Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> Although Extreme El Niño Events (EENE) have always impacted hydrology in South America, their intensification by global warming and their association with changes in human activities and land cover may lead to the acceleration of sediment transfers in river systems and dam reservoirs. This situation may threaten soil and water resources in a region highly dependent on freshwater originating from large dams. In this study, we investigated the sediment sequence accumulated in the Poechos reservoir (Northern Peru) and provided a retrospective reconstruction of the interactions of El Niño-Southern Oscillation (ENSO), agricultural practices and vegetation cover changes on sediment dynamics (1978–2019). To this end, a sediment core was dated and characterized by physical and chemical analyses (e.g. scanner tomography, X-ray fluorescence, particle size analysis) for estimating the evolution of sedimentation rates and changes in sediment sources during the last five decades. Sediment tracing results indicated the occurrence of changes in sediment sources associated with positive and negative phases of the Central and East Pacific Index (C and E index), with a greater contribution of the lowland dry forest biome in comparison to that of the Andean source to sediment during the warm periods (El Niño events, ENE), (mean contribution of 76 %, up to 90 % during the Coastal El Niño Events (CENE) of 2016–2017). Overall, the development of agriculture within the fluvial plains of the dry forest biome upstream of the Poechos dam induced an acceleration of sedimentation rates by 140 % after 1997, with an accumulation of coarser sediment in the reservoir during ENE and EENE. This study suggests that the expansion of agriculture (e.g. agriculture in floodplains, deforestation) associated with a higher frequency of extreme rainfall events amplified the consequences of ENE, EENE and CENE on the quantity and quality of sediment transported by this river system, which will significantly decrease the lifespan of the dam reservoir essential to meet freshwater demands of the farmers and the populations living in this region.
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