Only just over one-third of the world’s 242 long rivers — that traverse a distance of 1,000 km or more — are free-flowing. The rest are disrupted by dams and reservoirs, adversely affecting biodiversity and the benefits of the watercourses, an international team of researchers has found.

Most of the free-flowing rivers are restricted to remote regions in the Arctic, and the Amazon and Congo basins, said the scientists who assessed 12 million kilometres of rivers worldwide, in a study published in the prestigious journal Nature on Wednesday.

More importantly, only 23 per cent of the free-flowing rivers now connect to the oceans, indicating the extent to which estuarine and marine environments are being deprived of nutrients and sediments coming from the land, the study showed.

The study was carried out by 34 researchers from different countries, including India, and was led by Guenther Grill and Bernhard Lehner of the McGill University in Canada.

Free-flowing rivers create ecosystems with greatest biodiversity and dynamics, comparable to tropical rainforests and coral reefs. They also support millions of people by providing freshwater, irrigated agriculture, fishing and hydropower. In recent years human demands have led to the natural courses of rivers being altered and managed with infrastructural development, including dams and levees.

The study estimated that there are around 60,000 large dams worldwide, and over 3,700 new hydropower dams are currently under construction or planned. They are often planned and built at the individual project level, making it difficult to assess their real impact across an entire basin or region.

‘Right recipe’

“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. This first-ever map of the world’s remaining free-flowing rivers will help decision makers prioritise and protect the full value rivers give to people and nature,” said Thieme, who is also an author of the study.

The scientists feared that climate change would threaten the health of rivers further. On one hand, rising temperatures are already impacting flow patterns and water volumes. On the other, the increased focus on low-carbon economies will prompt countries increase their hydropower portfolio.

“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,” said 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 team, which developed a new method to comprehensively evaluate river connectivity, said there are six pressure indicators of human impact that sever the natural flow connectivity of rivers. These are river fragmentation, flow regulation, sediment trapping, water consumption and two measures of floodplain infrastructure development, namely road density and urbanisation.

The study also estimated that there are about 2.8 million dams along the 12 million km of rivers studied. “This leads to the fragmentation of the rivers and has serious impact on the whole river system,” said Christiane Zarfl, an applied geoscientist with Eberhard Karls University of Tübingen in Germany.

N LeRoy Poff, a researcher with Colorado State University, in a commentary article, also appeared in Wednesday’s issue of Nature, said: “…the flow of water occurs not only down the river channel , but also laterally onto floodplains and vertically through river bed and adjacent groudnwater. The different pathways of connectivity allow the exchange of nutrients, organic matter and organisms in all directions.”