The Rufiji River lies entirely within Tanzania. It is also the largest and longest river in the country. 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, in Pwani Region. Its principal tributary is the Great Ruaha River. It is navigable for approximately 100 kilometres (62 mi).