The Rufiji River lies entirely within Tanzania. The river is formed by the confluence of the Kilombero and Luwegu rivers. It is approximately 600 kilometres (370 mi) long, with its source in southwestern Tanzania and its mouth on the Indian Ocean opposite Mafia Island across the Mafia Channel. Its principal tributary is the Great Ruaha River. It is navigable for approximately 100 kilometres (62 mi). The Rufiji is approximately 200 kilometres (120 mi) south of Dar es Salaam. The river's delta contains the largest mangrove forest in eastern Africa.