Earth's rovers are running dry.

• Author: Postel, Sandra

In 1922, American naturalist Aldo Leopold journeyed by canoe through the great delta of the Colorado River. What he reported seeing there was a verdant waterscape, where for millennia the river had been depositing its rich silt and building up a diverse ecosystem before entering the Sea of Cortez--known north of the border as the Gulf of California. He saw deer, quail, raccoon, bobcat, vast amounts of waterfowl, and even jaguar. The meandering river, slowing as it spread out through countless green lagoons, led Leopold to muse, "For the last word in procrastination, go travel with a river reluctant to lose his freedom in the sea."

Leopold never returned to the delta for fear of finding this wilderness badly altered. His fears were justified; today, the Colorado's freedom has been lost to a degree Leopold scarcely could have imagined. Except in years of unusually high precipitation, the Colorado River no longer reaches the sea at all--it literally disappears into the surrounding desert. Much of the abundant wildlife is gone. Off the coast to the south, the once-productive fisheries in the Sea of Cortez have declined dramatically. In striking contrast to Leopold's experience, author Philip Fradkin more recently characterized what is now a desiccated place of mud-cracked earth, salt flats, and murky pools as "the most inhospitable terrain on the North American continent."

What has happened to the Colorado is but an extreme example of a disturbing worldwide trend. More and more rivers are running dry as dams and diversions siphon water off for burgeoning cities and thirsty farms. In Arizona, the Salt and Gila rivers used to converge west of Phoenix; now, they dry up east of the city because of extensive diversions for irrigated farm's. In California, approximately 22 miles of the San Joaquin River have been dewatered so permanently that thickets of trees have sprung up in the dry riverbed, sand and gravel are mined from it, and developers even have proposed building houses between the banks. In China, about 30 miles south of Beijing, villagers say the Heaven River dried up 20 years ago. In the water-deprived Middle East, where surface streams extensively are overtapped, the lower stretches of the Jordan River have dwindled to a salty trickle.

It is the decline of the world's larger rivers that most graphically conveys the magnitude of the problem. The Nile, Ganges, Amu Darya and Syr Darya, Huang He (or Yellow River), and Colorado are so dammed, diverted, or overtapped that, for parts of the year, little or none of their freshwater reaches the sea. Their collective diminution portends not only worsening water shortages and potential conflicts over scarce supplies, but mounting ecological damage--which, in turn, places the economies and people who depend on them in jeopardy.

Human efforts to control rivers date back thousands of years. It was not until this century, though, that engineering schemes began to alter natural water courses on a massive scale. The construction of dams to...