Of all the mapmakers’ imaginary lines inscribed on North America, the most famous is the one curling north and south to the poles, one hundred degrees west of Greenwich, England. In the United States, the hundredth meridian runs from the Dakotas through Nebraska, Kansas, the panhandle of Oklahoma, and the heart of Texas. It became famous after John Wesley Powell, explorer of the Grand Canyon, observed that this meridian roughly coincides with the twenty-inch annual rainfall line: land to the east receives more than twenty inches of rain per year, enough to grow crops without irrigation, while nearly all land to the west receives less, and thus requires irrigation. We should therefore be cautious, Powell argued, in how we settle the arid region beyond the hundredth meridian. Because this caution has been ﬂagrantly ignored, western aquifers, such as the Ogallala, are being rapidly depleted, and rivers such as the Colorado are being drained.