Sedimentary geochemistry mediated by a specific hydrological regime in the water level fluctuation zone of the Three Gorges Reservoir, China

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 compact 345-cm-long sediment core was extracted near the base water level (145.3 m) from the WLFZ of the TGR and sectioned at 5-cm intervals. Extracted sediment subsamples were analyzed for 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). The sediment core chronology was determined using 137Cs elemental analysis. 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). Afterward (2011–2013), although upstream inflowing suspended sediments and reservoir water level were comparable, sediment deposition and concentrations of sedimentary geochemical constituents showed considerably fewer variations. Seasonal variations in sediment deposition and geochemical composition were also observed during the rainy (October–April) and dry (May–September) seasons, in addition to annual variations. Grain size, OM, and other sediment geochemical constituents all had significant correlations with each other and with sediment core depth. The concentrations of geochemical elements in various sediment stratigraphic layers exhibited staggering associations with each other and were dependent on each other in several ways. The arrangement of geochemical elements in various stratigraphic layers of the extracted core illustrated amalgamation with inputs from upstream seasonal suspended sediment dynamics and 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 the distributions of geochemical elements. Interim mediations in geochemical element concentrations are associated with seasonal distal flash floods and local terrace bank collapses, which generate significant amounts of distal sediment loads that are quickly deposited and are not sorted hydrodynamically. Overall, although a specific mechanism was devised to circumvent the siltation process, a considerable amount of sediment is trapped at pre-dam sites. In addition, siltation caused nutrients and geochemical elements’ enrichment.

But the riverine fluxes have been altered considerably at a global scale both by climate change and human activities over recent decades (James and Marcus 2006;Milliman et al. 2008;Syvitski et al. 2005;Vorosmarty and Sahagian 2000;Walling and Fang 2003;Ye et al. 2003). Among these, dam infrastructure delivers the most significant anthropogenic disturbance to the rivers by dramatically increasing channel fragmentation and reducing fluvial hydrological connectivity. Dam operations trap a large proportion of inflowing suspended sediment and associated terrestrial materials. Streamflow is intra-annually reallocated, resulting in many on-site consequences for the dammed reaches and off-site effects on downstream channels and estuarial ecosystems (Nilsson et al. 2005;Petts and Gurnell 2005).
The Three Gorges Dam on the upper Yangtze River is the world's largest hydropower project, supplying a variety of services such as hydropower production, flood control, navigation improvement, and recreation (Fu et al. 2010;Li et al. 2017;Wu et al. 2004;Zhang and Lou 2011). The TGR has considerably altered fluvial connectivity and trapped a major proportion of the inflowing suspended sediment load (Chen et al. 2008;Li et al. 2011;Yang et al. 2014Yang et al. , 2006. The dam operates on the principle of "impounding turbid water and releasing clear water," with water impoundment to a peak level of 175 m for hydropower generation during the dry season (October-April), followed by water release to a base level of 145 m for flood management during the rainy season (May-September) (Cojean and Caï, 2011). The reservoir water level herein fluctuates between the base and peak levels on a hydrological year (October-September) basis (Fu et al. 2010;Li et al. 2014a;Wu et al. 2004), contriving a transitional landform stretching from the base to peak water level which is called the WLFZ (Bao et al. 2015). Following a damming of the prevailing riverine flow caused by reservoir operation, sediment from upstream rivers and tributaries is typically confined to the reservoir (Tang et al. 2014). Systematic flow regulation and hydrological regime modifications have consequential repercussions for inherent riverine flow and the ecosystem (New and Xie 2008).
Our previous studies have elaborated conspicuous sediment redistribution patterns regarding the magnitude, processes, and genesis of the WLFZ in the context of hydrological regime alterations following flow regulation and in-stream seasonal suspended sediment dynamics (Tang et al. 2016(Tang et al. , 2014(Tang et al. , 2018. But the extent of geochemical sequestration in this special landscape in response to regular dam operations and hydrological regime manipulation is unknown. To address this issue, the present study focuses on determining the sedimentary geochemical dynamics associated with sediment accretion along a sediment core profile extracted from the WLFZ and exploring its linkage to the specific hydrological regime and flow manipulation. The main objectives of this study are (1) to define the sedimentary geochemical properties and depositional patterns of proximal and distal sediments in the WLFZ of the TGR and (2) to comprehend the alterations in the concentrations of sedimentary geochemical elements in response to alterations in flow regulation mechanisms and episodic inundation periods.

Study area
The TGR extends 663 km from Yichang to Chongqing and impounds a surface water area of 1080 km 2 . It has a full water storage capacity of 39.9 billion m 3 and a flood regulation capacity of 22.4 billion m 3 (Fu et al. 2010;Wu et al. 2004). Larger channel widths with gentle bank slopes prevail in the upper and middle reaches of the reservoir while hillslopes with steep bedrock and narrow channel are the stream morphology in the lower reaches. Water level starts decreasing after February to maintain downstream flow and increase reservoir capacity for upcoming floods and continues till the end of May each year (Li et al. 2017(Li et al. , 2014b, subsequently leading to the creation of an artificial WLFZ spanning an area of 349 km 2 with a vertical height of 30 m (Zhong and Qi 2008). Zhong County in the middle reaches of the TGR at the Chongqing Province contained a large number of such WLFZs, among which a representative WLFZ was selected as the study site. It was present on the left bank of the Yangtze River, having a comparatively flatter surface of former terraces submerged after the TGR inundation at an elevation of 145.3 m (Fig. 1). Previously, this area was dominated by arable lands with gentle slope gradients. The process of reservoir inundation started in October 2006, and the water level rose to 156 m during the initial submersion year (Tang et al. 2016).

Sediment sampling
The selected sampling site was situated near the base water level of the reservoir. The coring method was challenging since the water level was unstable and the sediment coring process was frequently interrupted by changing water levels. The water level in the reservoir during August 2013 remained relatively at the base level, and sediment coring was performed. One compact sediment core having a length of 345 cm was successfully extracted using an Eijkelkamp piston corer and subsequently sectioned at 5 cm intervals, resulting in a total of 69 subsamples.

