The western United States is, famously, in the grips of its worst megadrought in a millennium. The Colorado River, which supplies water to more than 40 million Americans and supports food production for the rest of the country, is in imminent peril. The levels in the nation’s largest freshwater reservoir, Lake Mead, behind the Hoover Dam and a fulcrum of the Colorado River basin, have dropped to around 25% of capacity.