The Congo River basin and its drainage network.Encyclopædia Britannica, Inc.
The Congo River is some 2,900 miles in length. Its many waterfalls and rapids cause its valley, like that of the Nile, to lose elevation quickly. The river’s course is often constricted by gorges. The best-known are the Boyoma (Stanley) Falls at Kisangani, where the river swings through an arc to flow westward; in fact, the Boyoma Falls are no more than a series of unevenly spaced rapids at no great height, extending along a 60-mile stretch of the river. Downstream from Kisangani, the Congo is joined first by the Ubangi from the right and then by the Kasai—which rivals the Ubangi in the size of its drainage basin—from the left. Below its confluence with the Kasai, the main river cuts through the Cristal Mountains in a deep gorge, which at one point expands into Malebo (Stanley) Pool, a shallow lake measuring 22 miles in length and 14 miles in width. The Congo enters the sea through a swampy estuary that is about 6 miles wide at its mouth.
Enya (Wagenia) fishing in the rapids of the Congo River near Kisangani, Democratic Republic of the Congo.SuperStock
Zambezi basin
The Zambezi River is about 2,200 miles in length and occupies a basin with an approximate area of 463,000 square miles. Originally, there were two rivers, corresponding to the upper and lower courses of the present river; the valley of the lower section eroded toward the headwaters until it captured the waters of the upper section. Although there are stretches of the river where the gradient is very gentle—a drop of only about three inches to the mile—the valley as a whole has a fairly steep gradient. There are numerous waterfalls, the most spectacular of which is the Victoria Falls. After these falls, the river winds through a number of deep gorges cut out of basalt and, after flowing through a broad valley, enters Kariba Gorge, which is more than 16 miles in length and is cut through paragneiss (a gneiss, or coarse-grained rock, in which bands rich in granular minerals alternate with bands containing schistose minerals, formed out of sedimentary rock). The Kafue and the Luangwa, the two main tributaries, which both flow through gorges, join the Zambezi on its left bank downstream from Kariba. At the mouth of the main river is a delta about 37 miles wide.
The Zambezi River basin and its drainage network.Encyclopædia Britannica, Inc.
Orange basin
The Orange River is the longest in South Africa. Flowing across almost the entire width of the country, it makes its way from the highlands in the east through the Kalahari depression in the west to empty into the South Atlantic Ocean. Its major tributary, the Vaal River, is one of its northern headwaters; the two rivers together have a combined length of about 1,300 miles. Together with other major rivers on the continent, the Orange–Vaal river system shares the characteristic of flowing over steep gradients for numerous stretches of its course. The largest drop (about 400 feet) occurs at the Augrabies Falls.
The Orange River basin and its drainage network, one of the prominent physical features of southern Africa.Encyclopædia Britannica, Inc.
Chad basin
The Chad basin constitutes the largest inland drainage area in Africa. Lake Chad, a large sheet of fresh water with a mean depth between 3.5 and 4 feet, lies at the centre of the basin but not in its lowest part. Lake Chad is fed by three major streams, the Komadugu Yobe, Logone, and Chari, but these are in danger of having their waters captured by the drainage systems of rivers that flow in opposite directions. Lake Chad itself, with an area of only some 5,000 square miles, was formerly much more extensive.
For a detailed discussion of Lake Chad, the Congo River, the East African lakes, the Niger River, the Nile River, the Orange River, the Sénégal River, the Suez Canal, and the Zambezi River, see individual articles. Soils Soil types
In general, soil types on the African continent may be divided into five or six broad categories. There are desert soils; chestnut-brown soils, which border the deserts; and chernozem-like soils (dark black soils rich in humus and carbonates), which are found immediately south of the chestnut soils from Sudan westward to just beyond the Niger Bend (the bend in the middle course of the Niger River) and pockets of which are also found in East Africa, Zambia, Zimbabwe, and South Africa. In addition, there are black soils (often grouped with chernozems), and found on the Accra Plains of Ghana; red tropical soils and laterites (leached red iron-bearing soils), which occur in the tropical wet-and-dry and equatorial climatic zones; and Mediterranean soils, found in the Atlas Mountains of North Africa and the Cape region of South Africa.
Distribution of African soil groups as classified by the Food and Agriculture Organization (FAO).Encyclopædia Britannica, Inc.
The most important factors that affect soil formation are climate, parent material, relief, drainage, vegetation cover, and the passage of time. Where the land has been generally stable and fairly flat for prolonged periods, as in Africa, the climate becomes the major determinant of the soil groups. The different rocks are deeply weathered and are broken down into their common component elements to produce broadly similar soils under the same climatic conditions. Given sufficient time under a tropical climate, the differences in humus content of the great soil groups, which are introduced by vegetation types, are minimized. But within these groups there will naturally be differences in soil types as a function of local differences in physical factors. Desert soils
These soils are characterized by the general lack of organic content; by the types of rock reflected in them, the chemical weathering of which has been inhibited by the lack of water; and by the crusts or concretions of soluble salts on or just below their surface. While these crusts are in general thought to have been formed as a result of evaporation, it is nevertheless possible that they may have been formed under a wetter climate during the Pleistocene Epoch. Chestnut-brown soils
In the semiarid areas bordering the desert, increased rainfall makes grass vegetation more plentiful, results in rocks becoming more weathered than in the desert, and produces better developed soils with a higher humus content. It is the humus content that, according to the amount present, gives the chestnut soils their characteristic light or dark brown colour. Chestnut soils also differ from desert soils because they receive enough water to wash out some of the salt accumulations either on the surface or immediately below it. Chernozem-like and black soils
An unfailing characteristic of the chernozem is the presence of a subsurface zone of calcium carbonate, sometimes accompanied by calcium sulfate, which is left behind after all the soluble salts have been washed out. Grouped with them are the black soils, which should, perhaps, be differently classified, for their black colour is not necessarily due to high humus content but rather to the presence of certain minerals, as in the black soils of the Accra Plains, in Ghana. Red tropical soils and laterites
The majority of tropical soils have shades of colour varying from yellow and brown to red. The reddish colour reflects the presence of iron oxides that form as a result of chemical weathering. At one time all tropical red earths or soils were indiscriminately referred to as laterites, but it is now clear that the term laterite should be confined to those tropical soils with large concentrations of iron and aluminum sesquioxides (insoluble compounds) that have formed a hard pan at or just below the surface. At the most advanced state of laterization, bauxite, from which aluminum is extracted, is formed. Most tropical soils are in varying stages of laterization, which is to say they are at various stages of accumulating insoluble compounds as the soluble elements are leached out. The compounds accumulate more readily in areas with a pronounced dry season and where the water table is not too far below the surface. If the top horizons (layers) of the soils should erode, the subsurface concentrations of sesquioxides are then exposed to the atmosphere, whereupon they crystallize irreversibly to form true laterite concretions. Mediterranean soils