Fluvial system dynamics of Sudano-Sahelian zone during the Late Holocene. The Yamé River (Dogon Country, Mali)

Abstract

In West Africa, most palaeoenvironmental studies have focused on the role of climatic change on the environment. Because of the richness of its deposits, the Yamé River (Mali), a tributary of the Niger River, offers an opportunity to reconstruct the evolution of this fluvial system in comparison with both climatic and anthropogenic changes during the Late Holocene. To investigate more closely the spatio-temporal response of the fluvial system, sedimentary analyses were conducted on seven reaches distributed along the 137 km length of the Yamé valley. This approach testifies of the possibility for each time slice and river reach (1) to reconstruct the fluvial style and processes and (2) to estimate the sediment storage volume reflecting sediment distribution patterns. Results reveal a wide variability of the sedimentary cascade suggesting contrasting responses to external and local controls. Humid (4200–2900 cal. BP; 450–24 cal. BP) or arid (2350–1700 cal. BP) phases have been recorded in fluvial archives while an intensification in erosion and sediment supplies, even during the arid period (2900–2350 cal. BP and the 20th century), may be associated with an increase in human pressure. Two other periods are related to both climate and anthropogenic factors. During 1700–1400 cal. BP phase huge sediment supplies can be explained mainly by the reactivation of both hydrological processes and human occupation after an intense and long arid event. Dispersed sediment sinks recorded during the 450–24 cal. BP period originate from the combination of intensification of colluvial processes and a wet phase context originating from global climatic change. Thus, this study provides evidence that climate is the strongest driver for the fluvial response of these semi-arid and tropical rivers while human disturbance appear as a secondary factor due to the high sensitivity of the environment to climate variability in such areas.