The Yangtze, China's longest river, is "cancerous" with pollution, reports in the country's state media have said.
The Yangtze supplies water to 200 cities along its banks
Environmental experts fear pollution from untreated agricultural and industrial waste could turn the Yangtze into a "dead river" within five years.
That would make it unable to sustain marine life or provide drinking water to the booming cities along its banks.
The Yangtze rises in China's western mountains and passes through some of its most densely populated areas.
The government has promised to clean up the Yangtze, which supplies water to almost 200 cities along its banks.
But experts speaking in China's state media say that unless action is taken quickly, billions of tonnes of untreated industrial and agricultural waste and sewage are likely to kill what remains of the river's plant and wildlife species within five years.
China's rapid economic development means that many of the nation's waterways are facing similar problems.
Last year the authorities announced that the country's second-longest river, the Yellow River, was so polluted that it was not safe for drinking.
Correspondents say that 300 million people in China do not have access to safe drinking water.
They say that government efforts to clean up the country's polluted lakes and waterways are being thwarted by lax enforcement standards.