Droughts will pose a much larger threat to U.S. power plants with once-through cooling systems than scientists previously suspected, a new study shows. If surface waters warm 3 degrees Celsius and river flows drop 20 percent, drought-related impacts will account for about 20 percent of all shutdowns or capacity reductions at these plants. Retrofitting the plants with recirculating cooling systems will significantly reduce their vulnerability to costly impacts from both drought and environmental regulations.