37% of the world’s longest rivers remain free-flowing

Only 37% of the world’s longest rivers remain free-flowing, according to a new study published 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.

Nearly 60,000 large dams exist worldwide, with more than 3,700 currently planned or under construction, the study points out. Climate change is a growing threat to river health worldwide, both from direct impacts and as countries increasingly turn to hydropower as a renewable energy option.

"Indian rivers are exposed to sustained pressure from fragmentation and loss of river connectivity, constraining their capacity to flow unimpeded, affecting many fundamental processes and functions characteristic of healthy rivers and leading to the rapid decline of biodiversity and essential ecosystem services. A total 409,245 km of rivers was analyzed in India," the study states.

A team of 34 international researchers from McGill University, WWF, and other institutions assessed the connectivity status of 12 million kilometers (around 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 that only 21 of the world's 91 rivers longer than 1,000 km ( around 600 miles) 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.

“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," he was quoted in the study published in WWF-India website.

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,” said Michele Thieme, lead freshwater scientist at WWF and global leader of WWF’s free-flowing rivers initiative. “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.”

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 river connectivity often diminishes or even eliminates these critical ecosystem services.