Burundi is a country mainly of mountains and plateaus, with a western range of mountains running north–south and continuing into Rwanda. The highest point is Mt. Heha at 2,670 m (8,760 ft). The only land below 914 m (3,000 ft) is a narrow strip of plain along the Ruzizi River (about 800 m/2,600 ft), which forms the western border north of Lake Tanganyika. From the mountains eastward, the land declines gradually, dropping to about 1,400 m (4,600 ft) toward the southeastern and southern border. The average elevation of the central plateau is about 1,525 to 2,000 m (5,000 to 6,500 ft). The major rivers form natural boundaries for most of the country. The Kanyaru and the Kagera separate Burundi from Rwanda along many sections of the common border. The Kagera and the Ruvubu are important as the southernmost sources of the Nile. Most of Burundi's southern border is formed by the Malagarasi River. The principal lakes are Tanganyika, Cohoha, and Rweru.