The system of tributaries with an all-year-round constant run-off
This lies in the areas close to the equator in the East African Highlands
with their big lakes, such as Lake Victoria, Lake Albert, Lake Edward and Lake
Kyoga. Here the rainfall peaks are in spring and in autumn, but the run-off is
levelled by the big lakes, so that the discharge of the Nile tributaries is balanced
over the year. The annual discharge of Bahr el-Djebel, which runs through
Juba in southern Sudan, is 26.5 billion m3
. North of it the Nile enters the large
swamp region of the Sudd in which the river loses about half of its water
through evapotranspiration. Bahr el-Ghazal, a south-western tributary of Bahr
el-Djebel, loses about 95 per cent of its water in the Sudd swamps, so that it
discharges merely 0.6 billion m3
 annually into that river. The balance of the
annual discharge from this southern part of the river basin is 14.9 billion m3
,
measured south of the Sobat mouth (El Saiyad and Saudi 1966: 64).