More people are on the move in China than ever before in human history. Twelve million people are leaving the countryside for the cities every year, and, within a generation, there will be more people living in the towns and cities than in the countryside. With China already home to a fifth of the world's population, the Chinese government is building 400 new cities over the next 20 years, each housing over half a million residents. New towns and settlements are springing up from nowhere. Others are witnessing an explosion in their populations, stretching their capacity to deliver essential services to breaking point. This second City Life programme tells the story of how one provincial capital - Chengdu, in South West China - has reversed appalling environmental pollution and improved the lives of some of its poorest inhabitants.

The Chengdu basin has been inhabited continuously since pre-history. The regions owes much of its success to the steady stream of cold, clear, pure water that has flowed this way, uninterrupted for million of years. The Min River, a tributary of the Yangtze, feeds the Chengdu basin and comes from the Himalayan Mountains. From early on, the Chinese people learned to harness the river. In 256 BC, scholar and engineer Li Bing built the Dujiangyan Irrigation System, channelling the Min river through Chengdu in what is still recognised as a triumph of hydraulic engineering. It was this amazing accomplishment that spurred Chinese agriculture and enabled the Chinese population to expand. But it's not just an historic monument: it's still irrigating the plain today.

But the irrigation system was neglected and abused during the rapid industrial development of the 1970s, resulting in massive pollution and floods. Today, prompted by a campaign started in 1985 by Chengdu schoolchildren, the municipal government has succeeded in reversing the damage, turning what had become an urban nightmare into a model of modern day planning.

The Fu and Nan Rivers that encircle the inner city of Chengdu were once collectively known as the Brocade River. Silk Brocade was the famed material exported on the long and torturous Silk Road. Legend has it that the name came about because the river was so clean that when the fabric was washed in the river it would come out brighter and more lustrous. By the 1980s this was certainly not the case.

Da Pengfu teaches natural sciences at the Long Jiang Lu elementary school situated right beside the river. He has been using Chengdu's Fu-Nan rivers system as a living laboratory with his students since he began teaching. In 1985, his students became the catalyst that set in motion a process which is now transforming the city. "When we were students we could swim in the river whenever we wanted, we really enjoyed it, but when I was teaching in 1985 there was no way you could swim in this river and I realised this was a problem. . . I felt that if the children had a deep understanding of the river that it would nurture a feeling of respect. So I organised the 'Little Guards of the Fu-Nan Rivers'."

The schoolchildren also became involved in the issue and appealed to the authorities. Hu Han was one of the students whose activism in 1985 led to the river clean up. "When I was a student I didn't think that this project would be so big! This river passes by our school, by our homes, is right near by. We should care for it. It should be clean and beautiful. That's why we wrote the letter."

The students' appeal set off a long chain of events. It took several years of planning and massive efforts to build public consensus, but in 1992 Chengdu began a five-year plan called the Fu-Nan Rivers Revitalisation Project. Central to the plan was the removal and relocation of the factories responsible for the pollution, replacing the dilapidated shanties that had sprung up haphazardly along the riverside, and completely restoring the banks with public spaces.

Zhang Jihai, Secretary General of the Chengdu Communist Party, explains: "We had three very large problems to solve; one: where does the financing come from? Two: how do we handle the fact that so many businesses and residents have to be relocated, and three: what is the vision for this project? We needed a concept on how we wanted to design the renovations and we needed a consensus to make it happen."

A thousand businesses and as many as 100,000 residents had to be relocated. This was difficult, but most residents were glad to go: there was no running water, no private toilets - and the roofs leaked. "When it rained hard it leaked a lot, when it rained a little it leaked a little," says Grandfather Xiang with a smile.

Over 600 wastewater outlets that once emptied into the river were removed. The entire length of the river was dredged and its width increased by half. Forty-two kilometres of the river-bank were reconstructed and 20,000 trees were planted as the city added 25 hectares of green public spaces. The cost was huge - US$ 330 million over five year. But the municipality recovered some of the money by selling some higher-priced accommodation to people who could afford it.

Along the river, they built the Living Water Garden Wetlands, with the help of an American artist, Betsy Damon. This innovative public space is a recreational park, a water treatment facility and a classroom that teaches everyone who goes there the way nature cleans water. "We need in our urban environment to live with natural systems and to have a deep understanding of the natural systems, rather than try to eliminate them or exclude them," says Betsy, who runs the charity 'Keepers of the Waters'. "For a lot of reasons - not only to understand them but because of the quality of life that it brings back in the city, the freshness, the biodiversity that returns."

The designer of the Living Water Garden, Huang Shida, is now working on a much larger project on the outskirts of the city, and officials from other Chinese municipalities have asked his advice on how to create Living Water Gardens in their cities.

The government is aware of the environmental dangers of rapid economic growth and industrialisation, and some lessons have been learnt from the big Chengdu clean-up. "We as a developing country need to develop our economy, but we absolutely at the same time must do this environmentally, says Secretary General Zhang. "I think that polluting first and then cleaning-up is an extremely uneconomic and irresponsible way of doing things."

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