Reconstruction of the Evolution of the Hydro-Sedimentary Signal to the Sea from the Study of the Sedimentary Archives: Case of the Wadi Cheliff, Algeria

This work focuses on the study of several fluvial cores to help better understand the contribution of rainfall and large dams in the decline of sedimentary inputs of the Wadi Cheliff at its outlet. Therefore, three sediment cores, sampled in the lower Cheliff valley, downstream of dams, were studied through the paleohydrological approach. Then, the granulometric and geochemical profiles obtained were correlated with ¹³⁷Cs profiles, hydrological data of the Wadi Cheliff at the station closest to the Sea, Sidi bel Attar, the evolution of the rainfall signal and the data on the large dams. Such an examination aims, on the one hand, to establish a chronology of depositional accumulation and, on the other hand, to evaluate the ability of the information contained in the sedimentary archive to transcribe the evolution of the hydro-sedimentary signal and the fluctuations of the controlling factors. The results reveal a strong variation in the granulometric distribution of the deposits and a progressive decline in the rate of sediment accumulation. Thus, the upper part of the core is mainly made of particles belonging to the silt and clays granulometric classes that have accumulated with an average rate of about 1.31 cm.y⁻¹, contrary to the deposits in the lower part of the core composed of a succession of sand and finer sedimentary layers, and showing an accumulation rate much more superior, which value is evaluated to 16 cm.y⁻¹. However, the fluctuations observed in the granulometric composition, and the accumulation rate of the deposits correlated strongly with the evolution of the rainfall signal and/or the multiplication of the number of large dams. Indeed, frequent sand deposits and a higher accumulation rate correspond to the wet period before 1980. Then, the decrease in rainfall has been accompanied by a lower accumulation rate, and deposits composed mainly of clay and silt particles. In addition, the recently built dams have a drastic effect on the deposition process. Thus, the accumulation rate has been strongly slowed, and the deposits are short of the sand fraction. This study shows that the deposition process is closely linked to the hydro-sedimentary yield of the Wadi Cheliff to the sea, as it shows that the information present in the sediment archive accurately reflects the evolution of rainfall signal and the effect of large dams on the decline of sedimentary inputs from the Wadi Cheliff to the sea.