No river forms a boarder to the southern side of the US state of Louisiana. Louisiana's southern border is the coast of the Gulf of Mexico.
Texas and Arkansas each border Louisiana. All three can be described as as "southern States".
Texas and Louisiana
Texas and louisiana
California, Arizona, New Mexico, and Texas border Mexico. Texas, Louisiana, Mississippi, Alabama, and Florida border the Gulf of Mexico.
Texas borders Louisiana on the west.
Texas borders the south of Louisiana.
The southern border of Texas is the Rio Grande River. The river is owned by both the United States and Mexico.
Texas and Louisiana
It is the southern border for Texas; conversely, it is the northern border for Mexico.
The Coushatta moved frequently along the border between Louisiana and Texas to avoid conflict.
Oklahoma, Arkansas, and Louisiana
Bordering Texas and Louisiana is the Sabine River, the Red River borders Texas and Oklahoma, and between Texas and New Mexico/Mexico is the Rio.