The U.S.-built border wall cuts a decisive line through the middle of the Sonoran Desert. In this picture, Mexicali, a city of seven hundred thousand people, sits on the south side of the fence, while California’s Imperial Valley agricultural region lies to the north.

Despite its exceedingly dry climate, the Imperial Valley is one of the most productive farming areas in the United States, thanks to the All-American Canal and the water it delivers from the Colorado River, more than eighty miles to the east. In recent decades, enormous population growth in the Southwest has placed higher demands on the river, while a twenty-year mega-drought has left it with far less water to give. The Colorado River no longer reaches the ocean, but effectively ends its journey here instead, its diverted water dispersed across five hundred thousand acres of produce.