Water levels are falling in America's largest reservoir. If it dries up, so could power and water for much of the Southwest.Imagine Nevada's Lake Mead, the largest reservoir in the United States, as a great sand pit, and imagine the population of the western United States as a colossal ostrich burying its head in the pit. And now, imagine the sand level dropping so fast that the willfully ignorant bird is forced to confront the fact that Lake Mead may actually become as dry as a sand pit in a decade.Lake Mead stores water from the Colorado River. When full, it holds 9.3 trillion gallons, an amount equal to the water that flows through the Colorado River in two years. The water from Lake Mead is used for many things. It irrigates a million acres of crops in the United States and Mexico, and supplies water to tens of millions of people. Its mighty Hoover Dam generates enough electricity to power a half-million homes. Additionally, the power from Hoover Dam is used to carry water up and across the Sierra Nevada Mountains on its way to Southern California.In 2000, the water level at Lake Mead was 1,214 feet, close to its all-time high. It's been dropping ever since. When Lake Mead was built during the 1920s and 1930s, the western United States was enjoying one of the wettest periods of the past 1,200 years. Even today, our so-called drought is still wetter than the average precipitation for the area averaged over centuries. In other words, for the last 75 years, we've been partying like it's 1929. Farmers grow rice by flooding arid farmland with water from Lake Mead; residents of desert communities maintain front lawns of green grass; golfers demand courses in areas where the temperature exceeds 100 degrees Fahrenheit during the summer.The combination of a changing climate and a strong demand for the lake's remaining water has resulted in 100 foot drop since 2000. While that's just 10 percent under the lake's high water mark in 1983, Lake Mead is like a martini glass-wide at the top and narrow at the bottom. That 10 percent dip represents a loss of half Lake Mead's water supply in nine years, from 96 percent capacity to 43 percent.Anyone who's gone on a diet knows this simple equation: if you burn fewer calories than you eat, you'll gain weight. But like a cheating dieter in Superman's Bizarro world, the Western United States has been sucking more water out of Lake Mead than the dwindling Colorado River can provide to replace it. When output is greater than input, the reservoir shrinks.And it continues to shrink. Lake Mead's water level fell 14 feet last year, and the Bureau of Reclamation has projected the level will drop 14 more feet this summer. That will bring it perilously close to 1,075 feet, the point at which the federal government can step in and declare a drought condition, forcing a reduction of 400,000 acre-feet drawn from Lake Mead per year. A typical Las Vegas home uses a half acre-foot of water per year, so such a reduction would be equal toturning the tap off for 800,000 households.In 2008, the Scripps Institute of Oceanography issued a paper titled "When will Lake Mead go dry?" which set the odds of Lake Mead drying up by 2021 at 50-50. No more water, no more electricity, no more pumping power."Today, we are at or beyond the sustainable limit of the Colorado system," concluded the paper's authors. "The alternative to reasoned solutions to this coming water crisis is a major societal and economic disruption in the desert southwest; something that will affect each of us living in the region."Conservation efforts are helping (Southern Nevada has significantly reduced its draw from 325,000 acre-feet a year in 2000 to 265,000 acre-feet today) but the Colorado River remains "oversubscribed." Millions of acre-feet are sent to California, Nevada, and Mexico annually, draining Lake Mead and neighboring Lake Powell faster than they can be replenished. Conservation solutions include "grass buyback" programs to encourage people to install drought-tolerant landscaping, tax incentives for pool-covers, and inevitable rate hikes.Frustratingly, Las Vegas residents tried to pass a bill that would allow homeowners to install graywater systems but the Southern Nevada Water Authority blocked it, offering up a piece of fuzzy math as a defense. Las Vegas Valley is alloted 300,000 acre-feet of water per year from the reservoir. The water that goes down drainpipes in Las Vegas gets pumped 12 miles back to a reclamation plant near Lake Mead. This returned water counts as a credit toward getting more fresh water from the lake. The Water Authority says if people start using graywater to water their lawns and gardens rather than using drinking-quality water, their lowered water bills will dissuade them from conserving water. In other words, the Water Authority believes that legalizing graywater will cause people to use more fresh water and return less dirty water to the reclamation plant.One of the more radical proposals involves pumping water from the eastern United States (where many regions are suffering the consequences of flooded rivers) over the Rockies to the West. In a Las Vegas Sun interview on May 1, Pat Mulroy, general manager of the Southern Nevada Water Authority, said, "We've taken water from the West now for a hundred years, maybe it's time to start taking water from the East, rather than from the West." Another speculative proposal lies beyond the shores of California, where there's an ocean of water available for desalinization. In April, the California Coastal Commission approved the West Basin Municipal Water District's plan to build a desalination system in Redondo Beach that can desalt 100,000 gallons of seawater per day.The power requirement for either proposal-desalting seawater or transporting water over great distance-is enormous. But if the only other alternative is a mass evacuation from the western United States, what other choice do we have?Mark Frauenfelder is the editor-in-chief of Make magazine and the founder of Boing Boing. He is currently writing a book on the do-it-yourself movement for Portfolio, an imprint of Penguin.Top photo by flickr user (cc) TimPearce. Bottom photo by flickr user (cc) jscatty.