Chapter 3
Despite its prevalently low rainfall, Africa is not lacking in lakes and wetlands, and contains the longest river in the world. However, water flow in rivers can vary hugely in response to seasonal and annual variation in rainfall. Lakes can form and dry, reacting not only to rainfall variation but also to rift valley subsidence and other tectonic movements affecting water flow over quite short periods. Rivers have also changed their courses in response to ructions of the land surface.
Rivers bring surface water to regions that may otherwise be waterless. Groundwater seeping from channels supports flanking woodlands, which may retain green foliage when the rest of the landscape is brown and dry. Nevertheless, water-dependent herbivores prefer to seek their surface water needs away from major rivers, in sandy channels retaining water below the surface and from temporary pools in depressions known as pans. These are less likely to house lurking crocodiles and have less surrounding vegetation cover where lions can hide in ambush.
The flow of rivers has further biological consequences for terrestrial animals. No river crossing Africa’s interior plateau is deep and wide enough to block the movements of hominins or other animals between the north-eastern and southern extremes of the continent; at least, not during low flow in the dry season. Even the biggest east-flowing river, the Zambezi (Figure 3.1B), is easily forded upstream of Victoria Falls at low water. The White Nile, flowing northwest from its source in Lake Victoria (Figure 3.1A), continues northward after joining the Blue Nile emanating from the mountainous highlands of Ethiopia. The mighty Congo and other rivers having substantial flow year-round all trend westward.
Figure 3.1
African rivers. (A) Nile River below Murchison Falls in Uganda, deep and fast-flowing; (B) Zambezi River below Kariba Dam, broad and deep; (C) Limpopo River spread over a vast sandy bed; (D) Mara River in Kenya with deeply cut banks; (E) Letaba River in Kruger NP, flowing gently over a sandy bed; (F) Kidepo River in northern Uganda, seasonally without surface water.
Rivers
Africa’s five major rivers, in terms of mean volume of water discharged at their mouths, are the Congo, Niger, Ogooue, Zambezi and Nile, in that order (Table 3.1). The Congo is by far the largest and second globally in water volume amalgamated from its tributaries draining the tropical forests of the Congo Basin. Nevertheless, the amount of water discharged from its mouth averages only 20 percent of that carried by the Amazon River in South America. Second in water flow in Africa is not the Zambezi, but rather the Niger, arising in the west in the Guinean highlands and curving eastward then southward to enter the Atlantic Ocean in the ‘armpit’ of Africa in Nigeria. The Ogooue River draining the forests of Gabon is also bigger than the Zambezi in terms of mean water flow. The Nile River comes fifth. It is the longest river in the world, but is way down the ranking in water flow. Its discharge listed in Table 3.1 is the amount entering the Aswan Dam, because only half of this actually reaches its mouth into the Mediterranean Sea.
Table 3.1African rivers listed in order of mean flow volume at their mouths (source: Wikipedia)
The next biggest African river flowing into the Indian Ocean, after the Zambezi, is the Rovuma, forming the border between Tanzania and Malawi, but it carries only 15 percent of the mean discharge from the Zambezi. Third largest in the east is the Limpopo, with considerably lower flow levels. All of the major rivers draining savanna regions show huge seasonal fluctuations in flow, with the lowest level in the dry season amounting to 5 percent or less of the maximum flow during the wet season (Table 3.1). Several large rivers have catchments in the Angolan highlands where the MAR is as high as ~1400 mm. The Kunene turns westward towards the Atlantic Ocean along the northern border of Namibia, while the Okavango flows south-east into a spreading alluvial fan in Botswana from which only a trickle flow emerges. The Kwanga flows north-west to enter the Atlantic Ocean near Luanda. South Africa’s two largest rivers, the Gariep (or Orange) flowing westward toward the Atlantic Ocean and the Tugela flowing eastward into the Indian Ocean, both have their catchments in the Lesotho highlands. Many other rivers originating in Africa’s interior plateau flow only seasonally, and merely retain pools in their sandy beds during the dry season (Figure 3.1F).
