The surface of the department, which lies about 1,000 feet (300 m) above sea-level, is almost flat with a very slight inclination north to Lake Chad. It forms part of what seems to be the basin of an immense lake, of which Chad is the remnant. The soil is clay .Numerous tributaries of the river Chari River flow through the department, but much of the water is absorbed by swamps and sand-obstructed channels, and seasons of drought are recurrent.