Swollen rivers may be the unexpected fallout of billowing smog. By intercepting sunshine and shading the Earth, polluting particles can stifle evaporation, leaving extra water to flood local waterways. Dirty air can load rivers with up to 25 percent more water than they would have under cleaner skies, researchers report October 5 in Nature Geoscience. “In a way, it makes sense,” says coauthor Peter Cox, a climate scientist at the University of Exeter in England. But many other factors also alter how water loops through the environment, including rainfall, large-scale irrigation and humidity, he says. “We weren’t sure —until we did this —on the overall impact,” of air pollution, he says