Rivers

Zambezi River

Content Cover Image

Victoria Falls on the Zambezi River. @ C.Michael Hogan

The Zambezi River, Africa's fourth largest after the Nile, Zaire and Niger rivers, exhibits a length of 2700 kilometers prior to discharge to the Indian Ocean in Mozambique.


Zambezi River with Zambia in foreground and Zimbabwe in background. @ C.Michael Hogan  

Zambezi River at the junction of Namibia, Zambia, Zimbabwe and Botswana. Source: Brian McMorrow

From headwaters in northwest Zambia, the river flows:

  • southeast through a portion of eastern Angola;
  • south through western Zambia;
  • east along the Zambia-Namibia (Caprivi Strip) border to the junction of Namibia, Zambia, Zimbabwe and Botswana;
  • east along the border between Zambia and Zimbabwe over the dramatic Victoria Falls and on to Lake Kariba
  • east into Mozambique and Lake Cahora Bassa
  • south west through Mozambique (where it is joined by the Shire River which drains Lake Nyaka/Lake Malawi) and on to the Indian Ocean.

Climate in the Zambezi Basin is influenced by surface winds arriving both from the Indian Ocean and Atlantic Ocean, with each alternative direction bearing potential precipitation, although the Indian Ocean is responsible for most of the moisture, due to the warmer ocean temperatures.

Numerous fish species, as well as avafauna and mammals utilize waters of the Zambezi. Adverse impacts of enhanced erosion, sedimentation and enhanced flood events have resulted from poor agricultural land use practises throughout the basin, alternatively called the Zambeze Basin.

Hydrology

Southern Africa’s longest trans-boundary river, the Zambezi, rises at 1,585 meters above sea level in north-western tip of Zambia. The River flows for some 2,700km through plains, gorges, rapids and cataracts before spreading out in deltoid form as it enters the Indian Ocean in the East Coast of Mozambique. The River carries more than 75% of the mean annual runoff of the region’s interior, and drains more than 40% of the landmass. Demand for water is increasing with population and economic growth. Along its meandering journey to the Indian Ocean, nurturing life in its waters, along its banks and beyond, the Zambezi is not only a source of water but also of food, electricity, transport, communication and recreation for millions of people. (ZRA, 2008)

The Zambezi River Basin is the fourth largest river basin of Africa, after the Congo/Zaire, Nile and Niger basins. The Basin covers some 1.3 million square kilometres spread over eight countries, namely, Zambia (40.7%), Angola (18.2%), Zimbabwe (18%), Mozambique (11.4%), Malawi (7.7%), Botswana (2.8%), Tanzania (2%) and Namibia (1.2%). Almost 33% of the total population of the riparian countries live in the basin. (ZRA, 2008)

Source Zambezi River Authority

When the river discharges to the Indian Ocean it has an average discharge rate of approximately 7100 cubic meters per second. Poor conservation and land use practises in the basin have led to adverse flooding impacts, and also less stable in stream retention of flow, such that groundwater recharge has been hampered over the last half century. There are numerous dams on the Zambezi to provide for agriculture and other human use, especially to provide water in drought years. Along the Zimbabwe stretch alone there are more than 150 dams, including two massive man-made lakes, Kariba and Cahora Bassa, at over 5000 and 2000 square kilometers respectively.

Sandwiched between Botswana and Zambia, the narrow neck of land that is the Caprivi Strip gives Namibia access to the Zambezi River. Defined by the Zambezi and Chobe Rivers, the eastern tip of the Caprivi Strip is usually under water during the flood season in March and April, as shown in the top image. Water is black in the false-color image, which was made with both infrared and visible light measured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite.

Interestingly, the flood confines itself to the gap between the two rivers, as if constrained by country borders. One might expect, in such a case, that water flowing from the Chobe River into the rain-swollen Zambezi is backing up onto the floodplain, but that is not the case. The land immediately behind the confluence is dry; the floodplain ends several kilometers west of the confluence. Why? The detailed view of the eastern edge of the flood plain (second image) provides some clues.

