That remoteness, all the same, is part of the appeal. For 118 miles on the park's southern border, and dividing the United States from Mexico, the Rio Grande nourishes a ribbon-thin greenbelt in an endless desert. The international border, however, is more a suggestion than a wall. On a warm spring day, a Mexican family rows across the river to buy groceries in the store at Rio Grande Village. Americans, in turn, hail rowboats on the Mexican side to take them across for lunch in the villages of Boquillas and Santa Elena.
Nobody checks passports.
Even park rangers don't seem worried about illegal aliens. It's a long walk to civilization. The easiest way to see the river is to hike into Boquillas Canyon in the east, or into the yawning gorge of Santa Elena Canyon in the west. At Boquillas Canyon, the Rio Grande is so narrow you can pick up a rock and throw it into Mexico. The river sometimes runs so shallow at Santa Elena that a group of early-teen schoolgirls wades to the foreign side to cap off a recent sunny day.