The main geographical feature of Sudan is the Nile River, which with its tributaries (including the Atbara, Blue Nile, and White Nile rivers) traverses the country from south to north. The Nile system provides irrigation for strips of agricultural settlement for much of its course in Sudan and also for the Al Gezira plain, situated between the White Nile and the Blue Nile, just south of their confluence at Khartoum. In the extreme north, the Nile broadens into Lake Nasser, formed by the Aswan High Dam in Egypt.
Much of the rest of the country is made up of an undulating plateau (1,000–2,000 ft/305–610 m high), which rises to higher levels in the mountains located in the northeast near the Red Sea, as well as in the central and western portions of the country. Rainfall diminishes from south to north in Sudan; thus, the south is characterized by savanna and grassland which becomes desert and semidesert in the center and north.