The Advanced Land Imager on NASA’s Earth Observing-1 (EO-1) satellite captured the lower image several minutes before Terra MODIS acquired the top image. The extent of the EO-1 ALI image is outlined in white on the top MODIS image. This highly detailed true-color image shows the stark eastern edge of the floodplain. To the left of the edge, water covers everything. Deep blue channels wind among green, shallowly flooded plains. To the right of the edge, the land is dry. The city of Kasane is perched confidently along the edge of the flood plain. What stops the water from flooding the city and everything else east of the line?

The eastern edge of the flood plain is defined by the Mambova Fault, which elevated the land on its eastern side. The Zambezi and Chobe Rivers cut channels across the fault, but the shallow flood plain has not surmounted the elevated ground. The triangle between the two rivers and the fault creates Impalila Island. A channel of water, called the Kasai, connects the two rivers in the flood plain.

Floods on the Zambezi occur when heavy rains fall on the wetlands in Angola and Zambia. The water flows downstream and gets backed up at the Mambova fault. The river expands over the flat floodplain behind the fault until the waters meet the channel cut by the Chobe River in the south. During the annual flood, the build up of water from the Zambezi River overcomes the Chobe, and water begins to flow south into Lake Liambezi. At the height of the flood, water occasionally flows directly into Lake Liambezi from the Zambezi River through the Bukalo Channel, as it did on May 8, 2010. (Source: NASA)

Victoria Falls

Victoria Falls is one of the most famous tourist sites in subsaharan Africa. Details of the Falls are visible in this image taken with the 800 mm lens by Astronaut Edward Lu from the Space Station on September 4, 2003.

A major river in south-central Africa, the Zambezi River flows from western Zambia to the Indian Ocean in Mozambique. In the sector imaged here, it flows southeast (top left to bottom right) in a wide bed before plunging suddenly 130 meters over the Victoria Falls into a narrow gorge. The falls and their famous spray clouds are 1700 m long, the longest sheet of falling water in the world. The falls appear as a ragged white line. The small town of Victoria Falls in Zimbabwe appears just west (left) of the falls, with smaller tourist facilities on the east bank in Zambia. The international railroad bridge over the second gorge (between Zimbabwe and Zambia) can be seen in the detailed view on the bottom.

The positions of the falls are controlled by linear fault lines in the underlying basalt rocks. The falls have moved upstream (bottom to top) by intense river erosion, elongating the zig-zag gorge in the process. Prior positions of the strongly linear falls can be detected. The earliest on this cropped view may have been the longest (dashed line). The zig-zags represent subsequent positions, all with the characteristic water-worn lip on the upstream side. The falls will continue to erode northward.

(Image and text source: NASA)

Also see: Victoria Falls National Park, Zimbabwe

Water quality

Near the river mouth the mean concentrations of total dissolved solids is 113 milligrams per liter; the corresponding concentration of total suspended solids is 90 milligrams per liter. Thus the annual discharge of total solids to the Indian Ocean is 2.5x107 tons. The poor land management practises, chiefly of poor indigenous farmers of the basin, have contributed excessive sedimentation via extreme soil erosion throughout much of the Zambezi basin.

Nutrient levels in the Zambezi River are relatively low, especially in the upper Zambezi; in that reach, above Victoria Falls, most of the catchment drains Kalahari sands, whose nutrient levels are inherently low due to their aeolian formation; moreover, agricultural fertilizer addition throughout the Zambezi watershed is low, due to the shortage of capital available to farmers of this region. Nitrate levels (as nitrogen) in the upper Zambezi are typically in the range of .01 to .03 milligrams per liter. Correspondingly electrical conductivity of the upper Zambezi is on the order of 75 micro-S per centimeter, due to the paucity of ion content. From the Luangwa River downstream nitrate levels elevate to .10 to .18 milligrams per liter, and electrical conductivity rises to a range of two to four times the upper Zambezi levels.

