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The Sesan 2 dam in Stung Treng, Cambodia. Photo: Jason South/Fairfax Media/Getty Images

Earth's rivers are increasingly dammed, disrupted by development and fragmented — all of which are threatening food and clean water sources that hundreds of millions of people depend on, a new study finds.

What's new: A first-of-its-kind study published Wednesday in Nature provides a global census of the world's rivers, and seeks to answer the question of how many are still free-flowing.

By the numbers:

  • 2.8 million: The estimated number of dams constructed worldwide.
  • More than 3,700: Hydropower dams currently planned or under construction worldwide, particularly in Asia.
  • 15 gigawatts: The amount of hydropower capacity added in Asia during 2016 alone. The study highlights the Balkans, Amazon, China, and the Himalayas as hotspots of hydropower construction.
  • 37%: Share of rivers longer than 1,000 km (about 620 miles) in length that remain free-flowing.
  • 41%: Share of global river volume that still flows freely into the ocean.
  • 77%: Share of rivers greater than 1,000 km in length that have seen the connection from their source region to the sea severed.

The study finds large contiguous river networks with intact natural connectivity are limited to places where few humans live, including the Arctic, Amazon Basin, and Congo Basin in Africa.

  • Dams and reservoirs are the top contributors to major connectivity loss of sections of rivers.
  • Consumptive water use and infrastructure development also rank high on the list of river disturbances.

"The Mekong River is a prime example of a river that has been severed by several hydropower dams," study co-author Michele Thieme tells Axios via email. "Two more dams planned on the lower Mekong river, the Sambor and Stung Treng hydropower projects, could be the final straw..." This could deprive communities downstream of fish, imperiling livelihoods.

What they did: Researchers used satellite imagery and global hydrological data to compile an atlas of rivers around the world. They examined rivers' longitudinal connectivity, hydrological alterations of river flow, and the exchange of water between the atmosphere and groundwater.

They then used this data to rank rivers' intact natural connectivity.

  • The study finds the last 2 remaining very long free-flowing rivers in Asia — the Irrawaddy and Salween rivers — provide more than 1.2 million metric tons of fish catch per year, and help nurture coastal agriculture where more than 30 million people live.

What's next: The study's authors recommend river connectivity be considered in future development decisions, and note that the study illustrates how fragile and important free-flowing rivers are in providing benefits to humans and natural systems.

What they're saying:

"Rivers provide abundant fisheries that feed millions, nutrients to downstream floodplains and agriculture, sediments that help stop deltas from sinking, refuges for biodiversity in a rapidly changing world, and healthy floodplains and wetlands that can act as a buffer against extreme weather events."
— Co-author Michele Thieme of the World Wildlife Fund

Go deeper: Map of the world's free-flowing rivers.

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