BEIJING -- A section of the Yellow River turned red and smelly on Sunday in Lanzhou, capital of northwest China's Gansu Province, and the local environment watchdog has launched an investigation.

Residents were alarmed to see a sewage pipe pouring red water into the country's second longest river between 3:00 p.m. and 6:00 p.m. Sunday. The discharge occurred near a park by Nanbinhe Road in downtown Lanzhou. About a kilometer of the river was discolored.

The city's environment protection bureau and the Yellow River water environment monitoring center took samples of the river water for laboratory testing at 5:00 p.m.

Kang Mingke, an official with the city's environment protection bureau, said test results will determine what the red discharge was and whether it is detrimental to the river's ecology. He said the test results were not yet available Monday morning.

Kang said the red water could be from central heating systems, as there are no chemical plants located nearby.

Many citizens suspect the same, adding that heating companies sometimes dye the water in their pipe to prevent people from using it to cut their own water bills.

Many boilers that provide central heating are being tested for leaks as they will go into operation in mid November .