The Red River of the South, its mouth in the Rapides Parish in Texas, was once part of an inland sea, and the sea evaporated, leaving the salt.
The Red River, or sometimes the Red River of the South, is a major river in the Southern United States. Texas, Oklahoma, Arkansas and Louisiana About 250 million years ago, an inland sea blanketed parts of what is now those states. As time passed, that sea evaporated, leaving salt deposits  mostly sodium chloride. Tributaries include the Little Red River, Prairie Dog Town Fork Red River, Salt Fork Red River, North Fork Red River, Pease River, Washita River, Kiamichi River, Wichita River, Little Wichita River, Little River, Sulphur River, Loggy Bayou (through Lake Bistineau and Dorcheat Bayou) as well as the Ouachita River (also known as the Black River at that point) not far (at Acme, Louisiana) from the mouth. 
