The Pontotoc ridge separates the drainage system of the Mississippi from that of the Tombigbee; extending from the northeastern part of the state southward, this ridge divides in Choctaw county, the eastern branch separating the drainage basin in the Pascagoula from that of the Pearl, and the western branch separating the drainage basin of the Pearl from that of the Big Black and the Mississippi.
North and parallel with its course is a low watershed, which separates its drainage basin both from that of the Sao Francisco and from that of the Parnahyba, the northern confluent of the Parana.
The central and greater part of the state, however, is included in the drainage basin of the upper Sao Francisco.
Spain set up no claim to the region, and when Robert Cavalier, Sieur de la Salle, came down the river in 1682 from the French possessions to the north, he took possession in the name of France, which hereby gained her first title to the vast drainage basin of the Mississippi.
In the extreme north-east corner of the republic where the Brazilian Guiana plateau slopes toward the Atlantic there is a small area lying outside the drainage basin of the Amazon.