The Rio Grande (known in Mexico as the Río Bravo del Norte, or simply Río Bravo) is a river that flows from southwestern Colorado in the United States to the Gulf of Mexico. Along the way it forms part of the Mexico – United States border. Its length varies as its course changes. According to the International Boundary and Water Commission, its total length was 1,896 miles (3,051 km) in the late 1980s. Depending on how it is measured, the Rio Grande is the fourth or fifth longest river system in North America. It serves as a natural border between the U.S. state of Texas and the Mexican states of Chihuahua, Coahuila, Nuevo León, and Tamaulipas. A very short stretch of the river serves as the boundary between the U.S. states of Texas and New Mexico. The tremendous water use of big cities and vast irrigated acreage along the river has taken a heavy toll on the river's flow; less than a fifth of its historical discharge reaches the sea today. Near the river's mouth, the heavily irrigated Rio Grande Valley is an important agricultural region.

The Rio Grande's watershed covers 182,200 square miles (472,000 km²) Many endorheic basins are situated within, or adjacent to, the Rio Grande's basin, and these are sometimes included in the river basin's total area, increasing its size to about 336,000 square miles (870,000 km²).