Texas border map

The Rio Grande has only been an international line since 1848. Mexico held that the border of Texas was the Nueces River, more than 100 miles further north; the Treaty of Guadalupe Hidalgo settled the argument in favor of the Rio Grande, and the Republic of Texas claim was absorbed into the United States. The remaining northern claims were sold off in the Compromise of 1850, which is why the line with New Mexico is a survey line and not a river.

A river makes an awkward boundary because it moves. The channel below El Paso shifted south over decades, leaving several hundred acres of what had been Mexican soil on the Texas bank — the Chamizal dispute. It was settled by treaty in 1963 and the transfer was completed in 1967, after the river was moved into a concrete channel. Since then the International Boundary and Water Commission has fixed the line even where the water wanders.

Fourteen Texas counties touch Mexico. From the Gulf they are Cameron, Hidalgo, Starr, Zapata, Webb, Maverick, Kinney, Val Verde, Terrell, Brewster, Presidio, Jeff Davis, Hudspeth and El Paso. Four of them — Terrell, Brewster, Jeff Davis and Hudspeth — have no crossing at all, because the river there runs through desert canyons with no road on either bank. Brewster County alone accounts for a long stretch of river inside Big Bend National Park, where the Rio Grande cuts Santa Elena, Mariscal and Boquillas canyons.

The busiest crossings are at the two ends. Laredo handles the largest volume of truck freight of any U.S. land port, funneled onto I-35 from the World Trade Bridge; El Paso's four bridges carry the largest pedestrian traffic. In between, the Del Rio and Eagle Pass bridges connect US-90 and US-57 to the Mexican highway network toward Monclova and Saltillo.