river, the largest in Tanzania, East Africa, draining most of the southern part of the country and navigable for about 60 mi (100 km). Formed by the confluence of the Kilombero and the Luwegu rivers, it flows for about 175 mi northeast and east to enter the Indian Ocean, opposite Mafia Island. The river has major potential for irrigation and hydroelectric power development. Its principal tributary is the Great Ruaha.