8 (No. 98 GEOGRAPHY streams which descend from the Nile-Congo watershed and Dar Fertit hills, unite at Lake No ( 9° 30 ' N. ) to form the White Nile . They furnish practically the whole summer discharge of the Nile, but only one twentieth during the food period. At Lake Ño the Bahr el-Jebel discharges about 300 cubic metres per second at all seasons, the Bahr el-Ghazal only 12 to30, whilst the White Nile at Khartum , above the Blue Nile confluence, discharges 300 cubic metres per second at the low-water period, but 1,600 to 1,800 at the top of the flood, this increase being chiefly provided by the Sobat,the most southerly of its great flood tributaries.

( ii ) The most important of these flood tributaries is the Blue Nile ( Bahr el-Azrak ). This is the name given to the lower course of the Abbai , which drains practically the whole centre of the Abyssinian table land. Issuing from Lake Tsana (5,770 ft . above sea level), it flows for 535 miles through Abyssinia, receiving numerous tributaries. It enters the Sudan at Kiri (35° E. , 11° 12' N. ) , and joins the main river at Khartum . A mere trickle in the winter, it becomes during the summer and autumn a swift torrent, bringing down an immense volume of water and silt from the Abyssinian tableland. Its maximum discharge at this time is 10,000 to 12,000 cubic metres per second, and it contributes a quantity of water and fertilising matter greatly exceeding that of all the other branches andtributaries of the Nile system . The difference between the flood and low -water levels is 20-23 ft.

The Atbara, which also rises in the neighbourhood of Lake Tsana, enters the Sudan at Gallabat and the Nile near El Damer . The river is only a succession of pools in winter ; but it begins to rise in June, and by August discharges 3,000 cubic metres of water per second , heavily charged with silt. The Sobat River (Bahr el-Asfar, or Yellow River) is formed by the con Àuence of the Baro and its tributaries , draining the southern part of the Abyssinian plateau, with the Pibor and its tributaries, draining the plains and -