The upper Nyong basin belongs to a morphological unit called “the Southern Cameroonian Plateau” (Figure 1). This is a vast smoothly undulating surface, with elevation between 650 and 850 m; the characteristic landscape, exhibiting interfluves and depressions, corresponds to a more or less eroded “half-orange” original morphology. Some inselbergs and crystalline blocks (as near Yaoundé, with highest point at 1295 m) dominate the surrounding plateau.
The Nyong River rises eastern to Abong Mbang, at an elevation of about 690 m; it flows then in a wide channel weakly sloped towards West, where the confluence with the So’o River, a left side tributary, is at 28 km downstream to Mbalmayo. The So’o watershed, including three of our studied stations, forms a notable excrescence in the Southern part of the Nyong basin, whose shape appears clearly elongated in the EW direction. After the plateau break line, at 50 km downstream to Olama, the Nyong River flows in the coastal plains; the river course, presenting a strong slope increasing (2.5‰), becomes irregular, with rapids and falls. At its mouth, the length of the Nyong River is 690 km, the surface area of the basin being 27,800 km2. On the upper 200 km, the river sinuates through wide swampy depressions. In its middle part, the river course appears clearly influenced by the 20°N important fault, even if it does not correspond to a major structural direction at the scale of the country. The experimental Mengong catchment (0.60 km2), near the Nsimi village, presents elevations from 669 to 705 m, and rather sloped sides; the swampy depressed zone represents 19% of the watershed surface area.