BioEd Online

The last load of concrete is due to be poured at China's Three Gorges project on 20 May, completing the main wall of the world's biggest dam. Although the huge structure promises to control flooding on the Yangtze River and provide copious hydroelectric power, many fear that it could also have damaging social and environmental repercussions.

How big is this dam?

Massive. At 2.3 kilometres long, it is expected to cost around US$25 billion by the time it is fully functional in 2009. About 27 million cubic metres of concrete have gone into the structure since work began in 1993; that's more than eight times as much as has been poured into the Hoover dam on the Colorado River.

More than a million people have already been relocated to make way for the growing reservoir of water building up behind the dam, and 300,000 more will follow over the next two years. But while many move away, others will be able to stay put, and hopefully be safer for it. Thousands of people have died over the past few decades during dramatic floods on the Yangtze, which the dam is expected to quell.

Are there any other dams on the Yangtze?

Only about 50,000. Almost all of these, which mainly plaster tributaries of the 6,000-kilometre-long main river, have been built in the past 50 years, largely to create freshwater reservoirs.

The freshwater storage capacity has skyrocketed from 0.06 cubic kilometres in 1950 to 180 cubic kilometres in 2002. When the reservoir behind the Three Gorges Dam is full, it will add up to 40 cubic kilometres to that total.

When will Three Gorges start generating power?

It already is. Electricity started to flow from the dam's first turbine in mid-2003.

But when all 26 turbines are running, it should produce a mind-boggling 18 billion watts of power, which is roughly equivalent to five large coal-fired power stations. The Hoover dam, by contrast, has 17 main turbines producing a maximum 2 billion watts of power.

That sounds like a good power source

Absolutely. Proponents of the project say that China's exploitation of its hydroelectric potential is helping to save the planet; after all, they could be digging even deeper into their enormous coal reserves instead, adding to the world's burden of greenhouse gasses.

Yet, despite the enormous power output, Three Gorges will be only a tiny drop in the ocean of China's energy needs. China is currently increasing it's generating capacity by the equivalent of the United Kingdom's entire consumption every year, and this dam will only contribute a few per cent of China's total energy requirements.

So is it a 'green' project?

Any construction project on this scale is also going to have environmental repercussions. Silt is being trapped behind the dam, which means that less material is being carried to the Yangtze's delta near Shanghai. This is causing erosion of the wetland habitat there, which provides nurseries for fish and resting areas for migratory birds and is considered one of the world's most important wetland ecosystems¹. A similar effect was seen on the Nile after the Aswan Dam in Egypt was completed. Water evaporating from the reservoir's huge area could also have an impact on local climate².

Is all that being taken into consideration?

The China Yangtze River Three Gorges Project Development Corporation, in charge of construction, has admitted that the dam has some negative environmental impacts, but says it can mitigate them. They have already built a $2.5-million vessel, for example, to clear the tonnes of garbage that accumulate behind the dam each year.

So what's next?

Leaping Tiger. No, not a character from an action movie, but a place that some say would be perfect for the next big Chinese dam. Tiger Leaping Gorge is a narrow pass on the upper reaches of the Yangtze, deep in the Tibetan foothills and 1,500 kilometres away from Three Gorges. The area is still being surveyed, but construction could begin as soon as 2008.

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