There are four geographical regions. The western lowlands (rising from sea level to 600 m/2,000 ft) extend along the Gulf of Guinea coast and average about 100 km (60 mi) in width. The northwestern highlands consist of forested volcanic mountains reaching over 2,440 m (8,000 ft) in height. Mt. Cameroon (4,095 m/13,435 ft), which stands isolated on the coast to the south, is the nation's only active volcano and the highest peak in West Africa. The central plateau region extends eastward from the western lowlands and northwest highlands to the border with the Central African Republic and northward to the Bénoué (Benue) River. It includes the Adamawa Plateau, at elevations of 900 to 1,500 m (2,950 to 4,920 ft). This is a transitional area where forest gives way to savanna. The northern region is essentially a vast savanna plain that slopes down to the Chad Basin. Of the two main rivers, the Bénoué is navigable several months during the year, and the Sanaga is not navigable. Part of Lake Chad is in Cameroonian territory.