Laboratory analysis
Wet sediment samples were air-dried at room temperature in the laboratory. Using a mortar and pestle, samples were disaggregated gently and sieved through a 2-mm mesh. Malvern Instruments laser granulometer with a Hydro-2000 sample injection unit was used to determine the absolute particle size composition. A 0.1 g of sample was taken and pretreated for 24 h with 10 ml of 30% H 2 O 2 to remove organic matter and to remove calcium carbonates with 5 ml of 10% HCl for 10 min. Next, 500 ml of ultrapure water was added to pretreated samples in the Malvern Hydro 2000 unit, and 2 min of ultrasonic dispersion was performed immediately before analysis (Pulley et al. 2015). These bulk samples were grouped into sand, silt, and clay fractions, and median particle size was also determined. Using a UV/visible spectrophotometer, the organic matter was recorded using low-temperature digestion of K 2 Cr 2 O 7 -H 2 SO 4 . Gamma-ray spectrometry with Ortec EG&G hyper-pure Ge γ detectors at 662 keV was used to determine 137 Cs concentration. The counting times consistently exceeded 33,000 s. The analytical precision of 90% confidence intervals at ± 5% was retained. Elemental analysis was performed using the inductively coupled plasma-mass spectrometry (ICP-MS) analytical technique. Samples were prepared using simple HF-HNO 3 dissolution in screw-top Teflon bombs. External solutions with surrogate calibrations of four elements were utilized to measure ICP-MS instrumental sensitivity. The standard edition was used for matrix correction. To correct instrumental drift between the sample and spiked sample measurements, naturally occurring internal standards (Rb, Y, Ce, and Pb) were utilized, and data for elements (including K, Na, Mg, Ca, Fe, Mg, Cr, Co, Ni, Cu, Zn, and Pb) was extracted (Jenner et al. 1990).

An overview of the sediment core chronology
The initial impoundment phase of the TGR started in 2005, and the first maximum reservoir impoundment capacity was achieved in 2010. The maximum water level during each hydrological year from 2006 to 2010 remained at 156, 156, 172, 170.9, and 175 m, respectively, and subsequently achieved maximum impoundment capacity through each hydrological year till 2010 and onwards. Sediment deposition during the rainy season (May-September) was higher compared to the dry season (October-April) in each hydrological year. The amount of sediment extracted during the rainy season from 2006 to 2013 remained at 15, 10, 40, 40, 75, 20, 25, and 15 cm, respectively (4.35, 2.9, 11.6, 11.6, 21.74, 5.8, 7.25, and 4.35%, respectively). During the dry season, sediment deposition was 5, 15, 10, 20, 30, 10, and 15 cm, respectively (1.45, 4.35, 2.9, 5.8, 8.7, 2.9, and 4.35%, respectively). Sediment deposition during the rainy season was two times more when compared to the dry season. During rainy season, it was 69.6%, and during dry season, it was 30.4%. Meanwhile, 23.2% of the total deposition occurred in rainy seasons of 2008, 2009, and 21.7% in 2010's rainy season.
Sediment core chronology using local suspended sediment, an extracted sediment core profile, local soil analysis, and upstream suspended sediment has been established in our previous studies by elaborating depth distributions of grain-size properties and 137 Cs distribution, and a correlation has been established for the sediment origin. The primary source of sedimentation in the WLFZ is suspended sediment transported by the river regime, followed by distal Fig. 1 Map of the Three Gorges Reservoir Region and location of the study area sedimentation originating from erosion processes. There are considerable variations in sediment deposition between the rainy and dry seasons. Absolute grain size analysis revealed that sediment was coarser during the dry season and relatively finer during the rainy season. Sediments from the upstream section of the reservoir were coarser compared to onsite sediment samples. The distribution of 137 Cs established a link between the origin and deposition of sediment. Its distribution was more closely related to finer elements of particle size distribution, organic matter, and nitrogen. 137 Cs activity was higher in stratigraphic layers having a finer particle size distribution and lower in layers with relatively coarser particles. Sediment layers containing higher 137 Cs content were attributed to distal sources of sedimentation while lower 137 Cs concentration was attributed to proximal sediment distribution caused due to local bank collapsing and erosion processes. A comparison between suspended sediment and bank materials was conducted, and it was established that suspended sediment predominately originates from upstream sources rather than proximal ones during the rainy season. During dry seasons, sediment originates from proximal sources due to rill and sheet erosion caused by storm events. During the dry season, the main source of sediments is proximal, while distal sources are responsible for sediment influx during rainy seasons at or near the sampling site (Tang et al. 2016). Based on previously established assumptions, a sediment chronology can be established for the stratigraphic properties of the extracted sediment core profile components.
Recalling the previously elaborated considerations regarding sediment origins, 137 Cs activity in association with seasonal flood variations and upstream sediment inputs into the reservoir can categorically explain the sediment source tracing. Since coarser sediments are incapable of traveling long distances with water movements due to their heavier weight and decreased water affinity due to gravity, finer sediments are usually transported as suspended sediments. This process is intermingled with local storm events, erosion processes, terrace bank collapsing, and surface wave erosion, inducing local sedimentation usually consisting of coarse sediments and, as mentioned earlier, deposit proximally. These sediments are derived from lower or deeper soil layers, resulting in lower or no 137 Cs activity. These processes transpire concurrently and cause the mixing of distal and proximal sediments. It can be asserted that distal and proximal sediments constantly muddle, yet sediment stratigraphic layers characterizing lower 137 Cs activity and/ or relatively coarser sediments principally accredit proximal sedimentation. Alternatively, higher 137 Cs activity accompanied by relatively finer sediment stratigraphy can be ascribed to distal sediment genesis ( Fig. 2) (Tang et al. 2016).
The concentration of organic matter (OM) varied from 9.02-31.04 g/kg with a mean value of 18.61 g/kg. OM concentrations were higher in the upper, newer sediment layers and gradually decreased in the deeper, older sediment layers. From 2006 to 2013, OM concentrations were higher during the rainy season and lower during the dry season. TN concentration followed the OM trend, as did 137 Cs and grain size distribution. TN levels ranged from 0.42 to 1.42 g/ kg, with a mean value of 1.0 g/kg (Fig. 2). OM was strongly correlated with TN and highly significant statistically (r = 0.810, p = 0.0). The concentration of OM was strongly correlated and statistically highly significant with respect to silt, specific surface area (SSA), and 137 Cs (r = 0.624, Median Particle Size (µm) p = 0.0; r = 0.569, p = 0.0; and r = 0.407, p = 0.001, respectively) It was inversely correlated and highly significant statistically with depth, clay, sand, median particle size, Na, Mg, and Ca (r = − 0.310, p = 0.009; r = − 0.413, p = 0.0; r = − 0.486, p = 0.0; r = − 0.327, p = 0.006; r = − 0.331, p = 0.006; r = − 0.520, p = 0.0; and r = − 0.335, p = 0.005, respectively). It was inversely correlated and statistically significant with Cu and Zn (r = − 0.288, p = 0.016, and r = − 0.303, p = 0.011, respectively). The concentration of TN was inversely correlated and statistically highly significant with clay, sand, median particle size, Na, Mg, Ca, and Cu (

