Lake Mead is the largest man-made reservoir in the country. It's the huge pool created where the Colorado river collects behind the Hoover Dam and it supplies nearly all the water for Las Vegas and more than a few toilet flushes for LA, San Diego and Phoenix. But it's in trouble. Snow runoff has been down in recent decades and according to studies there's a 50/50 chance that Lake Mead could run dry by 2021. Climatologist Tim Barnett co-authored the research, and he speaks with Faith Salie on Fair Game.