State media quote environmental protection experts as saying the fluoride content in the Yuexi River was "double" the national standard. There were also "excessive levels" of nitrogen and phenol in the water.
A power plant in the province of Sichuan was apparently responsible for the pollution, which affected a 100-kilometer stretch of the river around the city of Yibin.
The city government arranged a fleet of water tankers to bring in emergency drinking-water supplies, but reports say the supply wasn't able to meet demand.
An environmental protection worker first noticed the discharge of yellow liquid from the power plant on 14 February.
The incident is the latest in a series of major chemical spills into Chinese rivers in recent months.