The Niger River is the main river of West Africa, extending about 4,180 km. Its drainage basin is 2,117,700 km2 (817,600 sq mi) in area. Its source is in the Guinea Highlands in southeastern Guinea near the Sierra Leone border. It runs in a crescent through Mali, Niger, on the border with Benin and then through Nigeria, discharging through a massive delta, known as the Niger Delta (or the Oil Rivers), into the Gulf of Guinea in the Atlantic Ocean. The Niger is the third-longest river in Africa, exceeded only by the Nile and the Congo River (also known as the Zaïre River). Its main tributary is the Benue River.