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.
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.