Abstract
The Okavango Delta in Northern Botswana as well as its drainage basin in Namibia and Angola is located in the Kalahari sand basin. It is an alluvial fan where fine sand brought in from upstream settle in the maze of branched streams that are filled up forcing abandonment and formation of new river channels. This gives the Delta a slightly convex shape but the gradients are very small at around 1:3,000. Although almost endorheic, the Delta is a freshwater system. This is caused by removal through evaporative concentration and precipitation of Ca-Mg-bicarbonates and silica complexes in vegetated islands into which stream water is flowing. It causes the creation and growth of more than 150,000 islands that usually start as termite mounds. The remaining water with more soluble ions, mainly Na and Cl, flows into the centre of islands, concentrate further until the density is becoming critically high when the water sinks down into deep groundwater due to a density fingering process.
An annual flood pulse arrives to the Delta in March-April caused by rains in the Angola highlands 4–5 months earlier. This causes a seasonal maximum flooding of 14,000 km2 with large variations between years. Over historic times 28,000 km2 has been flooded and this area is usually defined as the Okavango Delta. The Okavango fan is however 40,000 km2 and was created over longer geologic time.
The dynamic geologic-hydrologic features create a wetland landscape under constant change with phases of flooding and desiccation from annual to intervals of decades and with all spatial scales from a few square meters to hundreds of km2. The specific habitat units of which there are 500,000 in the Delta have however a mean 0.05 km2. The inflowing water and the Delta itself is very nutrient poor caused by the substrate of well withered sand; and the nutrients, nitrogen and phosphorus, are mainly kept in organic matter dead and alive. Both flooding and desiccation cause therefore a release of nutrients with sudden patchy outbursts of biological productivity, recognized in the aquatic phase by the congregation of fish eating birds and in the drying phase by flood plains grazers in large numbers. A bottle neck nutritious resource is provided during the dry winter season June - August causing a grazers biomass 10 times higher than in similar savanna systems. Many of the mammal species have adopted their reproductive cycle to this unusual seasonality but there are no endemic species. The biodiversity is however fairly high caused by the dry-wet mix of habitat types.
Recently large-scale irrigation has started in the drainage basin in Angola and Namibia. This may lead to eutrophication, blue-green algal blooms, growth of denser reed-beds and expansion of the floating weed Salvinia molesta all with far reaching consequences for biodiversity of the fauna. It is however not unlikely that the two unique and vital processes of flood switching and desalination will be hampered as well. If so, a destruction of the Delta in its present form is likely.
References
Bauer P. Flooding and salt transport in the Okavango Delta, Botswana: key issues for sustainable wetland management [dissertation]. Zurich: Swiss Federal Institute of Technology; 2004.
Bonyongo MC. The ecology of large herbivores in the Okavango Delta, Botswana [dissertation]. Bristol: School of Biological Sciences, University of Bristol; 2004.
Cotterill F. Reducine antelopes of the Zambezi basin. In: Timberlake J, editor. Biodiversity of the Zambezi wetlands. Famona, Bulawayo: Biodiversity Foundation for Africa; 1998. p. 145–99.
Gumbricht T, McCarthy TS. Spatial patterns of islands and salt crusts in the Okavango Delta, Botswana. S Afr Geogr J. 2003;85:164–9.
Gumbricht T, McCarthy J, McCarthy TS. Channels, wetlands and islands in the Okavango Delta, Botswana, and their relation to hydrological and sedimentological processes. Earth Surf Process Landf. 2004;29:15–29.
Heinl M, Frost P, VanderPost C, Sliva J. Fire activity on drylands and floodplains in the Southern Okavango Delta, Botswana. J Arid Environ. 2007;68:77–87.
Hogberg P, Lindholm M, Ramberg L, Hessen DO. Aquatic food web dynamics on a floodplain in the Okavango Delta, Botswana. Hydrobiologia. 2002;470:23–30.
