Scientists worked together to examine and report the effects of removing two large dams from the Elwha River in Washington State, the largest dam-removal project in U.S. history.
Scientific Portrait of the Largest Dam Removal in U.S. History
The effects of dam removal are better known as a result of several new studies released in February 2015 by government, tribal, and university researchers. The scientists worked together to examine and report the effects of removing two large dams from the Elwha River in Washington State, the largest dam-removal project in U.S. history. New findings suggest that dam removal can change landscape features of river and coasts, affecting ecosystems downstream of former dam sites.
“These studies not only give us a better understanding of the effects of dam removal, but show the importance of collaborative science across disciplines and institutions,” said Suzette Kimball, Acting Director of the U.S. Geological Survey (USGS).
Five peer-reviewed papers, with authors from the USGS, the Bureau of Reclamation, the National Park Service, Washington Sea Grant, National Oceanic and Atmospheric Administration (NOAA) Fisheries, the Lower Elwha Klallam Tribe, and the University of Washington, provide detailed observations and insights about changes in the river’s landforms, waters, and coastal zone during the first 2 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; more than a third of the 27 million cubic yards of reservoir sediment, equivalent to the volume of about 3,000 Olympic-size swimming pools, was eroded into the river during the first 2 years, even though the river’s water discharge and peak flows were moderate compared with 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.
“The expansion of the river mouth delta is very exciting, because we are seeing the rebuilding of an estuary and coast that were rapidly eroding prior to dam removal,” said USGS research scientist and lead author of the synthesis paper Jonathan Warrick.
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 effects downstream of the former dam sites. These effects include a renewal of 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.
“These changes to sediment and wood supplies are important to understand because they affect the river channel form, and the channel form provides important habitat to numerous species of the region,” stated USGS research scientist and river study lead author Amy East.
The final stages of dam removal occurred during the summer of 2014. Some erosion of sediment from the former reservoirs will likely continue. Research teams are continuing to monitor how quickly the river returns to its long-term restored condition.
“We look forward to seeing when the sediment supplies approach background levels,” said Bureau of Reclamation engineer and co-author Jennifer Bountry, “because this will help us understand the length of time during which dam-removal effects will occur.”
The five new papers have been published in Elsevier’s peer-reviewed journal Geomorphology, and they focus on the following aspects of the large-scale dam removal on the Elwha River, Washington:
- Erosion of reservoir sediment (“Large-scale dam removal on the Elwha River, Washington, USA: Erosion of reservoir sediment”)
- Fluvial (river) sediment load (“Large-scale dam removal on the Elwha River, Washington, USA: Fluvial sediment load”)
- River channel and floodplain geomorphic change (“Large-scale dam removal on the Elwha River, Washington, USA: River channel and floodplain geomorphic change”)
- Coastal geomorphic change (“Large-scale dam removal on the Elwha River, Washington, USA: Coastal geomorphic change”)
- Source-to-sink sediment budget and synthesis (“Large-scale dam removal on the Elwha River, Washington, USA: Source-to-sink sediment budget and synthesis”)
A public lecture about some of these findings was presented in February 2015 at the USGS campus in Menlo Park, California, by USGS research geologist Amy East. Read about it in “Undamming Washington’s Elwha River—Public Lecture on Largest Dam Removal in U.S. History,” Sound Waves, this issue.