A shrunken Lake Powell shows the impacts of drought and illustrates the consequences of human thirst on the Colorado River, which has been dammed and diverted so much that it now runs dry long before reaching its estuary at the Sea of Cortez.
About 30 million people, some as far away as Los Angeles, depend on the Colorado for drinking water. The river also nourishes a major agricultural breadbasket. Water shortages in the basin aren't just a future worry—they're part of the present.
"We're in a depletion situation in the Colorado River Basin, and not just during a drought year," Postel said. "The average ten-year demand is now higher than the average ten-year supply so we're using more water than the river usually has in it basin-wide."
Ninety years ago the Colorado River Compact divided water use among seven states. But demand has soared since then, and the river doesn't hold as much water as planners once thought. "In 1922, the year the compact was signed, it was the end of a pretty wet period," Postel said. "We're not likely to see that again any time soon for any length of time."
(See photos of rivers forced underground.)