Relationship between particle size distribution, OM, and sedimentary elements
Statistical correlations between OM and TN have been narrated in the antecedent segment. Clay proportion ranged from 6.6 to 12.7% with a mean value of 9.9%, and it was inversely correlated and highly significant statistically with median particle size distribution (r = − 0.501, p = 0.0). It was inversely correlated and statistically significant with sand (r = − 0.304, p = 0.011). It was positively correlated and statistically significant with depth, Mg, and Cu (r = 0.272, p = 0.024; r = 0.273, p = 0.023; and r = 0.284, p = 0.018, respectively). Silt percentage ranged from 57.5 to 91.4% with a mean value of 83.4%. It was strongly correlated and highly significant statistically with respect to SSA and 137 Cs (r = 0.893, p = 0.0 and r = 0.482, p = 0.0, respectively). It was inversely correlated and statistically highly significant with sand, median particle size, Mg, Ca, and Cu (r = − 0.965, p = 0.0; r = − 0.796, p = 0.0; r = − 0.441, p = 0.0; r = − 0.358, p = 0.003; and r = − 0.327, p = 0.006, respectively). The sand component ranged from 0.8 to 35.9% with mean value of 6.7%, and it was positively correlated with median particle size and Mg, and was statistically significant. (r = 0.891, p = 0.0, and r = 0.349, p = 0.003, respectively). It was inversely correlated and highly significant statistically with respect to SSA and 137 Cs (r = − 0.842, p = 0.0, and r = − 0.415, p = 0.0, respectively). It was positively correlated and statistically significant with Ca (r = 0.291, p = 0.015). There were no specific correlations between the sand component and other geochemical elements, aside from the previously mentioned correlations. The median particle size distribution ranged from 7.1 to 36.7 µm with a mean value of 10.3 µm, and its statistical correlations exhibited behavior similar to that of sand. Except for clay, silt, sand, SSA, OM, TN, and 137 Cs, no significant correlations were observed with other sedimentary constituents. Disparately, SSA ranged from 0.136 to 0.314 m 2 /g with a mean value of 0.220 m 2 /g and was correlated and highly significant statistically with silt, OM, TN, and 137 Cs (r = 0.893, p = 0.0; r = 0.569, p = 0.0; r = 0.779, p = 0.0; and r = 0.441, p = 0.0, respectively). It was inversely correlated and highly significant statistically with respect to sand, median particle size, Na, Mg, Ca, and Cu (r = − 0.842, p = 0.0; r = − 0.544, p = 0.0; r = − 0.319, p = 0.008; r = − 0.508, p = 0.0; r = − 0.383, p = 0.001; and r = − 0.416, p = 0.0, respectively). It was negatively correlated and statistically significant with depth, Pb, and Zn (r = − 0.255, p = 0.034; r = − 0.305, p = 0.011; and r = − 0.257, p = 0.033, respectively) (Fig. 3). Clay, sand, and silt proportions of sediments portray interdependence with each other as well as with grain size properties, OM, and TN in addition to intermittent interaction with other sedimentary geochemical elements in various stratigraphic layers of extracted sediment core profile. The affiliation of sediment texture (sand, silt, and clay percentages) with grain size properties, OM, and TN were considerable in contrast to other stratigraphic components. Correspondingly, elemental concentrations for most of the elements acted as a function of OM and TN concentrations in the form of inverse correlation. SSA is also extremely sensitive to OM concentrations. As a result, variables such as OM, TN, SSA, and element distributions deposited in various sediment layers established various correlations with one another, and the abundance of one variable is a function of the abundance of the other variables in various sediment layers.

Detailed description of sedimentary geochemical elements
Stratigraphic distribution of geochemical elements demonstrated intelligible amalgamation concerning abundance in numerous layers of the extracted sedimentary core. Curve formation revealed a close association of distinct segments of geochemical elements with each other in addition to associations with OM, TN, 137 Cs, and particle size distribution. A perspicuous analysis of curve formation for each examined parameter revealed that the majority of the fluctuations in curve formation are either associated with or antithetical to one another. These variations were visible throughout the length of the extracted sediment core and were especially noticeable during rainy seasons, though they were most noticeable during the rainy seasons of the initial reservoir impoundment years (2006)(2007)(2008)(2009)(2010).
At the inception of each season (rainy or dry), the stratigraphic distribution of geochemical elements proliferated thenceforth gradually decreased till season cessation, and this cycle repeated for every season regardless of variations depending upon seasonal upstream sediment input fluctuations and contemporary reservoir water levels. Multivarious chemical elements displayed curve formations similar to each other despite the other elements' manifested dissimilarities. Furthermore, the curve formation of some elements was compared during one season and contrasted during the other. Mg, Na, Zn, Ni, and Pb concentrations showed excessive speckle in curve formation in -2007Ca, Na, Zn, Fe, and Ni concentrations during 2008;and, except Zn, all other elements' concentrations speckled significantly during the 2010 rainy season, followed by relatively minor deviations during the same year's dry season. The distribution of elements comparatively stabilized (although there were some variations) during the preceding years (Figs. 4, 5, and 6).

Fig. 5
Sedimentary core depth profile exhibiting sediment geochemical properties of Zn (mg/ kg) and Mn (mg/kg) at sedimentary core depth from 0 to 345 cm during years 2006-2013 in the WLFZ of TGR 69.10 (mg/kg), respectively (Figs. 5 and 6). Except for Zn, these concentrations were statistically highly significant with each other. The statistical correlations of these metals with the foregoing sediment core constituents have already been discussed in the preceding sections (Fig. 3). The formation of the curve for these metals revealed a slight increase with increasing core depth (except for Zn), though the curve was highly speckled in between various stratigraphic layers during divergent rainy seasons of hydrological years. Distinctively, the Zn concentration curve stretched initially (during 2006-2008) and thenceforth decreased in the later stratigraphic core profile (Fig. 5).
The interrelation of OM, TN, 137 Cs, and grain size properties has been demonstrated in the preceding sections, and similar interactions have also been perceived for other geochemical elements. The stratigraphic distribution of geochemical elements in the sediment core profile revealed rugged curve formation following the commencement of seasonal floods during each hydrological year, which predominately manifested during the rainy season. When this curve formation is compared to suspended sediment load from upstream rivers and concurrent reservoir water levels ( Fig. 7), it is reasonable to conclude that seasonal variability in input sediment origin from upstream rivers and tributaries, as well as seasonal reservoir water level fluctuations, is the primary causes of depth variability of geochemical elements in the extracted sediment core profile (Figs. 4,5,and 6).

