The Rio Grande forms much of the border between the U.S. state of Texas and Mexico.
The river rises in Colorado's San Juan Mountains, flows through New Mexico, and reaches Texas near El Paso. From there, it runs generally southeast between Texas and the Mexican states of Chihuahua, Coahuila, Nuevo León, and Tamaulipas before reaching the Gulf of Mexico.
The international boundary follows the river's deepest channel, or thalweg, under the 1848 Treaty of Guadalupe Hidalgo. Because the river meanders and changes course, later agreements have addressed cutoffs, islands, and shifting boundary locations. Major paired cities along it include El Paso–Ciudad Juárez, Laredo–Nuevo Laredo, and Brownsville–Matamoros.
A common mix-up is the Colorado River: it forms part of the U.S.–Mexico border farther west, but it does not border Texas. In Mexico, the Rio Grande is commonly called the Río Bravo or Río Bravo del Norte.