The first time I saw the Rio Grande, I was with a nun. She took breaks in the afternoon and walked along the north bank of the river near the city of Hidalgo, looking for signs of people who needed help.

She found immigrants, who were hungry, wet, and often alone, and offered them comfort. They knew neither the law nor the language of the United States, and were hiding, desperately, in the canebrakes.

“That river really divides us, doesn’t it?” I asked.

“Oh, no,” she said. “The river is what sews us together. We are two countries but one people.”