484 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1908. 



If we consider the longitudinal section of the Nile, which has now 

 been leveled from the Mediterranean to the Victoria Lake (with the 

 exception of two short reaches, together not more than 140 miles in 

 length out of a total length of about 3,500), we find that both the 

 Victoria Nile and the Semliki River descend very rapidly from the 

 equatorial plateau in several series of rapids, interrupted by level 

 reaches, until Gondokoro or Mongalla on the Bahr el Jebel. Then 

 there follows a length of 1,060 miles in which the river falls only 

 180 feet, or at the average rate of only 2 inches per mile. Beyond 

 this j)oint the slope again increases, and the river descends some 

 800 feet in 1,200 miles in passing the several cataracts between Khar- 

 toum and Aswan, after wdiich it flows through Egypt, with a fall of 

 from 5 to 6 inches per mile. Thus there is first an unnavigable por- 

 tion, then a long and easy waterway which is separated by the 

 cataract portion, Avhich is partially navigable, from the valley of 

 Egypt where the Nile has furnished the best means of communi- 

 cation. The comparatively recent movement of blocks of the earth's 

 crust on the equatorial plateau is shown by the very moderate 

 amount of weathering which has as yet taken place, and by the very 

 incomplete development of the drainage systems there; lakes, 

 marshes, and river reaches of low slope, which are choked with reeds 

 and water plants, alternate with rapids and rocky stream beds, down 

 which the water rushes to deposit its load of detritus in another 

 lake or plain tract lower down. In this way the water which flows 

 over the Ripon Falls pours down 60 miles of rapids, and then joins 

 the still waters of Lake Choga ; at Foweira 50 miles of rapids begin, 

 wdiich end at the Murchison Falls, 120 feet high; and immediately 

 beyond these the material eroded from the rocky bed and brought 

 in by tributary streams is forming extensive mud flats where the 

 Victoria Nile enters Lake Albert. The Semliki River, after flowing 

 northward for some 50 miles from Lake Albert Edward, plunges 

 down a series of rapids until it reaches the level of the Albert Lake. 

 Thus far, then, the Nile may be said to be in its mountain tract, as 

 it scours its way down the gorges which it is carving out in the 

 masses of granite and gneiss which form the plateau. One hundred 

 and thirty miles beyond the Albert Lake the Nile again plunges down 

 90 miles of rapids, the last step of the lake plateau, after which it 

 flows gently through the plains of the Sudan. 



The Sobat, the Blue Nile, the Atbara, and the Mareb, or Khor el 

 Gash, all rise on the Abyssinian Plateau at altitudes of from 6,000 

 to 8,000 feet above sea level, and pour down their deejj-cut gorges 

 until they reach the plains of the Sudan, where they have excavated 

 meandering channels in the alluvial plain. 



The quantity of water supplied by these two great gathering 

 grounds would be very large but for certain geographical conditions 



