Lake Chad
Lake Chad is an endorheic freshwater lake located at the junction of four countries: Nigeria, Niger, Chad, and Cameroon, in western and central Africa respectively, with a catchment area in excess of. It is an important wetland ecosystem in West-Central Africa. The lakeside is rich in reeds and swamps, and the plain along the lake is fertile, making it an important irrigated agricultural area. The lake is rich in aquatic resources and is one of the important freshwater fish producing areas in Africa.
Lake Chad is divided into deeper southern parts and shallower northern parts. The water source of the lake mainly comes from rivers such as the Chari River that enter the lake. The water level varies greatly seasonally, and the area of the lake also changes dramatically. During the African humid period, the lake's area reached. Due to the increasingly arid climate, the lake surface gradually shrank. In the 19th century, it still had an area of. However, due to climate change and human water diversion, it has shrunk significantly since the mid-1970s, and its area has fluctuated between.
Prehistory and history
The Chad Basin was formed by the depression of the African Shield. The floor of the basin is made of Precambrian bedrock covered by more than of sedimentary deposits. For most of the Quaternary, the basin had abundant water sources. 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. The area of Lake Chad experienced four heydays between 39,000 BC and 300 BC, leaving thick diatomaceous earth and lacustrine deposits in the strata. This has been called Mega-Chad. The maximum depth of Mega-Chad exceeded and it covered an area of approximately, it flowed into Benue River through the Mayo Kébbi, which drains into the Atlantic Ocean through the Niger River.The vast waters formed during the African humid period provided conditions for the emergence of lakeside fishermen's settlements, and the Nilo-Saharan ethnic group also migrated to Lake Chad during this period. Agriculture also emerged in the Sahel region at this time. By 1800 BC, a pottery culture known as Gajiganna had emerged, initially as pastoralists, but, starting around 1500 BC, living in settled hamlets at the side of the lake. The archaeological discovery revealed wild grasses, mostly of the tribe Paniceae, and wild rice together with the earliest domesticated Pearl millet in the Lake Chad region, dating to 1200–1000 cal BC. One of the oldest domesticated Pearl millet in West Africa was found in the Chad Basin, charred together with wild grasses, and their era can be traced back to 800–1000 cal BC.
Permanent villages were established to the south of the lake by 500 BC, and major archaeological discoveries include the Sao civilization. According to the records of Claudius Ptolemy in the mid-2nd century AD, the Romans of the 1st century AD had already come into contact with Lake Chad through their connections with Tunisia, Tripolitania, and Fezzan. By the 5th century AD camels were being used for trans-Saharan trade via the Fezzan, or to the east via Darfur. After the Arabs conquered North Africa during the 7th and 8th centuries, the Chad Basin became increasingly linked to the Muslim countries.
Trade and improved agricultural techniques enabled more sophisticated societies. Around 900 AD, the Kanem people who spoke the Kanuri language unified numerous nomadic tribes and established the Kanem Empire in the northeast of Lake Chad. At the beginning of the founding of the country, the Kanem people continued to live a nomadic life until the 11th century, when they were Islamized and settled in Njimi. Through trans-Saharan trade, the power of the Kanem Empire reached its peak in the 13th century, but as the empire declined in the 14th century, its southwestern vassal state of Bornu began to rise, causing the power center of the empire to shift to Bornu around 1400. In 1574 the Ottoman Empire had invaded Fezzan and the Oasis, and even reached into Lake Chad. This was the deepest Ottoman incursion into the interior of Africa before the nineteenth century. The Ottoman Empire extended its influence on central region of the Central Sahel during the reign of Murad III, And Lake Chad becoming a part of Ottoman sphere of influence, and in the second half of the 16th century, the Bornu Empire began importing firearms from the ottoman Empire, consolidating its military hegemony. The Bornu Empire declined in the 18th century, and later lost its western region to the Sokoto Caliphate during the early 19th century. It was later colonised by European powers in the 20th century.
Following the growing interest in Africa among European academic and business communities, the Lake Chad area was extensively described by Europeans in the 19th century. Three scientific expeditions were conducted between 1898 and 1909. During the Berlin Conference in 1884–1885, Africa was divided between the European colonial powers. By the second decade of the 20th century, Lake Chad had been colonized and occupied by Britain, France, and Germany, defining boundaries that are largely intact with the present post-colonial states. At the beginning of independence, the countries surrounding Lake Chad not only had a poor economic foundation, but also had more complex ethnic, religious, and political conflicts. Nigeria and Niger, which had just gained independence, experienced continuous coups, while Chad also experienced ongoing civil war. The inability of countries along the lake to consider the protection of Lake Chad has led to a series of environmental problems.
