GEOGRAPHICAL ASPECTS OF THE NILE LYONS, 491 



ing them is due to the fact that they flow ahnost at the same level 

 as their flood plains, and, being free from suspended matter, are not 

 building them up. Conditions are therefore favorable to the growth 

 of marsh vegetation, which extends wherever the water reaches, and 

 which is often able to choke the smaller channels by its growth. 



In such country the inhabitants are naturally hunters and fishers 

 or cattle owners, only a very small amount of ground near the vil- 

 lages being cultivated. During the rains they move with their cattle 

 awaj^ from the rivers and marshes to higher ground between the 

 drainage lines, and later when the wells and ponds there dry up they 

 return to the rivers and the larger lagoons. 



In the lower reaches of the Bahr el Ghazal and the Bahr el Jebel 

 the flood plains lie lower, and the marshes are inundated for a con- 

 siderable part of the year, from June to December; but this is due, 

 not so much to the local rains as to the flood in the Sobat River. The 

 country is here so flat that the rise of water level of 7 or 8 feet in 

 the Sobat at flood stage raises the water level upstream for many 

 miles in the AAliite Xile, Bahr el Jebel, and Bahr el Ghazal rivers 

 and in their lagoons ; this facilitates the detachment from the bottom 

 by storms of wind of the plants growing in the marshes. AVlien 

 once set free, these masses of vegetation drift into the main river 

 channel, where they may be arrested at a narrow part or at a sharp 

 bend. More masses are constantly arriving, and soon the block 

 extends across the channel, and in time may completely close it. 

 These sadd-blocks have occurred principally in the last hundred 

 miles of the Bahr el Jebel immediately above Lake No, a few only 

 having been formed near Ghaba Shambe, where the wide marshes 

 in which the Bahr el Zaraf takes its rise cause a rise of the river 

 level of the Bahr el Jebel in the rainy season, and so facilitate the 

 setting free of the grass, reeds, and water plants which mnj then 

 form the sadd-block. A rise of water level in the lagoons is there- 

 fore an important cause of sadd-blocks, while stormy weather and a 

 narrow meandering river furnish the rest of the necessary conditions. 



In the Bahr el Ghazal marsh region the rivers are for the most 

 part shallower, and the vegetation which blocks them is oftener 

 growing on the bed of the stream than drifted into it as loose material 

 derived from the lagoons. But for all this region from Meshra el 

 Rek to the mouth of the Sobat, there is no doubt that the flood in the 

 latter river is the important geographical factor. Although the 

 rains cease on the Abyssinian Plateau after September, the level of 

 the Sobat in its lower reaches slowly rises until the end of November, 

 and only begins to fall toward the end of December, This is due to 

 the water which floods the plains to the south of the Sobat, through 

 which the Pibor River flows. Miles of country are flooded to a 

 depth of about 2 feet and are slowly drained off into the Sobat as 



