Desert Environments of Republic of Chad - Combating Desertification in Asia, Africa and the Middle East - page 164

to the Djourab, acted as an overflow canal; since Independence in 1960, climatic
conditions have made overflows impossible.
North and northeast of Lake Chad, the basin extends for more than 800 km,
passing through regions characterized by great rolling dunes separated by very deep
depressions. Although vegetation holds the dunes in place in the Kanem region,
farther north they are bare and have a fluid, rippling character. From its low point in
the Djourab, the basin then rises to the plateaus and peaks of the Tibesti Mountains
in the north. The summit of this formation - as well as the highest point in the Sahara
Desert - is Emi Koussi, a dormant volcano that reaches 3,414 m a.s.l. The basin's
northeastern limit is the Ennedi Plateau, whose limestone bed rises in steps etched
by erosion.
East of the lake, the basin rises gradually to the Ouaddaı Highlands, which mark
Chad's eastern border and also divide the Chad and Nile watersheds. Southeast of
Lake Chad, the regular contours of the terrain are broken by the Guera Massif, which
divides the basin into its northern and southern parts.
South of the lake lie the floodplains of the Chari and Logone rivers, much
of which are inundated during the rainy season. Farther south, the basin floor
slopes upward, forming a series of low sand and clay plateaus, called koros ,which
eventually climb to 615 m a.s.l. south of the Chadian border, the koros divide the
Lake Chad Basin from the Ubangi-Zaire river system.

2.2 Rivers

Chad's major rivers are the Chari and the Logone and their tributaries, which
flow from the southeast into Lake Chad. Both river systems rise in the highlands
of Central African Republic and Cameroon, regions that receive more than 1,250
millimeters (mm) of rainfall annually. Fed by rivers of Central African Republic, as
well as by the Bahr Salamat, Bahr Aouk, and Bahr Sara rivers of southeastern Chad,
the Chari River is about 1,200 km long. From its origins near the city of Sarh, the
middle course of the Chari makes its way through swampy terrain; the lower Chari
is joined by the Logone River near N'Djamena. The Chari's volume varies greatly,
from 17 cubic meters per second (cusecs) during the dry season to 340 cusecs during
the wettest part of the year.
The Logone River is formed by tributaries flowing from Cameroon and Central
African Republic. Both shorter and smaller in volume than the Chari, it flows
northeast for 960 km; its volume ranges from five to eighty-five cusecs. At
N'Djamena the Logone empties into the Chari, and the combined rivers flow
together for 30 km through a large delta and into Lake Chad. At the end of the
rainy season in the Fall, the river overflows its banks and creates a huge floodplain
in the delta.
The seventh largest lake in the world (and the fourth largest in Africa), Lake
Chad is located in the sahelian zone, a region just south of the Sahara Desert. The
Chari River contributes 95 % of Lake Chad's water, an average annual volume