches. To solve these problems, people are beginning to remove dams. Since 1999, more than 500 have been removed, reconnecting habitat and restoring natural flows in every region of the United States (American Rivers 2010). Removals have occurred nationwide but perhaps the most dynamic hotspot is the Pacific Northwest, where major rivers are expected to produce millions of salmon as well as millions of megawatts of electricity. This opens new opportunities for scientists as well as fish.
The biggest and best-known dam removal is happening on the Elwha River, in Washington’s Olympic Peninsula. In historic times, the Elwha saw ten runs of anadromous Pacific salmonids (Oncorhynchus spp.), including biennial returns of an estimated 250,000 pink salmon (O. gorbuscha) (NPS 2012) and gigantic Chinook (O. tshawytscha) that legendarily weighed 45 kilograms. In the early twentieth century, 32-meter Elwha Dam and 64-meter Glines Canyon Dam were built near the river mouth. The dams were impassible and salmonid populations dwindled. In 2011, removal began and the next spring the lower Elwha dam was gone. Glines Canyon is projected to be out by mid-2013. There are now at least four species of salmonid recolonizing upstream habitat they have not accessed since 1913 (USGS 2012).
The Elwha is well studied, but few other removal sites are. Dam removal’s impacts extend far beyond anadromous fish. The restoration of natural flows builds estuaries, beaches, and instream habitat below the dam as well as restoring upstream access. It also reconnects a river to its floodplain, with ripple effects far beyond the channel. Reservoirs, often a difficult environment for lotic taxa, are replaced by free-flowing habitat. While dam removal is not without problems – invasive species can move upstream and into the former impoundment, and it is challenging to manage the release of stored-up silt – there are few other restoration techniques that reconnect habitat on a system-wide scale. The issue for scientists is that removals’ span many disciplines and many years. What is needed is for more integrated research (Bushaw-Newton et al. 2003; M. Doyle et al. 2005) over longer time periods, to understand removal’s impacts over generations and along the length of the riparian corridor.
There will be many more dams considered for removal in the near future – roughly 85% of American dams will be past their useful lives by 2020 (Doyle et al. 2004). Dam removal is expensive and politically controversial, so good science is needed to inform these future decisions. The novel dam removal phenomenon presents an array of opportunities and challenges for scientists, as hydrologists, engineers, field ecologists, and modelers need to collaborate with managers and political stakeholders to ensure a successful project. Dams and rivers are idiosyncratic, and findings from individual cases can only have limited applicability. Going forward, we need strong pre- and post-removal monitoring to structure future projects and to provide broadly applicable science on dam removal and ensure effective ecological restoration.
American Rivers. (2010). 60 Dams removed to restore rivers in 2010.
Bushaw-Newton, K. L., Hart, D. D., Pizzuto, J. E., Thomson, J. R., Egan, J., Ashley, J. T., Johnson, T. E., et al. (2003). An integrative approach towards understanding ecological responses to dam removal : the Manatawny Creek Study. Journal Of The American Water Resources Association, 38(6), 1581–1599.
Doyle, M., Stanley, E., Orr, C., Selle, A., Sethi, S., & Harbor, J. (2005). Stream ecosystem response to small dam removal: Lessons from the Heartland. Geomorphology, 71(1-2), 227–244.
Doyle, M. W., Stanley, E. H., Selle, A. R., Stofleth, J. M., & Harbor, J. M. (2004). Predicting the Depth of Erosion in Reservoirs Following Dam Removal Using Bank Stability Analysis. International Journal, 18(2), 115–121.
Pejchar, L., & Warner, K. (2001). A river might run through it again: criteria for consideration of dam removal and interim lessons from California. Environmental Management, 28(5), 561–75.