The Niger River is the main river of western Africa. It is over 4180 km (2500 miles) long. It has a crescent shape and it goes through Guinea, Mali, Niger, on the border with Benin and then through Nigeria. Finally, it reaches the sea at a large delta, called the Niger Delta of the Oil Rivers. This part of the Atlantic Ocean is called the Gulf of Guinea. The Niger is the third-longest river in Africa. Only by the Nile and the Congo are longer. Its main tributary is the Benue River.