At the narrow waist of the Americas, where human ingenuity once bent geography to commerce's will, nature is reasserting its terms. The Panama Canal — carrying 5 percent of the world's maritime trade and 40 percent of American container traffic — will reduce daily vessel transits by 18 percent this October, as an El Niño drought of historic severity drains the freshwater reservoirs that make the locks run. What began as a feat of engineering now reveals its oldest vulnerability: dependence on rain.