This winter , for the first time in a quarter century, California was drought free—but not everyone was thrilled. On the Palos Verdes Peninsula in Los Angeles County, where some seventy-seven thousand live atop rugged coastal bluffs and steep hills, the near-biblical rains that have inundated the region in recent years pose an existential threat.

Overlooking the Pacific Ocean, the peninsula is some of the most coveted real estate in Southern California. It contains the wealthiest suburb in Los Angeles, with an average household income of $367,000. There are sprawling mansions, winding horse trails, six golf courses, and private clubs boasting initiation fees as high as $300,000.…