Bulgaria consists of a number of roughly parallel east-west zones. They are the Danubian tableland in the north, the Balkan Mountains (Stara Planina) in the center, and the Thracian Plain, drained by the Maritsa River, in the south. The Rhodope, Rila, and Pirin mountains lie in the southwestern part of the country. The average elevation is 480 m (1,575 ft), and the highest point, in the Rila Mountains, is the Musala, at 2,925 m (9,596 ft). The Danube (Dunav), Bulgaria's only navigable river, forms most of the northern boundary with Romania.