So far, this winter has brought ample snows to the Sierra Nevada, the spine of mountains that runs along California’s eastern flank. That’s good news for Californians, because the range’s melted snow provides 60 percent of the state’s water supply. Anyone in the United States who likes fruit, vegetables, and nuts should rejoice, too, because water flowing from the Sierra’s streams and rivers is the main irrigation source for farms in the arid Central Valley, which churns out nearly a quarter of the food consumed here.