Boundaries

The ecoregion lies in the southern portion of Central African Republic (CAR), and includes the western boundary of the Democratic Republic of Congo (DRC) along the Oubangui and Congo Rivers, and the western border of the Republic of Congo (ROC). The northern border of this ecoregion marks the divide between the Congo River and Lake Chad basins and the eastern border is the divide between the Congo and Nile basins. The ecoregion encompasses the entirety of the Oubangui basin (with the exception of the Uelé basin [Ecoregion 874]) and a portion of the mainstem Congo River from its confluence with the Oubangui downstream to Malebo Pool. Located between major river basins, the Oubangui’s freshwater fauna is considered to be transitional between the Nilo-Sudanian and Congolian faunas (Roberts 1973; Bailey 1986; Hughes & Hughes 1992). 

Freshwater habitats

The headwaters of the Oubangui River begin in the Central African Republic. The Chinko River and its affluent the Vivado, the Ouarra and its affluent the Goangoa, the Kerre, and the Mbokou rivers all begin in the far southeastern corner of CAR and flow down gentle slopes to join the Mbomou River. Floodplains are abundant in this section of the basin and follow the riverbanks for a total of over 1,320 km along these rivers, inundating more than 6,500 km² of land during the wet season. The Mbomou flows along the border of CAR and DRC for several hundred kilometers until it is joined by the Kotto River; below this confluence the river is called the Oubangui. Forested floodplains line the Oubangui as it flows west along the border. The floodplain contracts and the river valley narrows where the river flows over the Kouimba Rapids (4°37’N/20°27’E). Rapids are also located upstream along the Mbomou, Uelé, and Kotto Rivers. Several tributaries join the north bank of the Oubangui River, including the Kouma, Tomi, and Ombella, M’Poko, M’Bali, and Lobaye, from east to west. Inundated swamp forests cover approximately 2,310 km² of the floodplains of these tributaries.

In western CAR the river turns south and flows along the border of the ROC and DRC through virtually unbroken primary rainforest into the Cuvette Centrale. The Oubangui River is slow-flowing and wide (4-15 km) with islands in its channel. Northeast of its confluence with the Congo, the Oubangui is joined by the Giri River and spreads out across a large floodplain, forming the Giri or Bangala Swamp. These swamps have black, acidic waters (pH 3.5-5.2) derived from the surrounding floodplain forests (Roberts 1973; Bailey 1986; Hughes & Hughes 1992). The Oubangui eventually joins the Congo, which, at this point, is a slow flowing, braided maze of alluvial islands, sand banks, and floating beds of grass. Right-bank tributaries of the Congo River below its junction with the Oubangui include the Likouala aux Herbes, Sangha, Likouala, Alima, Nkeni, and Lefini rivers. Swamp forests predominate on the floodplains in this section, creating over 6,800 km² that include parts of the Likouala aux Herbes swamps (Beadle 1981; Hughes & Hughes 1992). The Likouala-aux-Herbes drains Lac Telé, a shallow lake possibly formed by a meteorite crater. The massive Kasai River joins the left bank of the Congo about 150 km upstream of where this ecoregion ends at Malebo Pool.

Terrestrial habitats

A variety of vegetation types covers the terrestrial landscape of the Oubangui ecoregion. In the Sudanian zone in the north, savanna predominates with some semi-humid forest, cropland, and gallery forest interspersed. Primary tropical rainforest blankets the landscape in the southern equatorial latitudes; dense evergreen forest occurs in the west-central areas and freshwater swamp forest occurs along the lower course of the Oubangui River (Sayer et al. 1992).

Description of endemic fishes

The lower course of the Oubangui River is characterised by lower levels of endemism than other rivers within the inner Congo basin. However, upstream areas and rapids, which are often located in savanna regions harbor some endemics, though these species are known only from small collections. There are twelve endemic fishes known from this ecoregion, including rivulines, cichlids, mochokids, and cyprinids. Recent collections in the Sangha River drainage indicate that some fish species previously considered to be endemic to the Oubangui ecoregion (e.g., Haplochromis oligacanthus) are in fact shared with the Sangha basin.

Justification for delineation

High habitat diversity and hydrogeographic barriers have shaped the ichthyofauna of the Oubangui drainage. Numerous waterfalls separate upstream and downstream sections of the main river and its tributaries. Some of these rapids may have been formed after river capture of Nilo-Sudanic catchments by the Oubangui drainage. Evidence for river capture comes from several Nilo-Sudanic fish species or subspecies occurring in the Oubangui drainage that are otherwise absent in the Congo basin, except in catchments further northeast with a similar history (e.g., the Lindi and Aruwimi). Nilo-Sudanic species include the cichlid fishes Sarotherodon galilaeus galilaeus and Tilapia zillii. It should be noted that the Uele River, a tributary to the Oubangui, defines a separate ecoregion [536] because isolation by rapids and waterfalls has created a distinct fauna there.

Level of taxonomic exploration

Fair