No African river is navigable by boat very far upstream from its mouth, due to sedimentation in lower reaches and rapids or waterfalls in upper courses. Before reaching the sea, the Congo River cascades over the Inga Falls and associated rapids, with the drop of 96 m over 15 km blocking navigation through this barrier. Boat travel resumes upstream from Kinshasa and Brazzaville as far as Kisangani well into the interior. River travel up the Zambezi had been blocked by the Cahora Bassa Gorge and associated rapids in Mozambique, before this section was submerged beneath the waters of a huge dam. The Zambezi plunges 108 m over Victoria Falls on the border between Zimbabwe and Zambia, with its width becoming constricted from 1708 m to merely 110 m at this point. The breadth of its curtain of falling water at high flow, masked by shifting clouds of spray, is greater than that of any other waterfall worldwide. By the late dry season, it gets attenuated to remnant trickles over sections of its width (Figure 3.2). Below the falls, the Zambezi continues through zigzag gorges representing the locations of previous waterfalls cut through the local basalt. The White Nile traversing Uganda becomes similarly restricted through the Murchison Falls gorge where it enters the Western Rift depression. The Tugela Falls, dropping in total 983 m over the Drakensberg escarpment in South Africa, is rivalled in height only by the Angel Falls in Venezuela.
Figure 3.2
(A) Victoria Falls in July with highest flow following the end of the wet season; (B) Victoria Falls in November with lowest flow at the end of the dry season.
The courses followed by many African rivers have changed over geological time, in response to tectonic movements and headward erosion. Following the breakup of Gondwana, the Karoo and Kalahari rivers flowing westward from the southern African interior became joined to form the lower section of the Gariep River.1 At that time the Limpopo accumulated the flow of several rivers emanating in the Angolan highlands, including the upper Zambezi. It was vastly bigger than its present meagre flow, as shown by its wide valley (Figure 3.1C).2 The Okavango River flowed into a vast Lake Mababe in Botswana until as recently as 8.5 ka, before tectonic blockages generated the Okavango Delta.3 The Savuti channel, branching from a bend where the Kwando River emanating from Angola hits the fault line, has fluctuated in its onward flow towards the Savuti Marsh, formerly part of Lake Mababe, over the past few centuries. Fossil drainage lines in the Kalahari, currently infilled with sediment and bone dry, testify to the former presence of quite large rivers in this currently arid region.
In eastern Africa, the Pliocene uplift diverted rivers that had flowed westward from the region of the Kenya–Uganda border to drain instead into the depression filled by Lake Victoria, forging a connection with the Nile. In western Africa, the Niger River was formed by the joining of two previously separate rivers. One had ended in a lake north-east of Timbuktu, while the other had its original source south of Timbuktu. The Congo River earlier had its mouth located about 200 km north of its current position.
Although many of the lesser African rivers cease surface flow during the dry season, water generally remains in their sandy beds. Many more would have flowed only seasonally during glacial advances when rainfall was generally lower than at present. This reinforced the lack of barriers to animal dispersals between eastern and southern Africa. Only the lower Zambezi River currently remains formidably deep during the dry season. South Africa’s biggest river, the Gariep (or Orange), was readily forded by ox-waggons at designated ‘drifts’. Thus, many animal species, or their geographic replacements, have distributions extending across Africa from the Cape to the Maghreb region bordering the Mediterranean coast.
