In the well-watered western part of equatorial Africa the total average annual flow of the Congo River is enormous: some 44 trillion cubic feet. River flow at the lower end of the basin has two maxima: one that corresponds with the rainy season north of the equator, the other with the rainy season that occurs when it is summer in the Southern Hemisphere. The waters in the lower reaches of the river are slightly acid after traversing the large swamps situated in the centre of the basin.
East Africa’s many lakes stretch along the East African Rift Valley from the Red Sea to the mouth of the Zambezi River. Evaporation from most of them exceeds their surface rainfall, and in consequence their outflow is less than the quantities brought in annually by their tributaries. They often govern river flow by acting as storage reservoirs—decreasing the flood flow and increasing the dry-season flow. A number of the rift valley lakes are situated in closed basins and contain high percentages of dissolved salts. The largest of these are Lakes Rudolf (Turkana), Natron, and Eyasi.
Rainfall over much of Southern Africa is small, and the majority of the rivers originating there have an intermittent flow. Some large perennial rivers (e.g., the Okavango, the Zambezi, and the Orange) flow from areas of abundant rainfall into the drier zones. Groundwater
The conditions under which groundwater is found and the quantity and quality of groundwater reserves are closely related to geologic structure. Large inland depressions in Africa’s basement rock, having been filled with sedimentary layers of continental origin, sometimes form important groundwater reservoirs, notably those in the Taoudeni–Niger region, in the central Sahara between the Atlas and Ahaggar mountains; in the Libyan Desert; and in Chad, the Congo basin, the Karoo area of South Africa, and the Kalahari.
The East African plateaus usually contain little or no quantities of groundwater, and aquifers (geologic formations containing water)—generally of local importance—are found only in humid areas where the crystalline rock is weathered or fractured.
The chalky shales (rocks of laminated structure formed by the consolidation of clays) and dolomitic limestones (those containing calcium magnesium carbonate), which sporadically cover the basement rock, may contain important aquifers; those in Zambia and South Africa are major sources of water.
In the Sahara a rock stratum called the Continental Intercalary series, which dates from the early Cretaceous Period and which includes the Nubian sandstones of southern Egypt, is the most important water-bearing layer. It extends over very large areas and reaches a thickness of more than 3,000 feet; in Egypt and Algeria it is a major source of artesian water. In Sudan it sometimes lies directly on the Precambrian bedrock and contains underground water layers of local importance. Overlaying the Continental Intercalary series, but generally separated from it by a thick marine deposit, is a younger Tertiary layer called the Continental Terminal, which is the second largest aquifer in this area. Both these layers contain “fossil” water—i.e., water that entered the layers when rainfall in and around the Sahara was much more abundant than today. Near the surface, aquifers are found in such geologically recent deposits as alluvial deposits and sand dunes.
In the coastal areas of Senegal, Côte d’Ivoire, Ghana, Togo, Benin, Nigeria, Cameroon, Gabon, the Republic of the Congo, Angola, Mozambique, the East African countries, and Madagascar, aquifers are found in sandstone, limestone, and sand and gravel sediments. Intensive exploitation, however, may result in saltwater intrusions.
The Jurassic limestones of the mountainous area of the Maghrib are much more abundant in water sources than are dolomitic limestones. Around the cape in South Africa, sandstones and limestones contain very little water.
Yields from aquifers with good porosity, such as coastal sedimentary rocks or alluvial deposits, vary from a few cubic feet per hour in the fine-grained sands found in many parts of the continent to 35,000 cubic feet (990 cubic metres) per hour in the coarse gravels of the Nile delta. The capacity of wells in the Continental Terminal is generally somewhat lower, and those in the Continental Intercalary and the Karoo formations can also deliver moderate to high yields.
In North Africa limestones containing many cracks and fissures may yield thousands of cubic feet of water per hour, while in the Democratic Republic of the Congo, Zambia, and South Africa large yields are drawn from dolomitic limestones.
The harder sandstones, sandy shale, and quartzites of Precambrian and Paleozoic age are not generally very porous, and water is obtained only from fractured or weathered deposits. Western African, Angolan, and Tanzanian wells in these formations produce only a few cubic feet per hour. Crystalline and metamorphic rocks are almost impermeable except where fractured or weathered. Volcanic rock, especially the basalts, may yield up to 1,060 cubic feet per hour.
Most of the exploited groundwater is generally fit for consumption, because the dissolved minerals in water from shallow wells, particularly in the sandy aquifers of western Africa, are quite low. Groundwater from deeper marine layers, however—such as occurs in parts of North Africa, Mozambique, Ethiopia, and South Africa—may have a high content of dissolved salts. In moist tropical countries the water from Precambrian rocks generally contains only small amounts of dissolved minerals, whereas in the volcanic areas of East Africa groundwater may have so high a content of fluorine as to make it unfit for human consumption. There, and elsewhere in Africa, hot (possibly medicinal) springs with high mineral contents occur. Availability for human use
The pronounced seasonal character of rainfall and the fact that many rivers stop flowing during the dry season have necessitated the development of groundwater for human use, and the tapping of local aquifers has become important in many parts of the continent.
Large-scale irrigation has long been practiced mainly in North Africa, Egypt, Sudan, South Africa, Mali, Zimbabwe, and Mozambique. Medium-scale irrigation projects have been operated in Madagascar, Senegal, Somalia, and Ethiopia. In Côte d’Ivoire, Burkina Faso, Kenya, Nigeria, Ghana, and Zambia, medium- to small-scale projects have been constructed.
More than 50 river and lake basins are shared by two or more countries, and the development of their resources requires the cooperation of the basin states and several intergovernmental agencies—such as the Organization for the Development of the Sénégal River, the Niger Basin Authority, and the Lake Chad Basin Commission.
Several large reservoirs were built in the late 20th century, such as the Aswan High Dam, Roseires, and Khashm al-Qirbah reservoirs in the Nile basin, Kainji on the Niger, Akosombo on the Volta, Kariba on the Zambezi, Cahora Bassa on the Zambezi in Mozambique, Kossou on the Bandama in Côte d’Ivoire, Kafue on the Kafue in Zambia, and Inga I and II on the Congo River in the Democratic Republic of the Congo. At a number of man-made lakes, research centres have been set up to study resettlement problems, the full use of ecological conditions, and the control of health hazards that sometimes occur. Biological resources
Africa’s naturally occurring biological resources—its immensely varied vegetational cover, vast insect life, and diverse animal life—have been described above. When combined with cultivated crops and domestic animals, these resources represent the great bulk of the continent’s economic wealth. Botanical resources
The two most economically important types of vegetation are forests and grasslands. Among the forested areas, the tropical forests contain much of the valuable timber. The vast equatorial lowland rainforest has the greatest variety of tree species, but the species most commercially in demand are found in the zones of broad-leaved woodlands and tropical highland forests. The true value of the forested areas, however, cannot be ascertained exactly, as original forests are progressively being converted to farming areas, and few governments have undertaken comprehensive land-use surveys to determine their present extent.