Sedimentary geochemistry mediated by a specific hydrological regime in the water level fluctuation zone of the Three Gorges Reservoir, China
Authors: Khurram Dil , Bao Yuhai , Tang Qiang , He Xiubin , Li Jinlin , Nambajimana Jean de D. , Nsabimana Gratien 
Published: March 2022

Abstract
The water level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) acts as an important sink for inflowing suspended sediment loads over the inundation periods following regular dam operations. This study depicts the sedimentary geochemical dynamics along a sedimentary profile based on the determined chronology and explores its links to the specific hydrological regime created by dam flow regulation and riverine seasonal suspended sediment dynamics. A 345 cm sediment core was extracted and sectioned at 5 cm intervals. Grain size composition, organic matter (OM), total nitrogen (TN) and geochemical elements (Na, K, Ca, Mg, Pb, Zn, Ni, Co, Mn, Cr, Fe and Cu) were determined. Sedimentary geochemistry and grain size properties of extracted sediment core exhibited greater variations during initial submergence years till the first complete impoundment of the TGR (2006–2010). Thereafter, despite upstream input suspended sediments and reservoir water level remained analogous, both sediment deposition and concentrations of sedimentary geochemical constituents demonstrated relatively fewer variations. Besides annual variations, seasonal variations were also observed during rainy (October–April) and dry seasons (May–September). Sediment deposition and sediment core constituents demonstrated substantial variations during rainy seasons compared to dry seasons. Grain size properties, OM and TN unveiled clear relations with sediment core depth and significant correlations were observed with each other together with other geochemical elements. The concentrations of geochemical elements in various sediment stratigraphic layers exhibited staggering associations with each other and were dependent severally. Arrangement of geochemical elements in various stratigraphic layers of extracted core illustrated amalgamation with input upstream seasonal suspended sediment dynamics and adjacent reservoir water levels. During shortened submergence periods and higher input sediment loads, geochemical elements demonstrated impulsive distributions. Alternatively, during longer submergence periods, elemental distributions were relatively uniform attributed to higher settling time to deposit according to grain size and geochemical affinities. Higher suspended sediment loads in association with seasonal floods also resulted in rough sediment deposition patterns imparting variations in distributions of geochemical elements. Interim mediations in concentrations of geochemical elements are associated with seasonal distal flash floods and local terrace bank collapse deriving substantial amounts of distal sediment loads which are promptly deposited and not sorted hydrodynamically. Overall, although a specific mechanism was devised to circumvent the siltation process, still a considerable amount of sediment is trapped at pre-dam sites. In addition, siltation caused enrichment of nutrients and geochemical elements.