The country's main river is the Volta, which is formed in the center of the country by the confluence of the Black Volta and the White Volta. The Volta enters the Gulf of Guinea at Ada in southeastern Ghana. Beginning in 1961 the construction of the Akosombo Dam on the Volta formed Lake Volta. The lake covers an area of 8,482 sq km (3,275 sq mi), making it one of the world?s largest artificial lakes. The two major tributaries of the Volta are the Oti and Afram rivers. Together, the rivers drain the Volta Basin. Ghana?s other significant river systems are the Densu, Birim, Pra, and Ankobra. All empty into the Gulf of Guinea. Ghana?s rivers are navigable only by small crafts, with the exception of the Volta. Located in the Ashanti uplands, Lake Bosumtwi is Ghana?s only natural lake.