Correlation between sets of variables
Sedimentary geochemistry was significantly affected by flow regulation. A significant correlation was observed between various sedimentary properties and components of sediments in various sedimentary layers of the extracted sedimentary core. However, no correlation was also observed within a few segments of sedimentary components. A positive correlation was observed between TN, OM, SSA, and 137 Cs activity. A positive correlation was also observed between Cr, Co, Fe, K, Ni, Pb, Mn, and Cu. There was also a positive correlation between Ca, Na, Mg, and Zn. Meanwhile a negative correlation of TN, OM, SSA, and Cs was observed with Ca, Na, Mg, and Zn (Fig. 8).

Discussion
Concentrations of sedimentary geochemical elements substantially varied throughout the depth profile of the extracted sediment core. Suspended sediment loads from major upstream sources and contemporary reservoir water level fluctuations are depicted in Fig. 7. Incoming suspended sediment loads and reservoir water levels varied perpetually throughout the hydrological year, during rainy as well as dry seasons. Upstream suspended sediment loads gradually proliferate each year during the rainy season and recurrently decline following the dry season, when upstream suspended Fig. 6 Sedimentary core depth profile exhibiting sediment geochemical properties of Co (mg/kg), Fe (g/kg), Ni (mg/kg), Cu (mg/kg), Cr (mg/kg), and Pb (mg/kg) at sedimentary core depth from 0 to 345 cm during years 2006-2013 in the WLFZ of TGR sediment loads are low. In the interim, water levels during the rainy season are typically maintained at the base reservoir levels to indemnify incoming floods, followed by the dry season where the reservoir water level is preferably maintained at peak reservoir capacity conducive to regular dam operation for hydropower production. During the initial impoundment session (2006)(2007), the reservoir water level was low (156 m), and the incoming suspended sediment load was substantial. The pioneer lands to be submerged under the WLFZ, comprised of cultivable terraces and terrace ridges, were not designed to withstand massive water loads. Some of these ridges collapsed, and these eroded 2 0 0 6 -1 0 2 0 0 7 -2 2 0 0 7 -6 2 0 0 7 -1 0 2 0 0 8 -2 2 0 0 8 -6 2 0 0 8 -1 0 2 0 0 9 -2 2 0 0 9 -6 2 0 0 9 -1 0 2 0 1 0 -2 2 0 1 0 -6 2 0 1 0 -1 0 2 0 1 1 -2 2 0 1 1 -6 2 0 1 1 -1 0 2 0 1 3 -2 2 0 1 2 -6 2 0 1 2 -1 0 2 0 1 3 -2 2 0 1 3 -6 parts intermingled with incoming water loads, while others sustained themselves and acted as an extended basin for incoming sediment from turbid upstream water loads. This process was recommenced until the initial maximum reservoir impoundment capacity was achieved in 2010. This phenomenon is analogous to our observations: axiomatic during the rainy seasons of 2006, 2007, and 2010 and later noticeable during other rainy seasons and, conversely, during dry seasons, where relative coarsening and fining of sediment, as well as other geochemical elements, exhibited relatively fewer variations in relative abundance. Pragmatically, coarser sediment particles are unable to travel longer distances with water due to gravity and weight restrictions and are typically deposited at proximal locations determined by stream velocity. Sediments originating from the terrace bank collapsing are overruled from settling time since the input sediment load is too bulky to deposit congruously. Finer sediments, on the other hand, have more endurance and can linger in water for longer periods, depending on the size of individual sediment particles. Such suspended sediments travel longer distances with water and deposit at distal locations. Impending sections endeavor to explain how these factors fraternize the geochemical distribution of sedimentary constituents in association with flow regulation manipulation and seasonal suspended dynamics in the WLFZ of a reservoir.

Vertical distribution of physical variables arbitrated by flow regulation mechanisms
The properties of physical variables in a depositional environment are significantly manipulated following an alteration in the flow regime through reservoir operation mechanisms. The composition of the hydrological regime containing sedimentary solutions is reflected in sediment deposition. The physical and chemical properties of deposited sediment are fundamentally determined by physical and chemical processes during weathering, erosion, and transport of sediments (Bjørlykke 2015;Jehlicka 2009;Nesbitt and Young 1989). Flood season onset in upper catchments discharges considerable sediment loads primarily consisting of suspended sediments. During the flood recession period, locally eroded sediment deposition and redeposition are frequently prompted by surface and subsurface wave and bank erosion caused by navigation traffic, winds, and occasional terrace bank collapse. Prolonged submergence sequels terrace bank erosion and degradation. Terrace ridges at surface and subsurface water levels are persistently prone to wave erosion. Ridges above the reservoir water level are subject to collapse when lesser support is available downslope due to these processes accumulating distal sedimentation (Chen et al. 2010;Gao et al. 2009;Liu et al. 2016Liu et al. , 2015Yuan et al. 2013). Terraced topography in the upper-middle reaches of the Three Gorges Reservoir Region fortified this process by accumulating diversified sediments (Schönbrodt-Stitt et al. 2013) in addition to prolonged water residence time (Vörösmarty et al. 2003). Occasional and seasonal rainfalls in the middle reaches of the Yangtze River cause sheet and gully erosion at hillslopes, vegetation, and agricultural lands in the upper distal areas escorting locally eroded sediments and causing variable inundations. Red soil is characteristic of the pioneer lands, while sediments deposited in the WLFZ were typically olive gray. Obvious distinctive layers of red soil were also identified during the extraction of the sediment core profile, which may have been eroded due to some major local storm events.
Generally, erosion residual solid particles are hydrodynamically sorted out during river channel transport based on size, shape, density, mineralogy, and chemical composition. When there is excessive rainfall and erosion, the input sediments are too high to settle hydrodynamically and are deposited onsite to compensate for more sediment input from upslope erosion. After this flash flood sediment accumulation is complete, some sediments at the top undergo a hydrodynamical sorting process and intermingle with distal suspended sediments. During sample collection, the margins of different colored sediments portrayed the mixing of distal and proximal sediments. Some sediment layers contained relatively finer particles, illustrating hydrodynamic sorting during sediment accumulation. Because of the rough sediment deposition and higher input sediment load, some other sediment layers were coarse, especially the ones deposited during the onset of the rainy season. This blended with abovementioned red soil sediment layers at points. Presumably, periodically higher sediment loads in the upper reaches of the Yangtze River resulted in higher sediment loads in addition to higher water inputs. This led to a rough accumulation of sediments, exempting them from the hydrodynamic sediment sorting process. These observations are also supported by sediment and water load data in the upper Yangtze River and tributaries compared with contemporary grain size distribution and curve formation for geochemical elements (Figs. 2, 4, 5, and 6).
Considering the foregoing, it is possible to conclude that sediment deposition in the TGR's WLFZ is an exceptional case study in many ways and that it differed from normal sedimentation processes due to a specific prevailing hydrological regime in the Three Gorges Reservoir Region. This process follows general sediment depositional trends to some extent but is fundamentally transformed as a consequence of the systematic reservoir operation. The main factors presumably responsible for this mediation include water level fluctuations due to normal reservoir operation, variable sediment, and water loads in the upper Yangtze River and tributaries, seasonal floods, and erosion processes at distal and proximal points, topography, vegetation, wave, bank, and gully erosion, and strong water currents produced as a result of navigational traffic passing through the Yangtze River.

