Last year, when a group of journalists and historians offered a list of the 100 biggest news stories of the 20th century, the Beatles' appearance on "The Ed Sullivan Show" was ranked 58th. Completion of Hoover Dam didn't make the cut. You sort of expect this from celebrity-infatuated mass culture: when it comes to fundamental achievements that make contemporary civilization work, a stifled yawn. Water, dams, aqueducts, irrigation, hydroelectricity — how borrrrrrring! Really? Los Angeles, world headquarters of celebrity culture, has measured as little as 5 in. (13 cm) of rainfall in a year. And despite occasional monsoons, Southern California is so chronically arid that it couldn't sustain a third of its current population without sucking billions of liters a day out of Lake Mead, the distant Colorado River reservoir formed by Hoover Dam.

One would think the creation of modern Los Angeles, which is what Hoover Dam allowed, would make that structure newsworthy. Actually, its historic significance is of more cosmic proportions. The first of the world's great dams, Hoover inaugurated an Age of Dams, which has spanned the past three-quarters of a century. The dam-building mentality has pretty much expired in the U.S. — one reason is, we've run out of dam sites-but it's still prevalent throughout much of the world. In China, which is erecting the Three Gorges Dam, the biggest (and, at $25 billion, the most expensive) hydroelectric project in history, one senses outright resentment against rivers running free.

Almost everyone has some appreciation of how water projects have altered the course of civilization in ways we (perhaps foolishly) call benign. Dams and reservoirs permit unimaginable numbers of people to inhabit forbiddingly arid regions — as well as floodplains where cities would be washed away without upstream protection. Sacramento, Calif., for example, is dryer than North Africa, but the Sacramento River, on whose banks it sits, spread 30 miles (50 km) wide during the wettest California winter on record, in 1862, before dams and levees tamed the river. Dams produce more clean energy than nuclear reactors. Irrigation agriculture, largely dependent on reservoirs, grows 40% of the world's food on a much smaller fraction of its farmland.

What we're just beginning to understand is how water development has, like nuclear energy, amounted to a Faustian bargain between civilization and the natural world — which, as it happens, supports civilization. Hydroelectricity from Grand Coulee Dam in Washington State smelted enough aluminum during World War II to build tens of thousands of warplanes, with enough surplus power to make plutonium for the first atom bombs. But now, in the form of devastated salmon fisheries, Grand Coulee (along with countless other dams) is extracting an awful price for its creation.

It seems ironic that nuclear energy is widely regarded as a greater environmental threat than dams, even though fissio — with the jarring exceptions of Chernobyl and Three Mile Island-has caused relatively little harm. There may be huge calamities in its future, and its fiercely toxic fission products still have no demonstrably safe burial place. But dams, for all their material blessings, are responsible for some of the worst environmental tragedies in history.

In Central Asia, the aral sea, originally only somewhat smaller than Lake Superior, has shrunk to less than half its former size since the 1960s. The two big rivers that used to replenish it, the Amu Darya and Syr Darya, are now ceaselessly diverted to irrigate millions of hectares of cotton in Uzbekistan. Inspired by Joseph Stalin, this grand-scale hydrologic theft continues despite the dissolution of the Soviet Empire. It has destroyed one of the world's richest inland fisheries; abandoned boats rust 50 miles (80 km) from the sea's latest shoreline. Dust storms, poisoned by heavy metals and pesticides that washed into the disappearing lake, have combined with diseases that flourish in withered, polluted rivers to create a public-health catastrophe.

It's increasingly obvious that one of the worst features of communism was a relentlessly thuggish assault against nature. ("Let the Aral Sea die a beautiful death," a Soviet planner proclaimed in 1987. "It is useless.") But in the realm of water, the record of democratic capitalism, especially during the cold war, wasn't much better. As environmental tragedies go, the destruction of the Columbia River salmon fishery in the American Northwest runs a close second to the ruining of the Aral Sea.