Only a third of Earth’s longest rivers still run free

Mapping millions of kilometers of waterways allowed scientists to make the new calculation

Coco River in Brazil

BRANCHES AND BRAIDS  In central Brazil, the Coco River (left) empties into the Araguaia River, or “River of the Macaws,” one of the world’s remaining free-flowing long rivers.

© Day's Edge Productions, WWF-US

Free-flowing rivers are an endangered species on Earth. Only about a third of the world’s longest rivers still flow freely along their entire lengths, unchained by dams or reservoirs, scientists report in the May 9 Nature.

The study is the first global map of river “connectivity,” the ability of river water to move freely downstream, across floodplains and into and out of aquifers throughout the year. Connectivity signals river health, and is vital to protect freshwater biodiversity, support fish stocks and deliver sediment to coastal regions threatened by rising seas.

The team, led by geographer Günther Grill of McGill University in Montreal, used satellite data to map 12 million kilometers of rivers around the globe. Of the planet’s 246 rivers that are longer than 1,000 kilometers, only 37 percent still run free, the team found.

Most of the remaining free-flowing rivers are in more remote parts of the world, such as Canada’s Liard River in the Arctic and Zambia’s Luangwa in the Congo Basin.

Cambodian fishermen
FISHING EXPEDITION The Mekong River, the world’s 12th-longest river, has many dams upstream, but much of the lower Mekong still flows freely, allowing sediments, fish and nutrients to move freely. But two proposed hydropower projects in Cambodia could be the “final straw,” hampering the lower Mekong’s flow, researchers say. The river is a key source of fish for people in countries such as Laos (shown) and Cambodia.© Nicolas Axelrod, Ruom, WWF-Greater Mekong
coastal shrimp farms
RIVER RELIANT Mangroves and sustainable shrimp farms ring Isla Escalante in Ecuador, near where the Rio Guayas, the national river of Ecuador, flows west into the Pacific Ocean. The farms rely on the river’s health and its ability to flow naturally. © Antonio Busiello, WWF-US
hydropower dam
WATER POWER This newly constructed hydropower dam on the Congo River, the second-longest in Africa after the Nile, may be a sign of things to come, as several large hydropower projects are planned for the fast-flowing river. © Jaap van der Waarde, WWF-Netherlands
ELEPHANT CROSSING The Luangwa River in Zambia, now one of the longest free-flowing rivers in Africa, winds through South Luangwa National Park, a haven for myriad wildlife such as elephants, leopards, giraffes, hippopotamus and buffalo. The river basin is also home to 25 chiefdoms. But a proposed dam and reservoir would flood about 30 percent of the length of the river within the park, including one entire chiefdom, and would cut through a wildlife corridor connecting the park with the Lower Zambezi National Park. Geoff Gallice/Wikimedia Commons (CC BY 2.0)

Carolyn Gramling is the earth & climate writer. She has bachelor’s degrees in geology and European history and a Ph.D. in marine geochemistry from MIT and the Woods Hole Oceanographic Institution.

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