This story was co-published with The New York Times. Wedged between Arizona and Utah, less than 20 miles up river from the Grand Canyon, a soaring concrete wall nearly the height of two football fields blocks the flow of the Colorado River. There, at Glen Canyon Dam, the river is turned back on itself, drowning more than 200 miles of plasma-red gorges and replacing the Colorado’s free-spirited rapids with an immense lake of flat, still water called Lake Powell, the nation’s second largest reserve. Decommissioning Glen Canyon Dam, however, could offer a solution that politicians cannot afford to ignore — a cheap, immediate, and significant new source of water where it is most desperately needed. The argument has logical weight because both reservoirs have been struggling to remain half full, and may not ever refill. Researchers at Scripps Institution of Oceanography at the University of California San Diego gave Lake Mead a 50 percent chance of running dry by 2021.