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What Happens When You Remove a Dam?

And what comes in their wake.

By Francie Diep

Nearly 90 years after it was dammed, New York’s St. Regis River is now running free again. Leading the project was the Mohawk Tribe, the first American Indian tribe to decommission a federally licensed dam, the Associated Press reports.


The St. Regis River. (Photo: Wikimedia Commons)

St. Regis’ Hogansburg Dam is just one of more than 1,000 dams—some of them dating to colonial times—that the United States has torn down over the past century. More than 500 dams have come down since 2006 alone. And while the Mohawk Tribe’s work is a historic feat, the Mohawks aren’t the first tribe to advocate for, and benefit from, dam removal. The Penobscot Indian Nation of Maine was one of several parties — which included conservation groups, government agencies, and a power company — to sign the Lower Penobscot River Settlement Accord in 2004. The accord provided for the removal of two dams, which the Penobscot Nation considered to be a step toward restoring the sacred river, the Portland Press Herald reports. In the 1990s, the Lower Elwha Klallam Tribe in Washington launched an effort to remove the Elwha River’s two dams, arguing that, because the structures kept salmon from running in the river, they interfered with the tribe’s right to salmon catches, the Seattle Times reports. The last of the Elwha’s second dam came down in 2014.*


A Chinook salmon leaps through white water in the Rapid River in Idaho as it attempts to clear a migration barrier dam. (Photo By Bill Schaefer/Getty Images)

In an article published in the journal Science in 2015, federal researchers summarized what science has learned from decades of watching how rivers respond to undamming. Many consequences remain unpredictable, including some unwanted ones, such as the spread of contaminants. In general, however, scientists know that freed rivers often carve out a stable channel within a few months or years. Fish also swiftly return. In the Penobscot, ale fish numbers more than tripled, while shad increased nearly 100-fold. In the Elwha, Chinook salmon began swimming past the old lower dam site within days of its destruction, and the number of Chinook nests in the river more than tripled between 2013 and 2015. The St. Regis River will now be open to American eel, lake sturgeon, and Atlantic salmon from the connecting St. Lawrence River.


In 2011, contractors used explosives to blow a 15-foot hole in the Condit Dam in Washington in order to to drain its reservoir and allow young salmon to enter the river and head to sea. (Photo: Wikimedia Commons)

The U.S. still has a lot of dams left. “We as a nation have been building, on average, one dam per day since the signing of the Declaration of Independence,” Frank Magilligan, a professor of geography at Dartmouth College, told PBS in 2015. Many of those structures are aging and becoming expensive to maintain and re-license, which might make them tempting to remove. But it would take a lot of work and political will to take down America’s largest dams, and it’s not clear that determination exists for all of them. In addition, in regions where climate change will slash rainfall in the future, communities may want to keep dams to store fresh water.

*Update—January 11, 2017: A previous version of this article stated that more than 500 dams had come down since 2015; in fact, that number includes all dams that have come down since 2006.