Editing Geography of Chad (section)

==Water systems==
{{Further|List of rivers of Chad|List of lakes of Chad}}
[[File:ShrinkingLakeChad-1973-1997-EO.jpg|thumb|[[Lake Chad]] in a 2001 satellite image. On the top, the changes from 1973 to 1997 are shown, with the lake shrinking.]]
[[File:Ounianga Lakes from ISS.jpg|thumb|This astronaut photograph features one of the largest of a series of ten mostly fresh water lakes in the Ounianga Basin in the heart of the Sahara Desert of northeastern Chad.]]
Permanent streams do not exist in northern or central Chad.<ref name=":0" /> Following infrequent rains in the Ennedi Plateau and Ouaddaï Highlands, water may flow through depressions called enneris and wadis.<ref name=":0" /> Often the result of flash floods, such streams usually dry out within a few days as the remaining puddles seep into the sandy clay soil.<ref name=":0" /> The most important of these streams is the Batha, which in the rainy season carries water west from the Ouaddaï Highlands and the Guéra Massif to Lake Fitri.<ref name=":0" />
[[File:Chari River.jpg|thumb|left|Chari River]]
Chad's major rivers are the [[Chari River|Chari]] and the [[Logone River|Logone]] and their tributaries, which flow from the southeast into Lake Chad.<ref name=":0" /> Both river systems rise in the highlands of Central African Republic and Cameroon, regions that receive more than 1,250 millimeters of rainfall annually.<ref name=":0" /> 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 kilometers long.<ref name=":0" /> 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.<ref name=":0" /> The Chari's volume varies greatly, from 17 cubic meters per second during the dry season to 340 cubic meters per second during the wettest part of the year.<ref name=":0" />

The Logone River is formed by tributaries flowing from Cameroon and Central African Republic.<ref name=":0" /> Both shorter and smaller in volume than the Chari, it flows northeast for 960 kilometers; its volume ranges from five to eighty-five cubic meters per second.<ref name=":0" /> At N'Djamena the Logone empties into the Chari, and the combined rivers flow together for thirty kilometers through a large delta and into Lake Chad.<ref name=":0" /> At the end of the rainy season in the fall, the river overflows its banks and creates a huge floodplain in the delta.<ref name=":0" />

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.<ref name=":0" /> The Chari River contributes 95 percent of Lake Chad's water, an average annual volume of 40 billion cubic meters, 95% of which is lost to evaporation.<ref name=":0" /> The size of the lake is determined by rains in the southern highlands bordering the basin and by temperatures in the Sahel.<ref name=":0" /> Fluctuations in both cause the lake to change dramatically in size, from 9,800 square kilometers in the dry season to 25,500 at the end of the rainy season.<ref name=":0" />

Lake Chad also changes greatly in size from one year to another.<ref name=":0" /> In 1870 its maximum area was 28,000 square kilometers.<ref name=":0" /> The measurement dropped to 12,700 in 1908.<ref name=":0" /> In the 1940s and 1950s, the lake remained small, but it grew again to 26,000 square kilometers in 1963.<ref name=":0" /> The droughts of the late 1960s, early 1970s, and mid-1980s caused Lake Chad to shrink once again, however.<ref name=":0" /> The only other lakes of importance in Chad are Lake Fitri, in Batha Prefecture, and Lake Iro, in the marshy southeast.<ref name=":0" />