> Up close, the Delta doesn’t look like much: a huge expanse of flat agricultural land, with relatively few signs of human habitation. On Google Earth, it resem

> Up close, the Delta doesn’t look like much: a huge expanse of flat agricultural land, with relatively few signs of human habitation. On Google Earth, it resembles a triangular green jigsaw puzzle. The principal puzzle pieces are five or six dozen irregularly shaped islands, which are separated from one another by seven hundred miles of sloughs and meandering waterways. The islands are actually what the Dutch called polders; they’re landforms that farmers created, beginning in the nineteenth century, by draining natural wetlands. Most of the islands cover thousands of acres. All are surrounded by dikes, which are known locally as levees; their purpose is to keep water from flooding back in. The cultivated fields inside the levees have gradually subsided, and in some places are now twenty-five feet below sea level. One consequence is that Delta farmers, in addition to siphoning irrigation water from the channels that surround their islands, have to pump water out—a chore familiar to anyone who has used a sump pump to keep a basement dry. > The main threat to the Delta is saltwater intrusion. If an earthquake caused a major levee failure, the sunken islands would flood, drawing salt water from the Pacific into waterways that are now kept fresh by the pressure of inflows from the Sacramento. “Instantly, your fresh water turns to sea water,” Mulroy said—and, at that moment, a resource that millions of Californians depend on for drinking and irrigation would be unusable. A month before my interview with Mulroy, I had met with Bradley Udall, who had just joined Colorado State University as a senior water-and-climate-research scientist. During our conversation, he described the Delta to me as “the biggest potential water disaster in the United States.” That was eight years ago. In the meantime, the drought has continued, making all the problems worse.