TheChad Basin is the largestendorheic basin in Africa, centered approximately onLake Chad. It has no outlet to the sea and contains large areas of semi-arid desert andsavanna. Thedrainage basin is approximately coterminous with the sedimentary basin of the same name, but extends further to the northeast and east.

The basin spans four modern nations, including most ofChad and a large part ofNiger,Nigeria andCameroon.^[1] A combination of dams, increased irrigation,climate change, and reduced rainfall are causing water shortages. Lake Chad continues to shrink.

The geological basin, which is smaller than the drainage basin, is aPhanerozoic sedimentary basin formed during the plate divergence that opened the South Atlantic Ocean.The basin lies between theWest African Craton andCongo Craton, and formed about the same time as theBenue Trough.It covers an area of about 2,335,000 square kilometres (902,000 sq mi).^[2]It merges into theIullemmeden Basin to the west at the Damergou gap between theAïr andZinder massifs.^[3]The floor of the basin is made ofPrecambrian bedrock covered by more than 3,600 metres (11,800 ft) of sedimentary deposits.^[2]

The basin may have resulted from the intersection of an "Aïr-Chad Trough" running NW-SE and a "Tibesti-Cameroon Trough" running NE-SW.^[2]That is, the two deepest parts are an extension of the Benue Trough that runs northeast to the margin of the basin, and another extension running from below the present lake to below theTénéré rift structure to the east of the Aïr massif.The southern part of the basin is underlain by another elongated depression.^[3]This runs in an ENE direction and extends from the Yola arm of the Benue trough.^[4]

At times, parts of the basin were below the sea. In the northeastern part of the Benue Trough where it enters the Chad Basin there are marine sediments from theLate Cretaceous (100.5–66 Ma^[a]). These sediments seem to be considerably thicker towards the northeast. Boreholes underMaiduguri have found marine sediments 400 metres (1,300 ft) deep, lying over continental sediments 600 metres (2,000 ft) deep.^[4] The sea seems to have retreated from the western part of the basin in theTuronian (93.5–89.3 Ma). In theMaastrichtian (72.1–66 Ma) the west was non-marine, but the southeast probably was still marine. No marine sediments have been found from thePaleocene (66–56 Ma).^[4]

For most of theQuaternary, from 2.6 million years ago to the present, the basin seems to have been a huge, well-watered plain, with many rivers and water bodies, probably rich in plant and animal life. Towards the end of this period the climate became drier. Around 20,000-40,000 years ago,eolianite sand dunes began to form in the north of the basin.^[4]During theHolocene, from 11,000 years ago until recently, a giant "Lake Mega-Chad" covered an area of more than 350,000 square kilometres (140,000 sq mi) in the basin.^[5] It would have drained to theAtlantic Ocean via theBenue River. Stratigraphic records show that "Mega-Chad" varied in size as theclimate changed, with a maximum about 2,300 years ago. The remains of fish and molluscs from this period are found in what are now desert regions.^[6]

The Chad Basin covers almost 8% of the African continent, with an area of about 2,434,000 square kilometres (940,000 sq mi). It is surrounded by mountains. TheAïr Mountains and theTermit Massif in Niger form the western boundary. To the northwest, in Algeria, are theTassili n'Ajjer mountains, including the 2,158 metres (7,080 ft)Jebel Azao. TheTibesti Mountains to the north of the basin includeEmi Koussi, the highest mountain in the Sahara at 3,415 metres (11,204 ft). TheEnnedi Plateau lies to the northeast, rising to 1,450 metres (4,760 ft).^[7] TheOuaddaï highlands lies the east.^[6] They include theMarrah Mountains inDarfur at up to 3,088 metres (10,131 ft) in height. TheAdamawa Plateau,Jos Plateau,Biu Plateau, andMandara Mountains lie to the south.^[7]

To the west the basin is separated by a watershed from theNiger River, and to the south it is separated by a basement dome from theBenue River.^[8]Further east, watersheds separate it from theCongo Basin and the riverNile.