Flow regulation: manipulating the concentrations of OM and TN in the sediments
Concentrations of OM and TN are directly related to each other in freshwater ecosystems and behave incongruously compared to other sedimentary constituents. Topographic features of the sediment genesis are the main factors determining the presence of OM in the sediments. In the upper 15-cm sediment layer below the sediment-water interface, OM conversion takes place due to the presence of benthic organisms. In contrast, in deeper layers, OM is mostly stable, and its mineralization takes place due to bacterial activity in addition to nitrogen, which is a requirement for decomposition (Kemp 1971). An association exists between OM and clay proportions of sediments, OM input into the basin, and sedimentation rate (Thomas 1969). Higher sedimentation rates result in OM dilution with inorganic sediments (Trask and Patnode 1942), while lower sedimentation rates result in OM oxidation during settling through the water column (Kaplan and Rittenberg 1963). OM distribution in a sedimentary geochemical environment is a complicated process. The movement of OM depicts the flow of nutrients within an ecosystem. The topography of the Three Gorges Reservoir Region consists of green mountains, fertile agricultural landscapes, and forests. Higher rainfalls and water availability support various types of vegetation at different altitudes. Greener landscapes input larger amounts of OM input into the Yangtze River. The process of bacterial decomposition requires nitrogen (Middelburg 1989). The distribution of OM and TN in the extracted sediment core was strongly correlated and statistically highly significant. OM was inversely correlated with depth distribution, and its concentration gradually declined towards deeper stratigraphic layers of the sediment core. In the presence of TN, OM decomposes due to the activity of soil microorganisms and benthic organisms. Therefore, it is normal that the concentrations of TN and OM decline from the top to the bottom layers of the extracted sediment core. The formation of the curve indicated that other factors were also involved. At the onset of the rainy season, a substantial increase in OM and TN concentrations was observed and gradually declined toward the dry season during each hydrological year. When the input suspended sediment loads and the reservoir water level were high, larger variations were observed. Other factors (mentioned in Sect. "Vertical distribution of physical variables arbitrated by flow regulation mechanisms") related to local erosion processes also contribute to occasional OM distribution drift. Diminished 137 Cs activity in some stratigraphic layers in association with OM and TN deviations indicates local sedimentation. Higher vegetation in the WLFZ as well as on the upper hillslopes accompanied increased OM inputs. Local bank collapse loads contained diminished OM levels. Uniform sediment and OM depositions associated with relatively higher and stable 137 Cs activity indicate sediment deposition from upstream catchments. Variations in sedimentation rates, input OM in sediments, and sediment source topography each play a part in determining OM concentrations in sediments. The variations in TN concentration in Fig. 2 were also similar. Configurations of OM and TN in addition to grain size properties and geochemical elements showed nearly similar curve formations supported by statistical analysis. Incongruous distributions were observed during the rainy and dry seasons. At the onset of the rainy season, curve speckles in OM distributions complement geochemical elements, followed by the dry season showing diminished concentrations during each hydrological year.

Flow regulation mechanisms influencing the concentrations of sedimentary geochemical elements
Geochemical elements present in sedimentary rocks or soil layers undergo processes of weathering and erosion and are transported to the sedimentary solutions in various fractions and shapes like colloidal material, river solutes, suspended sediments, gravels, boulders, and bed sediments along with the hydrological regime (Martin and Meybeck 1979;Viers et al. 2009). These sediments deposit in a sedimentary environment and undergo continuous erosion and deposition until finally deposited and are still prone to disruption by a subsequent change in the hydrological regime (Chamley 2013). This process is strengthened due to the construction of the TGR, which created a permanent WLFZ and altered the normal sedimentary geochemical mechanisms (Tang et al. 2014). The main contribution of a reservoir operation to sediment geochemistry is by reducing water velocity, resulting in reduced sediment transport capacity. Thus, the sediment wash load starts settling with large particles, followed by smaller particles and suspended sediments (Annandale 2006). Seasonal floods and flow regulation mechanisms further complicate the process (Tang et al. 2016). Compared to natural riparian zones, this particular WLFZ increased the settling time for sediment deposition due to reduced water velocity and higher water levels. Therefore, a significant amount of sedimentation has taken place since the TGR started operation.
Putting in place the specific hydrological regime, there are two main possibilities for the settling process of geochemical elements. (1) Higher sediment input from upper tributaries during rainy and flood seasons results in the settling of raw sediment inputs with comparatively less settling time available. Thus, sediments and sedimentary geochemical elements settle in place mostly as it is, and there are fewer chances of sorting process of elements based on their chemical affinities. The higher input sediment load keeps coming, and previous sediments are simply buried below the new sediments. This rough sediment accumulation (detailed in Sect. "Vertical distribution of physical variables arbitrated by flow regulation mechanisms") is also true for seasonal as well as upper hillslope floods. This status quo is visible and supported by our results, especially during the initial rainy seasons until the first complete impoundment (2006)(2007)(2008)(2009)(2010) and the year afterwards (which will be further discussed in the later sections). (2) The majority of the sediment had already been deposited during flood recession periods; unsettled sediment settled hydrodynamically according to geochemical properties. The geochemical elements in these sediments settled according to their chemical affinities and characteristics. These possibilities were true and visible in the graphical illustrations at various depths of the sediment core profile. Despite higher water levels and abundant settling time, the sediment deposition was relatively finite. Under such settings, the concentrations of geochemical constituents declined and displayed comparatively fewer variations (Figs. 4, 5, and 6). The geochemistry of deposited sediments was manipulated as a result of a specific hydrological regime as well as seasonal variability in sediment genesis. Systematic reservoir operations integrated with variable seasonal water and sediment loads from the upper reaches of the Yangtze River result in the alteration of the normal hydrological cycle, accordingly changing the geochemical configuration of deposited sediment.