Huntsman-Mapila P, Ringrose S, Mackay AW, Downey WS, Modisi M, Coetzee SH, Tiercelin J-J, Kampunzu AB, VanderPost C. Use of geochemical and biological sedimentary record in establishing palaeo-environments and climate change in the Lake Ngami basin, NW Botswana. Quat Int. 2006;148:51–64.
Junk WJ, Brown M, Campbell IC, Finlayson M, Gopal B, Ramberg L, Warner BG. The comparative biodiversity of seven important wetlands: a synthesis. Aquat Sci. 2006;68:400–14.
Lindholm M, Hessen DO, Mosepele K, Wolski P. Food webs and energy fluxes on a seasonal floodplain: the influence of flood size. Wetlands. 2007;27:775–84.
McCarthy TS. The Okavango Delta and its place in the geomorphological evolution of southern Africa. S Afr J Geol. 2013;116:1–54.
McCarthy TS, Ellery WN, Ellery K. Vegetation-induced, subsurface precipitation of carbonate as an aggradational process in the permanent swamps of the Okavango (delta) fan, Botswana. Chem Geol. 1993;107:111–31.
Mendelsohn J, El Obeid S. Okavango River. The flow of a lifeline. Cape Town: Struik Publishers; 2004.
Mendelsohn J, VanderPost C, Ramberg L, Murray-Hudson M, Wolski P, Mosepele K. Okavango Delta: floods of life. Windhoek: RAISON (Research & Information Services of Namibia); 2010.
Mitsch WJ, Nahlik A, Wolski P, Bernal B, Zhang L, Ramberg L. Tropical wetlands: seasonal hydrologic pulsing, carbon sequestration and methane emissions. Wetl Ecol Manag. 2010;18:573–86.
Murray-Hudson M, Wolski P, Brown M, Kashe K. Disaggregating hydroperiod: components of the seasonal flood pulse as drivers of plant species distribution in floodplains of a tropical wetland. Wetland. 2014;34:927–42.
Ramberg L. A pipeline from the Okavango River? Ambio. 1997;26:129.
Ramberg L, Wolski P. Growing islands and sinking solutes: processes maintaining the endorheic Okavango Delta as a freshwater system. Plant Ecol. 2008;196:215–31.
Ramberg L, Wolski P, Krah M. Water balance and infiltration in a seasonal floodplain in the Okavango Delta, Botswana. Wetlands. 2006a;26:677–90.
Ramberg L, Hancock P, Lindholm M, Meyer T, Ringrose S, Sliva J, Van As J, VanderPost C. Species diversity of the Okavango Delta, Botswana. Aquat Sci. 2006b;68:310–37.
Ramberg L, Lindholm M, Hessen DO, Murray-Hudson M, Bonyongo C, Heinl M, Masamba W, VanderPost C, Wolski P. Aquatic ecosystem responses to fire and flood size in the Okavango Delta: observations from seasonal floodplains. Wetl Ecol Manag. 2010;18:587–95.
Siziba N, Chimbari MJ, Mosepele K, Masundire H. Spatial and temporal variations in densities of small fishes across different temporary floodplain types of the lower Okavango Delta, Botswana. Afr J Aquat Sci. 2011;36:309–20.
Stanistreet IG, McCarthy TS. The Okavango Fan and the classification of subaerial fan systems. Sediment Geol. 1993;85:115–33.
White F. The vegetation of Africa. A descriptive memoir to accompany the UNESCO/AETFAT/UNSO vegetation map of Africa. Paris: Unesco; 1983. 336 p.
Wolski P, Ramberg L, Magole L, Mazvimavi D. Evolution of River Basin management in the Okavango system, Southern Africa. In: Ferrier RC, Jenkins A, editors. Handbook of catchment management. Blackwell Publishing Ltd: John Wiley & Sons Ltd…West Sussex UK; 2010. p. 457–75.
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Ramberg, L. (2018). Okavango Delta, Botswana (Southern Africa). In: Finlayson, C., Milton, G., Prentice, R., Davidson, N. (eds) The Wetland Book. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-4001-3_104
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DOI: https://doi.org/10.1007/978-94-007-4001-3_104
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