In dry winters like the one California just had, less fresh water flows down from the mountains into the Sacramento River, the state's largest. That allows saltier water from Pacific Ocean tides to push further into the state's main water hub, known as the Delta. It helps supply water to two-thirds of the state's 39 million people and to farms that grow fruits and vegetables for the whole nation, playing a key but sometimes underappreciated role in the state's economy.

A drought that scientists say is part of the U.S. West's driest period in 1,200 years plus sea level rise are exposing the fragility of that system, forcing state water managers, cities, and farmers to look for new ways to stabilize their supply of fresh water. The Delta's challenges offer a harbinger of the risks to come for critical water supplies elsewhere in the nation amid a changing climate.