Lakes and Wetlands
Lake levels in Africa have fluctuated hugely, contributing to local swings between arid and moist conditions. Lake Victoria, currently Africa’s largest lake in area (Figure 3.3A), is quite shallow and has dried up completely several times, most recently between 18 and 14 ka after the LGM.4 At other times it covered an even larger area than at present, extending into the western corridor of Serengeti NP. Lake Chad, in north-central Africa, was larger in area than current Lake Victoria as recently as 5 ka and had an outlet to the Atlantic Ocean then. Its progressive contraction in recent years indicates the increasing aridity in its catchment. Lakes Afar, Omo, Turkana and Baringo formed after 5 Ma within the northern section of the rift valley extending from Ethiopia into Kenya.5 Lake Tanganyika, located in the Western Rift, is currently the second deepest lake in the world and third largest in water volume, with parts of its floor below sea level. Lake Malawi is nearly as deep, but nevertheless became reduced to shallow puddles around 0.75 Ma.6 Other large lakes in the Western Rift include Kivu, Edward (Figure 3.3B), George and Albert. Lakes situated in the Eastern Rift, including Bogoria, Nakuru, Elmenteita, Magadi, Natron, Manyara and Eyasi, are shallow and highly saline. Lake Naivasha is exceptional in containing fresh water, presumably due to a hidden outlet.
Figure 3.3
African lakes. (A) Lake Victoria in Tanzania, vast but shallow; (B) Lake Edward in the Western Rift in Uganda, huge and deep; (C) Lake Nakuru in Eastern Rift in Kenya, shallow and saline; (D) Lake Malawi in the southern end of the Eastern Rift in Malawi, deep.
The Okavango Delta (strictly, an alluvial fan), located in northern Botswana, constitutes one of the six major wetlands of the world (Figure 3.4A,B).7 It was formed after a seismic fault blocked onward flow of water from the Okavango River, perhaps as recently as 120 ka. The river entering its ‘pan-handle’ branches into numerous distributaries. Only a variable trickle flows onward via the Botete River into the Makgadikgadi depression, now mostly a dry salt pan. The remainder of the water entering the Delta evaporates. Water flowing through the Delta is exceptionally clear, because it comes from the Angolan highlands where the Kalahari sand cover lacks clay. Due to the delay in water passage through the various channels, the main inflow of water from the catchment during January–February generates peak flooding within the Delta only in July, during the middle of the dry season. Sediment accumulation in the form of sand eventually causes former channels bounded by papyrus to become islands. In earlier times, the paleo-lakes Makgadikgadi, Mababe and Ngami combined may have exceeded Lake Victoria in their total area, perhaps as recently as 17 ka.3,7,8 A wetland exists in Namibia across its border with Botswana where the Kwando River hits a fault line and changes its flow direction to become the Linyanti and eventually join the Zambezi.
Figure 3.4
African wetlands. (A) Okavango Delta interfaced with savanna, Botswana; (B) Okavango Delta islands formed around termite mounds, Botswana; (C) Sudd Swamp bordering Nile River, Sudan (image from UNEP); (D) Busanga Plains in Kafue NP, Zambia (photo: Dean Polley, image provided by Wilderness Safaris); (E) aerial view of pan with elephants approaching, Botswana; (F) large pan kept full by pumping, with elephants, in Hwange NP, Zimbabwe.
The Nile River flows through a vast swamp called the Sudd upstream of Khartoum in Sudan (Figure 3.4C). Within Mali, the Niger River forms a region of braided streams, marshes and lakes before reaching Timbuktu. Wetlands elsewhere in Africa include the Kafue, Barotse and Busanga flats (Figure 3.4D) as well as Lake Bangweulu in Zambia, the Gorongoza floodplain in Mozambique, and Rukwa-Katavi floodplain in Tanzania.
Seasonally waterlogged valleys called dambos (or vleis in South Africa) are a feature especially of south-central Africa where miombo woodlands prevail on basement granitic soils. They can develop into quite extensive wetlands where their drainage is blocked, for example at Nylsvley in the northern Limpopo Province of South Africa.
