BEIJING - A half-mile section of China's Yellow River turned "red and smelly" after an unknown discharge was poured into it from a sewage pipe, state media said Monday. The incident in Lanzhou, a city of 2 million people in western Gansu province, follows a string of industrial accidents that have poisoned major rivers in China over the last year, forcing several cities to shut down their water systems. It wasn't immediately clear what was tainting the section of the Yellow River. Environmental protection officials took samples and were trying to determine whether the sewage was toxic, the official Xinhua news agency said. "Residents were alarmed to see a sewage pipe pouring red water into the country's second longest river" on Sunday between 3 p.m. and 6 p.m., the agency said. A news photo from the local paper showed a resident in the city center by a stretch of the river — a drinking water source for millions — that was rose-colored instead of the usual milky brown. Other photos showed patches of bright red and pink. An official from Yellow River Water Resource Committee in Lanzhou confirmed the pollution. He said they were still analyzing the sample and had not determined what caused it. Like many Chinese officials, he gave only his family name, Wang. Environmental protection has taken on new urgency for Chinese leaders following a November 2005 chemical spill in the Songhua River in northeastern China which forced the city of Harbin to shut down its water supply for days and sent toxins flowing into Russia. China's cities are among the world's smoggiest, and the government says its major rivers, canals and lakes are badly polluted by industrial, agricultural and household pollution. Hundreds of millions of people live without adequate supplies of clean drinking water. Throughout the country, protests have erupted over complaints by farmers that uncontrolled discharges by factories are ruining crops and poisoning water supplies. "The Yellow River is the mother river of our country," said one bulletin board posting Monday on Sina.com, a major Chinese news Web site. "See how it has been ruined!" Said another: "Let the mayor of Lanzhou drink the water and then they will immediately have measures in place to deal with the environmental pollution." Kang Mingke, an official with the city's environment protection bureau, said there were no chemical plants located nearby, according to Xinhua. He said the red water could have come from central heating companies who dye their hot water to prevent people from diverting it for their own use, the news agency said.