River New Mexico is a term that refers to two rivers in the state of New Mexico. The GILA River is a tributary of the Colorado River, and is approximately 649 miles long. It is the sixth longest river in the United States and is known for its scenic beauty. The Pecos River is a tributary of the Rio Grande, and is approximately 926 miles long. It is the longest river located entirely within the state of New Mexico and is known for its rugged terrain.