Geochemical perspective on large dams changing the downstream sediment sources

Abstract

Human activities play an increasingly important role in shaping Earth surface and changing river geochemistry. This study presents elemental concentrations of suspended sediments collected from the water depth profiles in two hydrological years in the lower Changjiang (Yangtze) River, aiming to discern how intense dam constructions (e.g., Three Gorges Dam) homogenize the source signals of downstream sediments in a large river basin. Our results show that sediment sources indicated by elemental ratios of Cr/Th, Co/Th and europium anomaly (Eu/Eu*) displayed seasonal changes in 2010–2011, while the seasonal variability disappeared in 2014–2015. After excluding the effects of hydrodynamic sorting and weathering processes, the waning seasonal variation can be best explained by the shift of downstream sediment source and sink roles with dam constructions. Different from the traditional view that the middle basin became the dominant source of sediment into the sea after dam constructions, we suggest that the middle basin actually act as a transition or buffer zone for the upstream-derived sediments. From a geochemical point of view, the sediments from the Changjiang River still feature a source signal of its upper basin above the Three Gorges Dam. Our study sheds new light on great and comprehensive influences of human activities on river sediment geochemistry, and highlights the need to carry out integrated studies on physical and chemical processes during sediment transfer from land to sea in fasting changing world.