South Cameroon is located in a tropical and tectonically quiescent region, with landscapes characterized by thick highly weathered regolith, indicative of the long-term predominance of chemical weathering over erosion. Currently this region undergoes huge changes due to accelerated mutations related to a growing population and economical developments with associated needs and increasing pressures on land and natural resources. We analysed two of the main south Cameroon rivers: the Nyong River and Sanaga River. The Sanaga catchment undergoes a contrasted tropical climate from sub-humid mountainous and humid climate and is impacted by deforestation, agriculture, damming, mining and urbanization, especially in the Mbam sub-basin, draining the highly populated volcanic highlands. By contrast, the Nyong catchment, only under humid tropical climate, is preserved from anthropogenic disturbance with low population except in the region of Yaoundé (Méfou sub-basin). Moreover the Nyong basin is dam-free and less impacted by agriculture and logging.We explore both denudation temporal variability and the ratio between chemical and physical denudation through two catchmentaveraged erosion and denudation datasets. The first one consists of an 11-year long gauging dataset, while the second one comes from cosmogenic radionuclides [CRNs, here beryllium-10 ( 10 Be)] from sand sampled in the river mainstreams (timescale of tens to hundreds of thousands of years). Modern fluxes estimated from gauging data range from 5 to 100 m/Ma (10 to 200 t/km 2 /yr); our calculations indicate that the usual relative contribution of chemical versus physical denudation is 60% and 40%, respectively, of the total denudation. Beryllium-10 denudation rates and sediment fluxes range from 4.8 to 40.3 m/Ma or 13 to 109 t/km 2 /yr, respectively, after correction for quartz enrichment. These fluxes are slightly less than the modern fluxes observed in Cameroon and other stable tropical areas. The highest 10 Be-derived fluxes and the highest physical versus chemical denudation ratios are attributed to anthropogenic impact.

Total suspended sediment (TSS) data for 1960-1970 and from recent investigations (1990-2000) are used to evaluate the variability in sediment yield of the Sanaga catchment (Cameroon) and the equivalent rates of erosion. At the annual and seasonal time scales, total suspended sediment concentrations for the Mbam sub-catchment are three to four times higher than for the Sanaga basin, reflecting the higher sensitivity of the former to erosion. Classical clockwise hysteresis loops are observed in both sub-catchments, despite a significant increase in human-induced catchment changes. At the multi-year time frame (over a 40-year period), it appears that the second half of the 1990s is marked by a downward trend in TSS. This shift is due to the control infrastructures (river impoundments and ponds) installed in certain parts of the whole catchment, combined with a drop in annual rainfall and river discharge.

The Sanaga River is one of Sub-Saharan Africa's largest and greatly regulated rivers. Available flow data for this hydrosystem largely cover the pre-and post-regulation periods. From comparisons between unregulated (hypothetical) and observed scenarios, it has been possible to separate and to quantify hydro-climatic (groundwater þ rainfall) change effects from anthropogenic impacts (especially dam-related alterations). To appreciate shifts in the river regime, discontinuity detection tests and the IHA model were applied to discharge data series reflecting average and extreme flow conditions, respectively. Results obtained principally from the Hubert segmentation method reveal that a major discontinuity occurred in 1970-1971 separating a surplus phase between 1945-1946 and 1969-1970, and a deficient and much contrasted one, from 1971/1972. This implies that the Sanaga catchment is dominantly affected by hydro-climatic changes. However, wide land cover/land use changes experienced here since 1988 have resulted in an increase in surface runoff. Additional quickflows linked to these changes may have partly compensated for the substantial decline in the dry season rainfall and groundwater inputs observed from this date. Although at the monthly scale, dam-related impacts on average flows increase with stage of regulation, the seasonal variability of the river regime remains generally unaffected. A comparison of the IHA statistics, calculated from unregulated and observed streamflow data, show that hydrologic shifts occurring in maximum and minimum discharges are mostly significant from 1971/1972 and are mainly due to the action of dams. Minimum flows appear, however, widely impacted, thus reflecting the prime objective assigned to the existing reservoirs, constructed to supplement flows for hydroelectricity production during the dry season.

Abstract. Le système Chari-Logone est principal cours d'eau drainant l'Extrême-Nord du Cameroun. À l'image des autres parties du bassin versant de ce cours d'eau, cette région a connu plusieurs inondations catastrophiques. Cet article examine la dynamique des extrêmes hydrologiques du Logone et du Chari à travers les stations de Bongor et N'Djamena en prenant en compte le contexte des changements climatiques et l'occurrence des risques hydro-climatiques. Les analyses portent essentiellement sur les données journalières maximum (Qmax) et minimum (Qmin) sur la période 1960–2015. Les logiciels XLStat et ChronoStat ont permis de calculer un ensemble d'indices. Il en ressort que les Qmax présentent une rupture en 1971 aux deux stations marquant la fin de la période humide. Les Qmin de Bongor ont connu deux ruptures successives en 1994 et en 2007. Les Qmin de N'Djamena par contre ont connu une rupture en 1986. Le coefficient de tarissement est globalement faible (0,020/jour en moyenne). L'occurrence des inondations catastrophiques est très contrastée, car seulement 30 % s'inscrivent dans les années excédentaires. Cela témoigne du rôle important que jouent les autres facteurs de construction du risque comme la pédologie, la pluviométrie et la vulnérabilité socioéconomique.

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