Nyasa, Lake (nĪăsˈə) [key], or Lake Malawi məläˈwē, Port. Niassa, c.11,600 sq mi (30,040 sq km), c.360 mi (580 km) long and from 15 to 50 mi (24–80 km) wide, E central Africa, in the Great Rift Valley. Lake Nyasa, the third largest lake in Africa, is bordered by Tanzania in the north and northeast, by Mozambique in the east, and by Malawi in the south and west. The lake is bounded by steep mountains, except in the south. Its main tributary is the Ruhuhu River in the northeast; the Shire (in the south), a tributary of the Zambezi, is the lake's sole outlet. There is regular steamship service on the lake. Overfishing has depleted the lake's fish stocks. Lake Nyasa was visited and named by the Scottish missionary David Livingstone in 1859. The northern portion of the lake is disputed between Malawi and Tanzania, with Malawi claiming all of it and Tanzania claiming the eastern half.