Boundaries

The ecoregion largely follows the boundaries of the catchment of the Kasai River, which encompasses a vast area (900,000 km²) within southwestern Democratic Republic of Congo (DRC) and the northeast corner of Angola. 

Topography

The headwaters of the Kasai River begin in a series of highland massifs along the northern rim of the Lunda Plateau (with sources at elevations between 1,000 and 1,500 m asl) (Hughes and Hughes 1992). The land tends to slope gently towards the central Congo basin, but as a result of numerous scarps located in the plateau, most of the rivers traverse falls and rapids before reaching the Cuvette Centrale (Marlier 1973; Hughes and Hughes 1992). The Cuvette Centrale is the low-lying portion of the Congo basin that once made up part of an ancient inland lake formed during the Pliocene (Beadle 1981).

Freshwater habitats

Tributaries to the Kasai originate at elevations up to 1,400 m asl in Angola and flow north into the DRC across a plateau of about 1,000 m asl. Most of the Kasai River’s rapids are located upstream from the city of Tshikapa at about 500 m asl (Kamdem-Toham et al. 2003). Above this elevation, the upper reaches of the Kasai and its tributaries are interrupted by falls and rapids along the edge of the Lunda plateau (Marlier 1973). The rivers flow off the plateau and eventually flow into the mainstem Kasai. The Kasai descends to about 300 m asl at its lowest point within this ecoregion and the mainstem river flows through the swamp forests of the central Congo Basin before entering the mainstem Congo River. The mainstem Kasai attains mean widths of 4 km for stretches of up to 700 km, and in its lower reaches it is studded with islands that become inundated in periods of high water. Major tributaries of the Kasai from east to west are the Kwango (Cuango), Wamba, Kwilu (Cuilo), Loange, Lulua, and Sankuru Rivers.

The Kasai has a flooding regime that is unimodal in its upper reaches and becomes bimodal approaching its junction with the Congo River. The annual amplitude of flooding is about 3 m in its lower reaches (Marlier 1973). Seasonally and permanently inundated swamp forests grow in the lowest part of all river valleys in the ecoregion, in strips 100 m to 10 km wide. For example, along the Kwilu River and its tributaries there are 11 major swamps with a total area of about 1,550 km². Near Sandoa, there is a large wetland of 4,500 km² associated with the Lulua River (Hughes and Hughes 1992).

Terrestrial habitats

With the exception of swamp forests in the lowermost river reaches, the vegetation of the ecoregion is primarily savanna with gallery forests lining the river valleys. A Julbernardia-Brachystegia complex dominates the vegetation of savannas in the south. Gallery forests are tall and dense and include species such as Anthocleista vogelii, Mitragyna ciliata, and Phoenix reclinata (Hughes and Hughes 1992). These forests become more extensive as the Kasai flows towards the central Congo basin. In addition to gallery forests, extensive peat bogs exist in some valleys and shallow depressions adjacent to the rivers. Permanent swamp forests include the oil palm Uapaca guineensis, and in deep narrow valleys and ravines, Mitragyna stipulosa and Sarcophrynium schweinfurthianum dominate swamp forests directly adjacent to streams (Hughes and Hughes 1992).

Description of endemic fishes

As an example, Angolan tributaries on the Lunda Plateau harbor an impressive array of endemic fishes, including the dwarf distichodontid Dundocharax bidentatus and one or more poeciliids, cyprinids, mormyrids and cichlids (Poll 1967). The Fwa River, a short (20 km) tributary of the Lubi located in the middle of the ecoregion, also possesses five endemic cichlid species. Two of the endemic cichlids (Cyclopharynx fwae and C. schwetzi) belong to a genus only found in the Fwa (Roberts and Kullander 1994). The Fwa’s stable environment might have facilitated speciation.

Other noteworthy fishes

In rapids of the lower Kasai River, specialised fish assemblages known only from a few historical specimens include the cichlids Steatocranus rouxi and Teleogramma monogramma. Interestingly, these taxa often have their closest relatives in Pool Malebo and the rapids of the Lower Congo, but not in other parts of the Kasai drainage. 

Justification for delineation

The ecoregion largely follows the boundaries of the catchment of the Kasai River. The many falls and rapids along the course of the Kasai River and its tributaries restrict the movement of fish. This may explain the high level of endemism in this ecoregion (Beadle 1981). Rapids have prevented the dispersal of species to the lower reaches of rivers as well as headwater colonization by potentially competing species (Lévêque 1997). A few of the uppermost tributaries of the Kasai are included within the Zambezian Headwaters ecoregion [555] due to their historic connection with the rivers of that ecoregion and the Zambezian affinity of their fauna (Poll 1967).

Level of taxonomic exploration

Poor