Construction of large dams in the U.S. mostly came to a halt in the 1970s. Many are now unsafe, inefficient or no longer needed, requiring removal—events that geologists and biologists alike will follow closely to observe how these unimpeded rivers and their wildlife respond.

by the numbers

538

Dams removed in the 90 years before 2005.

548

Dams removed from 2006–2014.

10 million

Cubic meters of stored sediment released on removal of two dams (64- and 32-meter-high structures) in Washington State last year, the largest release to date.

SOURCES: “1000 DAMS DOWN AND COUNTING,” BY J. E. O'CONNER ET AL., IN SCIENCE, VOL. 348; MAY 1, 2015 (dam removals); “THE REMAINS OF THE DAM: WHAT HAVE WE LEARNED FROM 15 YEARS OF US DAM REMOVALS?” BY GORDON E. GRANT AND SARAH L. LEWIS, IN ENGINEERING GEOLOGY FOR SOCIETY AND TERRITORY, VOL. 3. EDITED BY GIORGIO LOLLINO ET AL. SPRINGER, 2015 (sediment release)