River Ganges near the point of its origin in Gangotri. © S. Priyadarshini

Only one-third of the world’s longest rivers, those longer than 1,000 kilometres, remain free flowing, and these are restricted to remote regions in the Arctic and in the Amazon and Congo basins, a study reveals1. It also shows that human activities, such as the building of dams and reservoirs, disrupt rivers’ free flow.

Using satellite imagery and other data, a team of 34 international researchers, including one from the World Wide Fund for Nature (WWF) in New Delhi, India, assessed the connectivity status of 12 million kilometers of rivers worldwide.

They found that about half of all river reaches globally show diminished connection to seas. The fragmentation effect of dams is the dominant pressure factor in more than two-thirds of the affected rivers, followed by flow regulation, affecting one-quarter, and sediment trapping, affecting almost 5 per cent of rivers.

Free-flowing rivers longer than 500 kilometres are largely absent from the mainland United States, Mexico, Europe and the Middle East, as well as parts of India, southern Africa, southern South America, China and much of South-east Asia and southern Australia.

Of special concern, the researchers note, is the loss of connectivity of very long rivers to the sea because they are vital sources of water, nutrients, sediments and species with deltas, estuaries and the ocean. For instance, the Irrawaddy and Salween rivers, the last two very-long free-flowing rivers remaining in South-east Asia, provide more than 1.2 million tonnes of catch annually, and sustain agriculture in a region inhabited by more than 30 million people.

The researchers say that their methods and results can play a critical role in finding sustainable solutions to close the gap between irrigation demand and extreme water stress.

[Nature News & Views: A river that flows free connects up in 4D

Nature Podcast ]


1. Grill, G. et al. Mapping the world’s free-flowing rivers. Nature (2019) doi: 10.1038/s41586-019-1111-9