The drainage basin of the Nile covers 3,254,555 square kilometres (1,256,591 sq mi), about 10% of the area of Africa.
The two great tributaries join at Khartoum. The White Nile starts in equatorial East Africa, and the Blue Nile begins in Ethiopia. Both branches are on the western flanks of the East African Rift, the southern part of the Great Rift Valley. Below this confluence the only major tributary is the Atbara River, roughly halfway to the sea, which originates in Ethiopia north of Lake Tana, and is around 800 kilometres (500 mi) long. The Atbara flows only while there is rain in Ethiopia and dries very rapidly. During the dry period of January to June, it typically dries up. It joins the Nile approximately 300 kilometres (200 mi) north of Khartoum.
The course of the Nile in Sudan is distinctive. It flows over six groups of cataracts, from the first at Aswan to the sixth at Sabaloka (just north of Khartoum) and then turns to flow southward before again returning to flow north. This is called the "Great Bend of the Nile".
North of Cairo, the Nile splits into two branches (or distributaries) that feed the Mediterranean: the Rosetta Branch to the west and the Damietta to the east, forming the Nile Delta.
The Nile basin is complex, and because of this, the discharge at any given point along the mainstem depends on many factors including weather, diversions, evaporation and evapotranspiration, and groundwater flow.
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