Restoration of the Elwha River system, Washington, through dam removal

Most of the world's rivers contain one or more dams. On streams and rivers in the U.S. alone there are more than 75,000 dams large enough to store more than one year's runoff capacity, and many more dams smaller than that. Dams are built usually for one of three purposes: water storage, flood control, and hydropower generation. Although these provide useful services for human societies, dams also cause substantial environmental impacts. Dams change rivers' physical and ecological characteristics by altering flow magnitude and timing (the amount of water and timing of high and low flows), water temperature, sediment transport, and habitat quality. Many large dams built in the early 1900s are now aging, often requiring expensive repairs. In some cases instead of repairing an aging dam, it is economically favorable to remove the dam instead. Dam removal is increasingly being used as a means to restore watersheds and their ecosystems.

The Elwha River, Washington, USA, is the site of the largest dam removal to date. The Elwha watershed occupies a largely undeveloped, pristine area in Olympic National Park, and was formerly the site of legendary salmon and steelhead runs. The construction of two large dams in the early 1900s to provide hydropower to a paper and timber mill company (32-m-high Elwha Dam in 1913, and 64-m-high Glines Canyon Dam in 1927) blocked fish access to most of the watershed and trapped the river's sediment supply in two reservoirs, forming large sediment deltas in the lakes behind the dams. The loss of fish habitat caused by the dams contributed to steep decline in native fish populations and habitat quality. The Lower Elwha Klallam Tribe, whose culture is closely connected to the salmon runs and surrounding land, was negatively impacted by the dams. In the 1990s federal legislation was passed that required removal of both dams; the paper and timber company began obtaining electric power from the local grid that did not depend on the Elwha River dams.

Removal of both dams on the Elwha River began in September 2011 (Fig. 1). Dam removal proceeded gradually over the next two years, in carefully-timed phases designed to minimize short-term harmful effects of a large sediment release. The river is naturally eroding sediment that had been trapped in the two reservoirs for nearly a century (Fig. 2). The reservoir sediment now moves downstream by flooding during winter storms and spring snowmelt, a process that will continue for decades as the river adjusts to dam removal.

Scientists from many disciplines are studying the changes in the reservoir sediment deltas on the Elwha River, and changes in the downstream river channel and coastal system, during dam removal. Geomorphologists use topographic surveys, sediment grain-size measurements, aerial photography, and airborne and terrestrial lidar (light detection and ranging) data to characterize the Elwha channel and coastal zone before, during, and after dam removal. Biologists and ecologists study plant and animal communities, nutrient abundance, and food-web structure to monitor the effects of restoring upstream sediment supply to the lower river and coastal zone, and restoring salmon migration access to the upper Elwha watershed.

The first winter after dam removal began (2011–2012) saw deposition of new mud downstream of the Elwha Dam site in deposits tens of centimeters thick, and by fall 2012 new sand, pebble and gravel deposits covered areas of the riverbed below both dam sites where coarse cobbles had been previously (Fig. 3), an indication that the caliber of spawning gravel used by salmon could soon be present over much of the riverbed. In summer 2012 adult salmon and steelhead were observed upstream of the Elwha Dam site, having migrated from the ocean into a section of the river that had formerly been blocked by the lower of the two dams. The Elwha river and coastal system will continue to change rapidly in the coming years in response to restored upstream sediment supply, and the ecosystem (including marine-derived nutrients from salmon, and feeding habits of animals from invertebrates to black bears) will begin to adapt to the restoration of salmon migration and spawning in the upper watershed.

• Draut, A.E., Logan, J.B., and Mastin, M.C., 2011, Channel evolution on the dammed Elwha River, Washington, USA: Geomorphology, v. 127, p. 71–87.
• Duda, J.J., Warrick, J.A., and Magirl, C.S., eds., Coastal habitats of the Elwha River, Washington—Biological and physical patterns and processes prior to dam removal: U.S. Geological Survey Scientific Investigations Report 2011-5120, http://pubs.usgs.gov/sir/2011/5120/.
• Graf, W.L., 1999. Dam nation; a geographic census of American dams and their large-scale hydrologic impacts. Water Resources Research 35 (4), 1305–1311.
• Schmidt, J. C., and P. R. Wilcock (2008), Metrics for assessing the downstream effects of dams, Water Resources Research, vol. 44, W04404, doi:10.1029/2006WR005092.
• Warrick, J.A., Duda, J.J., Magirl, C.S., and Curran, C.A., 2012, River turbidity and sediment loads during dam removal: Eos, v. 93, no. 43, p. 425–426, DOI: 10.1029/2012EO430002. http://www.nps.gov/olym/naturescience/elwha-ecosystem-restoration.htm

For more information:

National Park Service web site on Elwha River dam removals
http://www.nps.gov/olym/naturescience/elwha-ecosystem-restoration.htm

Lower Elwha Klallam Tribe's information on the dam removals
http://www.elwha.org/damremoval.html