Changes in sediment, nutrients and major ions in the world largest reservoir: Effects of damming and reservoir operation

Abstract

River damming and reservoir operation alter hydrological process, raising concerns over far-reaching influence on fluvial water, biogeochemistry and sediment fluxes due to the common occurrence of fragmented river landscapes around the world. Here, we investigated carbon, nutrients, and geochemical conditions in the world's largest reservoir, the Three Gorge Reservoir (TGR) in China, under three different dam operations. We also analyzed long-term trends of sediment and geochemistry in response to the reservoir building and operation. We found a drastically reduced average sediment load of 32 ± 29 Mt/y during the post -TGR, which accounts for only 6.4% of the average annual sediment load (501 Mt/y) during the period, of 1950–2000. Cumulatively, over 2004–2018, sediment inflow to the TGR totaled 2090 Mt and sediment outflow (i.e. downstream of the TGR) was 480 Mt, resulting in a total of 1610 Mt (or 77%) sediment trapped in the Reservoir, equivalent to 107 Mt/y. The reservoir water is geochemically dominated by Ca2+ and HCO3− with low-moderate mineralization and excess nutrients. Concentrations of carbon, nutrients and major ions differ notably in response to reservoir operations with the highest nitrogen level under the sluicing period (SP). Major cations concentrations are low in the low water level period (LWLP), while major cations and Cl− and SO42− concentrations reach high in the SP. Concentrations of Cl−, SO42−, and Na++K+ increase 2-3-fold, increasing their relative abundance by a factor of 1.5–2. Nutrients show overall increases with considerable alteration of dominance for dissolved and particular species due to damming and anthropogenic inputs. Furthermore, dissolved inorganic nitrogen increases 8-fold during the last 50 years. The findings imply great effects of river damming and reservoir regulation on sediment transport, nutrient levels and major ion chemistry.