The tension already seethes. In the clogged alleyway around one of Nehru Camp's last functioning taps, which run for one hour a day, a man punches a woman who cut in line, leaving a purple welt on her face. "We wake up every morning fighting over water," says Kamal Bhate, a local astrologer watching the melee. This one dissolves into shouting and finger-pointing, but the brawls can be deadly. In a nearby slum a teenage boy was recently beaten to death for cutting in line.

As the rivers dwindle, the conflicts could spread. India, China, and Pakistan all face pressure to boost food production to keep up with their huge and growing populations. But climate change and diminishing water supplies could reduce cereal yields in South Asia by 5 percent within three decades. "We're going to see rising tensions over shared water resources, including political disputes between farmers, between farmers and cities, and between human and ecological demands for water," says Peter Gleick, a water expert and president of the Pacific Institute in Oakland, California. "And I believe more of these tensions will lead to violence."

The real challenge will be to prevent water conflicts from spilling across borders. There is already a growing sense of alarm in Central Asia over the prospect that poor but glacier-heavy nations (Tajikistan, Kyrgyzstan) may one day restrict the flow of water to their parched but oil-rich neighbors (Uzbekistan, Kazakhstan, Turkmenistan). In the future, peace between Pakistan and India may hinge as much on water as on nuclear weapons, for the two countries must share the glacier-dependent Indus.

The biggest question mark hangs over China, which controls the sources of the region's major rivers. Its damming of the Mekong has sparked anger downstream in Indochina. If Beijing follows through on tentative plans to divert the Brahmaputra, it could provoke its rival, India, in the very region where the two countries fought a war in 1962.

For the people in Nehru Camp, geopolitical concerns are lost in the frenzied pursuit of water. In the afternoon, a tap outside the slum is suddenly turned on, and Chaya, smiling triumphantly, hauls back a full, ten-gallon jug on top of her head. The water is dirty and bitter, and there are no means to boil it. But now, at last, she can give her children their first meal of the day: a piece of bread and a few spoonfuls of lentil stew. "They should be studying, but we keep shooing them away to find water," Chaya says. "We have no choice, because who knows if we'll find enough water tomorrow."