The lowest part of the basin is not Lake Chad, but theBodélé Depression, at a distance of 480 kilometres (300 mi) to the northeast of the lake. The Bodélé Depression is just 155 metres (509 ft) above sea level in its deepest portion, while the surface of Lake Chad is 275 metres (902 ft) above sea level.^[7]

The basin spans parts of eight countries. These are:^[9]

The northern half of the basin is desert, containing theTénéré desert,Erg of Bilma andDjurab Desert.South of that is theSahel zone, drysavanna and thorny shrub savanna.The main rivers include riparian forests,flooding savannas and wetland areas.In the far south there are dry forests.^[7]Rainfall varies widely from year to year. The amount of annual rainfall is very slight in the north of the basin, increasing to 1,200 millimetres (47 in) in the south.^[10]

As late as 2000, the basin has remained home to large populations of wildlife. In the Sahel these include antelopes such as theaddax anddama gazelle, and in the savanna there arekorrigum andred-fronted gazelle. Theblack crowned crane and other waterbirds are found in the wetlands. There are populations ofelephants,giraffes, andlions. Thewestern black rhinoceros was once common but is now extinct. Elephants almost became extinct by the end of the nineteenth century due to European and American demand for ivory, but stocks have since recovered.^[11]

The seasonalKorama River in the south of Niger does not reach Lake Chad. Nigeria includes two sub-basins that drain into Lake Chad. The Hadejia - Jama'are - Yobe sub-basin in the north contains theHadejia andJama'are rivers, which supply the 6,000 square kilometres (2,300 sq mi)Hadejia-Nguru wetlands. They converge to form theYobe, which defines the border between Niger and Nigeria for 300 kilometres (190 mi), flowing into Lake Chad. About .5 cubic kilometres (0.12 cu mi) of water reaches Lake Chad annually. Construction of upstream dams and growth in irrigation have reduced water flow, and the floodplains are drying. The Yedseram - Ngadda sub-basin further south is fed by theYedseram River andNgadda River, which join to form an 80 square kilometres (31 sq mi) swamp to the southwest of the lake. There is not any significant water flow from the swamp to the lake.^[9]

TheCentral African Republic (CAR) contains the sources of the riversChari andLogone, which flow north into the lake. The volume of water entering Chad annually from the CAR has decreased from about 33 cubic kilometres (7.9 cu mi) before the 1970s to 17 cubic kilometres (4.1 cu mi) during the 1980s. A further 3 cubic kilometres (0.72 cu mi) to 7 cubic kilometres (1.7 cu mi) of water annually flows fromCameroon into Chad via the Logone River. The Chari-Logone system accounts for about 95% of the water entering Lake Chad.^[9]

The basin in the Nigerian section contains an upper aquifer ofEarly Pleistocene alluvial deposits that are often covered by recent sand dunes, varying in thickness from 15 to 100 metres (49 to 328 ft). It consists of interbedded sands, clays and silts, with discontinuous clay lenses. The aquifer recharges from run-off and rainfall. The local people access the water with hand-dug wells and shallow boreholes, and use it for domestic use, growing vegetables and watering their livestock. Below this aquifer, separated from it by a sequence of grey to bluish-grey clays from theZanclean, is a second aquifer at a depth of 240 to 380 metres (790 to 1,250 ft). Due to intensive pumping, since the start of the 1980s the water levels in both aquifers has been lowered, and some wells no longer function.^[12] There is a third, much lower, aquifer in Bima Sandstones that lies at a depth of 2,700 to 4,600 metres (8,900 to 15,100 ft).^[13]

Both the current and historical areas of the basin and the mega-basin contain concentrations of fossils. Fossil-fuels deposits in the area are estimated to exceed a trillion barrels of reserves.

The Lake Chad Basin Commission was established in 1964 by Cameroon, Chad, Niger and Nigeria, the four countries that contain parts of Lake Chad.^[10][14]About 20% of the basin, lying in these countries, is termed the Conventional Basin.TheLake Chad Basin Commission manages use of water and other natural resources in this area.^[9]Although the lake fluctuates considerably in size from one year to another, the general trend has been for water levels to decrease.There has been a proposal to supply water from theCongo Basin via a canal 2,400 kilometres (1,500 mi) long,but major political, technical, and economic challenges would have to be overcome to make this practical.^[9]

Humans have lived in the inner Chad Basin from at least eight thousand years ago, and were engaging in agriculture and livestock management around the lake by 1000 BC. Permanent villages were established to the south of the lake by 500 BC at the start of theIron Age.^[15] The Chad Basin contained important trade routes to the east and to the north across the Sahara.^[9]By the 5th century AD camels were being used for trans-Saharan trade via theFezzan, or to the east viaDarfur, where slaves and ivory were exchanged for salt, horses, glass beads, and, later, firearms.^[16] After the Arabs conqueredNorth Africa during the 7th and 8th centuries, the Chad Basin became increasingly linked to the Muslim countries.^[15]

