490 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1908. 



plains have a greater extent, and reach from the latitude of Gondo- 

 koro to the Sobat River, broken only here and there by a knoll of 

 granite. In the rainy season these jolains are flooded for miles, and 

 the water is slowly drained oif by the meandering channels which, 

 half choked with rank vegetation, afford an inadequate escape for 

 the water, of VN'hich a large proportion is removed by evaporation. 

 The Bahr el Jebel, which receives a constant supply from Lake Albert, 

 maintains an open channel and a steady flow throughout the year; 

 many of the others, which are bank full in the summer months, soon 

 fall as the rain diminishes, and by the winter consist for the most 

 part of almost stagnant pools. Between these streams the country, 

 which is largely flooded in the rains, can only be crossed with diffi- 

 culty in the dry season on account of the lack of water. 



The marshes of the Bahr el Jebel and the Bahr el Ghazal, though 

 very extensive, are not so vast as they were formerly represented to 

 be, and those of the Bahr el Jebel in particular have been much 

 reduced by the results of recent surveys. This river flows in a very 

 shallow valley from 5 to 15 miles wide, in which at flood time the 

 water in the lower reaches is on a level with the surface of the 

 flood plain, and even in the upper reaches is but little below it. The 

 abrupt change from the steeply sloping bed above Gondokoro to 

 the level plain below it causes most of the suspended matter to be 

 deposited in the upper reaches, and little if any sedimentation is yet 

 taking place in the middle and lower reaches. Consequently the 

 sides of the flood plains are still occupied by large lagoons, which 

 are filled in the rainy season and slowly evaporate during the other 

 half of the year; former bends and branches of the river, which 

 changes of the main channel have left as isolated depressions, stand 

 full of water and furnish suitable places for the growth of papyrus 

 and other marsh vegetation. Five hundred miles of such a marsh- 

 grown valley can not only take the rainfall of 30 to 35 inches which 

 falls on it, but can receive also all the water that flows out of the 

 main stream by numerous branches and side channels ; much is taken 

 up by the dense growth of marsh plants, and the dry winds which 

 blow from November to April off the parched plains of the Sudan 

 rapidly carry off a vast quantity of moisture; thus it is that the 

 trebling of the volume discharged at Gondokoro in the rainy season 

 has no effect on the volume which leaves the river to join the W^iite 

 Nile at Lake No. The descriptions of the A^ast marshes, the rank 

 vegetation which blocks the river bed, the difficulty of recognizing 

 the true river channel, have given rise to an impression that the 

 Bahr el Jebel has no defined bed, but is a shallow stream losing 

 itself in the lagoons. But this is far from being the general case, 

 for all the rivers of these plains excavate for themselves well-defined, 

 steep-sided channels in which they flow. The difficulty of recogniz- 



