SEATTLE, WA, Feb. 18, 2015 -- New studies released this week by government, tribal and university researchers have shed new light on the effects of dam removal, revealing that it can change landscape features of river and coasts, which have ecological implications downstream of former dam sites. These scientists worked together to characterize the effects of the largest dam removal project in U.S. history, which occurred on the Elwha River in Washington State (see "Largest dam removal in U.S. history nears completion in WA").
Five peer-reviewed papers, with authors from the U.S. Geological Survey (USGS), Reclamation, National Park Service, Washington Sea Grant, NOAA Fisheries, the Lower Elwha Klallam Tribe, and the University of Washington, provide detailed observations and insights about the changes in the river's landforms, waters and coastal zone during the first two years of dam removal. During this time, massive amounts of sediment were eroded from the drained reservoirs and transported downstream through the river and to the coast.
One finding that intrigued scientists was how efficiently the river eroded and moved sediment from the former reservoirs. Over a third of the 27 million cubic yards of reservoir sediment (equivalent to about 3,000 Olympic swimming pools filled with sediment) was eroded into the river during the first two years, although its water discharge and peak flows were moderate compared to historical gaging records.
This sediment release altered the river's clarity and reshaped its channel while adding new habitats in the river and at the coast. In fact, the vast majority of the new sediment was discharged into the coastal waters of the Strait of Juan de Fuca, where the river mouth delta expanded seaward by hundreds of feet.
Although the primary goal of the dam removal project is to reintroduce spawning salmon runs to the pristine upper reaches of the Elwha River within Olympic National Park, the new studies suggest that dam removal can also have ecological implications downstream of the former dam sites. These implications include a renewal of the sand, gravel and wood supplies to the river and to the coast, restoring critical processes for maintaining salmon habitat to river, estuarine and coastal ecosystems.
The final stages of dam removal occurred during the summer of 2014. Some sediment erosion from the former reservoirs will likely continue. The Elwha Project and research teams are continuing to monitor how quickly the river returns to its long-term restored condition.
See also:
"American Rivers reports 72 dam removals for 2014, sets goal to 75 for 2015"
"Sixty-five dams in 19 states removed to restore rivers in 2012"
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