Vox: It's a good time to revisit the slow-motion water crunch in the American Southwest. Last week, Lake Mead -- a key reservoir that helps supply water for 25 million people in Nevada, Arizona, and California -- shrunk to its lowest level ever. And the question of how to grapple with water scarcity is making headlines yet again. Back in the 20th century, the United States built an army of dams across the West to tame rivers, generate electricity, and store water in reservoirs for cities and farms. This...