The Colorado River, which provides drinking water to 40 million people in seven U.S. states, is drying up, straining a water distribution pact amid the worst drought in 12 centuries, exacerbated by climate change. California split from the six states of Arizona, Colorado, Nevada, New Mexico, Utah and Wyoming on Tuesday in the face of a U.S. government deadline to negotiate their own supply cuts or face possible mandatory cutbacks by the federal government.