The Rio Grande- Spanish for "big river-" has its source in the mountains of Colorado, runs through New Mexico, Texas, and the country of Mexico, and empties into the Gulf of Mexico. Along the way, this river marks part of the border between the United States and Mexico.
In the past, Mexico territory included much of what is today the southwest United States. For much of history, the Rio Grande did not represent a national border, rather a geographic border between the northerly and central parts of Mexico. In 1848, the United States acquired much of the American Southwest through the Treat of Guadelupe, which meant reorganizing national borders. The Rio Grande was a natural choice for demarcating the border because of its geographic significance.