The two longest rivers in Texas are the Rio Grande and the Red River. The Rio Grande is the longest, stretching approximately 1,900 miles in total, with about 1,250 miles of that serving as the international border between Texas and Mexico. It originates in the mountains of Colorado and flows all the way to the Gulf of Mexico. The Red River is the second longest, running for about 1,360 miles. It forms the northern border between Texas and Oklahoma and continues through Arkansas into Louisiana. In 2026, these rivers are vital not just for their length, but as critical water sources for agriculture and metropolitan centers. Close behind these two is the Brazos River, which is the longest river with its entire source and mouth located within the state of Texas, making it a "truly Texan" river of roughly 1,280 miles, often causing confusion in "longest river" rankings depending on whether you count bordering rivers.