Río Rico, Tamaulipas

Río Rico is a village located along the Rio Grande in the Mexican state of Tamaulipas. It includes a portion of the Horcón Tract, a narrow 461-acre (1.87 km²) piece of land (including former riverbed) that was part of the United States until 1977.

The Horcón Tract was originally north of the meandering river—and thus part of the U.S. state of Texas—until an unauthorized diversion of the river to its north cut it off from the rest of the state in 1906. Río Rico was founded years later, in 1929, and developed as a de facto Mexican settlement.

In 1967, an American geography professor discovered what had happened and it was determined that parts of Río Rico were legally on U.S. territory, making those who were born there American citizens by birthright. The international border was moved under the terms of the Boundary Treaty of 1970, placing the tract and the village in Mexico, effective in 1977.