Okavango Delta

• Category: Inland Aquatic Biomes.
• Geographic Location: Southern Africa.
• Summary: One of the world’s largest inland deltas is surrounded by desert and is an indispensable seasonal home to savanna wildlife.

The Okavango Delta is located in the northwest of the Kalahari Desert, in the Republic of Botswana (district Ngamiland). It is one of the largest sites in the Ramsar list of Wetlands of International Importance, although size estimates vary greatly among authors. Gumbricht and collaborators estimate the total area, including permanently dry islands, to be 11,000 square miles (28,000 square kilometers). The delta is situated in an endorheic basin, meaning the drainage of the river is closed and landlocked, rather than flowing into a lake or ocean.

The two main tributaries of the Okavango River, the Quito and Cubango, both rise in the central part of the neighboring state Angola, draining catchments of 25,000 and 44,000 square miles (65,000 and 115,000 square kilometers), respectively. They flow through the Caprivi strip (of neighboring Namibia) into the so-called Panhandle and subsequently into the delta, which spreads out among three major fault lines: the Gumare fault in the northwest, and the Kunyere and Thamalakane faults in the southeast. These lines of weakness in the basement rock represent an extension of the East African rift system. Between them, the crust of the Earth has collapsed, and by subsequent deposition of Okavango river sediments into this depression, a gently sloping, slightly conical alluvial fan has developed. McCarthy and collaborators determined a gradient of 1:5,570 in the Panhandle region, and a gradient of 1:3,400 on the delta.

Seasonality and Volume

The dry season in Botswana lasts usually from May until October. The climate is semiarid, with potential evapotranspiration exceeding rainfall in all months of the year. Both in the Angolan catchment and the delta, rainfall usually occurs from December to February and peaks in January. Inflow into the Panhandle at Mohembo peaks four months later, in April, and peak outflow from the delta into the Thamalakane River at Maun occurs in August.

The distance between Mohembo and Maun is only about 100 miles (250 kilometers), but the flood wave spreads outward as it leaves the confinement of the Panhandle, and generally proceeds slowly due to the low topographic gradient and relief. Authors also disagree on the amount of surface inflow, and indeed rainfall and water discharge vary greatly among years, decades, and in space. Ellery and collaborators give a mean figure of 14 billion cubic yards (11 billion cubic meters) per year for total inflow via the Okavango, and 6.5 billion cubic yards (5 billion cubic meters) per year of summer rainfall over the Okavango Delta.

Most of the river system drains terrain covered by Kalahari sand. Although the concentration of dissolved solids—mostly silica and calcium and magnesium bicarbonates—is very low, the sheer volume of water introduced ensures that they are found in considerable quantities. Considering that more than 96 percent of inflow plus rainfall is lost to the atmosphere by evapotranspiration, while only 2 percent leaves the delta as surface outflow and presumably less than 2 percent exists as subsurface outflow, it is remarkable that the system maintains drinking water quality without human intervention.

Two ecological processes probably account for this phenomenon: Continuous spatial and temporal shifts in flooding regimes assist in the regeneration of soils and water bodies, and a multitude of islands develop continuously. Initially, any elevation above the level of flooding, such as termite mounds, enables growth of woody plants. Dissolved salts are either taken up by these woody plants via transpiration, or else precipitate as silica and carbonates (calcite) in the soil—resulting in vertical and lateral expansion of islands—or concentrate in the groundwater beneath the island center. Because the water table is generally beneath that of the surrounding floodplain, no lateral movement of salt water occurs away from the island.

Three Zones

Ecologists broadly subdivide the delta into three zones: The panhandle comprises a meandering river system flanked by permanent swamps, dominated by up to 13-foot-high (four-meter-high) stands of papyrus sedge (Cyperus papyrus). Further vegetation types which receive and require a very steady water supply predominate in the panhandle, the delta apex, and northeast of Chief’s Island. Frequently flooded communities are characterised by the sedge Schoenoplectus corymbosus and wireleaf daba grass (Miscanthus junceus). Open water and permanent swamp communities contain, besides papyrus, fully or partly submerged plants such as common hornwort (Ceratophyllum demersum) and hippo grass (Vossia cuspidata).