Geography
The Chad Basin includes Chad, Nigeria, Cameroon, Niger, Sudan, and the Central African Republic. It is an extensional fault depression type rift basin, which can be divided into four secondary structural units: southern depression, northern depression, central uplift, and eastern slope. The southern depression of the basin is characterized by an asymmetric fault depression composite rift with steep slopes in the east and gentle slopes in the west on the profile, and is distributed in an NNW direction on the plane. There are two large basin‐bounding normal faults developed on both sides of the basin, with a graben style fault and depression in the middle. The east and west sides are outward dipping low angle gentle slope areas. The eastern boundary fault is steep with a dip angle of about 55°, while the western fault has a dip angle of about 45°. The overall thickness of the inner layer in the slope area is relatively thin. In the central area of the basin, the thickness of the sedimentary strata is large, and the thickness of the sedimentary center zone reaches over. The northern part of the basin appears steep in the west and gentle in the east on the profile. Five fault structural zones parallel to the basin‐bounding faults have developed from west to east.Lake Chad is divided into north and south parts by a shallow sill called the Great Barrier, with the bottom of the northern basin at an altitude of and the bottom of the southern basin at. When the water level in the south exceeds above sea level, it will flow into the north. In the south, there is continuous open water at the mouth of the Chari River, and the western part of the water is covered by reed swamps, and the sand dunes that are not completely submerged in the eastern waters form an archipelago. The average depth of the southern lake basin is between, that of the northern lake basin is between 0 and, and that of the eastern archipelago is between 0 and.
The climate of the Lake Chad region is strongly influenced by continental and maritime air masses. The maritime air mass moves northward during the summer, producing seasonal precipitation. In late summer, continental air mass dominate again. The average annual precipitation in the Lake Chad area is, with an average annual precipitation of on the south bank and about on the north bank. The highest temperature in the rainy season is, and the highest temperature rises to more than when October and November enter the dry season. The temperature difference between day and night is almost twice that of the rainy season, and the lowest nighttime temperature sometimes drops to in December and January. April is usually the hottest month of the year, with temperatures occasionally reaching, the lowest water levels appear in June to July, and the highest water levels in November to December, with surface water temperatures ranging from.
Hydrology
The Chad Basin covers an area of about, and is injected by the Chari, Logone, and Yobe Rivers. The water supply of the lake is seasonal. Most of the precipitation comes from the Adamawa Plateau in the south of the basin, which is transported to the lake basin through the Chari River and the Logone River. The two contribute 95% of the total inflow of Lake Chad, while the Yobe River only contributes less than 2.5%. The lake seeps through the underground to the lowest point of the Chad Basin, the Bodélé Depression, approximately northeast of Lake Chad, with the deepest point reaching an elevation of only above sea level. This takes away most of the salinity and maintains the low salinity of Lake Chad. The southwestern waters of Lake Chad being freshwater, and the water in the northeast is only slightly salty.The water volume of most large lakes in Africa depends on rainfall and evaporation, which means that temperature and precipitation are crucial for regulating the water balance of these bodies of water, and any fluctuations can cause significant changes in their water level and area. Lake Chad is a shallow inland lake, and the rainfall in the Chad basin is very sensitive to small changes in atmospheric circulation, so the surface area of Lake Chad is greatly affected by climate change. Dry climate due to vegetation loss from overgrazing and deforestation and large-scale irrigation projects that diverted water from the rivers that feed the lake are the main reasons for the shrinkage of Lake Chad. The Atlantic multidecadal oscillation and the El Niño–Southern Oscillation have affected precipitation in the Sahel region. From the early 1960s to the mid-1980s, the lake water level decreased by compared to the average level from 1900 to 2010.
In 1870, the area of Lake Chad was about. The lake was able to flow out of the Bahr el-Ghazal during the rainy season. At the turn of the 20th century the area of Lake Chad shrank briefly, and reached a new high in the middle of the 20th century and overflowed from the Bahr el-Ghazal again. A major drought started in the Sahel region in the late 1960s and caused severe damage in 1972 and 1984. It was thought to be related to vegetation loss, global warming, and sea surface temperature anomalies. During this period, Lake Chad shrunk considerably and fluctuated in the range of thereafter.
From June 1966 to January 1973, the area of Lake Chad shrank from, further shrunk to in 1975, and only in February 1994. Since then, the area of Lake Chad has entered a relatively stable stage with a slight increase. From 1995 to 1998, it fluctuated within the range of. The area once reached in 2000, and the average area of surface water from 2013 to 2016 was about, with the largest area being in July 2015.