Not surprisingly, pH, calcium ion concentration, bicarbonate and electrical conductivity are all higher in portions of the catchment where limestone soils predominate compared to granite.

Geologic history

Delta sediment core data show a decrease of lithogenic deposits to the delta shelf region after about 16,000 years before present, produced by a landward retreat of the Zambezi River delta system during the period of sea-level rise induced by glacial melt. At the same time interval, deposition of fluvial mud shifts northward associated with associated progressive flooding of the Zambezi shelf and inception of northward shelf currents. From that same late Pleistocene era, pollen records prove an increase of mangrove and coastal vegetation, implying an extension of coastal wetlands during flooding of the broad, low-gradient Zambezi inner shelf area.

Aquatic biota

caption Pristis microdon. Source: J.Patrick Fischer There are a total of 190 fish species present in the Zambezi River, including eel and shark taxa. The largest native demersal species present are the 117 centimeter (cm) long tiger fish (Hydrocynus vittatus), the 175 cm African mottled eel (Anguilla bengalensis labiata), the 120 cm Indonesian shortfin eel (Anguilla bicolor bicolor), the 200 cm Giant mottled eel (Anguilla marmorata), the 150 cm African longfin eel (Anguilla mossambica), the 183 cm Sampa (Heterobranchus longifilis), the 150 cm Cornish jack (Mormyrops anguilloides) and the 700 cm largetooth sawfish (Pristis microdon).

caption Hippopotamus exiting the Luangwa River, a tributary of the Zambezi. @ C.Michael Hogan The largest native benthopelagic fish in the Zambezi are the 170 cm North African catfish (Clarias gariepinus), the 146 cm common carp (Cyprinus carpio carpio), the 150 cm Indo-Pacific tarpon (Megalops cyprinoides) and the introduced 120 cm rainbow trout (Oncorhynchus mykiss). The native reef-associated taxa are the 400 cm bull shark (Carcharhinus leucas), the 120 cm crocodile snake eel (Brachysomophis crocodilinus) and the 38 cm silver sillago (Sillago sihama).

There are two native pelago-neritic (coastal brackish water) species in the Zambezi: the 131 cm tenpounder (elops machnata) and the 20 cm orangemouth anchovy (Thryssa vitrirostris).

Certain mammalian species are found in the Zambezi River. Notably, the dugong (Dugong dugon) is found in the delta region, where discharge to the Indian Ocean takes place. Further upriver in Zambia, Zimbabwe, Namibia and Botswana, hippopotami (Hippopotamus amphibius) are found wading in the shallower waters.

Terrestrial ecoregions

The upper Zambezi River drains the Central Zambezian miombo woodlands, an ecoregion of dense forest woodland that bisects Africa directly south of the Congo Basin and East African savannas. These woodlands are dominated by trees of the subfamily Caesalpinioideae, particularly species belonging to the genera Brachystegia, Julbernardia, and Isoberlinia, which seldom occur outside miombo. In this ecoregion, mature miombo woodland trees are usually 15 to 20 meters high, with a broadleaf shrub and grass understory. Although trees here are mostly deciduous, this locale exhibits a richer fraction of evergreen tree species than drier Zambezian miombo, about 24 percent compared to nine percent in the Zimbabwean woodlands. Diverse in species, this ecoregion, mapped as wetter Zambezian miombo woodland, includes virtually all miombo dominants, including Brachystegia floribunda, B. glaberrima, B. taxifolia, B. wangermeeana, Marquesia macroura, Julbernadia globiflora, J. paniculata, and Isoberlinia angolensis.

caption Zambezian and mopane woodlands (in yellow). Source: World Wildlife Fund The Zambezian and mopane woodlands occupy a considerable fraction of the lower Zambezi catchment. These mopane woodlands are dominant in the ecoregion. Mopane often forms pure stands to the exclusion of other species, but is generally associated with several other prominent trees and shrubs, such as Kirkia acuminata, Dalbergia melanoxylon, Adansonia digitata, Combretum apiculatum, C. imberbe, Acacia nigrescens, Cissus cornifolia, and Commiphora spp.

