Water shortages due to ongoing drought affect the U.S. power supply as power plants become overheated and shut down or run at lower capacity, analysts say. Because they are completely dependent on water for cooling and make up about half the water usage in the United States, power plants can become casualties of droughts, says Barbara Carney of the National Energy Technology Laboratory in Morgantown, W. Va. Nuclear power suffers particularly, since the average nuclear plant that generates 12.2 million megawatt hours of electricity requires far more water to cool its turbines than other power plants. If water levels in the rivers that cool the plants drop too low, the power plant won\'t be able to draw in enough water. In addition, if the cooling water discharged from a plant raises river temperatures above certain levels, U.S. environmental regulations require the plant to shut down. At least four nuclear plants had to shut down in July for these reasons, and nationwide, nuclear generation is at its lowest in a decade with the plants operating at only 93 per cent of capacity, NewScientist.com reported.