New Study in Nature: Just One-Third of the World’s Longest Rivers Remain Free-Flowing

Posted on 09 May 2019

First ever global assessment of the location and extent of the planet’s remaining free-flowing rivers highlights severe degradation, and offers a method for tracking the status of free-flowing rivers over time. Mekong, Irrawaddy and Salween included in study.
Bangkok, Thailand (9 May 2019) Just over one-third (37%) of the world’s 246 longest rivers remain free-flowing – including rivers in the Greater Mekong region -- according to a new study published today in the scientific journal Nature. Dams and reservoirs are drastically reducing the diverse benefits that healthy rivers provide to people and nature across the globe.

A team of 34 international researchers from McGill University, WWF, and other institutions[i] assessed the connectivity status of 12 million kilometers of rivers worldwide, providing the first ever global assessment of the location and extent of the planet’s remaining free-flowing rivers[ii].

Among other findings in ‘Mapping the World’s Free-Flowing Rivers’, the researchers determined only 21 of the world's 91 rivers longer than 1,000 km that originally flowed to the ocean still retain a direct connection from source to sea. The planet’s remaining free-flowing rivers are largely restricted to remote regions of the Arctic, the Amazon Basin, and the Congo Basin.

They also include the Irrawaddy and Salween in Myanmar – which are Southeast Asia’s last free flowing rivers; and only the lower part of the Mekong in Southern Laos, Thailand, Cambodia and Vietnam. The Mekong, Irrawaddy and Salween are threatened by infrastructure development, and the Lower Mekong especially is under threat from the massive proposed Sambor Dam and the smaller but equally worrying Stung Treng Dam.

“Free flowing rivers are essential to the future of the Greater Mekong region’s economy, food security, livelihoods and biodiversity but they are under constant threat from unsustainable development, especially dams,” said Marc Goichot, a co-author of the study and WWF-Greater Mekong’s Water Lead. “We have to ask ourselves if we will dam them, sand mine them and degrade them out of existence or if we will value them and invest in alternative renewable energy sources like solar and wind and keep our rivers as free flowing as possible.”

“The world’s rivers form an intricate network with vital links to land, groundwater, and the atmosphere,’’ said lead author Günther Grill of McGill’s Department of Geography. ‘’Free-flowing rivers are important for humans and the environment alike, yet economic development around the world is making them increasingly rare. Using satellite imagery and other data, our study examines the extent of these rivers in more detail than ever before.”

Dams and reservoirs are the leading contributors to connectivity loss in global rivers. The study estimates there are around 60,000 large dams worldwide, and more than 3,700 hydropower dams are currently planned or under construction. They are often planned and built at the individual project level, making it difficult to assess their real impacts across an entire basin or region.

“This first ever map allows us to prioritize and protect the world’s remaining free-flowing rivers, as these are lifelines for wildlife and people across the globe,” said Michele Thieme, WWF Freshwater Scientist and co-author of the paper. “Rivers provide diverse benefits that are often overlooked and undervalued. Decision makers must consider the full value of rivers when they plan new infrastructure.”

Healthy rivers support freshwater fish stocks that improve food security for hundreds of millions of people, deliver sediment that keeps deltas above rising seas, mitigate the impact of extreme floods and droughts, prevent loss of infrastructure and fields to erosion, and support a wealth of biodiversity. Disrupting rivers’ connectivity often diminishes or even eliminates these critical ecosystem services.

Protecting remaining free-flowing rivers is also crucial to saving biodiversity in freshwater systems. WWF’s Living Planet Report 2018 recently revealed that populations of freshwater species have experienced the most pronounced decline of all vertebrates over the past half-century, falling on average 83 percent since 1970. This week, IPBES’ 2019 Global Assessment Report on Biodiversity and Ecosystem Services highlighted threats to the world’s freshwater ecosystems and called for the protection and restoration of free-flowing rivers.

The study also notes that climate change will further threaten the health of rivers worldwide. Rising temperatures are already impacting flow patterns, water quality, and biodiversity. Meanwhile, as countries around the world shift to low-carbon economies, hydropower planning and development is accelerating, adding urgency to the need to develop energy systems that minimize overall environmental and social impact.

“While hydropower inevitably has a role to play in the renewable energy landscape, countries should also consider other renewable options,” said Thieme. “Well-planned wind and solar energy can have less detrimental impacts on rivers and the communities, cities, and biodiversity that rely on them.”

With the historic coming together of key decisions on environment, climate and sustainable development, 2020 provides an unmissable opportunity for world leaders to protect and restore free flowing rivers as part of a New Deal for Nature and People – an agreement which would aim to halt and reverse the loss of nature, and protect our planet.


Notes to Editors:

For further information, contact:
Lee Poston, WWF-Greater Mekong, 0918832290,

[i] Contributing Institutions:
McGill University, WWF-US, WWF-NL, WWF-UK, WWF-Mediterranean, WWF-India, University of Basel, Joint Research Centre (JRC), WWF-China, WWF-Canada, WWF-Zambia, WWF-Greater Mekong Programme, The Nature Conservancy, University of Nevada, WWF-Malaysia, IHE Delft, WWF-Germany and HTWG Konstanz, King’s College London, Umeå University, Swedish University of Agricultural Sciences, University of Washington, Harvard University, University of Wisconsin-Madison, Conservation International , WWF-Mexico, WWF-International, Stanford University, Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Freie Universität Berlin, WWF-Brazil, Eberhard Karls University of Tübingen.

[ii] First ever science-based definition of a free-flowing river:
Rivers where ecosystem functions and services are largely unaffected by changes to fluvial connectivity allowing an unobstructed exchange of water, material, species, and energy within the river system and with surrounding landscapes.

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© Nicolas Axelrod / Ruom / WWF-Greater Mekong
© Nicolas Axelrod / Ruom / WWF-Greater Mekong
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