The Texas Border River is composed of two rivers, the Sabine and the Rio Grande. The Sabine River is a 6-letter river that forms part of the border between Texas and Louisiana. It is a tributary of the Red River and is about 910 miles long. The Rio Grande is a 9-letter river that forms the border between Texas and Mexico. It is the fourth longest river in North America and is about 1, 900 miles long.