Large parts of China's longest river, the Yangtze, have been irreversibly polluted, state media quotes a report as saying.
Billions of tons of waste end up in China's waterways each year
Around one-tenth of the 6,200km-long river is in a "critical condition" and nearly 30% of major tributaries are seriously polluted, the report found.
Even a huge reservoir created by the Three Gorges Dam has become heavily polluted.
China's environment has suffered as a result of the country's economic boom.
The government has pledged to clean up the Yangtze, which supplies water to almost 200 cities along its banks and accounts for 35% of the country's total fresh water resources.
But correspondents say attempts to clean up China's polluted lakes and waterways have been thwarted by lax enforcement standards.
The first comprehensive study into the health of the Yangtze found that 600km of the river were in a critical condition.
Around 14bn tons of waste are believed to be dumped into the river each year.
The river's aquatic life had been seriously affected, with the annual harvest of aquatic products falling from 427,000 tons in the 1950s to 100,000 tons in the 1990s, the report found.
A huge reservoir created by the Three Gorges Dam - the world's largest hydro-power project - had also been seriously polluted with pesticides, fertilisers and sewage from passenger boats.
"The impact of human activities on the Yangtze water ecology is largely irreversible," Yang Guishan, of the Nanjing Institute of Geography and Limnology, which helped compile the report, said.
"It's a pressing job to regulate such activities in all the Yangtze drainage areas and promote harmonious development of man and nature."
The report said a comprehensive management system needed to be put in place to stop further parts of the river from becoming critically polluted.