LAKE CACHUMA, Calif. — At the marina Monty Keller manages amid sloping mountains here, business is down by half. For hours every workday, he stares sadly at the reason.

Lake Cachuma, a giant reservoir built to hold Santa Barbara County’s drinking water, has all but vanished in California’s historic drought. It reached an all-time low this summer — 7 percent capacity, which left a thick beige watermark that circles the hills framing the lake like an enormous bathtub ring. “We’re just amazed,” Keller said. MORE