Chad's geography: a lake held above the floor of its own basin

Why Lake Chad sits above its basin floor

Chad's frontiers enclose most of a single structure. Britannica describes the country, in its physical structure, as a large basin bounded on the north, east and south by mountains, and the Wikipedia article on the lake identifies that basin as a downwarped section of the Precambrian African Shield whose floor is buried under more than 3,600 metres of sediment. Water gathers in the south-west, in Lake Chad on the western frontier. The ground goes on falling beyond it.

The record places Chad's lowest point in the Bodélé Depression, at 155 metres above sea level. Lake Chad stands at about 280 metres: Britannica gives 281 metres in its article on the country and 280 in its article on the lake, and the Wikipedia article on the geography of Chad gives 282. The floor of the basin therefore sits more than 120 metres below the surface of the lake, about 480 kilometres to the north-east by the distance both Britannica and Wikipedia print.

The sources disagree on exactly where the bottom is. Britannica names the Djourab Depression and puts it at 175 metres. The CIA World Factbook figures reproduced on Wikipedia, from the 2025 edition, give the Bodélé at 160 metres. The geography article on Wikipedia spreads the low ground across both, with the Bodélé in the north-centre of the country and the Djourab in the north-east.

What holds the lake in place is sand. The great stationary dunes of Kanem, ergs fixed by their vegetation, rise between the lake and the depression and act as a dam. There is one channel through them: the Bahr el Ghazal, which leaves the north-eastern corner of the lake and runs towards the Djourab. Wikipedia's geography article dates the last overflow along it to the 1870s and states that the climate since independence in 1960 has made one impossible, while the Wikipedia article on the lake records the water spilling into the Bahr el Ghazal again in the middle of the twentieth century.

Where the lake's water goes instead

It leaves into the air and into the ground. Britannica estimates that evaporation and the transpiration of water plants take about 2,500 millimetres of the lake's depth a year, and that up to a further 250 millimetres a year passes into the groundwater of the Manga and Kanem lowlands and moves as underflow along the line of the Bahr el Ghazal. The Wikipedia article on the lake describes the same seepage towards the Bodélé as what removes most of the salt.

The chemistry follows the slope. Britannica calls the salt content of Lake Chad unusually low for a tropical dryland lake with no outlet, and records the highest values along the north-eastern shoreline, the side facing the depression. Wikipedia describes the south-western water as fresh and the north-eastern water as only slightly salty.

What rims the basin on three sides

Britannica lists the rim in order: the volcanic Tibesti Massif to the north, the sandstone peaks of the Ennedi Plateau to the north-east, the crystalline mountains of Ouaddaï to the east and the Oubangui Plateau to the south. The semicircle closes in the south-west with the Adamawa and Mandara mountains, which lie mostly beyond the frontier in Cameroon and Nigeria. Inside the bowl the Guéra Massif, rising to about 1,500 metres, divides the basin into a larger northern part and a smaller southern part that falls almost entirely within Chad.

Why Emi Koussi has four heights

North and north-east of Lake Chad the basin floor runs for more than 800 kilometres through rolling dunes separated by deep hollows. In Kanem plants hold the dunes in place; farther north, Wikipedia's geography article notes, they are bare and move. From the Djourab the ground then climbs to the plateaus and peaks of the Tibesti.

The summit is Emi Koussi, a dormant volcano. The record gives it 3,445 metres. Britannica and the Factbook give 3,415 metres, the Wikipedia article on Chad and the land section of its geography article give 3,414, and the opening paragraph of that same geography article gives 3,100. Four figures circulate for one mountain, and the catalogue prints the record's.

What closes the basin to the east

The Ennedi Plateau closes the basin to the north-east. Britannica and UNESCO describe it as sandstone, and the Wikipedia geography article as a limestone bed rising in steps cut by erosion. UNESCO, which inscribed the Ennedi Massif on the World Heritage List in 2016, describes a plateau carved by water and wind into canyons and valleys with cliffs, natural arches and pitons. In the largest canyons water is present all year, and that water sustains plants, animals and people. Thousands of images are painted and carved into the rock of caves, canyons and shelters. The Wikipedia article on Chad records a small surviving group of West African crocodiles in the Ennedi, one of the last colonies known in the Sahara.

The Ouaddaï Highlands mark the eastern border and divide the Chad watershed from the Nile's. Britannica describes their slopes as carrying red, iron-bearing tropical soils on the exposed folds and mounds, and Wikipedia places them, with the Ennedi, in the East Saharan montane xeric woodlands ecoregion.

How the rain divides Chad from north to south

The country runs about 1,800 kilometres from north to south and averages about 800 kilometres across, between latitudes 7° and 24° north. Over that distance the climate moves from wet and dry tropical to hot arid. The mechanism is a front: each year humid Atlantic air pushes north under the dry continental air along the intertropical convergence zone, which at the height of the rains can reach Kanem, and by the middle of the dry season it has withdrawn south of Chad and taken the rain with it.

