FROM THE RIM of Lake Powell, you can’t see the danger line. There isn’t a flashing gauge or a chart on the canyon wall that warns visitors when one of the most important machines in America begins to fail. There’s only an invisible number: 3,490 feet above sea level.
Engineers call it “minimum power pool.” Above that elevation, Lake Powell can still push water through Glen Canyon Dam’s massive penstocks, spin its turbines, and send electricity across the Southwest with no trouble. Below it, the dam loses the function that helped justify its existence. The river still wants to move, but the machine starts running out of ways to move it.
