On weekday evenings, millions of workers return to their homes across the American Southwest and turn on their air conditioners, microwaves and televisions. From Tucson to Burbank, power needs surge. Meeting this demand begins at 5 or 6 A.M. inside the Glen Canyon Dam, the chip of concrete that plugs the Colorado River just above the Grand Canyon. At noon, an average peak of 14,000 cubic feet of water per second is churned through eight turbines, then released.