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The Mobaye outlet on the Ubangi River is located 151 km from the confluence of the Mbomu River (CAR) (Central African Republic) and the Uele River (DRC) (Democratic Republic of the Congo), the confluence from which the Ubangi River originates at Kemba (Figure 6.1). The Ubangi is the largest right‐bank tributary of the Congo and a natural border between the CAR to the north and the DRC to the south. The Ubangi at Mobaye has a watershed of 403,800 km². The SE (Uele Highlands) and N (Kotto Highlands) ends of the basin are dominated by mountains with respective altitudes of 1,700 m and 1,300 m, nested between two plateaus (the Uele Sandstone Plateau in the south and the Central African Surface in the north), then the plain of the Ubangi foothills, which is quite narrow, confines the downstream valleys of the Mbomu, Bili, and Uele rivers up to Mobaye, including the downstream valley of the Kotto. In our study, the Bili River of only 21,400 km² is integrated into the Uele basin (i.e. Orange, 1996).

Figure 6.1 Topographic map of the Ubangi basin at Mobaye, and of the hydropluviometric network studied, with the limits of the five studied sub‐watersheds (at the outlets of Bondo for the Uele River, Zemio and Bangassou for the Mbomu River, and Bria and Kembe for the Kotto River).

Source: Based on Orange et al., 1994.

Figure 6.2 Vegetation map of the Ubangi basin at Mobaye, the geological bedrock and the hydropluviometric network studied, with the limits of the five studied sub‐watersheds (at the outlets of Bondo for the Uele River, Zemio and Bangassou for the Mbomu River, and Bria and Kembe for the Kotto River).

Source: Based on Orange et al., 1996.

The rainfall gradient decreases from south to north, from 1,700 mm on the Uele to 1,000 mm on average over the upper Kotto basin (Nguimalet & Ndjendolé, 2008). The basin of the Ubangi at Mobaye is a well‐marked latitudinal stratification of the edaphic stages (Figure 6.2). From south to north, one observes the tropical rainforest domain of the equatorial forest with 15% primary forest (Global Forest Watch, 2019), followed by the wooded savannah zone, successively wooded, shrubby, and grassy in the extreme north (Nguimalet, 2017). The latest 2018 surveys by Global Forest Watch confirm that in the Central African Republic and Northern Congo the forest area has decreased very little (–1.5% between 2001 and 2018), with a slightly greater loss of primary forest (–1.9%). This study considers that the forest area has not changed. Thus, according to Orange & Ghiloufi (1996), the Uele basin (including Bili) is 70% covered by tropical forest and 30% by wooded and tree‐covered savannah, the Mbomu basin has only 10% tropical forest and 30% wooded savannah (according to Orange, 1996), and the Kotto basin is representative of wooded and tree‐covered savannah (Figure 6.2). The humid forest thus represents 30% of the Ubangi basin at Mobaye.

In the region, population growth is low: the population in Bangassou on the Mbomu River, the regional capital, increased from 902,205 inhabitants in 2006 to 1,126,730 in 2015, an increase of only 2.5% per year. The entire Ubangi basin at Mobaye thus remains a sparsely populated area, with a population density varying between 10 inhab/km² and 3.3 inhab/km². In the Uele basin, the population density was 7 inhab/km² in 2008 (Haut‐Uélé, 2020). Human activities are essentially primary (slash‐and‐burn agriculture, artisanal mining, production of firewood and charcoal, etc.), concentrated around the Ubangi, Uele, and Mbomu valleys; these activities have changed little since 1970 (Haut‐Uélé, 2020; Nguimalet & Orange, 2019; Orange & Ghiloufi, 1996).