Two-Thirds of World's Longest Rivers Are No Longer Free-Flowing, Study Says

By Pam Wright

May 10 2019 01:05 PM EDT


Aerial view the Hoover Dam over the Colorado River, where infrastructure has made the once flowing river unrecognizable.
(Scott Williams/EyeEm/Getty Images)

At a Glance

  • Instead of free-flowing, rivers are often broken up by dams that provide drinking water and electricity for millions of people.
  • Only 37 percent of the longest rivers remain free-flowing.
  • Unhindered, free-flowing rivers transport water, vital nutrients and species that "sustain biodiversity and benefit millions of people."

Two-thirds of the world's 246 longest rivers are no longer free-flowing, which drastically reduces the benefits that healthy rivers provide to humans and nature, a new study says.

A team of 34 international researchers with the World Wildlife Federation (WWF), Canada's McGill University and other institutions used satellite imagery and other data to map out 7.5 million miles of rivers across the globe. They found that only 21 of the world's 91 rivers longer than 1,000 kilometers (621 miles) that originally flowed to the ocean still "retain a direct connection from source to sea."

They also found that only 37 percent of the 246 longest rivers in the world remain free of obstructions like dams, dikes, roads and other infrastructure.

Instead of free-flowing, rivers are often broken up by dams that provide drinking water and electricity for millions of people.

"But when built in the wrong place — for instance, a river’s main stem — they can impede a river’s flow, causing drastic declines in biodiversity and affecting fish migration, agriculture and livelihoods," the WWF said in a press release.

Unhindered by dams and other development, free-flowing rivers transport water, vital nutrients and species like river dolphins and migratory fish that "sustain biodiversity and benefit millions of people."

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Unobstructed rivers swell and shrink naturally with rainfall and snowmelt and help to replenish groundwater sources, the WWF notes.

There are about 60,000 dams worldwide, according to the study, and that number is growing rapidly as the demand for hydropower increases.

One current hot spot for dam development is in Southeast Asia, where 50 new dams are planned along the Mekong River, the 12th longest river in the world that meanders some 2,700 miles through several countries.

"The fear is that the river, which is the lifeblood of most of Southeast Asia, will gradually become so fragmented that it will lose function, and thus no longer support the huge diversity of wildlife and millions of people that depend on it,” study co-author Zeb Hogan of the University of Nevada said in a press release.

Back in the United States, dams and fragmentation have "totally altered the flow, water temperature, fish, plants and other aquatic animals on the Lower Colorado River Basin in Arizona, Nevada, and California," Hogan said.

"The river – in many locations – would be unrecognizable to people living along its banks 100 years ago. On the Columbia River, due to dams and other factors, populations of wild salmon have declined by about 95 percent; and many stocks and runs – like those that migrated into northern Nevada – have disappeared entirely," Hogan said.

Hogan noted that more than 70 percent of Europe's anadromous fish, which are fish that migrate from the ocean to freshwater, are now gone, while the migratory fish of major tropical rivers like the Amazon, Congo and Mekong are under great threat from hydropower development.

WWF says it is working with governments, industries and developers to consider alternative options for meeting water and energy supply needs, including solar and wind sources.

The researchers noted that 97 percent of the world's shortest rivers — no longer than 62 miles in length — remain free-flowing.

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