Just one-third of the world’s long rivers run free


More than 60,000 dams obstruct water flow and disrupt ecosystems. Nick Carne reports.


The Congo is one of the few big river systems that still has a direct connection from source to sea.

Andrew McConnell / robertharding/Getty Images

Just 37% of the world's 242 longest rivers remain free-flowing, a new study shows, and most of these are restricted to remote regions of the Arctic and the Amazon and Congo basins.

The first global assessment of the location and extent of free-flowing rivers (FFRs) paints a stark picture of how dams and reservoirs, in particular, are drastically reducing their number and value.

Only 21 of the 91 rivers longer than 1000 kilometres that originally flowed to the ocean still retain a direct connection from source to sea.

''Free-flowing rivers are important for humans and the environment alike, yet economic development around the world is making them increasingly rare,'' says lead author Günther Grill, from Canada’s McGill University.

“Using satellite imagery and other data, our study examines the extent of these rivers in more detail than ever before.”

More detail, and great distance. A 34-strong international team led by McGill and the World Wildlife Fund (WWF) assessed the connectivity status of 12 million kilometres of waterways.

The researchers report that there are 60,000 large dams worldwide, and more than 3700 hydropower dams planned or under construction, but say it is difficult to assess the impact across entire regions because these are often planned and built at the individual project level.

What they do make clear, however, is that disrupting river connectivity diminishes or even eliminates critical ecosystem services such as supporting fish stocks, delivering sediment that keeps deltas above rising seas, and mitigating the impact of floods and droughts.

The one upside to all of this, says WWF freshwater scientist Michele Thieme, is that producing a map of remaining FFRs “will help decision-makers prioritise and protect the full value rivers give to people and nature".

In theory, at least, they are ready to do so. The international community is committed to protect and restore rivers under Agenda 2030 for Sustainable Development, which requires countries to track the extent and condition of water-related ecosystems.

The new study, which is published in the journal Nature, aims to provide the data and methods to do just that.

An important part of this work was to develop a new definition of FFRs.

“Without an existing scientific definition of FFRs, practitioners and scholars have in the past used the term ‘free-flowing’ to describe rivers that are ‘unimpounded’ or ‘unregulated’ by the presence of dams or by flow alterations downstream of reservoirs,” the researchers write.

“Expanding on this traditional view, which focused mostly on longitudinal connectivity, we here propose a more comprehensive definition based on four dimensions of connectivity, explicitly recognising that connectivity is necessary within all of those dimensions for a river to flow freely.”

Five pressure factors relate to those four connectivity dimensions: river fragmentation, flow regulation, sediment trapping, water consumption, and infrastructure development in riparian areas and floodplains.

As with so many things, climate change is an underlying issue in trying to determine the best way forward.

Rising temperatures are impacting on river flow patterns, water quality and biodiversity, but there is pressure also from attempts to mitigate climate change. As countries shift to low-carbon economies, hydropower planning and development is accelerating.

"Renewable energy is like a recipe: you have to find the right mix of ingredients to have both a sustainable energy grid and a thriving natural world," says Thieme.

"While hydropower inevitably has a role to play in the renewable energy landscape, well-planned wind and solar energy can be more viable options for rivers and the communities, cities, and biodiversity that rely on them."

The WWF maintains a website providing information on free-flowing rivers and an interactive map.

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  1. https://www.un.org/sustainabledevelopment/development-agenda/
  2. https://doi.org/10.1038/s41586-019-1111-9
  3. https://www.worldwildlife.org/pages/free-flowing-rivers
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