Nearly a kilometre of the river, known as China’s mother river, had turned red near a park in downtown Lanzhou, capital of the northwestern province of Gansu, Xinhua news agency said.
The change of colour could have been caused by water from central heating systems, a local environmental protection official was quoted as saying.
"Test results will determine what the red discharge is and whether it is detrimental to the river’s ecology," Xinhua said.