A stretch of China's Yellow River turned red for the second time in a month because of pollution, media says.
The same stretch of Yellow River turned red a month ago
Waste water from a heating station near the city of Lanzhou contaminated 1km of the river, the country's second longest, according to Xinhua.
A similar spill occurred in the same area at the end of October.
China has some of the world's most polluted rivers, and is accused of overlooking the protection of the environment to develop its economy.
Local environmental officials said the red plume of water "very possibly" came from a station that provides heating for Lanzhou residents.
The plant had added a red dye to its water to prevent people from siphoning off the hot water from the pipes it used, officials were quoted by Chinese media as saying.
Tests found the dye was not toxic, and the spill lasted for around an hour, Xinhua news agency reports.
The spill came almost exactly a month after the same section of the river turned red. Officials are still said to be determining the cause of that event.
The latest spill occurred on the same day China's environmental watchdog, the State Environmental Protection Agency, said tributaries feeding the Yellow River are seriously polluted.
Some 21,000 chemical factories are believed to be located along China's rivers and coastline - more than half on the Yellow and Yangtze rivers, which are relied upon by millions of people.
At the beginning of the year, the country's environment chief warned that more than 100 of those chemical plants posed safety threats.
A year ago, an explosion at a chemical plant in eastern China caused a major toxic spill in the Songhua river.
Water supplies to Harbin city were cut off for four days, and rivers in Russia's Far East were affected.