Boundaries

Located within southwestern Zambia, southeastern Angola, the Caprivi Strip of Namibia, and the northern edge of Botswana, the Upper Zambezi Floodplains ecoregion is defined by the floodplains of the Zambezi and Cuando Rivers above Victoria Falls. The ecoregion begins at the confluence of the Lungwebungu, Zambezi, and Kabompo rivers. The Zambezi River then flows south through the Barotse floodplains and turns east at the Caprivi Strip before descending the 98 m high Victoria Falls above the Batoka gorge. The falls form an impassable barrier between the middle and upper portions of the Zambezi River (Davies 1986).

Freshwater habitats

Along the upper Zambezi long stretches of low-gradient river with associated floodplains and swamps alternate with extensive rapids. Rapids are present in several areas between Nangweshi and Katima Mulilo (176km), and from Mombava to the Victoria Falls (72km) (Roberts 1975). The Gonye Falls about 300 km upstream from Victoria Falls has a maximum height of 21 m and is the only waterfall, other than Victoria Falls, on the main river that is high enough to constitute a barrier to the movement of fish in the dry season (Bell-Cross 1972).

Regular flooding events inundate the extensive floodplains and swamps of this ecoregion and are essential to maintaining an abundant aquatic flora and fauna. Floodplain grasses within the wetter areas include Acroceras macrum, Brachiaria arrecta, Digitaria sp., Echinochloa pyramidalis, and Oryza longistaminata, among many others. In deeper channels and lagoons, the reed Phragmites mauritianus is the most abundant species. Vegetation stands also include Cyperus papyrus, Hibiscus diversifolius, Urena lobata, Persicaria senegalensis, Aeschynomne uniflora, and the swamp grasses Echinochloa pyramidalis, E. stagnina, and Vossia cuspidata. The floodplains are inundated for long periods, while low flows are of relatively short duration.

The Upper Zambezi is an important ‘reservoir river’, seldom exhibiting large variations in height because of its gentle slope and the fact that floodwaters are not confined to a channel, but spread out across the vast floodplains. The average flood height of the river is 5.2 m (Jackson 1961, 1963; van der Waal & Skelton 1984).

Three floodplains are of major importance for the ecoregion’s freshwater biodiversity. Along the Zambezi the Barotse (or central Barotse) floodplain extends from Lukulu in the north to Nangweshi in the south, and is approximately 240 km long and up to 35 km wide. In flood, it covers an area of about 7,500 km². The eastern Caprivi (or southern Barotse) floodplain is approximately 100 km in length and is located between Sesheke and Maramba (Hughes & Hughes 1992). Marginal vegetation is abundant and provides cover for small fish and juveniles. The eastern Caprivi floodplain is contiguous with the eastern portion of the Chobe-Linyanti floodplain system, which begins at the point where the Cuando River enters Botswana (van der Waal & Skelton 1984). The Chobe flows in an east-north-easterly direction to join the Zambezi, at which point the Chobe floodplain system becomes part of the southern Barotse floodplain. The Linyanti swamp is about 300 km² in area, but its size varies according to the extent of flooding in the upper Zambezi (Marshall 2000). The floods in the Linyanti system are dampened by the moderating effect of the vast Phragmites-Typha-Cyperus swamps in the river valley, delaying the flood peak in the Linyanti until August. Lake Liambezi, which periodically dries up, lies at the end of the Linyanti swamp, has an open water surface of 100 km² when full, and is bordered by a swamp of 200 km² (van der Waal & Skelton 1984). Total estimates of the extent of the swamp and floodplain in the east Caprivi area vary from 250 km² to over 1,000 km², and from 1,670 to 1,870 km², respectively (Timberlake 2000a). The water is nutrient-poor and unproductive since the rivers flow over belts of Kalahari Sands and constantly change course owing to erosion and deposition.

Terrestrial habitats

The terrestrial vegetation of the ecoregion is predominantly miombo (Brachystegia/Julbernardia) woodland, with extensive wetlands and floodplains around the Zambezi River and important grasslands on the Liuwa plain. The Barotse floodplain is clearly separated from the surrounding woodland, whereas in eastern Caprivi the floodplains form a mosaic with the woodlands (Timberlake 2000a). Extensive and stable seasonally-flooded grasslands grow on the Barotse floodplains, along with some swamp vegetation. The Cuando (Kwando or Mashi) River is bordered by floodplain and riparian woodland (Timberlake 2000a).

Description of endemic fishes

The ecoregion contains several threatened species of fish. Clariallabes platyprosopos has an extremely restricted range, with two of its four known localities occurring in this ecoregion at Katima Mulilo and Impalila. The striped killifish Nothobranchius sp., which is endangered, is known from two small rainwater pans at Gunkwe and Bunkalo in the Caprivi Strip (Marshall 2000).

Other noteworthy fishes

The ecoregion contains several threatened species of fish. Clariallabes platyprosopos has an extremely restricted range, with two of its four known localities occurring in this ecoregion at Katima Mulilo and Impalila. The striped killifish Nothobranchius sp., which is endangered, is known from two small rainwater pans at Gunkwe and Bunkalo in the Caprivi Strip (Marshall 2000).

Ecological phenomena

The floodplain fishes are adapted to migrate to and from the floodplain with the flood cycle beginning with the first floods each year, in November/December. Floodplain spawning is advantageous to juvenile fish because of the abundant food, well-oxygenated conditions, and cover from predators. Fish on the floodplains exhibit a wide variety of life histories and behavioral patterns (Bowmaker et al. 1978). Fish migration is species-specific; some fish exhibit seasonal migratory behavior, using the floodplain temporarily in the wet season, while others are semi-permanent residents, depending on the availability of water (Winemiller 1991; Winemiller & Kelso-Winemiller 1994; Merron & Bruton 1995; van der Waal 1996; Winemiller & Kelso-Winemiller 1996).

Justification for delineation

The floodplain fauna of the Upper Zambezi Floodplains ecoregion has similarities with the floodplain faunas of the Okavango [12] and Kafue [8] (Skelton 1993; Marshall 2000). The biogeographic affinities of Zambezian fish could best be explained as a result of a previous separation of the Upper Zambezi basin from the Middle/Lower Zambezi with consequent different origins of much of their faunas. Prior to the Pleistocene, it is postulated that the Upper Zambezi may have constituted a common faunal arena that probably drained first into the Atlantic via the southwest and later into the Indian Ocean via the Limpopo valley. The Upper Zambezi was later captured by the headwaters of the Middle Zambezi and changed direction to flow northeast to reach the Indian Ocean via the Lower Zambezi system (Bell-Cross 1972; Beadle 1981; Skelton 1994; Marshall 2000).

Level of taxonomic exploration

Fair. The quality of ecological data is moderate, but more research is needed in all aspects pertaining to the ecology of this ecoregion and the impacts of fisheries activities. The report Biodiversity of the Zambezi Basin Wetlands provides a comprehensive summary of current knowledge of biodiversity in the basin (Timberlake 2000b).