492 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1908. 



the supply from the plateau diminishes, so that the level in the main 

 river is maintained. The effect of the Sobat flood is felt even as far 

 as Meshra el Rek on the Bahr el Ghazal, for here, too, the variation 

 of the water level follows that of the Sobat exactly in rising very 

 slowly from June, in attaining its maximum level at the beginning 

 of December, and in falling rapidly at the end of that month. The 

 effect of this regimen on the discharge at Taufikia is shown in fig- 

 ure 1. 



The Sobat, which rises on the southern portion of the Abyssinian 

 Plateau at an altitude of some 7,000 feet, descends very rapidly to the 

 low-lying Sudan plains, through which it flows in a well-defined 

 channel. The slope here being low, most of the suspended material 

 is deposited in the middle reaches, to form sand banks which render 

 navigation difficult at the low stage. It is characterized by the late 

 maximum level which has been already alluded to, and which notably 

 augments the volume of water available for Egypt in January and 

 February. In April and May its supply is small, but in favorable 

 years it is a valuable addition to the White Nile, which may be in- 

 creased by flood waves, due to occasional winter rain storms falling 

 on the plateau. 



Although the country of the Bahr el Jebel and the lower Sobat is 

 a vast plain having a very slight inclination, the flattest portion of 

 the Nile Valley is that between the mouth of the Sobat and Khartoum. 

 Here, the river falls, at low stage, only 2G feet in a distance of 515 

 miles, or 1 in 107,000, equivalent to little more than half an inch a 

 mile; at high stage, when the Blue Nile has risen 26 feet, the water 

 of the A^liite Nile is held up so that the water slope from Taufikia to 

 Khartoum is only 11 feet in the same distance, or 1 in 255,000, which 

 corresponds to about a quarter of an inch per mile only ; but this slope 

 occurs in the southern portion only, for from Renk to Khartoum, a 

 distance of about 300 miles, the river presents a level surface, and this 

 portion of the valley is a vast reservoir held up by the flood water of 

 the Blue Nile." 



The river here flows through a vast plain, with the low hills of 

 Kordofan on the west and those of the center of the Gezira on the 

 east, which divide it from the basin of the Blue Nile. Both these hill 

 masses consist of granites, gneisses, and other crystalline rocks, rep- 

 resenting the worn-down stump of a hill range which in former times 

 was of much greater importance; long-continued erosion has worn 

 them away, and streams have distributed the material to form the 

 alluvial plains of the White Nile. On these numberless herds of 

 cattle and sheep are raised by the tribes which inhabit them, and in 



[o gee " Tlie longitudinal section of the Nile," Geographical Journal, July, 

 1908.] 



