During the winter holidays, Carmelo Sandoval rows hundreds of tourists across the Rio Grande from southwest Texas to the Mexican town of Boquillas del Carmen. The remote crossing in Big Bend National Park is within the largest U.S. Customs and Border Protection sector amid more than 500 miles of border without fencing or a wall. Melissa and Hal Epperson, tourists from Kissimmee, Fla., boarded Sandoval's rowboat for their first trip to the park. Melissa was nervous, but her husband was excited about the crossing. "I’ve never been to Mexico," he said, "so this was my chance."