The water level of Lake Mead, the country’s largest reservoir, has dropped more than 130 feet since the beginning of 2000, when the lake’s surface lapped at the spillway gates on Hoover Dam.

Twenty-one years later, with the Colorado River consistently yielding less water as the climate has grown warmer and drier, the reservoir near Las Vegas sits at just 39% of capacity. And it’s approaching the threshold of a shortage for the first time since it was filled in the 1930s.