THE EVEN GREATER WALL OF CHINA

IN THE final hours, it was a fleet of giant dumper trucks which sealed mankind's mastery over the world's third longest river. One by one, their tyres dwarfing nearby Chinese labourers, the vehicles off-loaded massive boulders into the narrowing gap, gradually throttling the flow of water. Suddenly the sound of firecrackers, ships' horns and a military brass band rolled across the valley, and flares soared towards the sky chased by thousands of balloons. Thus was the natural course of the Yangtze blocked for ever more.

That was last November when, under the proud gaze of China's leaders, the Yangtze river was diverted so that work could start on the main wall of the Three Gorges Dam, the world's biggest hydro-electric power project. The completion of the preliminary "coffer dam" across the Yangtze at Sandouping forced the river to pass instead through a man-made channel excavated to the south. On the dry riverbed downstream from the coffer dam, work has now started on the foundations for a curtain of concrete, 175 metres high and 2.3 kilometres wide, which will sweep across the valley.

Depending on one's point of view, it is either a triumph for China's ability to remodel nature, or the communist world's last misguided Stalinist folly. It could prove a visionary power and flood-control scheme, or hubris of unforgiveable proportions, drowning the livelihoods of 1.2 million people who will be forced to relocate, submerging the famed scenery and archeological relics of the middle reaches of the Yangtze, and taking a huge environmental gamble.

The vista from the riverbank is like a scene from Gulliver's Travels, as if some giant child has been playing in a stream, building barriers and gouging out channels to see how the water will run. For a dam of this magnitude is, during the first decade of construction, primarily an exercise in brute earthmoving on an unimaginable scale. Peer down into the chasm which has been sliced through the rock on the north bank in preparation for a ship lift and locks, and the workers at the bottom look like termites frozen in a deathly shadow. Gaze up at the two markers high on the hills on either side of the river, and the realisation dawns that they mark what will be the top of the dam wall - and the upstream level of the future reservoir.

By completion, some 103 million cubic metres of soil and rock will have been excavated, 29 million cubic metres of landfill deposited, and 27 million cubic metres of concrete set. "You wouldn't expect to see another concrete-pouring project as big as this in 50 years," said Xiao Chongqian, the official in charge of dam equipment. The estimated total price tag is 203 billion yuan (pounds 16 billion). It has taken three years to build the coffer dam, the first electricity will be generated in 2003, and final completion is set for 2009. Foreign companies are taking their share of the business; last year saw contracts for turbines and generators worth a total of pounds 460 million awarded to foreign consortiums.

The Three Gorges Dam will have a generating capacity of 18,200 megawatts, some 50 per cent more than the world's present biggest hydro-electric scheme, the Itaipu Dam on the Brazil-Paraguay border. President Jiang Zemin, gazing last November at what has become a national symbol of China's emerging industrial might, declared that this "remarkable feat in the history of mankind to reshape and exploit natural resources [would] greatly inspire confidence in the Chinese people". Or not, sceptics might add, if the consequences are deemed too painful.

The arguments rumble on, almost silently inside China where public debate is stamped on, and more audibly among environmentalists elsewhere. But, last November marked more than just an engineering turning point. Barring a radical change of Chinese government, or perhaps a war, the Three Gorges Dam is going to be built. The question is, at what cost, socially and environmentally?

The mountainous geography of this part of the Yangtze means that a reservoir 630 kilometres long and 1.1 kilometres wide on average will snake through China's heartland, submerging 632 square kilometres of land - approximately equivalent in area to the Isle of Wight. Thirteen cities, 140 towns, 1,352 villages, and 1,600 enterprises will disappear. It is difficult to imagine how a project of this magnitude would ever survive the process of public scrutiny in a more open political system. But China's authoritarian government does not have to worry about environmental protesters like Swampy digging themselves in. Reservoirs in modern China have already forced the relocation of more than 10 million people in recent decades.

It is also true that something needs to be done to stop the ruinous annual floods in the lower Yangtze, which in 1931, 1935 and 1954 alone killed a total of 317,000 Chinese. Environmentally, there is one plus: the Three Gorges Dam will reduce China's rising coal consumption by 40 million tonnes a year, and produce the energy of a dozen nuclear-power stations.

But try telling that to the residents of Fengjie, a busy river port 160 kilometres from the dam site, or Fuling, famous for its pickles, a further 340 kilometres upstream. The populations of these far-flung cities will have to leave when the water level of the Yangtze starts to rise in 2003. Inland, new cities are under construction along the path of the future reservoir, with roads, railways, bridges and ports. But complaints are already rife that relocation money is disappearing into the pockets of corrupt officials and promised jobs will not materialise. For the peasants along the river's banks, the situation is even worse; they will be forced to move to less fertile, higher slopes in a region which is already short of farmland.

Meanwhile, Chinese archaeologists have broken their silence and complained that too little is being done too late to save and excavate relics. The most famous sites, such as Fengjie's Baidecheng temple, the low-water record tablets near Fuling, and the City of Ghosts in Fengdu will be relocated or safeguarded, but many hundreds of other sites will be lost.

Then there are the technical questions. The river's flow will slow once the dam is built, forming a trap for the one billion tonnes of sewage and industrial pollution which is discharged annually into this middle stretch of the Yangtze. Added to this is the 526 million tonnes of sediment and pebbles brought down each year by the Yangtze, which is likely to settle on the reservoir bed and behind the dam wall.

The Chinese government insists all possible problems have been "solved". But there are occasions when official certainties about the Three Gorges Dam stretch the credibility of even the most ardent supporter. The official "Environmental Impact State-ment" includes a section called "Analysis for Impacts of a Dam-breaking". It states: "A dam-breaking can only occur when the dam is directly hit by nuclear weapons." But even then, the report reassures, the steep, twisting valleys will help contain the flood as 40 billion cubic metres of water pour forth. "Thus no major damage is expected if a dam-breaking occurs," the report concludes.

That, at least, is what the government is telling the tens of millions of people who live downstream.