The Rio Grande ( or ; known in Mexico as Río Bravo del Norte, or simply Río Bravo) is one of the principal rivers (along with the Colorado River) in the southwestern United States and northern Mexico. The Rio Grande begins in south-central Colorado in the United States and flows to the Gulf of Mexico. After passing through the length of New Mexico along the way, it forms part of the Mexico–United States border. According to the International Boundary and Water Commission, its total length was in the late 1980s, though course shifts occasionally result in length changes. Depending on how it is measured, the Rio Grande is either the fourth- or fifth-longest river system in North America.