Although severe droughts had left their mark hundreds of years back in the paleologic record, the 20th century was abnormally wet. Consequently, what we think of as normal is, in fact, unnaturally wet, compared to the much longer lifetime of the Colorado River.

Throw in climate change, which will almost certainly result in a decrease in average flows for the Colorado River (though to what extent is still hotly contested), and things really start looking grim. The desert Southwest is facing the slow decay of the water supply upon which the region has been built.

"An 80 percent year is not that bad. But if we had 20 years of 80 percent runoff on the Colorado River, we'd drain Lake Mead and Lake Powell," Kuhn says. "We're so close between supply and demand that a long period of 80- or 85-percent years will bankrupt the system."