GEOGRAPHICAL ASPECTS OF THE NILE LYONS. 487 



idly ponds back that of the White Nile, and so long as the former is 

 discharging more than 180,000 cubic feet per second but little of the 

 White Nile water passes forward; it floods its own valle}^, forming 

 a reserve supply, which drains off in November and December, when 

 the level of the Blue Nile has fallen. Thus the equatorial plateau 

 has no effect whatever on the flood in the Nile Valley north of Khar- 

 toum, but furnishes the bulk of the low-stage supply. 



From Khartoum northward the discharge diagram takes a wholly 

 different form, for the Blue Nile is fed by the rains of Abj^ssinia, 

 which are strictly limited to a short season, and may be considered as 

 almost restricted to the months of June, July, August, and September, 

 in which 15 per cent, 30 per cent, 30 per cent, and 15 per cent of the 

 year's rainfall occur, respectively. Consequently the river rises 

 rapidly to its maximum level, which it reaches at the beginning of 

 September, and then falls almost as rapidly. The Atbara is supplied 

 by the same rains, and has a similar regimen, but falls somewhat 

 earlier, so that the maximum level at Wadi Haifa and Aswan is also 

 reached in an average year at the beginning of September. 



The Wadi Haifa diagram shows the resultant effect of the two 

 sources of supply — the Blue and White Niles. At the beginning of 

 the year the discharge of the Blue Nile has diminished to a very small 

 amount ; the Sobat is furnishing a considerable su^iply to supple- 

 ment the constant volume delivered by the Bahr el Jebel, and to in- 

 crease the water which had been stored in the White Nile Valley, and 

 which is now rapidly running off. From this time until May the 

 volume of the Blue Nile and that of the Sobat decreases rapidly, the 

 water stored in the White Nile has drained off by the end of January, 

 and all that is available for Nubia and Egypt is the water from the 

 equatorial plateau supplemented by such small supply as the Sobat 

 and the Blue Nile may still bring down, as well as by a certain 

 amount which drains back into the river from the flood plains and 

 the sandstone which forms the valley sides. This then is the time of 

 Egyi^t's greatest need, and increasing cultivation has necessitated the 

 construction of reservoirs and regulating dams by means of which 

 the surplus Avater of November and December can be stored up and 

 supplied during the period of deficiency which lasts through May, 

 June, and July. It is easy to see now what will cause deficiency in 

 the low-stage supply, since that which arrives from the equatorial 

 plateau is constant throughout the year; weak rains in Abyssinia, 

 which end earlier in the autumn than usual, will cause the Sobat and 

 the Blue Nile to fall to their minimum early in the spring months; 

 the low flood will have held up less water in the A^Hiite Nile Valley, 

 a less thickness of alluvium in the valley will have been saturated, so 

 that the variable sources of supply will be much reduced. If such 

 88292— SM 1908 32 



