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The largest dam removal in U.S. history was the subject of a public lecture by U.S. Geological Survey (USGS) research geologist Amy East on February 26, 2015, at the USGS campus in Menlo Park, California. East described changes to the landscape caused by the removal of two large dams—the 32-meter-tall Elwha Dam and the 64-meter-tall Glines Canyon Dam—from the Elwha River in Washington State. This was the largest dam removal ever undertaken, both in terms of the dams’ heights and in terms of how much sediment had accumulated behind them.

Staged deconstruction of the two dams began in September 2011 (see “Elwha Dam Removal Begins—Long-Planned Project Will Restore Ecosystem, Salmon Runs,” Sound Waves, November-December 2011) and ended in summer 2014. Numerous federal, tribal, state, and academic scientists are collaborating to examine and report the effects of this restoration effort. East is one of many USGS scientists participating in the collaboration. They began gathering baseline data on the Elwha and the coastal area around its mouth on the Strait of Juan de Fuca more than 5 years before dam deconstruction began, and they will keep studying the river system to understand its physical and biologic changes.

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After introducing her audience to the history of dams and dam removal in the United States, East focused on the Elwha and, specifically, on the effects of releasing massive amounts of sediment downstream during the first 2 years of dam deconstruction. Approximately 90 percent of this sediment made it to the river mouth, even though there were no floods during that 2-year period; in fact, the river’s water discharge and peak flows were moderate compared with historical gaging records. “This was probably the biggest surprise of the study so far,” she said—that the river could move so much material downstream without floods to push it along.

Additional effects documented by East and her USGS colleagues include a rise of about 1 meter in the elevation of the riverbed, the appearance of new channels, formation of new gravel bars, and a general decrease in bed-sediment grain size—a change that has improved spawning areas for fish. Other USGS scientists have documented significant enlargement of the coastal delta at the river’s mouth.

Details about these and many other findings were recently published in the journal Geomorphology in a series of papers about the first 2 years of dam removal (see “Scientific Portrait of the Largest Dam Removal in U.S. History”). This information about how the physical system has changed provides a basis for biologists to understand changes to habitats.

USGS scientists continue to monitor the river system, and, as East told her audience, “We expect to learn from the Elwha for years to come.”

Watch an archived video of East’s talk.