The Rio Grande forms the natural boundary between Texas and Mexico, flowing for over 1,200 miles.

One significant river that naturally separates Mexico and the United States (more especially, Texas) is the Rio Grande. It starts in Colorado's Rocky Mountains and flows for more than 1,200 miles, passing through New Mexico and creating the Texas-Mexico border. The environment, agriculture, and economy of the area are all significantly impacted by the river. Additionally, historically, it has played a significant role in forming both nations' political and cultural environments. Discussions on immigration, water rights, and the shared history of the United States and Mexico all revolve around the Rio Grande.