Tuesday, March 10, 2015

The Chad Basin

The Chad Basin is both a topographic and a geologic sedimentary basin that covers nearly 8% of the African Continent, stretching from southeastern Algeria to the northwestern Central African Republic and from central Niger to western Sudan; most of Chad, northern Cameroon and northeastern Nigeria also lie within the basin.  Lake Chad (elevation about 920 feet) lies at the center of the basin (though it is not the basin's lowest point).

Geologically, the Chad Basin is a broad depression of the Precambrian Shield upon which nearly 12,000 feet of Paleozoic, Mesozoic and Tertiary sediments have accumulated, deposited by inland seas or eroded from highlands that rim the basin.  Mega-Chad Lake, the precursor of Lake Chad, covered a large portion of the basin from about 40,000 to 3,000 years ago, draining to the Atlantic via the Benue and Niger Rivers; as the lake level fell, this outlet was lost and Lake Chad became a true basin lake, expanding and contracting with the regional climate.

Since the 1960s, Lake Chad has lost more than 90% of its volume, the result of dams on basin rivers, deforestation, overgrazing, increased water use for irrigation and global warming; desert and semiarid grasslands now cloak most of its floodplain, less than half of which is covered by open water (with an average depth of only 5 feet).  Almost all of the inflow now reaches Lake Chad via the Chari-Logone River system, rising in the Central African Republic.  While the Lake Chad Basin Commission was established to address this ecologic, economic and societal catastrophe, regional politics have, to date, derailed any significant progress.