Trade and improved agricultural techniques enabled more sophisticated societies, resulting in the early kingdoms of theKanem Empire, theWadai Empire, and theSultanate of Bagirmi.^[16] Kanem developed during the 8th century in the region to the north and east of Lake Chad. The Sayfuwa dynasty that ruled this kingdom had adopted Islam by the 12th century.^[15] The Kanem empire went into decline, shrank, and during the 14th century was defeated byBilala invaders from theLake Fitri region.^[17] The Kanuri people commanded by the Sayfuwa migrated to the west and south of the lake, where they established theBornu Empire. By the late 16th century the Bornu empire had expanded and recaptured the parts of Kanem that had been conquered by the Bilala.^[18] Satellite states of Bornu included theSultanate of Damagaram in the west and Baguirmi to the southeast of Lake Chad.

TheTunjur people initiated theWadai Empire to the east of Bornu during the 16th century. During the 17th century, theMaba people revolted and established a Muslim dynasty. At first, Wadai paid tribute to Bornu and Durfur, but by the 18th century Wadai was fully independent and had become an aggressor against its neighbors.^[16] To the west of Bornu, by the 15th century theKingdom of Kano had become the most powerful of theHausa Kingdoms, with an unstable truce with theKingdom of Katsina to the north.^[19] Both of these states adopted Islam during the 15th and 16th centuries.^[20] Both were absorbed into theSokoto Caliphate during theFulani War of 1805, which threatened Bornu itself.^[21]

During theBerlin Conference in 1884-85 Africa was divided between the European colonial powers, defining boundaries that are largely intact with the present post-colonial states.^[22] On 5 August 1890 the British and French concluded an agreement to define the boundary betweenFrench West Africa and what would becomeNigeria. A boundary was agreed along a line fromSay on the river Niger toBarruwa on Lake Chad, but leaving the Sokoto Caliphate in the British sphere.^[23]Parfait-Louis Monteil was given charge of an expedition to discover where this line actually ran.^[24] On 9 April 1892 he reachedKukawa on the shore of the lake.^[25] During the next twenty years a large part of the Chad Basin was incorporated by treaty or by force intoFrench West Africa. On 2 June 1909 the Wadai capital ofAbéché was occupied by the French.^[26] The remainder of the basin was divided by the British in Nigeria who captured Kano in 1903,^[27] and the Germans in Kameroun. The countries of the basin regained their independence between 1956 and 1962, retaining the colonial administrative boundaries.

The area is badly affected by theBoko Haram insurgency, which began in 2009 and is centred onBorno State in northeastern Nigeria.

As of 2011, more than 30 million people lived in the Chad Basin. The population is growing rapidly.^[9]Ethnic groups includeKanuri,Maba,Buduma,Hausa,Kanembu,Kotoko, Bagger,Haddad,Kuri,Fulani andManga. The largest cities areKano andMaiduguri inNigeria,Maroua inCameroon,N'Djamena inChad andDiffa inNiger.^[7]

The main economic activities are farming, herding and fishing.^[9]At least 40% of the rural population of the basin is impoverished and experiences chronic food shortages.^[28]Crop production based on rain is possible only in the southern belt. Flood recession agriculture is practiced around Lake Chad and in the riverine wetlands.^[10]Nomadic herders migrate with their animals into the grasslands of the northern part of the basin for a few weeks during each brief rainy season, where they intensively graze the nutritious grasses. When the dry season starts they move back south, either to grazing lands around the lakes and floodplains, or to the savannas further to the south.^[29]

During the period 2000-01, fisheries in the Lake Chad basin provided food and income to more than 10 million people, with a harvest of about 70,000 tons.^[28]Fisheries have been managed traditionally by a system where each village has recognized rights over a defined part of the river, wetland or lake, and fishers from elsewhere must seek permission and pay a fee to use this area. The governments only enforced rules and regulations to a limited extent.^[30]Fishery management practices vary. For example, on theKatagum river inJigawa State, Nigeria, a village will have a water management council that collects a portion of each fisherman's catch and redistributes it among the villagers, or sells it and used the proceeds for communal projects.^[31]Local governments and traditional authorities are increasingly engaged inrent-seeking, collecting license fees with the help of the police or army.^[32]

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