GEOGRAPHICAL ASPECTS OF THE NILE LYONS. 483 



Many of the views which were formerly held have been modified 

 as new information was accumulated, but we may now say with 

 some confidence that the regimen of the basin is well known in its 

 broad outlines, and although much detailed work remains to be done, 

 although many lines of investigation are as j'et almost untouched, 

 it is possible now to describe the more striking geographical char- 

 acteristics of the river, and to indicate some of the deficiencies in 

 our knowledge which those who have opportunity for observation 

 may be inclined to make good. 



There are three principal and characteristic factors of the Nile 

 regimen which exercise a marked effect upon it as a w^hole, and 

 hifluence to a greater or less degree each portion of the river system ; 

 out each portion also has its own peculiarities, which may greatly 

 alter its reach of the river, and may produce effects which in turn 

 influence otlier areas. It will be convenient to deal first with those 

 factors which are of wider influence, and afterwards with those which 

 affect a more limited area. Of the former there are three: 



Firstly, the plateau of the equatorial lakes and the Abyssinian 

 Plateau — for both of these receive a heavy rainfall and supply the 

 whole of the water which is carried by the Nile and its tributaries, 

 that which falls on the Sudan plains being so little that it may be 

 considered to be practically negligible as a source of supply for the 

 river ; secondly, the rainless, or at least arid, conditions which prevail 

 over a very large proportion of the basin, since even over the greater 

 portion of the Sudan plains precipitation is very moderate in 

 amount; thirdly', the very low slope of the basin, which is such that 

 an altitude of 1,500 feet above sea level is not reached on the AAliite 

 Nile until a point 3,000 miles from the Mediterranean, near Gon- 

 dokoro. 



Though each portion of the river has its own peculiar character, 

 these three factors, a hea\'y localized rainfall, aridity in other parts 

 of the basin, and a valley of low slope, are those which exert the 

 greatest and widest influence. 



A\Tien we begin to study the basin of the Nile as it is presented to 

 us to-day in the latest maps and in the descriptions penned by 

 travelers in recent years, we find that in some respects this great 

 river departs from the normal type, and exhibits peculiarities which 

 it is of interest to investigate. Rivers have usually a steep slope near 

 their source, which gradually becomes less as lower levels are reached 

 and the eroding power of the water diminishes ; beyond this follows 

 a region of deposition, where the inclination of the valley is very 

 slight, and the river flows slowly through alluvial plains which it 

 has formed in the course of ages; but this plain tract occurs in the 

 Nile system at two very distant points, the valleys of the Bahr el 

 Jebel and the White Nile, and the valley of Egypt. 