The rainy season lasts from May to October in the south and from June to September in the Sahel, according to the Wikipedia article on Chad. Britannica describes the dry season from December to February as relatively cool everywhere in the country.

The Saharan zone covers roughly the northern half of the country. Faya-Largeau averages less than 12 millimetres of rain a year and nearly 3,800 hours of sunshine, in the figures Wikipedia's geography article gives. Across much of the north the average daily maximum is about 32 °C in January and about 45 °C in May. On 22 June 2010 Faya recorded 47.6 °C, breaking a record set there in 1961. Even here the rain has an exception: in northern Biltine a district called the Mortcha receives 350 millimetres or more, mostly in July and August, and herders bring cattle and camels to its brief pasture before leaving with the end of the rains.

The Sahel forms a belt about 500 kilometres wide from Lac and Chari-Baguirmi east to the Sudanese frontier. N'Djamena records a maximum annual average of 580 millimetres, and Britannica gives daytime temperatures averaging more than 38 °C there between March and June. In the southern part of the belt rainfall permits millet and sorghum on unirrigated land.

The Sudanian zone in the south receives between 750 and 1,250 millimetres between April and October, and the Guinea zone in the south-western tip between 1,000 and 1,200 millimetres. In Moundou daytime readings run from 27 °C in January to about 40 °C in March, April and May.

Where that leaves the people

The population follows the rain. Wikipedia's geography article, in figures it does not date, gives densities from 54 people per square kilometre in the Logone basin to 0.1 in the Borkou-Ennedi-Tibesti region of the north, which is larger than France. Britannica puts the national average at 17.9 per square kilometre in its estimate for 2026. The record counts 19,319,064 people in 2023. The Factbook figures on Wikipedia classed 3.89% of the land as arable in 2012, and irrigated land covered 302.7 square kilometres in 2003.

Where the rivers rise and where they end

Northern and central Chad have no permanent streams. After the infrequent rain on the Ennedi and the Ouaddaï, water runs for a few days along depressions called enneris and wadis before seeping into the sandy clay. The Batha is the one that matters most: in the rainy season it carries water west from the Ouaddaï and the Guéra to Lake Fitri.

Everything else of consequence arrives from outside. The Chari and the Logone rise in the highlands of the Central African Republic and Cameroon, where rainfall exceeds 1,250 millimetres a year, and flow from the south-east into Lake Chad. Wikipedia and Britannica's country article give the Chari about 1,200 kilometres; Britannica's article on the river itself gives about 1,400. Its flow ranges from 17 cubic metres a second in the dry season to 340 in the wettest part of the year. Near Sarh it takes in the Aouk, the Kéita and the Salamat. The Salamat and the Aouk both rise in Darfur in Sudan, the Salamat draws water from Lake Iro on its middle course, and the Aouk forms part of the boundary with the Central African Republic.

The Logone is formed by the Pendé and the Mbéré, runs for part of its course along the Cameroon frontier and carries between 5 and 85 cubic metres a second over about 960 kilometres. The two rivers meet at N'Djamena and flow together for about thirty kilometres through a delta into the lake.

The sources disagree on how much of the lake this one system supplies. The Wikipedia geography article puts the Chari's share at 95%, an average of 40 billion cubic metres a year, almost all of it lost to evaporation, and the Wikipedia article on the lake gives the Chari and Logone together 95%. Britannica gives the Chari and Logone about four-fifths. The supply has shrunk: the Wikipedia article on the lake reports that the average annual inflow of the two rivers from 1970 to 1990 was 55% of the inflow from 1950 to 1970, and that since the 1980s about a third of their water has been taken upstream in the Central African Republic for irrigation and hydroelectric power.

How far the shore of Lake Chad has moved

The lake is very wide and very shallow, so a small change in level moves its edge a long way. Wikipedia gives average depths of half a metre to 2 metres in the southern basin. A sill called the Great Barrier separates the southern basin from the northern one, and water crosses it only when the southern level exceeds 279 metres. Britannica describes a low ridge formed during a drought at the beginning of the twentieth century that has at times completely divided the two pools. Within a single year the area peaks in late October or early November and has shrunk by more than half by late April or early May.

The Wikipedia geography article attributes the fall after 1963 to the droughts of the late 1960s, the early 1970s and the mid-1980s, and the article on the lake adds the water diverted from the rivers that feed it. By 1985, Britannica records, the lake had been reduced to a pool just north of the mouth of the Chari and Logone, and in the early twenty-first century its area was typically about 1,500 square kilometres. The Wikipedia article on the lake states that it is no longer estimated to be shrinking, as rainfall has partly recovered. Since the drought of the 1970s, farmers have planted the exposed lake floor as polders, with maize, cowpea, rice and sorghum.

The larger lake that came before

The present lake is the remnant of a much larger one, usually called Mega-Chad, and its figures vary by source. Britannica describes the lake in the early Holocene, possibly until about 7,000 years ago, standing at about 335 metres, some 55 metres above the present surface, up to 170 metres deep and covering about 336,700 square kilometres. The Wikipedia article on the lake gives four high stands between 39,000 BC and 300 BC, a maximum depth of more than 180 metres and an area of about 400,000 square kilometres, and the article on Chad gives 330,000 square kilometres 7,000 years ago.

