Above is a satellite image of part of Lake Mead, taken in August of 1985. You can see the Colorado River flowing in on the right.
This was the same part of Lake Mead in August of 2010:
Lake Mead is the reservoir that sits behind the massive Hoover Dam, where it generates electricity and provides water to most of the American Southwest. The water level in Lake Mead has pretty much always been up and down, depending on drought and other factors. You can see how this normal fluctuation has played out since the high point of 1941 in a graph on NASA's Image of the Day page.
The low you're looking at here is the lowest the Lake has been since 1956. Why? The National Park Service sums up the problem: "In an "average" year, the amount of water flowing out of Lake Mead exceeds the amount of water flowing into Lake Mead."
Now, that's a bit of a "duh" statement, but it's important to remember that the problem really is that simple. There's no black magic going on here, just basic math. Part of the problem is an ongoing 12-year drought that's limiting inflow from snow melt in the Rockies. But, as seen throughout Lake Mead's history, droughts come and go. The really worrying issue here is on the demand side.
Decades of population growth have led to increased water demand in the Southwest. Take, for instance, Las Vegas, which gets 90 percent of its water from Lake Mead. Back in the 1940s, fewer than 9,000 people lived there. In 2006, the population was estimated at more than 550,000, and growing. Rapidly.
Multiplied throughout the region, that added demand means the tolerance for expected drought fluctuation becomes more brittle. And if the cycle of drought and rain doesn't behave like it has in the past—a change some scientists say you can see happening now, and others say is likely under climate change scenarios going forward—it puts more people at risk for water shortage.
In a nutshell, that's what's got people freaking out about Lake Mead. It's not so much the current water level, but concern about what happens if the rains take a particularly long time to return. Or don't last as long as we'd like them to before another long drought.
Thanks to Mr. Bad Example, who sent the photos in via Submitterator.