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|Physical Geography of West Africa|
The 8 million square kilometers and 17 countries covered by this atlas encompass a wide range of landscapes from alluvial valleys in Senegal and Ghana, sandy plains and low plateaus across the Sahel, and rolling hills of Togo to rugged mountains with summits reaching over 1,500 m in Guinea and 1,800 m in Niger. Covering approximately one quarter of Africa, West Africa contains a broad range of ecosystems, bioclimatic regions, and habitats from rain forest to desert.
West Africa can be divided internally through its natural features. Geology, relief, climate, vegetation, soils, and the responses of people to the patterns of its biophysical resources through human land uses all tend to be arranged along east-west belts. Pastoralists in northern Senegal would likely find their livelihoods more similar to those of pastoralists 3,000 km to the east in Niger than to those of someone raising cattle just 300 km south in Guinea-Bissau. Likewise the mix of crops varies more within Nigeria — from the semiarid north to the wet southern coast — than it does from one end of the West African Sahel in Senegal to the other in Chad. The most dramatic transitions in natural features and land use occur as one moves north or south across these belts we call bioclimatic regions. To better understand the geography of West Africa and how it drives land use, we briefly examine the geology, topography, hydrography, climate, and vegetation through these broad bioclimatic regions.
West Africa is remarkable for its geological variety. Like most of Africa, the region is largely composed of ancient Precambrian rocks (at least 541 million years old; the oldest rocks may be about 3 billion years old), which have been folded and fractured over hundreds of millions of years. These rocks are exposed over about one-third of West Africa and are part of the vast continental platform of Africa, which in West Africa has an average elevation of 400 m (Church, 1966). Numerous series of Precambrian rocks of various ages and their eroded surfaces provided a fairly level floor for the advance and retreat of shallow Palaeozoic seas (a major geologic era after the Precambrian, spanning about 289 million years). As these seas came and went, they deposited and eroded new material that formed the sedimentary rocks that overlay the ancient Precambrian floor across the region. For example, a large sedimentary basin called the Senegalo-Mauritanian Basin extends across much of western Mauritania, two-thirds of Senegal, and into Guinea. It is composed of sediments deposited when the ocean covered this part of the African plate (Michel, 1973; Stancioff and others, 1986).
For most of West Africa, continental conditions have existed since the Eocene or Oligocene, that is, since the last 23 to 34 million years. Most of West Africa’s mountain massifs and highlands, such as the Aïr Mountains, the Tibesti Mountains, the Adrar des Ifoghas, and the Fouta Djallon, originated as Precambrian folds (Church, 1966). Much later, volcanic activity in many of these highlands deposited additional layers of igneous rock. Volcanic outpourings have occurred throughout West Africa’s geologic history, with major activity as recent as the Pliocene (2.5 to 3.6 million years ago), and even more recent activity in the Aïr and Tibesti Mountains.
During recent dry periods in the late Quaternary (0.5 to 1 million years ago), intensive weathering of sandstone formations produced much of the present day sand sheets that cover vast areas north of a line running approximately through Kano, Ouagadougou, Bamako and Dakar. These sand deposits fill in many irregularities of relief and mask much of the surface geology.
Relief on its own is not the source of great regional diversity in West Africa. For the most part, West Africa is relatively flat and low, which sets it apart from the other major regions of Africa. Nor does the relief do much to interrupt the zonal patterns and latitudinal belts of climate and vegetation, except in the mountainous regions of the Fouta Djallon, the Guinea Highlands, the Jos Plateau, and the Aïr Mountains. In these areas, rainfall is somewhat higher than in the low plains around them.
Several major rivers, including the Niger — West Africa’s longest river — originate in the Guinea Highlands, where rainfall is heavy. Other major rivers rise from Guinea’s Fouta Djallon, including the Gambia and Senegal. The Senegal River drains a major basin — the third largest in West Africa after the Niger Basin and the Lake Chad Basin. West Africa’s rivers experience great seasonal variations in river flow.
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|Major river basins of West Africa|
The Niger River is about 4,180 km long and passes through almost every climatic zone in West Africa. A vast inland delta has formed along its way in Mali, owing to the shallow slope of the river and sand accumulations that have obstructed its many channels. The Inland Niger Delta acts like a giant sponge, moderating the flow downstream and reducing the risk of flooding. Where the Niger arcs past Timbuktu in Mali’s northern Sahel, sand accumulations push it southward. In Nigeria, the Niger River is joined by the Benue, its major tributary, which drains much of northeastern Nigeria.
The Lake Chad Basin occupies a huge area, covering parts of Niger, most of Chad, Nigeria, Cameroon, and the Central African Republic. The catchment of the Chari and the Logone Rivers comprises the southern part of the Lake Chad Basin. They feed Lake Chad, which has shrunk to a small fraction of its 1960 size.
Many separate basins are defined by smaller rivers that drain the land between the Atlantic Ocean and the basins of the Senegal and Niger Rivers. Of these, two are worth mentioning: the Gambia, which drains central Senegal and the nation of The Gambia, and the Volta River, which starts at the confluence of the Nakanbé (White Volta) and the Mouhoun (Black Volta), and reaches into the Mossi Plateau in Burkina Faso. Ghana constructed the Akosombo Dam (completed in 1965) in a gorge where the Volta cuts through the Akwapim–Togo Range, creating the world’s largest artificial lake, Lake Volta.
Most of West Africa, from the southern Sahara to the humid coastal countries, has only one rainy season, which lasts from one to six months. The area of two rainy seasons, a long one and a shorter one, is limited to the southern portions of the coastal countries from Liberia to Nigeria. The climate is related to the advance and retreat of the intertropical front— the interface between two air masses — one hot and humid and the other cool and dry. This front migrates annually north and south, following the position of the sun, with a lag of 1 to 2 months. In the winter months (December to March), there is an anticyclonic high pressure area centered over the Sahara. It drives the Harmattan, a desiccating, dusty wind that blows rather persistently from the northeast, drying out landscapes all the way to the coast. In the summer the high pressure area is replaced by a depression, bringing warm, moist winds in from the Atlantic in the southwest (from the Gulf of Guinea) (Arbonnier, 2000; Zwarts and others, 2009). Generally, the dry season lengthens and annual rainfall decreases with increasing latitude. Conversely, in the southern latitudes, rainfall increases and the dry season shortens, often to just four months (December to March). Maximum temperatures and temperature ranges also increase with latitude. In the humid south, temperatures vary little, whereas in the arid north one temperatures range from 0˚C to more than 45˚C (Church, 1966).