Whatever the nature of the overture, what follows is the same.
A wall of concrete and steel -- one and a half miles in length and as high as a sixty-story building -- groans, shifts, heaves, and bursts asunder from the unthinkable pressure behind it. Through the rapidly-widening breach pours a torrent of water more massive than any flood in the memory of mankind. The colossal fluid juggernaut roars forward, unstoppable, driven by the force of the reservoir now unleashed -- a reservoir hundreds of miles long, holding almost ten cubic miles of water. Before it, defenseless, lies its prey: vast tracts of agricultural land among the most densely-populated on the planet, and the industrial heartland of a great nation.
Within hours, innumerable villages and several major cities have been scoured away as if they had never existed at all, and tens of millions of human beings are dead. Most of their bodies will never be found, for the torrent will not lose force appreciably until it has carried them out into the ocean a thousand miles downstream. Tens of millions more stand in mortal danger from the famine and disease which will ravage the gutted nation. As the news spreads worldwide, leaders in every capital tremble in anticipation of the great political upheaval which is sure to follow in the wake of the deadliest disaster in all of human history.
This nightmare scenario could actually materialize in the near future, thanks to a hideously-misguided engineering project of which most people in the Western world are only vaguely aware: the Three Gorges Dam in Hubei province, mainland China.
Under construction from 1994 to 2003, the gigantic dam (7,575 feet long and 607 feet high) is intended to generate electricity and control the disastrous floods to which the Yangtze has always been prone. When filled, its reservoir will be 410 miles long and contain 9.43 cubic miles of water (source), displacing 1,400,000 people. In effect, the project has created a lake in the deep interior of China -- a lake, however, restrained at one end only by a man-made barrier of concrete and steel.
China has suffered dam-related disasters before. In 1975, after a series of typhoons, a dam burst in Henan province; the resulting wave shattered the next dam downriver from it, and the one after that, and so on, until a total of sixty-two dams had been breached. The death toll from this event was estimated at between 80,000 and 230,000, while millions suffered from disease and famine. But those dams were not even close in scale to the Three Gorges Dam; it has been estimated that a breach in the latter would release a flow of water forty times the volume of that of the 1975 disaster.
The project has aroused a degree of outspoken opposition unusual in a totalitarian state, as hydrologists and engineers have drawn attention to the risks -- but the regime, as it typically does, has simply suppressed dissenting views.
Poor, totalitarian states are notoriously prone to corruption, which can lead to shoddy construction as builders and suppliers cut corners. There is already evidence that the Three Gorges Dam has suffered from this problem. There have been many reports of cracks in the dam (example here).
As if that weren't alarming enough in itself, there are six active earthquake faults near the dam, and there has been speculation that the tremendous weight of the water in the reservoir as it fills could actually trigger an earthquake.
The Yangtze valley is among the most densely-populated areas in China. Combined, the river's valley and its delta (which would also suffer at least some effects from flooding if the dam burst) contain one-third of China's population and account for almost one-half of the country's food production. The valley is also a major industrial region, including the industrial city of Wuhan (population eight million) a short distance downriver from the dam.
If disaster struck, there is obviously no hope that a country with a primitive infrastructure could evacuate a significant percentage of the endangered population in the few hours (at best) that would be available.
The Three Gorges Dam reservoir will be filled to capacity by the end of 2008.