Influence of the soubré dam on sediment dynamics and dissolved matter transport within the Sassandra River basin toward Gaoulou (Southwestern Côte d’Ivoire)

Climate change and changes in land use are substantially reshaping fluvial dynamics within watershed systems. This study aimed to assess recent changes in the transfer of suspended and dissolved matter from the Sassandra River to Gaoulou, and to compare current conditions with historical data in order to evaluate the impact of dam regulation on downstream fluxes. The approach combined daily water sampling to quantify total suspended solids (TSS) and monthly sampling to analyze major dissolved constituents. Fluxes were calculated by coupling concentration data with discharge records and compared with data from 2003 (pre-dam conditions). Results show that dam regulation reshaped the flood regime, with the June flood peak now reaching magnitudes comparable to September. Mean TSS concentrations declined from 15.59 mg/L in 2003 to 10.41 mg/L in 2019–2020 (−33.23%), while total sediment load decreased by 43.57%, reflecting reservoir sediment retention. Conversely, dissolved matter transport increased, reaching 1,335,825 tonnes, indicating enhanced relative contribution of chemical fluxes. Between September 2019 and August 2020, total material transport reached 1,544,746.31 tonnes, dominated by dissolved load (86.3%). The findings demonstrate a clear shift from particulate to dissolved matter dominance downstream of the Soubré Dam, with implications for watershed biogeochemistry and coastal sediment supply.