Drainage and soils
The Nile River system is the dominant physical feature, and all streams and rivers of Sudan drain either into or toward the Nile. It enters the country as the White Nile (Baḥr Al-Abyaḍ) in the southeast, about 60 miles (100 km) south of Kūstī, and maintains an extremely low gradient until it is joined by the Blue Nile (Baḥr Al-Azraq) at Khartoum. The Blue Nile, which rises in the Ethiopian Plateau, contributes much of the floodwaters of the White Nile. After the confluence of the White and Blue Niles at Khartoum, the river flows in a great northward-curving course and is known simply as the Nile (Nahr Al-Nīl). Throughout much of the country, however, drainage does not reach the Nile; the rivers of the southwest infrequently reach the Baḥr Al-Ghazāl system, and to the north most hill groups initiate seasonal watercourses that are lost in the surrounding plains.