Pan depressions varying in size are a common feature of drier regions of Africa, accumulating water run-off during the wet season and becoming dry sometimes quite late into the dry season (Figure 3.4E,F).9 In years with high rainfall, larger pans may retain water year-round, effectively sealed by impervious clay or calcrete crusts beneath. Numerous pans are a feature especially of the south-western Kalahari and may be self-perpetuating through the trampling activities of large herbivores.10 There were numerous small pans retaining pools for varying periods in the Hluhluwe-iMfolozi Park where I did my white rhino study. Following any well-trampled animal trail usually led me to one of these water sources. Which of the pans retained water longest varied from year to year. Pans can keep herbivore concentrations away from perennial water sources in rivers and lakes for much of the year.
The fluctuations in water levels in lakes suggest changing rainfall conditions in their local catchments, but respond also to earth movements. Lakes Turkana and Baringo filled and emptied repeatedly between 2.69 and 2.58 Ma and again around 1.7 Ma, during the early Pleistocene.5,11 Rapid swings between low- and high-water levels over relatively short timescales occurred, responding to the 21 kyr cycle driven by precession in the Earth’s axis of rotation. Paleo-lake Olduvai fluctuated widely in extent and depth through the early Pleistocene, eventually drying up around 1.15 Ma. The fossil site at Olorgesailie, in southern Kenya, was sometimes beneath a lake and at other times quite dry, as it is at present.12 Lake Malawi, currently very deep, shrank to a series of puddles between 135 and 75 ka.13 Water levels in various eastern African lakes were high during the period between 145 and 120 ka, then again from 80 to 65 ka.14 The water level in Lake Tanganyika dropped by over 400 m around 90 ka, more than took place during the LGM, but had risen back to its former level by 75 ka.15
While responding to climatic and tectonic variability, lakes in and near the Eastern Rift accumulated the sediments that preserved fossils, of human ancestors along with numerous other animals. Those in eastern Africa associated with the Western Rift were too deep to provide fossil accumulations.
Overview
Because of Africa’s relatively dry climate, its rivers carry less water than those in most other continents (except for Australia), and many cease flowing seasonally. Annual variation in peak river flow is also wider than in other continents.16 All of Africa’s easterly flowing rivers can readily be crossed in their upper reaches during the dry season and thus have not formed barriers to animal dispersals between the northern and southern sections of the continent. Some of its rift valley lakes are exceptionally deep while others are shallow and saline. Pools retained in seasonal rivers, pan depressions and wetlands play an important role in drawing concentrations of large herbivores away from major rivers and lakes. In South America, the enormous Amazon River plus its tributaries and the Orinoco River further north form major obstacles to north–south movements by animals not able to swim competently. Although Africa has several large wetlands, those formed in the Llanos of Venezuela and Pantanal of Brazil are far vaster. Europe and North America have numerous deep-flowing rivers navigable far upstream from the coast, plus many large lakes. Most of interior Australia is largely waterless and devoid even of long-lasting pools.
Water movements redistributing rainfall have further influences on soil fertility, in association with bedrock geology, as will be explained in the next two chapters.
SUGGESTED FURTHER READING
Fynn, RWS, et al. (2015) African wetlands and their seasonal use by wild and domestic herbivores. Wetlands Ecology and Management 23:559–581.
Maslin, M. (2017) The Cradle of Humanity. Oxford University Press, Oxford.
Walling, DE. (1996). Hydrology and rivers. In Adams, WM, et al. (eds) The Physical Geography of Africa. Oxford University Press, Oxford, pp. 103–121.
REFERENCES
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3.Burrough, SL, et al. (2009) Mega-lake in the Kalahari: a Late Pleistocene record of the Palaeolake Makgadikgadi system. Quaternary Science Reviews 28:1392–1411.
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11.Lupien, RL, et al. (2018) A leaf wax biomarker record of early Pleistocene hydroclimate from West Turkana, Kenya. Quaternary Science Reviews 186:225–235.
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13.Scholz, CA, et al. (2007) East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins. Proceedings of the National Academy of Sciences of the United States of America 104:16416–16421.
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