There is only one river left to slake the thirst of California, as the nation's most populous state keeps growing. The state's other rivers are tapped out. We need this last great river more than ever as global warming threatens to make longer, drier droughts the norm throughout the West. But you won't find California's last river on any map because it's a virtual river. It doesn't exist as a physical river, but that doesn't make it any less real.