California’s Sacramento-San Joaquin Delta is vital to water supplies for 25 million people and 4 million acres of farmland. It is linked to the Pacific Ocean via San Francisco Bay, which makes this water supply uniquely vulnerable to sea level rise. Yet understanding sea level rise in the Delta is complicated. The largest estuary on the West Coast of the Americas, it consists of some 70 islands and more than 1,000 miles of levees. It is also fed by California’s two largest rivers, which drain the Sierra Nevada range.