Boundaries

Situated within southern Namibia and northern South Africa, this ecoregion encompasses the western Orange River in its 1,200-km course downstream of its confluence with the Vaal River (Allanson et al. 1990). 

Freshwater habitats

The Orange River, like many South African rivers, has a large sediment load, especially during floods. This is partly a natural feature of draining a semi-arid region, and partly a consequence of poor land management and subsequent erosion. The foothills of Lesotho, central farmlands, and dunes of the Kalahari Desert all contribute sediment to the river (O\'Keeffe et al. 1992). The water is turbid and alkaline, with moderate amounts of dissolved solids (Agnew 1986). The Orange is a highly regulated river, with a series of major impoundments along its upper course, and one, Lake Boegoeberg, on its lower course (Allanson et al. 1990). The Augrabies Falls, at about 620 km from the mouth of the Orange River, is the only natural barrier to the movement of fish.

Most of the western Orange River lies below 1,000 m, while the lower section forming the boundary between South Africa and Namibia lies below 350 m. This downstream section of the river has only one tributary, the Fish River entering from the north, and there is therefore little further addition to the flow (Alexander 1985; Agnew 1986; Allanson et al. 1990). Littoral sands cover the coast near Alexander Bay. The large discharge of water through the mouth limits tidal exchange and causes most mixing to occur at sea; thus limiting the extent of estuarine wetland areas (Cambray et al. 1986).

Terrestrial habitats

Along the upper part of the western Orange, from Buchuberg to Kakamas, there are rich strips of alluvial soil on both banks, deposited by summer floods of the river over many millennia. These floodplains are no longer inundated by seasonal floods, but are irrigated by water abstracted from the Orange and transported via a network of canals and furrows (Midgley et al. 1994). Vegetation is classified as Karoo and Karoo shrub in the eastern portion of the ecoregion.

The western portion of the ecoregion is classified as desert and semi-desert with succulent steppe vegetation (Stuart et al. 1990). Below Augrabies Falls, the Orange flows through increasingly arid areas with arenosols, lithosols, and weakly developed shallow soils (Cooke 1964; Cambray et al. 1986; Stuart et al. 1990). Below the falls, the 620 km reach has a low gradient with tree-lined banks and few submerged aquatic macrophytes (Curtis et al. 1998).

Description of endemic fishes

The western Orange River contains some endemic species, including three crustaceans and one fish, but it generally supports an impoverished freshwater fauna. However, compared to the upper Orange, the section of river included within this ecoregion has a more distinctive biota (Agnew 1986; Skelton et al. 1995; Curtis et al. 1998).

The Namaqua barb (Barbus hospes) is considered rare and is endemic to the lower Orange River below the Augrabies Falls, and an isolated population of Mesobola brevianalis (the southernmost neoboline cyprinid population in Africa) is also found in this stretch of river. The Orange-Vaal river endemic Austroglanis sclateri is rare and listed as data-deficient on the Red List (IUCN 2002).  The channel-like nature of the gorge below the falls reduces habitat diversity to open flowing water interspersed with rocky rapids. Such conditions are likely to have been the major forces directing the anatomical adaptation of these two species to streamlined, active swimming forms (Skelton 1993).

Justification for delineation

This ecoregion is defined by the mainstem of the Orange River and its tributaries below the confluence of the Orange and Vaal rivers. The Orange River is the northern limit of the southern temperate (Cape) ichthyofauna and the southern limit of the tropical (Zambezian) fauna of southern Africa (Skelton 1986, 1994). Zambezian fish species in the Orange River include Barbus trimaculatus, Clarias gariepinus, Barbus paludinosus, Mesobola brevianalis, and Tilapia sparrmanii. The river is thought to have once flowed west to the Atlantic in the vicinity of the present-day Olifants River mouth, before the Orange River was captured by the Koa tributary of the Lower Orange to isolate the Olifants River system (Skelton 1986; Allanson et al. 1990; Skelton 1994).

Level of taxonomic exploration

Fair