Boundaries

This ecoregion encompasses the floodplains of the Okavango Delta. The ecoregion is situated in northern Botswana and includes one of few major inland endorheic deltas on the continent and one of the most important wetlands in southern Africa.

Topography

The Okavango River system, which has an average altitude of 900-1200 m, rises in the central highlands of Angola on the Benguela Plateau. It shares its watershed divide with the westward-flowing Cunene and Cuanza, and it flows south and east into the mainly arid Okavango basin to diverge into the many arms of the Okavango swamps (Beadle 1981).

Freshwater habitats

The Okavango Floodplains ecoregion contains the largest expanse of wetlands in southern Africa and supports a range of habitats and a rich bird fauna. The Okavango delta is one of the largest inland deltas in the world (Allanson et al. 1990). The permanent Okavango swamp covers 6,000-8,000 km2, but annual floods from the highlands of southern Angola inundate the floodplains, expanding the seasonally inundated area to 12,000-15,000 km2. Near the tributaries, the swamps are intersected with numerous deep, perennial channels, shallow floodwater channels, and backwaters or lagoons. Stony rapids are absent from the swamps and the water is clear throughout the year. The swamps in the delta vary in character mainly according to the perennial or intermittent occurrence of floodwaters. In the northwestern section of the delta, which is permanently flooded, the swamps are mostly covered with a dense growth of papyrus (Cyperus papyrus), stands of reeds (Phragmites), and bullrush (Typha). In the lower part of the delta, where the floodwater is more seasonal, the dry depressions are periodically filled with water and generally only have reeds and papyrus around the edges (Jubb & Gaigher 1971).

Terrestrial habitats

A region of thorn scrub / mopane (Colophospermum mopane) savanna is located in the north. It includes stretches of dense dry Baikiaea plurijuga forest, mopane and Pterocarpus woodland savanna, thorn scrub (Acacia/Sclerocarya) savanna, and semi-arid grassland. Forest savanna and dry woodland predominate in the drier southern portion of the ecoregion (Stuart et al. 1990).

Ecological phenomena

Lake Ngami in the Okavango Floodplains is noted for its global importance for wetland birds (Thieme et al. 2005). During times of flooding, more than one million birds have been documented at this site (Fishpool & Evans 2001).

Justification for delineation

This ecoregion is defined by the boundaries of the Okavango Basin, excluding the upper reaches of the river, which were historically connected with the river systems of the Upper Zambezi ecoregion [816]. The biogeographic affinities of the aquatic fauna of the Okavango are Zambezian, although it also shares a number of species with the southern tributaries of the Congo River, especially the Kasai. The faunal affinities of the region can be best explained by the theory that present day headwaters of the Congo and Cuanza were formerly part of the Okavango/Zambezi (Skelton 1994). As a result of tectonic faulting, the Okavango was impeded and deflected eastward to the Upper Zambezi and was ultimately captured by the middle Zambezi River (Beadle 1981). Skelton (1994) considers the Okavango to be a western-sector subunit of the Zambezian Province and to be the heart of the western Zambezian evolutionary arena.

Level of taxonomic exploration

Good. The state of ecological knowledge is reasonable, and the area is well known taxonomically. In June 2000, Conservation International completed a rapid ecological assessment of the delta (Alonso & Nordin 2003). In terms of biological investigation, fish assessments and investigations into how the system functions as a whole are needed (R. Bills, South African Institute of Aquatic Biodiversity, personal communication).