On an afternoon in late June, the San Luis Reservoir – a nine-mile lake about an hour southeast of San Jose, California – shimmered in 102-degree heat. A dusty, winding trail led down into flatlands newly created by the shrinking waterline. Seven deer, including a pair of fawns, grazed on tall grasses that, in wetter times, would have been at least partially underwater. On a distant ridge, wind turbines turned languidly.

That day, the reservoir, California’s sixth-largest and a source of water for millions of people, was just 40% full. Minerals deposited by the receding waters had turned the reservoir’s lower banks white, like the rings on a bathtub. Discarded clothing, empty bottles, and a lone shoe sat scattered across the newly exposed, parched ground. An interactive graphic in the visitor’s center reported that this year’s snowpack – which provides the water that travels from the Sacramento River Delta into the reservoir itself – was zero percent of the yearly average.