In 2011, heavy snows in the Rocky Mountains filled the Colorado River, lifting reservoirs—and spirits—in the drought-stricken U.S. Southwest. The following year, however, water levels dropped to nearly their lowest in a century, imperiling a supply vital to millions of people and dozens of ecosystems. Now, scientists say they may have come up with a potential early warning system for the Colorado’s water levels—by watching temperature patterns in the Atlantic and Pacific oceans, thousands of kilometers away.