Lake Chad is mainly in the far west of Chad, bordering on northeastern Nigeria. The Chari River, fed by its tributary the Logone River, provides over 90% of Lake Chad's water, with a small amount coming from the Yobe River in Nigeria/Niger. Despite high levels of evaporation, the lake is fresh water. Over half of the lake's area is taken up by its many small islands (including Bogomerom archipelago), reedbeds and mud banks, and a belt of swampland across the middle divides the northern and southern halves. The shorelines are largely composed of marshes.
Because Lake Chad is very shallow—only 10.5 metres (34 ft) at its deepest—its area is particularly sensitive to small changes in average depth, and consequently it also shows seasonal fluctuations in size of about 1 m every year. Lake Chad has no apparent outlet, but its waters percolate into the Soro and Bodélé depressions. The climate is dry most of the year, with occasional rains from June to December.