Okavango Delta, Botswana (Southern Africa)
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.
KeywordsOkavango Delta Southern Africa Alluvial fan Seasonal flood pulse River avulsion Flood switching Desalination Island growth Density fingering Carbon sequestering Biological productivity Biodiversity Irrigation threats
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