A new study published in Nature has found that only a third of the world’s large rivers are still free flowing with the rest fragmented and degraded due to dams and other man-made structures.
The research, which took 10 years to put together, looked at dams, levees, and other artificial banks and how they impact seasonal flow, the movement of sediment and wildlife, the Guardian reports.
Scientists from McGill University in Montreal, Canada examined 12 million kilometres of rivers and found that just 90 of the 246 rivers of more than 1,000 kilometres did not flow freely to the sea.
Free flowing rivers are crucial to maintaining healthy ecosystems and for wildlife to thrive. Billions of people rely on rivers for drinking water and for irrigation to grow food. Freshwater habitats are among the most damaged of the world’s natural environments and wildlife populations in rivers have dropped by 83% since 1970.
“Free-flowing rivers are important for humans and the environment alike, yet economic development around the world is making them increasingly rare. Two billion people take water from rivers for drinking, so it is important they remain a clean source,” said Gunther Grill who led the research.
River deltas have been particularly badly hit and many are sinking into the sea as upstream dams starve them of sediment.
Populated Areas the Hardest Hit
There are now hardly any free-flowing rivers in heavily populated areas, with most confined to sparsely populated areas of the world such as the Arctic, Siberia, the Congo, and the Amazon. Rivers that have been heavily damned and fragmented include the Nile, Yangtze, and the Danube.
The main reason for so many dams is the production of hydroelectricity, which with climate change is becoming an increasingly popular way of generating low carbon electricity and scientists have warned that the broader environmental costs of hydropower are not often taken into consideration. There are already around 60,000 large dams in the world, with another 3,700 under construction or in the planning phase.
A separate study in Britain carried out by scientists at the University of Swansea looked at the effects of smaller infrastructure in rivers, such as weirs and culverts, and found that 97% of UK rivers have their flow interrupted by manmade structures.
Carlos Garcia de Leaniz, who led the British research, thought the global study underestimated the extent of river fragmentation because it only considered very large dams. “We believe free-flowing rivers don’t exist anymore, at least in Europe,” he said.