Winding view of Elwha river, site of the former Lake Mills, south of the Glines Canyon Dam in 2012.

A lake in Washington state has been replaced by the free-flowing River Elwha, which was unleashed by a dam-removal project some two decades in the making. Chris Wilson/WP via Getty

Geophysics

River bounces back after world’s largest-ever dam removal

The Elwha River quickly cleared itself of debris after dams’ demolition.

Freed from two large dams, a small river in Washington state has efficiently flushed vast amounts of mud, sand and gravel towards the sea.

In the world’s largest dam-removal project so far, two obsolete barriers on the Elwha River — the 32-metre-tall Elwha Dam and the 64-metre-high Glines Canyon Dam — were dismantled between 2011 and 2014 to restore the river’s flow from source to mouth. Amy East at the US Geological Survey in Santa Cruz, California, and her team monitored river flow and topography before, during and after the dams’ removal.

The release of some 20 million tonnes of sediment that had been trapped in the reservoirs behind the dams substantially altered the shape of the river, filling pools and creating new sandbanks. But the major disturbance lasted no more than five months; by the end of that time, most of the debris reached the river’s mouth at the Strait of Juan de Fuca.

Rivers of sufficient stream power seem to be able to cope with large dam removals without serious harm, the authors conclude.