Study finds, only one-third of the world's longest rivers remain free flowing

Infrastructure development and other human-made changes are the leading contributors to connectivity loss in global rivers, resulting in disrupted ecosystem services, claimed researchers.

Just over one-third (37 per cent) of the world's 246 longest rivers remain free-flowing, according to a new study published in the journal of Nature.

A team of 34 international researchers from McGill University, assessed the connectivity status of 12 million kilometres (7.5 million miles) of rivers worldwide, providing the first-ever global assessment of the location and extent of the planet's remaining free-flowing rivers.

Among other findings, the researchers determined only 21 of the world's 91 rivers longer than 1,000 km (600 miles) that originally flowed to the ocean still retain a direct connection from the source to sea.

"The world's rivers form an intricate network with vital links to the land, groundwater, and the atmosphere. '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,'' said lead author Gunther Grill.

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.

"Rivers are the lifeblood of our planet. They provide diverse benefits that are often overlooked and undervalued. This first-ever map of the world's remaining free-flowing rivers will help decision makers prioritize and protect the full value rivers give to people and nature," said Michele Thieme, lead freshwater scientist.

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.

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.

"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. 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," concluded Thieme.