With or without a wall, the border isn’t where you think it is
In 1854, the Gadsden Purchase modified a short, 30-mile stretch of the western border to be the midline of the Colorado River. The Mexican and U.S. governments soon realized that when these rivers shifted across their floodplains, questions about national jurisdiction arose. For example, an exasperated U.S. agent reported that “the lower Colorado … alters its channel from time to time, cutting off a large stretch of land on one bank and depositing the soil on the other or leaving its old bed and tearing through a large piece of silty bottom land to form a channel some distance away.” The agent went on to complain that these movements made it difficult to determine which land fell on which side of the line…