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

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

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.

By Carolyn Gramling

May 10, 2019 at 7:00 am

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.