The flood water brought down by the Shari in December and January causes the lake to rise to a maximum of 24 ft., the water spreading over low-lying ground, left dry again in May or June.
Lenfant, also a French officer, succeeded in reaching the lake (which he circumnavigated) via the Benue, proving the existence of water communication between the Shari and the Niger.
In the north the country drains into Lake Chad through the Logone and Shari (q.v.).
North of the Congo basin and separated from it by a broad undulation of the surface is the basin of Lake Chad - a flat-shored, shallow lake filled principally by the Shari coming from the south-east.
There was also at one time communication between the Shari and the Bahr-el-Ghazal, so that the water of the firstnamed stream reached Chad by way of the Bahr-el-Ghazal.
Thus Pizigano's map of 1367 extends as far east as the Gulf of Persia, whilst the Medicean map of 1356 (at Florence) is remarkable on account of a fairly correct delineation of the Caspian, the Shari river in Africa, and the correct direction given to the west coast of India, which had already been pointed out in a letter of the friar Giovanni da Montecorvino of 1252.
There is still, however, a distinct current from the Shari delta to the east end of the lake - known to the natives, like the depression beyond, as the Bahr-el-Ghazal - indicative of the former overflow outlet.
Numerous tributaries of the Shari flow through the country, but much of the water is absorbed by swamps and sand-obstructed channels, and seasons of drought are recurrent.
Through the Tuburi marshes there is a water connexion between the Benue (Niger) and Shari (Lake Chad) systems.
Fed by the Shari and other rivers, the lake has no outlet and its area varies according to the season.