BIG BEND NATIONAL PARK, Tex. — There are places in the desert canyons of far West Texas where the border between the United States and Mexico amounts to an olive-green ribbon of water, so shallow that canoes scrape to a halt on the rocks. Here the Rio Grande — the border that has separated the two countries since 1848, with the signing of the Treaty of Guadalupe Hidalgo — narrows to a pinch. At times it is as wide as a school bus is long. At other times it is not even that wide. An owl can make the crossing with one or two flaps of its wings.

In these remote places in Big Bend National Park, the Rio Grande seems void of any power to divide. There are no boundary lines, no signs, no walls, no border agents on either side. To journey here to the vast, empty canyons of West Texas is to watch the border itself all but vanish as a physical and political space, an extraordinary feat in these times when the notion of the border often seems more a political construct than a geographic one.