Seasonal or temporary floodplains are characterised by a multitude of islands. The largest of these, several thousand square feet (several hundred square meters) in extent, are believed to be of tectonic origin, whereas smaller islands develop as described above. Shrubs, trees, and the Makalani fan palm (Hyphaene petersiana) grow on the highest elevations in older islands, often on a fringe around the islands, while the center of the islands turns barren, or supports only few salt-tolerant plant species. Between these islands and the main floodplain lies a grass-dominated zone, characterized by grazing-tolerant species such as Bermuda grass (Cynodon dactylon).

Wildlife and Human Needs

With 1,300 species of plants, 71 of fish, 33 of amphibians, 64 of reptiles, 444 of birds, and 122 of mammals identified so far, Ramberg and collaborators consider the species diversity of the Okavango Delta “normal for the southern African region.” Its great importance for wildlife lies in the extent of its water bodies, and the fact that these have their peak water level during the dry season. Unfortunately in this regard, Ngamiland was furnished with a cross-pattern of veterinary fences from the late 1950s onward in order to avoid contact and possible spread of diseases between wildlife and domestic herds, beef production being one of the mainstays of the economy of Botswana. Some of these fences have posed a severe threat to wildlife, obstructing migratory routes and access to the water resources of the delta.

There is also a constant demand for the water resources of the delta by humans. Various plans for extensive water extraction not only from the Botswana section of the Okavango system, but also from the tributary rivers in Angola and Namibia were proposed and, partly as a consequence of national and international impact assessments, shelved. The Permanent Okavango River Basin Commission (OKACOM), a joint commission of the three countries Angola, Namibia, and Botswana, approved of a Transboundary Diagnostic Analysis, which will serve as a basis for a joint management plan for the whole river basin.

Another threat that began in the 2020s is mining operations. ReconAfrica began drilling for oil and gas in the Namibian area of the Kavango Zambezi Transfrontier Conservation Area (KAVA), the conservation zone comprising Angola, Botswana, Namibia, Zambia, and Zimbabwe, in 2021. Drilling for oil was planned in Botswana. Environmentalists warned of the threat increased traffic and oil wells posed to the waters of the Kavango Basin.

Climate change linked to human activity is a looming threat to the area. Drought and fire are two of the biggest problems facing the region, made more intense by the effects of climate change. Historically, the rains that feed the area varied. However, sustained drought has characterized the region and negatively impacted wildlife. Higher temperatures have also caused increased evaporation, further creating a water shortage.

Bibliography

Alonso, Leeanne E. and Lee-Ann Nordin, eds. “A Rapid Biological Assessment of the Aquatic Ecosystems of the Okavango Delta, Botswana: High Water Survey.” RAP Bulletin of Biological Assessment 27 (2003).

Ellery, William N., Terence S. McCarthy, and J. M. Dangerfield. “Biotic Factors in Mima Mound Development: Evidence From the Floodplains of the Okavango Delta, Botswana.” International Journal of Ecology and Environmental Sciences 24, nos. 2–3 (1998).

Gheorghiu, Andy. "Threatening Africa's Eden--Oil and Gas Plans Loom Over the World's Largest Nature Conservation Area." Energy Transition, 12 Oct. 2021, energytransition.org/2021/10/threatening-africas-eden-oil-and-gas-plans-loom-over-the-worlds-largest-nature-conservation-area/. Accessed 28 Nov. 2024.

Gumbricht, Thomas, Jenny McCarthy, and Terence S. McCarthy. “Channels, Wetlands and Islands in the Okavango Delta, Botswana, and Their Relation to Hydrological and Sedimentological Processes.” Earth Surface Processes and Landforms 29, no. 1 (2004).

McCarthy, Terence S., M. Barry, A. Bloem, and William N. Ellery. “The Gradient of the Okavango Fan, Botswana, and its Sedimentological and Tectonic Implications.” Journal of African Earth Sciences 24, nos. 1–2 (1997).

Moses, Oliver, Ross C. Blamey, and Chris J. C. Reason. “Drought Metrics and Temperature Extremes Over the Okavango River Basin, Southern Africa, and Links with the Botswana High.” International Journal of Climatology, vol. 43, no. 14, 6463-83, 2023, doi.org/10.1002/joc.8215. Accessed 28 Nov. 2024.

Ramberg, Lars, Peter Hancock, Markus Lindholm, Thoralf Meyer, Susan Ringrose, Jan Sliva, Jo Van As, and Cornelis VanderPost. “Species Diversity of the Okavango Delta, Botswana.” Aquatic Sciences 68, no. 3 (2006).