There is no shortage of ways to see just how short of water Lake Mead is. You can count the white bathtub rings of mineral deposits on the bedrock walls of the sprawling, 250-sq.-mi. (647 sq km) reservoir, indicating the old high-water mark--now left nakedly exposed 100 ft. up. You can look at the docks that have been moved repeatedly, chasing the receding lake. Or you can simply read a line graph at the reservoir's visitor center, which tracks the water elevation of Lake Mead since it was created by the construction of the Hoover Dam in 1935. After years of relative stability, starting in 2000 the graph resembles the record of a stock-market crash. The visitor center's chart stops at 2006, but as a park ranger tells me, "It just keeps going down from there."
The worrying question is whether it will ever stop. A major, prolonged drought, combined with rapid population growth in nearby urban areas like Las Vegas, has stressed Lake Mead and the rest of the Colorado River Basin, which provides water to farmers and cities from Colorado to Southern California. Now there are fears that global warming could drastically reduce the Colorado River's flow--even as the Southwest continues to expand. Scientists at the Scripps Institution of Oceanography in La Jolla, Calif., last month estimated that there is a 50% chance that Lake Mead could be effectively dry by 2021 if the climate changes as forecast and water use is not curtailed. "I think we're at or beyond the level of water in the Southwest at which civilization can be sustained," says Tim Barnett, a research marine physicist at Scripps who co-authored the study.
The practical effects of climate change are notoriously difficult to predict on the regional level, and many experts criticize the Scripps study for failing to take into account improved water-management policies that could keep the lake wet well into the future. But it is as clear as those chalky white bathtub rings that Mead and the Colorado River are getting lower, and that could leave the states along the basin--whose populations grew 10% from 2000 to 2006, compared with the U.S. average of 5.6%--high and dry. "We don't think this is a regular drought," says Scott Huntley, a spokesman for the Southern Nevada Water Authority (snwa). "Something is going on. Something is happening."
Few urban areas are more vulnerable to those changes than Las Vegas, the dryest big city in America. Vegas takes 90% of its water from Lake Mead, although Nevada gets by far the smallest share of water among the seven states that border the Colorado--just 2% of the total. (Each state draws a fixed amount according to a deal hammered out in 1922, when the river was at an unusually high level.) Pat Mulroy, the powerful head of the snwa, says Las Vegas has worked hard to conserve water, paying residents to replace thirsty lawns with desert-appropriate landscaping. The city's overall water use has dropped since 2002, even as population and visitor numbers have continued to rise, and Mulroy thinks Las Vegas still has room--and water--to develop smartly. "It's not whether you grow but how you grow," she says.
If the rest of the Southwest can use its water more efficiently, it should be safe for decades. One solution could involve diverting more of the river's water away from agriculture--which claims 85% of the supply--in favor of the region's thirsty cities. That would be challenging politically, but something has to give. Still, while Lake Mead has shrunk to just 52% of capacity, the immense reservoir still contains an incredible 9 trillion gal. (35 trillion cu L) of water. But the dry sky above and the rock all around reinforce the inescapable fact that this land was a desert, is a desert and always will be a desert. When the American explorer J.C. Ives visited the present location of the Hoover Dam in 1857, he declared the land "worthless," adding, "There is nothing there to do but leave." Today's residents are hoping there's another choice.
GLOBAL DISPATCH For a new postcard from around the world every day, visit time.com