486 



ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. 



RIPON FALLS 



MURCHISON FALLS 



FAJAO 

 WADELAI 



GONDOKORO 



LAKE NO 

 BAHR EL JEBEL 



TAUFIKIA 

 DUEIM 



koro there is a well-marked maximum in September corresponding 

 to that already noted at the Murchison Falls, and is, like it, due to 

 the rainfall on the northern edge of the plateau. But little of this 

 water ever reaches the White Nile, for as the level of the Bahr el 

 Jebel rises, the plains of the valley are flooded, and the discharge 

 into Lake No, 500 miles to the north, hardly varies throughout the 



year. This 12,000 cubic 

 feet per second represents 

 the whole of the effective 

 supply furnished to the 

 TVHiite Nile by the rain- 

 fall of the equatorial pla- 

 teau, so that the run-off 

 at this point amounts ap- 

 proximately to one-tenth 

 per cent of the rainfall. 

 The Bahr el Ghazal and 

 the Bahr el Zaraf together 

 contribute from one-sixth 

 to one-third of that fur- 

 nished by the Bahr el 

 Jebel, and their combined 

 discharge represents the 

 whole of the supply which 

 is not derived from the 

 Abyssinian Plateau. 



In the Sobat Eiver we 

 have the effect of the 

 Abyssinian rains, but the 

 maximum is only reached 

 in December, owing to 

 the delaying effect of the 

 plains of the Pibor Eiver, 

 a branch of the upper 

 Sobat, which are flooded 

 by the summer rains, and 

 are only drained off grad- 

 ually. 

 The "Wliite Nile carries the volume supplied by the Bahr el Jebel, 

 Bahr- el Zaraf, and Bahr el Ghazal, together with whatever is sup- 

 plied by the Sobat — that is to say, from about 14,000 cubic feet per 

 second as a minimum to about four times that amount in flood time. 

 But the slope of this portion of the Nile north of its junction with 

 the Sobat is so low that the flood water of the Blue Nile rising rap- 



KHARTOUM 



ATBARA 



WADI HALFA 



Fig. 1. 



Volume discharged bv the Nile. 



