Stretch of China's Yellow River Turns Red
Nearly a km (half a mile) 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, on Sunday, 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.
Heating companies sometimes dye water in their pipes to prevent people from using it to cut their own water bills.
Spills into China's rivers are common.
An arsenide pollution case in Hunan province last month cut water supplies to 80,000 people, and an explosion at a chemical plant in northeast China last year poured toxic benzene compounds into the Songhua River, which supplies water to millions.
China's national environmental watchdog had blamed soaring energy consumption, unbridled construction investment and spotty enforcement of environmental due diligence for the increasing number of spills.
Last week, state media cited China's State Oceanic Administration as saying the Bohai Sea, the body of water between China and the Korean peninsula, was so polluted it would "die" within 10 years.