Scientists are calling it a “mega-drought” brought on by climate change. The latest U.S. Drought Monitor Map shows large areas of the Southwest are “exceptionally dry,” the worst category. It’s taking a dramatic toll on the Colorado River system that provides water to 40 million people in seven states — and may force the federal government to make a drastic and historic decision. For more than eight decades, the iconic 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.