California’s largest reservoir, Shasta Lake, sits where the dry Central Valley meets the rainier, mountainous northern part of the state. At its western edge is Shasta Dam, 602 feet high, built by the Bureau of Reclamation between 1938 and 1945 to help irrigate California. For decades, agricultural and municipal water districts have sought to heighten the dam to capture more water as it runs out of the Cascade Range through the McCloud, Pit and Sacramento rivers. Environmentalists have long rallied against the proposal, and state officials contend such a project would violate California law.