The longest river in the Southwest United States is the Rio Grande.

The Rio Grande flows through Colorado, New Mexico, and Texas, eventually forming part of the border between the United States and Mexico. It stretches for about 1,900 miles (3,060 km), making it one of the longest rivers in North America. Known for its importance to agriculture, water supply, and its cultural and historical significance, the Rio Grande has shaped the development and history of the southwestern United States. It plays a vital role in the ecosystem of the region, supporting diverse wildlife and plant species.