Californians are always talking about the coming Big One, but what if the big one is a flood, not an earthquake? With this recent cavalcade of rainstorms, there's been renewed interest in a 2011 USGS study on the so-called "ARkStorm." In it, the USGS lays out a case for a hypothetical "megastorm," one that could cause up to $725 billion in damage and impact a quarter of California's homes. The ARkStorm would bring with it catastrophic rains, hurricane-force winds and hundreds of landslides. Central Valley flooding alone is projected to span 300 miles. If that sounds far-fetched, there's historic precedent:...