Lake Mead, which serves almost 25 million people and cities such as Las Vegas, could drop to 20% full this year, which experts say is incredibly worrying. (Image credit: Christopher Moswitzer via Getty Images) Catastrophic water shortages are in store for the 40 million people living in areas fed by the Colorado River, even if cities in the region such as Denver, Phoenix and Las Vegas make dramatic cuts to their usage, recent research suggests. Water shortages could start as soon as this summer, as snowpack levels reached a record low over Lake Powell and the spring runoff into the Colorado River is expected to be minimal, experts told Live Science. And the region's major cities, which have already slashed their per capita water consumption by up to 58% between 2002 and 2025, can't solve the problem alone.…