The ecoregion nearest the Zambezi River mouth is the Zambezian coastal flooded savanna.  The plantlife of this wetland ecoregion contains both open grassland-dominated communities and mixed freshwater swamp forests. Dominant grass genera of seasonally flooded clayey depressions (tandos) include Hyparhenia, Ischaemum and Setaria, while species such as Panicum curatellifolia, Uapaca nitida and Syzigium guineense are common woody species of the ecoregion. The swamp forest component, including Barringtonia racemosa, Ficus verruculosa and Phoenix reclinata, generally borders rivers, lakes, and lagoons within this ecoregion. In more permanently waterlogged areas, reed swamps characterized by Phragmites australis and Typha capensis predominate, being replaced by mixed herbaceous and grass swamps under drier conditions. Associated vegetation types are numerous and include Borassus palm savanna, mangrove and dune forests as well as patches of mopane and miombo woodlands.

Prehistory

Stone age artifacts began to be recovered along the Zambezi, especially near Victoria Falls, since the 1930s. Along the Zambezi River in areas of present day Zimbabwe, conditions suitable to early sedentary agriculture were met by sufficient water availability and arable soils. Traces of such early sedentary farming lifestyles exist in the archaeological record dating to the first millennium AD, considered the Iron Age in this region, even though metal-working was a small part of the technology of these early cultures.. The majority of the land in this reach is mopane woodland underlain with the dominant grass Heteropogon contortus. Much of this area was considered only marginally suitable for agriculture in prehistorical times, due to the known high prevalence of tsetse fly populations, particularly unsuitable for livestock.

References

  • Zambezi River Authority (ZRA), Transboundary Issues on Sustainable Hydropower Development in the Zambezi River Basin in the Eyes of the Zambezi River Authority (September 2008)
  • B.Campbell, P.Frost and N.Byron. 1996. Miombo woodlands and their use: overview and key issues. pp. 1-10 in B. Campbell, editor. The Miombo in transition: woodlands and welfare in Africa. CFIOR, Bogor.
  • B.R.Davies. 1986. The Zambezi River system. in B.R.Davies & K.F.Walker (eds.) The Ecology of river systems. Dordrecht, Netherlands
  • K.M.Dunham. 1994. The effect of drought on the large mammal populations of Zambezi riverine woodlands, Journal of Zoology, v. 234, pp. 489–526
  • FishBase. 1999. Species in Zambezi
  • Mamdouh Shahin. 2002. Hydrology and water resources of Africa: Volume 1. Springer Publishers.  books.google.com 659 pages
  • E.Marshall and M.Maes. 1994. Small water bodies and their fisheries in southern Africa. books.google.comB. Food and Agriculture Organization of the United Nations. Committee for Inland Fisheries of Africa. 68 pages
  • G.Pwiti. 1996. Settlement and Subsistence of Prehistoric Farming Communities in the mid-Zambezi valley, northern Zimbabwe. South African Archaeological Bulletin. 51: 3-6
  • Rik Tjallingii. 2012. Sedimentary evolution and climate history of Zambezi River sediments since 44 ky before present. Nederlands Aardwetenschappelijk Congres
  • Bernard Wood. 2011. Wiley-Blackwell Encyclopedia of Human Evolution. 1264 pages - Google eBook
  • World Wildlife Fund. 2002. Zambezian coastal flooded savanna; Central Zambezian miombo woodlands.
Glossary

Citation

Hogan, C. (2012). Zambezi River. Retrieved from http://www.eoearth.org/view/article/51cbf2e97896bb431f6ab2de

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