Two-thirds of the world’s longest rivers no longer run free

Science | AAAS | 5/8/2019 | Staff
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Free-flowing rivers are increasingly threatened by dams, levees, and water diversions, such as for these shrimp farms in Ecuador. © ANTONIO BUSIELLO/WWF-U.S.

About two-thirds of the world’s longest rivers are no longer free flowing, compromising their ability to move sediment, facilitate fish migration, and perform other vital ecosystem services, according to a new study. And with more than 3700 large dams in the works, the future of free-flowing waterways looks even bleaker, researchers say.

Perspective - River - Conditions - Bernhard - Lehner

To get a global perspective on river conditions, Bernhard Lehner, a hydrologist at McGill University in Montreal, Canada, who for years has studied the effects of dams on entire watersheds, teamed up with researchers from the World Wildlife Fund (WWF), based in Washington, D.C., and elsewhere. Using aerial, satellite, and other data, the team examined 12 million kilometers of waterways, evaluating their flows in 4.5-kilometer segments.

Traditionally, researchers focused on dams when assessing a river’s free flow. But in this assessment, the team also considered the impacts on flow created by riverbank levees, other flood control structures, and water diversions for power, irrigation, or drinking supplies. “It’s a more comprehensive analysis of global hydrology than we have had before,” says N. LeRoy Poff, a hydroecologist at Colorado State University in Fort Collins who was not part of the project.

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Researchers - Rivers - Kilometers - Nile - Mississippi

In particular, the researchers focused on the 246 longest rivers encompassing more than 1000 kilometers of flowing water—think the Nile and Mississippi rivers—because of their huge ecological impact. Just 90 of those big rivers are still unencumbered, they report today in Nature. Most of the remaining unblocked rivers are in the Amazon, the Arctic, and Africa’s Congo basin.

“In the U.S., Europe, and more developed areas, these longer, free-flowing rivers don’t really exist,” Poff says. And those free...
(Excerpt) Read more at: Science | AAAS
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