The vast majority of the world's rivers are reeling from pollution, over-development and excessive extraction, and billions of dollars of investment by rich countries to avert water stress have damaged biodiversity, a study released on Wednesday said.
"Rivers around the world really are in a crisis state," said one of its authors, Peter McIntyre, a professor of Zoology at the University of Wisconsin at Madison.
The investigation, published by the journal Nature, looked at the health of the world's major rivers, assessing them for water security and the state of their wildlife.
Their probe covered 23 factors, including water extraction, types of agriculture and industry, pollution levels, habitat, wildlife, population growth and urban development.
The result makes for grim viewing.
"We find that nearly 80 percent of the world's population is exposed to high levels of threat to water security," the authors say.
Over 30 of the world's 47 largest rivers, which collectively account for half of the global runoff of freshwater, are under at least "moderate" threat, they say.
Eight of them are rated as being under very high threat in terms of water security for humans. Fourteen of them are rated as being under very high threat for biodiversity.
In contrast, the rivers of Scandinavia, Siberia, northern Canada and unsettled parts of the tropical zone in Amazonia and northern Australia have the lowest threat rating.
In rich countries, heavy investment in dams and reservoirs and diverting flows from wetlands has benefited 850 million people, reducing their exposure to extreme water scarcity by 95 percent.
But this has failed to address the cause of water stress itself and had the worst impact on wildlife, in some cases dramatically reducing habitat for aquatic species, says the paper.
"(This) underscores the necessity of limiting threats at their source instead of through costly remediation of symptoms," it says tartly.
In upper-middle income countries, investment has benefited 140 million people, reducing their risk of extreme water scarcity by 23 percent.