At that size the basin was open to the sea. Mega-Chad overflowed southward along the present-day Kébi River and over the Gauthiot Falls to the Benue, and through the Niger to the Atlantic. Its traces are still on the ground: older dunes flooded by the high lake form the long linear islands in the present one, diatomite and lake sediment fill the hollows between dunes for hundreds of kilometres to the east, and the soils south of the lake derive from clay laid down on the Mega-Chad floor. The country's name comes through the lake from the Kanuri word for lake.

The four states on the shore, Cameroon, Niger, Nigeria and Chad, set up the Lake Chad Basin Commission on 22 May 1964 with its headquarters in N'Djamena. The Central African Republic joined in 1996 and Libya in 2008.

Why permanent lakes survive in the northern desert

The north holds the opposite case to Lake Chad: lakes fed from below in a place with almost no rain at all. UNESCO inscribed the Lakes of Ounianga in 2012, eighteen lakes in two groups 40 kilometres apart, in a hyperarid part of the Ennedi region that receives less than 2 millimetres of rain a year. Groundwater replaces what evaporates, and UNESCO describes the water level as stable apart from a slight seasonal variation.

The two groups behave differently. Ounianga Kebir has four lakes, and Yoan, at 358 hectares and 27 metres deep, is so saline that only algae and some microorganisms live in it. Ounianga Serir has fourteen lakes separated by sand dunes. Lake Teli covers 436 hectares and is less than 10 metres deep, and some of the freshwater lakes hold fish. Floating reeds cover part of the Serir lakes and reduce evaporation; UNESCO's own page gives the share as almost half in one passage and about a third in another. The colours of the lakes, blue, green or reddish, follow their chemistry.

The basin that holds them was occupied by a much larger lake less than 10,000 years ago, according to UNESCO, which calls the system globally unique: permanent freshwater lakes kept full by an aquifer in the middle of a hyperarid desert. The property covers 62,808 hectares below the 450-metre contour of the lake watershed, with a buffer zone of 4,869 hectares, and Ounianga Serir, a village of about 1,000 people in 2012, sits inside it beside Lake Teli. A decree of 15 December 2010 designated the lakes a natural site and forbids mining there.

What lives where the water and the grass are

Plants and animals follow the three rain belts. In the Saharan north the only trees are the date palms of the oases. Palms and acacias grow in the Sahel, and the Sudanian south is grassland and woodland savanna, much of it now under cultivation. The Wikipedia article on Chad lists six terrestrial ecoregions and, without a date, at least 134 species of mammals, 509 of birds and more than 1,600 of plants.

Forest covered about 3% of the land in 2020, 4,313,000 hectares, down from 6,730,000 hectares in 1990. Planted forest made up 19,800 hectares of the 2020 total. At Lake Chad, Wikipedia records more than a million ruff counted on the water at one time, and the basin as a whole holds 179 species of fish.

The Greater Zakouma Ecosystem in the south-east, between the Sahara and the forests of Central Africa, spans 28,162 square kilometres of floodplain and perennial river, with Zakouma National Park at its centre covering 3,054 square kilometres. The government of Chad signed a management partnership for the park with African Parks in 2010, extended it in 2017 to Siniaka Minia National Park, the Bahr Salamat Faunal Reserve and the corridors between them, and renewed it in 2022. African Parks reports that the park's elephant population has grown by 40% since 2010 and that more than 6,300 visitors came in 2025.

The World Database on Protected Areas, in its October 2026 profile, counts 266,099 square kilometres of protected land and inland water, 20.96% of a land area it measures at 1,269,454 square kilometres.

How the frontiers were drawn across the basin

Britannica notes that Chad's frontiers are a heritage of the colonial era and coincide with neither natural nor ethnic boundaries. The record gives the country 1,284,000 square kilometres and six neighbours. The Factbook figures on Wikipedia, from the 2025 edition, divide that area into 1,259,200 square kilometres of land and 24,800 of water, and give 6,406 kilometres of land boundary: 1,556 with the Central African Republic, 1,403 with Sudan, 1,196 with Niger, 1,116 with Cameroon, 1,050 with Libya and 85 with Nigeria. There is no coast. Part of the eastern border follows the 24th meridian, so, as the Wikipedia geography article puts it, Chad technically has no single easternmost point.

The northern line was settled in court. Libya and Chad agreed in Algiers on 31 August 1989 to take their territorial dispute to the International Court of Justice if no political settlement came, and Chad filed its application on 3 September 1990. Libya argued that no boundary existed and asked the Court to fix one. Chad argued that a boundary already existed. In its judgment of 3 February 1994 the Court found that the Franco-Libyan treaty of 10 August 1955 had recognised a complete frontier, built from the Anglo-French Declaration of 1899, the Franco-Italian exchange of letters of 1902 and the Anglo-French Convention of 1919, and that a boundary agreed by treaty stands even after the treaty itself lapses.