Boundaries

The name of this ecoregion comes from the two main rivers that define it: the Middle Zambezi and the Luangwa. The ecoregion covers parts of Zambia, northern Zimbabwe, and western Mozambique; it extends from Victoria Falls to the Cahora Bassa Gorge on the Zambezi, and from the headwaters of the Luangwa River to its confluence with the Zambezi.

Freshwater habitats

The damming of the Zambezi River led to the creation of Lakes Kariba (5,364 km²) and Cahora Bassa (2,670 km²) and changed the character of the Middle Zambezi. The regulation of river flow, a reduction in flood volume (Magadza 2000), and the deposition of sediments in the reservoirs have changed downstream habitats whereas the creation of extensive new limnetic habitats has changed the composition of the fish populations (Timberlake 1997; Davies et al. 2000). Some specialised rheophilic forms such as the cyprinids Opsaridium zambezense and Barbus marequensis have disappeared completely from the area flooded by Lake Kariba while others, such as the labeos and distichodids that were formerly the dominant species in the river, have declined (Marshall 2000). Aquatic weeds, particularly water hyacinth(Eichhornia crassipes), water fern(Salvinia molesta) and water lettuce(Pistia stratiotes) proliferated soon after the lakes were created but their populations decreased as the lakes matured and they are now only of nuisance value. A small area of alluvial terraces exists in the Mana Pools region between the two dams (Hughes & Hughes 1992) and is apparently subject to increased lateral erosion as a result of the lack of sediment being carried to it by the Zambezi River (Guy 1980).

The Luangwa River, with a catchment area of 165,000 km², experiences flash flooding (Kasimona & Makwaya 1995), with the river rising rapidly to full flood during heavy rains. Important riverine habitats include oxbow lagoons, dambos, and the riparian fringe, the latter providing important habitat for terrestrial species. Seasonal flooding maintains this diversity of habitats. The river is very turbid during this period and the clay content of its water is apparently the cause of the very high clay content of the water in Lake Cahora Bassa, into which it discharges.

Terrestrial habitats

Both the middle Zambezi and Luangwa valleys are relatively fertile compared to the upper Zambezi (which mostly consists of nutrient-poor Kalahari sands) because they drain a geologically complex area with ancient volcanic rocks, metamorphosed sediments and relatively recent sedimentary systems as well as alluvial soils (Jachmann 2000). The natural vegetation gradually shifts from pure stands of the tree Colophospermum mopane that dominate the valley floor to miombo woodlands that stretch up the escarpments onto the plateaus to the north and south. Riparian forests line the rivers while open savannas, with Faidherbia albida being an important and conspicuous species, dominate the vegetation on the alluvial soils (Wild & Barboza 1967; Jachmann 2000).

Justification for delineation

The origins of the Zambezi system are complex, but it is likely that the Upper Zambezi was once separate and probably joined the Limpopo (Skelton 1994). It was captured during the mid-Pleistocene when the middle Zambezi cut back through the Batoka Gorge, a long (>100 km), east-west fissure through basalt, at the head of which is Victoria Falls, where erosion through similar limestone-filled fissures continues (Davies 1986). The Victoria Falls is presently the western boundary of the middle Zambezi system, separating it from the upper Zambezi. 

Evidence from the distributions of fish (Jackson 1986) and other freshwater animals such as the Odonata (Pinhey 1978) support this hypothesis about the Zambezi system. A striking example is the occurrence of nine Synodontis species, seven of which are found only in the Upper Zambezi and the other two only in the middle and lower sections. The fish fauna of the Middle Zambezi has several Zambian/Congo elements not found in the Upper Zambezi, such as two Distichodus species, Mormyrops anguilloides, Mormyrus longirostris, Heterobranchus longifilis, Malapterurus shirensis, and Protopterus annectens. In addition, the Middle Zambezi has some east coast species suggesting that the Middle Zambezi formed part of an eastern drainage basin that included the lower Zambezi, Lower Kafue, Luangwa, and Shire Rivers (Bell-Cross & Minshull 1988).

Level of taxonomic exploration

Fair. Most scientific investigations carried out in this area have been related to Lakes Kariba and Cahora Bassa; the former is particularly well-studied (see bibliography in Timberlake 1997, 2000)(Timberlake 1997; Timberlake 2000a). Almost nothing has been done on fish or other aquatic organisms in the riverine sections of the mid-Zambezi either above or below Kariba, or in the Luangwa. Limited studies on the Luangwa River and its associated floodplains have focused largely on larger mammals (Ansell 1965; Berry 1973a, 1973b) and birds (Feely 1964; Dowsett 1971; Scott 1993). Dunham (Dunham. K.M. 1989; Dunham 1990; Dunham 1991a; Dunham 1991b; Dunham 1994) (1989; 1991b; 1991a; 1994) has worked on the ecology of the Mana Pools area.