What is the importance of the border between the United States and Mexico in the nonfiction book, The Line Becomes a River?

The border between the United States and Mexico is a fundamental focus of all three parts of the narrative. Several of the physical, or geographical, components of the border (particularly the Rio Grande, which makes up part of the actual border, and the desert on the American side) serve as triggers for both psychological and physical suffering, for both the author and the migrants whose situations he considers.