State media said 11 tonnes of the chemical sodium cyanide leaked into the Luohe River after a lorry transporting it to a local gold mine was involved in a road accident late last week.
Two dams have been built across the river, about 75km (50 miles) upstream from the industrial city of Luoyang, and officials say so far they believe they have the situation under control.
One person reportedly became sick after eating sweet potatoes washed in river water but later recovered. A number of livestock animals were reportedly killed by poisoning.
The Luohe river is a tributary of the Yellow River, a major waterway in northern China.
Arrests
Ho Wai Chi, the Hong Kong director of the environmental pressure group Greenpeace, told BBC's East Asia Today programme that the incident was potentially dangerous to humans because many people in agricultural areas depend on rivers for drinking water.
But officials said there was no risk of the mainstream of the Luohe River being contaminated, and they said only a few kilometres of water had been affected.
Some 500 tonnes of disinfectant are reported to have been poured into the tributary in an attempt to nullify the affects of the cyanide, yet this is still likely to have damaging consequences for the local environment.
The spill is a reminder of the often low safety standards in the transporting of hazardous materials in China.
Six people are reported to have been arrested in connection with the accident, including the lorry driver and a representative from the gold mine.
Last year, an accident at a Romanian mine released thousands of cubic metres of cyanide-polluted water into the Somes River.
The poisoned water spread through 1,000km of waterways incuding the Danube, affecting drinking water in Romania, Hungary and Yugoslavia.
Worst hit was the River Tisza in Hungary where virtually the entire ecosystem was destroyed including hundreds of tonnes of fish.