Ubangi, also called the Oubangui, river of central Africa, the chief tributary of the Congo River, formed by the confluence of the Uélé and Mbomou rivers at Yakoma in northern Democratic Republic of the Congo (DRC, formerly Zaire). The river flows generally northwest, forming the border between the DRC and the Central African Republic. Near Fort de Possel, Central African Republic, the Ubangi turns southwest, and then flows between the DRC and the Republic of the Congo. The Ubangi, which is 1,130 km (700 mi) long, enters the Congo River near Lake Tumba in northwestern DRC. The Ubangi is navigable for 600 km (370 mi) from its junction with the Congo to Bangui, in the Central African Republic.