A punishing drought is gripping much of the western U.S. Scientists are calling it a “mega-drought” brought on by climate change. It’s taking a dramatic toll on the Colorado River system that provides water to 40 million people in seven states. For more than eight decades, the Hoover Dam has relied on water from Nevada’s Lake Mead to cover up its backside. But now at age 85 it finds itself uncomfortably exposed. Much of the water the dam is supposed to be holding back is gone.