Alkali metals
Na and K are abundant in nature, while Cs is considered a trace element. The main feature of alkali metals is their relatively large atoms, which result in quick loss of valence electrons and oxidize easily; thus, these oxides essentially transform into ionic compounds. Atomic weight plays a crucial role in determining the degree of ionic character; therefore, Cs forms most ionic bonds due to higher atomic weight. The behavior of alkali metals is different in geochemical processes. Possibly, K will increase over Na, and Cs will increase over K (Heier and Adams 1964). It was also coherent in results, where the abundance of K was higher than that of Na and that of Cs was higher than K (Fig. 4). Keeping flow regulation manipulations under consideration (mentioned in Sect. "Flow regulation mechanisms influencing the concentrations of sedimentary geochemical elements"), the deposition of alkali metals in the sediments was accompanied by the specific nature of the elements. Flow regime manipulations participated in the determination of the concentration of alkali metals. Therefore, it can be projected that the concentration of alkali metals in sediments was partially determined and controlled by the specific hydrological regime. On the other hand, chemical properties also played a part in determining the specific relative concentration of the elements in a depositional environment. The possible reason for this phenomenon could be the loss of Na due to the leaching process, and therefore, silts contain greater Na concentration compared to clay proportions. In the case of K, it contributes towards plant metabolism and is therefore abundant on surface sediment while in deeper layers, comparatively larger K ions undergo adsorption on another clay mineral called kaolinite. Moreover, the K/Na ratio comparison can also demonstrate Na loss relative to K because of the latter's larger ionic size (McLaughlin 1955). The graph for the K/Na ratio is demonstrated in Fig. 9. During the initial impoundment (2006)(2007), the deposited sediment layer was comparatively less. Therefore, the K/Na ratio was close to 1, and there were comparatively fewer variations because Na leaching and K adsorption were facilitated due to less sediment deposition and sorted out relative to chemical properties. This was also true in 2012-2013 because the deposited sediment layer was not as thick and behaved similarly to previous years; however, the K/Na ratio was close to 2. Manipulation in the flow regime greatly contributed during the periods of higher sediment accumulation (2008-2010); thus, subsequently greater variations were recorded comparatively, and the overall K/Na ratio remained close to 2. This scenario resulted from high sediment accumulation that greatly inhibited leaching and adsorption processes; thus, during this period, elements were not distributed according to their chemical characteristics.
Contemplating the hydration process of ions in the aqueous solution, alkali metal radii in the hydrated form are 2.80 Å and 1.90 Å for Na and K, respectively. Hydrated radii for Cs range from 2.95 to 3.21 Å which is so large that it is not hydrated appreciably (Welby 1958). Therefore, Cs concentration behaved differently, and flow regime manipulation posed higher effects on its concentration than K and Na.

Alkaline earth metals and Pb
Limited research is available on the behavior of alkaline earth metals and their geochemical properties in the sedimentary geochemical environment in the WLFZ of a reservoir. However, the sedimentary geochemical properties of these metals in riverine, marine, estuarine, wetland, and coastal environments are widely discussed (Alagarsamy and Zhang 2005;Ingri and Widerlund 1994;Kim et al. 1999;Nissenbaum and Swaine 1976;Obunwo et al. 2014;Prusty et al. 2009;Sarma and Rao 1999;Zhang and Wang 2001). Considering the particularities of these metals in the abovementioned environments, an attempt is made to define the sedimentary geochemical behavior of Ca and Mg. The concentrations of Ca and Mg are strongly related to each other (Fig. 4). Despite seasonal variations during 2006-2007, the behavior of these elements was strikingly similar throughout the core profile depth, as manifested statistically. Considering the flow regulation mechanisms, Ca and Mg concentrations also greatly varied from time to time with respect to changes in the flow regime. This observation is in line with Cox et al. (2002). They demonstrated that geochemical processes are typically associated with Ca and Mg concentrations.
Biological sources are also considered the main Ca and Mg intakes in freshwater ecosystems (Barbosa and Fearnside 1996). Meanwhile concentrations of these metals are also associated with OM concentrations in a sedimentary environment (Nissenbaum and Swaine 1976). The current study also depicted similar observations with highly significant statistical correlations. Kim et al. (1999) studied Ca and Mg distribution in a marine environment and observed that clay mineral dilution of carbonate and quartz mainly controls Mg distribution while Ca distribution is mainly associated with the presence of carbonate shells. Similarly, in a riverine environment, limestone bedrock plays a central role in the distribution of Ca and Mg, while weathering processes also contribute to hindering the process (Zhang and Wang 2001). Considering the proportions of sand, silt, and clay in the sediments, the concentrations of Ca and Mg were also mediated by the sand and silt in the sediments.
It can be summed up by stating that concentrations of Ca and Mg in the WLFZ of a reservoir are affected by multiple factors. The relative abundance of these elements in upstream sediment inputs, seasonal variations in sediment origin, alteration in flow regime mechanisms, OM concentrations, carbonate, limestone, and quartz all play a pivotal role in determining these elements' concentrations in a reservoir sedimentary environment.
Pb is usually considered a heavy metal and categorized as a post-transition metal, and it is widely used for sediment dating and evaluation processes (Appleby and Oldfieldz 1983). The main source of Pb in a sedimentary environment is industrial waste (Kamala-Kannan et al. 2008). Considering Pb from a heavy metal perspective, according to the Interim Sediment Quality Guidelines (ISQG), concentrations above 30.20 mg/kg are considered the threshold level above which biological effects are expected to be adverse (Looi et al. 2019;Yap et al. 2008). The average Fig. 9 Sedimentary core depth profile exhibiting K/Na ratio at sedimentary core depth from 0 to 345 cm during years 2006-2013 in the WLFZ of TGR Pb concentration in the extracted sedimentary core profile of the WLFZ was 69.10 mg/kg, ranging from 3.40 to 138.10 mg/kg. Pb concentrations were quite high, above the threshold levels, in various sedimentary layers. Pb concentrations showed an increasing trend during 2006-2009 and declined subsequently in the following years until 2013. It is worth noting that variations in concentration were significantly higher during the rainy season compared to the flood recession period. Moreover, during the initial impoundment years, concentrations varied greatly, and after the reservoir achieved its first complete inundation, Pb concentrations declined substantially during the preceding years. Although Pb concentrations declined during subsequent years, the concentrations were still above the threshold levels.

