Lake, west-central Africa.
Located at the juncture of the boundaries of Nigeria, Niger, Chad, and Cameroon, the lake covered 9,900 sq mi (25,600 sq km) in the mid-20th century. Its water level has since dropped—owing to severe drought, the desertification of the surrounding Sahel region, and irrigation projects—and, by the beginning of the 21st century, the lake had shrunk to 580 sq mi (1,500 sq km). The lake is fed by the Chari River.
![The Niger and Sénégal river basins and the Lake Chad basin and their drainage …
[Credits : Encyclopædia Britannica, Inc.]](https://web.archive.org/web/20090719095824im_/http://media-2.web.britannica.com/eb-media/64/5364-003-6BDD1A28.gif "The Niger and Sénégal river basins and the Lake Chad basin and their drainage …
[Credits : Encyclopædia Britannica, Inc.]")
freshwater lake located in the Sahelian zone of west-central Africa at the conjunction of Chad, Cameroon, Nigeria, and Niger. It is situated in an interior basin formerly occupied by a much larger ancient sea that is sometimes called Mega-Chad. Historically, Lake Chad has ranked among the largest lakes in Africa, though its surface area varies greatly by season, as well as from year to year. When the surface of the lake is approximately 920 feet (280 metres) above sea level, the area is about 6,875 square miles (17,800 square km); in the early 21st century, however, the area was typically about 580 square miles (1,500 square km). The hydrologic contributions and biological diversity of Lake Chad are important regional assets. The region is noteworthy for important archaeological discoveries, its role in trans-Saharan trade, and its association with historic African kingdoms.
The Chad basin is a downwarped section of the Precambrian African Shield. Most of the older crystalline rocks are covered by more recent deposits. The most significant physiographic influence on the basin was the ancient sea. At its maximum extent the sea was more than 600 feet (180 metres) deep, occupied an area of approximately 154,400 square miles (400,000 square km), and drained into the Atlantic Ocean through the Benue River system. It experienced four high stages between 41,000 and 2,300 years ago. The history of the sea is documented in the stratigraphic record, which includes thick layers of diatomaceous earth, lacustrine sands, terraced shorelines, and the remains of modern fish and mollusks in now-arid tracts of the basin. The floor of the basin dips to the northeast of the modern lake, reaching its lowest point in the Djourab Depression, some 300 miles (480 km) away. Lake Chad occasionally overflows into the generally intermittent El-Ghazal River leading into the depression, but it is usually confined by the dune fields of Kanem.
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