"What are you doing?" the security guard demands. "Nothing," replies the stocky woman lurking outside the gates of the paper mill, tucking her secret weapon—a handheld global positioning device—under her sweater. The guard eyes her for a minute, and the woman, a 51-year-old laid-off factory worker named Jiang Lin, holds her breath. When he turns away, she pulls out the GPS and quickly locks in the paper mill's coordinates.

As an employee of Green Camel Bell, an environmental group in the western city of Lanzhou, Jiang is following up on a tip that the mill is dumping untreated chemical waste into a tributary of the Yellow River. There are hundreds of such factories around Lanzhou, a former Silk Road trading post that has morphed into a petrochemical hub. In 2006 three industrial spills here made the Yellow River run red. Another turned it white. This one is tainting the tributary a toxic shade of maroon. When Jiang gets back to the office, the GPS data will be emailed to Beijing and uploaded onto a Web-based "pollution map" for the whole world to see.

For all of Lanzhou's pride in being the first and biggest city along the Yellow River, it is better known for its massive discharge of industrial and human waste. But even here there is a glimmer of hope: the first seedlings of environmental activism, which may be the only chance for the river's salvation. In the mid-1990s a mere handful of environmental groups existed in China. Today there are several thousand, including Green Camel Bell. Jiang Lin's 25-year-old son, Zhao Zhong, founded the group in 2004 to help clean up the city and protect the Yellow River. With only five paid staff, Green Camel Bell is a shoestring operation kept afloat by grants from an American NGO, Pacific Environment. The name they chose, after the reassuring bells worn by camels in Silk Road caravans, is meant to be "a sign of life," says Jiang. "The bell is supposed to give hope to everyone who hears it."

At long last Beijing appears willing to listen. After three decades blindly pursuing growth, the government is starting to grapple with the environmental costs. The impact is not simply monetary, though the World Bank calculates that environmental damage robs China of 5.8 percent of its GDP each year. It is also social: Irate citizens last year flooded the government with hundreds of thousands of official environmental complaints. Whether to save the environment or stave off social unrest, Beijing has adopted ambitious goals, aiming for a 30 percent reduction in water consumption and a 10 percent decrease in pollution discharges by 2010.