The first sign of serious trouble for the drought-stricken American Southwest could be a whirlpool. It could happen if the surface of Lake Powell, a man-made reservoir along the Colorado River that’s already a quarter of its former size, drops another 38 feet down the concrete face of the 710-foot Glen Canyon Dam here. … The normally placid Lake Powell, the nation’s second-largest reservoir, could suddenly transform into something resembling a funnel, with water circling the openings, the dam’s operators say. If that happens, the massive turbines that generate electricity for 4.5 million people would have to shut down — after nearly 60 years of use — or risk destruction from air bubbles. … Such an outcome — known as a “minimum power pool” — was once unfathomable here. Now, the federal government projects that day could come as soon as July.

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