Only a third of the world’s great rivers remain free flowing, due to the impact of dams that are drastically reducing the benefits healthy rivers provide people and nature, according to a global analysis.
Billions of people rely on rivers for water, food and irrigation, but from the Danube to the Yangtze most large rivers are fragmented and degraded. Untouched rivers are largely confined to remote places such as the Arctic and Amazonia.
The assessment, the first to tackle the subject on a worldwide level, examined 12m kilometres of rivers and found that just 90 of the 246 rivers more than 1,000km (621 miles) long flowed without interruption.
The scientists, whose research, published in the journal Nature, was led by Günther Grill, at McGill University in Canada, were particularly concerned to discover that only a quarter of long rivers that once flowed freely to the sea, rather than to an inland lake or other river, still had such a course.
Separate research in Britain, which included the effects of smaller infrastructure such as weirs, fords and culverts, suggests that 97% of the nation’s river network has been interrupted by human-built structures.
Thriving wildlife in rivers is crucial to keeping water clean but freshwater habitats were found to be the hardest hit of all the ecosystems, with wildlife populations having plunged by an average of 83% since 1970 due to dams, overuse of water and pollution.
“Free-flowing rivers are important for humans and the environment alike, yet economic development around the world is making them increasingly rare,” said Grill. “Two billion people take water from rivers for drinking, so it is important they remain a clean source.”
Annually, 12m tonnes of fish are caught from rivers, providing a vital source of food for hundreds of millions of people, but 500 million people are living on deltas that are sinking as dams starve them of sediments.
Grill said rivers needed more protection, but that their length made such habitats harder to protect than areas of land.
“Rivers are the lifeblood of our planet,” said Michele Thieme, lead freshwater scientist at WWF and one of the 34 researchers behind the analysis. “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 and biodiversity that rely on them.”
LeRoy Poff, at Colorado State University, in the US, who was not part of the team, said: “The research provides a crucial new perspective on the global status of rivers [and] suggests that their global sustainability is more precarious than currently recognised.”
Poff said population growth made it difficult to balance the conflicting demand for water, food and energy, with the need to keep rivers healthy and free flowing. “Attaining this balance will increasingly rely on rigorous, state-of-the art analyses, as exemplified this study.”
The data used in the study took 10 years to assemble and examined dams and their impacts on seasonal flow and sediment movement, as well as levees, other artificial banks, and general water use.
Great rivers that flow freely are now rare in populated areas. Heavily fragmented rivers include the Danube, Nile, and Euphrates, the Paraná and Missouri in the Americas, the Yangtze and Brahmaputra in Asia, and the Darling in Australia. The Congo and Amazon were found to be among the least affected.
The biggest impact comes from physical barriers created by dams, but reservoirs also seriously affect the natural seasonal flow of rivers.
“It can be really freaky sometimes, when the electricity is produced one hour on, one hour off, and the river goes up and down by a metre, which is very stressful to the ecosystems downstream,” said Grill.
The study estimates that there are about 60,000 large dams worldwide and 3,700 in planning or construction, in addition to millions of smaller dams.
Many rivers are also hemmed in by banks built by people to prevent flooding in urban areas, although flood waters can end up displaced downstream. These banks also stop natural replenishment of nutrients in soil on flood plains.
Carlos Garcia de Leaniz, a professor at Swansea University, who led the UK study, praised the global analysis but he said the closer examination carried out by his team in the UK revealed a still worse picture. “The [global] study grossly underestimates the extent of river fragmentation as [it] only considers very large dams. We believe free-flowing rivers simply don’t exist any more, at least in Europe.”
The negative environmental impacts of many large dams is well known, but the international drive to fight climate change by reducing the burning of fossil fuels means low-carbon hydropower is in demand. “We are in a boom phase,” said Grill, noting that this was particularly so in the Balkans, Amazonia, China and the Himalayas.
Grill added that the broader environmental costs of hydropower were often not taken into account. He said he hoped his team’s database would enable planners to make better decisions about the necessity, location and design of dams.