Transition metals
Many authors have included Zn, Ni, Co, Mn, Cr, Fe, and Cu in the category of heavy metals, and their effects and implications for the environment are discussed thoroughly (Hara and Sonoda 1979;Usmani 2012, 2013;Vaithiyanathan et al. 1993;Zhu et al. 2016). Many researchers have also characterized these metals according to their chemical properties and their characterization in the periodic table, but those studies are limited to marine sediments (Finney et al. 1988;Francois 1987;Graybeal and Heath 1984;Mudholkar et al. 1993). Wei and Jing (1990) also used a similar approach for suspended sediments in the Yangtze and Yellow Rivers. Like the research on alkali and alkaline earth metals, the research on transition metals from a similar perspective for reservoir sediments is scarce.
Sediment in the Yangtze River mainly originates from its upper reaches and tributaries, where sedimentary rocks, granite, and metamorphic rocks are widely distributed (Yun-Liang et al. 1985). The middle and lower reaches of the Yangtze River are densely populated areas where mining and industrial activities are quite common, and it is an important waterway linking East and West China (Wei and Jing 1990). So, the source of transition metals in the Three Gorges Reservoir Region is mainly from the upper reaches and tributaries of the Yangtze River and is mainly attributed to mining, industry, navigation traffic, and other anthropogenic activities. These factors, coupled with the altered flow regime of the Yangtze River, greatly influence the distribution and deposition of transition metals.
The behavior of transition metals was slightly different than that of alkali, alkaline earth metals, and Pb. The resultant concentration curves for transition metals revealed comparatively fewer fluctuations in concentrations with sediment depth. The resultant concentration curve for transition metals depicted fewer variations in concentration than other elements, in addition to a few exceptions. For example, the concentration curves for Zn and Ni displayed relatively higher variations in 2006-2007 compared to other transition metals. The remaining curve for Zn was quite different from the rest of the elements, and concentration declined sharply afterwards. Lesser availability of Zn in subsequent sediment loads from the upper reaches of the reservoir or its unique chemical properties may have declined the curve afterward, as Zn is excluded from transition metals in certain cases and can be classified as a post-transition metal due to different electronic configurations (Cotton et al. 1999;Jensen 2003). During the initial reservoir impoundment (2006)(2007)(2008), Zn was deposited randomly with rough sediment deposition and a higher sediment load, and during later stages, there was enough water level to allow it to deposit according to its chemical affiliations. It is also possible that the concentration of Zn declined in the latter input sediment loads. The second hypothesis also seems true for the reason that during the rainy season of the first full impoundment (2010), the amount of sediment was quite higher compared to subsequent years. Therefore, Zn concentration displayed a little recovery during this period, although this was not close to that observed during the rainy and dry seasons of 2006-2007. Conclusively, this pattern can be partially attributed to input Zn loads in the sediments and partially to the chemical properties.
Ni concentration was also higher than other elements during the flood season of -2007, and for the rest of the curve formation, it was in line with other transition metals. This exception can also be due to the reason mentioned above for Zn. Considering the rest of the curve formation for Ni, the chances for flow regime manipulations on element deposition become more obvious. The chemical properties of the transition metals are similar; the difference in depositional patterns can be more precisely attributed to flow regime manipulations and seasonal suspended loads.

Implications of the dam's role in fluvial geochemical sequestration
The water level in the TGR fluctuates between 175 and 145 m, during the dry and rainy seasons, respectively. Regular reservoir operation also leads to a gradual decline in water level over time as soon as the water is discharged from the reservoir for hydropower production. Seasonal floods due to rainfall in upstream catchment areas and hillslopes cause soil erosion (Chen et al. 2010;Gao et al. 2009;Liu et al. 2016), and when these floods reach reservoir areas, it causes additional fluctuations in the reservoir water levels (Li et al. 2017(Li et al. , 2014b. This mechanism transports additional sediment loads that integrate and incorporate with the previous sediment budget. Both suspended and deposited sediment subsequently deviate from normal sediment deposition under a constant water level decline process (Liu et al. 2015;Yuan et al. 2013). This process is further interrupted due to surface and subsurface water waves and swell or surface gravity waves due to heavy shipping traffic passing through the Yangtze River (Gleick 2009;Zheng and Yang 2016). These waves interrupt deposition mechanisms in various surface and subsurface wave erosion mechanisms. When the reservoir water level fluctuates from 145 to 175 m, these waves constantly erode, disrupt, resuspend, and relocate sediment deposited near the prevailing reservoir water level. Similarly, subsurface gravity currents also constantly interrupt submerged sediments (Anderson 1972;Rapaglia et al. 2011).
The topography of the Three Gorges Reservoir Region, previously consisting of fertile cultivable terraces, is crucial. The submergence of the terraces due to the TGR inundation caused terrace land degradation (Schönbrodt-Stitt et al. 2013). The erosion of terrace banks and the collapse of critical terrace structures incorporated proximal sediment budgets into the flow regime. Substantial water volume and a longer water residence time also complicated the process because these agricultural terraces were not designed to withstand substantial water loads and pressure (Vörösmarty et al. 2003). These factors are prominent in normal sedimentation processes to some extent, but in the case of a large-scale reservoir, these factors decisively determine fluvial geochemical sequestration. Considering the sediment depositional pattern, geochemical properties, and geochemistry of the extracted sediment core, reservoir operation and flow regulation greatly impacted the fluvial geochemical sequestration.
Fluvial sediment dynamics contribute to determining sediment geochemical properties. These properties in the sediment core profile were related to grain size and OM. For example, clay is an important sediment constituent, and it has complex affinities towards the geochemical elements in a depositional environment with a divergent depositional pattern (Gagnon et al. 1992). Due to its negative surface charge, elements are present in sediment solution as hydrated cations, i.e., Ca 2+ , Mg 2+ , Fe 2+ , Mn 2+ , Li + , Na + , and K + , thus preventing absorption by clay minerals (Bjørlykke 2015;Manahan 1993). Therefore, these hydrated cations remain in solution form for longer periods, and strong hydration prevents these cations' adsorption capacity. In the case of a large-scale reservoir, these cations remain in a flow regime for substantially longer periods. Cs + and K + adsorb more certainly on clay minerals due to comparatively less hydration and a more positive surface charge. Low soluble hydroxides, i.e., Fe, Mn, and Al, are present in solution form as Fe(OH) 3 , Mn(OH) 4 , and Al(OH) 3 , and in the presence of a longer residence time, these hydroxides ultimately accumulate in sediments (Velde 2013). Elements like Na, Fe, K, Cs, Mg, Zn, Ni, Co, Mn, Cr, Pb, and Cu are usually soluble during water-rock interactions and are subsequently partitioned between dissolved and solid phases; thus, deposition is partially attributed to water residence time and partially to the depositional pattern of sediments (Dupré et al. 1996). Comparatively, Ca and Co are insoluble elements and are essentially transported through a solid load; therefore, concentrations of these elements are more specifically dependent upon grain size (Bouchez et al. 2011). Solid sediment load from upstream rivers is related to seasonal floods and influenced by the flow regulation mechanisms of the reservoir, which ultimately implicate the deposition patterns of geochemical elements in the sedimentary environment. On the onset of the 2010 rainy season, the first full impoundment of the TGR was achieved with a substantial input of sediment. The fallout of this phenomenon was evident in sediment core profile constituents during the same period. Throughout different hydrological years, varying fluctuations in sediment properties during rainy seasons were comparatively obvious compared to the dry seasons.
Sediment transport and deposition is a vital process having multiple implications for biological, chemical, and geochemical processes on the earth's surface. There has been a significant drop in the sediment transport of the Yangtze River, from 472 million tons/year during 1950-1980 to 124 million tons/year in the subsequent years. With the construction of the TGR, sediment transport is further reduced (Chen et al. 2008). Understanding the geochemistry and sediment transport in the TGR area attains more attention because this reservoir is the biggest and situated in one of the most populated areas on the globe (Chetelat et al. 2008). The transport of the chemical sedimentary budget of the Yangtze River is significantly reduced toward the downstream river and is mostly retained in the reservoir area (Tang et al. 2014).
Recapitulating, construction, impoundment, and operation of the TGR created a specific hydrological regime in the WLFZ, and the geochemistry of sediments was greatly intervened. The proposed contributing factors in this process include flow regulation mechanisms, seasonal floods, erosion in upper tributaries and hillslopes, paddy terrace land degradation, and bank and wave erosion. Discernible changes in sediment geochemical properties are primarily associated with flow regulation and hydrological regime alterations, followed by seasonal changes in suspended sediment dynamics. The extracted sediment core articulated a clear understanding of sedimentary geochemical processes and sediment depositional patterns in a large-scale reservoir. The Three Gorges Reservoir Region is a vast geographical area, and topography is diverse throughout the region. Similarly, the WLFZ also extends to a vast geographical area and elevation, with a vertical height of 30 m. Samples were collected near the base water level of the Three Gorges Dam (145.4 m) from a relatively plain area. Cogitating, these articulations are the limitations of this study. The scope of the study can be broadened in any aspect of the abovementioned factors to provide a more detailed and comprehensive interpretation of sedimentary geochemical processes in this region, in addition to other geological and environmental studies. This study explains modifications in sediment depositional patterns and geochemical properties in response to the construction of a large-scale reservoir. Therefore, similar studies can be carried out in other reservoirs to estimate the geochemical and sedimentary implications of reservoir construction and seasonal alterations in upstream suspended sediment dynamics. The process of siltation also has wider implications for reservoir life and environmental processes. This study can also help understand this by explaining the constituents of deposited sediments in large-scale reservoirs and related geochemical processes.

