CHARACTERISTICS AND DISTRIBUTION OF LAKES 551 
little Shotts which present the same phenomena as the greater 
depressions in the Lower Sahara, The Arabs compare the moving sand 
to a net ; it occupies a fairly extensive zone in both the Lower and 
the Upper Sahara, but does not cover one-third of the whole Algerian 
Sahara. 
The Sebka of Gurara (lat. 29° N., long. 2° W.) is a saline depression, 
measuring about 60 miles in a N.N.E. to S.S.W. direction. It seems 
to be marked by more or less humidity throughout, the moisture 
being derived from the underground supply, fed by the drainage of the 
southern slopes of the Atlas, which here comes to the surface. In the 
course of the whole depression there are three main basins, known as 
the Shotts of Dahram, Shergi, and Gebli, though even in these there 
does not seem to be permanent water above the surface. 
Lake Chad (or Tsad) occupies the centre of a vast area of inland Sudan, 
drainage in the Sudan. It lies about 850 feet above sea-level, and 
varies in area from 7700 to about 20,000 square miles, according to 
the season. During the rainy season, when the lake is expanding, the 
water is fresh and limpid ; it becomes slightly salt only at the period 
of low water, i.e. in May and June. Numerous lagoons scattered 
along the shores of the lake are really salt marshes, and, according 
to Captain Dubois,^ plsiy a role in the economy of the lake analogous 
to that of Karaboghaz in the Caspian Sea, When the lake expands 
these lagoons receive an enormous quantity of water, which, once 
communication is cut off by the receding of the lake in the dry season, 
concentrates and finally disappears by evaporation, leaving behind 
a deposit of salts. In this way every year Lake Chad partially clears 
itself of dissolved salts automatically, and the concentration of the 
waters by evaporation in the main lake during the dry season is 
arrested. The chief inflow to the lake is the River Shari, entering 
from the south-east, but it has no outflow, except when it overflows 
into the lagoons in the manner mentioned above. 
The lake, according to Destenave,^ is constantly retreating towards 
the west, and so a continually increasing number of islands is forming 
in the east, which are becoming more and more populated by tribes 
formerly settled in Kanem, the arid district to the east of the lake. 
A wide valley, called Bahr el-Ghazal, stretches towards the north- 
east from the south-east extremity of the lake. This valley, which 
is now waterless, served formerly as an outflow from the lake at times 
of high floods, the water leaving the lake gradually losing itself by 
evaporation in the more arid regions of the Sahara. The same name, 
1 See Destenave, " Exj^loration des iles clu Tchad," Let, Geographie^ t. vii. 
p. 425, 1903. 
2 Log. cit. 
