As it is formed by the confluence of the Semefé (Bakoy) and Bafing rivers in Bafoulabé, it is considered a sweet water river. The Semefé and Bafing rivers have their mutual source in Guinea; the Bafing River flows through Mali and the Semefé is on the Malinese-Senegalese border.
Approaching its mouth, the Senegal passes through Biffeche and the island on which the city of Saint-Louis, Senegal is located, then turns south. It is separated from the Atlantic Ocean by a thin strip of sand called the Langue de Barbarie before it pours into the ocean itself. The river has two large dams along its course, the Manantali Dam in Mali, and the Maka-Diama dam on the Mauritania-Senegal border, near the outlet to the sea. The Manantali dam was build as a reservoir. The Maka-Diama dam prevents access of salt water into the inner country.