Conclusion
This study provides supplementary evidence on sediment sorting and geochemical elements' repartitioning in the WLFZ of a large-scale reservoir. Flow regulation mechanisms and flow regime manipulations, in association with varying levels of upstream suspended sediment and flood dynamics, primarily manipulate the grain size properties and concentrations of geochemical elements in the sedimentary depositional environment. Other factors involved in this manipulation included various types of floods, erosion, land degradation, and terrace bank erosion and degradation.
Sediment core chronology was established using 137 Cs activity analysis, and sediment deposition periods were recorded. Both seasonal and annual variations were observed in the sediment properties and distribution of the sedimentary geochemical elements. Annual variations were prominent during initial reservoir impoundment years until the first complete impoundment of the reservoir in 2010. After this period, annual variations in sediment core properties were comparatively less evident. Flow regulation manipulations associated with varying reservoir water levels principally contributed to annual variations in sediment properties. Seasonal variations in sediment core properties were similar to annual variations, with the difference that these variations were relatively higher during rainy seasons compared to the dry season of each hydrological year. Like annual variations, seasonal variations were prominent during initial impoundment periods until 2010, and afterwards, these variations declined substantially. Sediment deposition was more than twice as high during the rainy season as it was during the dry season. Seasonal and flash floods, varying upstream suspended sediment dynamics, land degradation, and terrace bank erosion and degradation were the primary contributors to seasonal variations.
The distribution of geochemical elements in various sediment stratigraphic layers is affected by the concentrations of other elements and sediment grain size properties, establishing various statistical correlations. OM concentrations declined from the upper to the lower sediment layers, and seasonal fluctuations were higher in rainy seasons compared to dry seasons. TN and 137 Cs concentrations also followed nearly similar trends. The statistical correlations of clay, silt, and median particle size with other geochemical elements were obvious compared to sand. Alkali metals (Na and K) were significantly correlated with each other, and with other sediment core constituents. Transition metals (Fe, Mn, Cr, Co, Ni, Cu, and Zn) and post-transition metal (Pb) concentrations correlated with each other as well as most of the previously mentioned sediment core constituents. The sand fraction of the sediment core was less likely to be correlated with other sediment core constituents. As mentioned earlier, seasonal as well as annual variations in the concentrations of the geochemical elements were obvious throughout the sediment core profile.
The process of natural hydrodynamic sediment sorting is interrupted due to varying input sediment budgets, local eroded sediment tranches, contemporary reservoir water levels, stream velocity, and available time. For normal dam operation, the water level is constantly managed and monitored at a specific level in response to contemporary rainy and dry seasons. Chronologically, the aforesaid factors are substantially influenced, engendering the stratigraphic deposition of sediments. There was a relationship between all of the sedimentary constituents, with nearly identical responses to changes in the flow regime and seasonal suspended sediment dynamics. Despite this point study highlighting the sedimentary modifications due to flow regime changes, further studies are required to better understand the vicissitudes on a larger scale. This study will add to our understanding of sedimentation and siltation processes and problems in large-scale reservoirs and invite further studies related to the environment and geochemistry.
Author contribution YB, QT, and XH designed and conceptualized the research, acquired the funding, and supervised the project. QT, DK, JL, JD, and GN analyzed the data. QT drafted the initial manuscript structure and DK wrote the original manuscript. QT, XH, and TB revised and approved the final manuscript. All authors read and approved the manuscript.
Funding This work was supported by the National Natural Science Foundation of China (U2040207, 41977075), Science Fund for Distinguished Young Scholars of Chongqing (cstc2021jcyj-jqX0026), Fundamental Research Funds for the Central Universities (SWU020013), and the Sichuan Science and Technology Program (2020YJ0202, 2020YFQ0002).

Availability of data and materials
The authors confirm that the data supporting the findings of this study are available within the article.

Declarations
Ethical approval This article does not involve any experimentation based on humans or animals therefore no ethical approval is required.

Consent for publication Not applicable.
Competing interests The authors declare no competing interests.