The last of 16 million tonnes of concrete will be poured in today, making Chairman Mao's dream of a reality, and giving China's current generation of engineers-turned-leaders the chance to proclaim another colossal step forward in the country's "harmonious development".

But the completion of the Three Gorges dam has been anything but harmonious. It is now being cited as a textbook example of how not to build a dam. Before it even starts operating, the giant hydro-electric scheme is threatened by silt - the solution to which is to pour yet more concrete into the Yangtse river.

Since construction began 15 years ago, more than 1 million people have been relocated from areas engulfed by the 370 mile-long reservoir that formed behind the wall of the dam. Another 80,000 will have to leave in the next few months.

The state has gone into overdrive to proclaim the achievements of the 1.4 mile-long dam, completed nine months ahead of schedule: what it will do for flood control, navigation safety, energy generation and the economy. With an output equivalent to a dozen nuclear power plants or the burning of 50 million tonnes of coal, it will power Shanghai and other cities on the fast-growing eastern seaboard.

As the waters behind the dam rise, increased pressure will allow it to generate more power and recoup the $22bn (£11bn) investment more quickly. But the output is not as significant as had been originally imagined. At first, it was envisaged the dam would supply at least a 10th of the country's energy, but electricity supply has grown rapidly along with the economy, and by the end of this year, it will provide less than a 30th.

And the dam's environmental record is, at best, mixed. Several species of freshwater fish are no longer able to reach their spawning grounds. Scientists warn that the upstream water quality has deteriorated because the flow is too slow to allow the river to clean itself. More than half of the sewage from Chongqing is pumped into the river untreated. New water treatment plants have been built, but this has failed to stop a slow stagnation. To minimise the loss of fertile land, farmers have been encouraged to dig up soil under the flood line and re-lay it on the tops of hills, but much biodiversity has already been lost under the reservoir.

Green activists urged the government to learn the lessons of the Three Gorges by allowing greater public participation in future projects. Consultation was absent from the approval debate for the dam. Even the National People's Congress was so enraged by the plan that a third of delegates either voted against or abstained - the closest to a rebellion that China's rubber stamp parliament has ever seen.