Treating the discharge of the TGR, riverbed souring, tributaries, and lake exchange as the “source” of sediment and the Yangtze Estuary as the “sink” of sediment. The processes through which sediments travel from “source” to “sink” were analyzed, and the impact of the sediment “source-sink” relationship change on the estuary was studied.

Starting from Yichang hydrographic station (S_Yichang ), we considered the influence of Dongting Lake (S_{Dongting Lake} ), Poyang Lake (S_{Poyang Lake} ), Hanjiang River (S_{Hanjiang River} ), and the inward import relationship of the interval on the sediment amount and the amount of sediment scouring or deposition of the riverbed (S) recharge from Yichang to Datong reaches ( Figure 2 ). Datong hydrographic station (S_Datong ) is the last hydrographic station of the Yangtze River mainstream. The calculation formula of riverbed recharge suspended sediment from Yichang to Datong reaches is as follows:

The sediment quantity of Dongting Lake (S_{Dongting Lake} ), into the Yangtze River mainstream is the difference between Chenglingji hydrographic station (S_Chenglingji ) and Three Outlets (S_{Three Outlets} ). The calculation formula is as follows:

The Three Outlets at which Dongting Lake diverts sediment from the Yangtze River are Songzikou (S_Songzikou ), Taipingkou (S_Taipingkou ), and Ouchikou (S_Ouchikou ). That is, the formula for calculating the amount of sediment at the three mouths of Dongting Lake is as follows:

The hydrographic station where Poyang Lake merges into the Yangtze River is the Hukou station (S_Hukou ). Namely:

The hydrographic station where the Hanjiang River merges into the Yangtze River is the Huangzhuang station (S_Huangzhuang ). Namely:

There is no sediment monitoring station in Datong–Xuliujing reaches. A topographic method should be used to calculate the suspended sediment recharge in this section. The river sediment test primarily aimed to analyze the suspended sediments, and there was an unmeasured area at a certain thickness at the bottom. This part of the sediment has traction load movement characteristics, part of which exchanges with the riverbed sand in the suspended sediment, and most of which is transported downstream along the riverbed surface (Wang et al., 2020; Yang et al., 2021b). The bottom sand exchange was measured by the bottom-supported system at the Yangtze Estuary and was concentrated in the deep-water waterway of the North Passage (Liu et al., 2011). Meanwhile, according to the estimate using the Rousse formula, the sediment content of the unmeasured area at the bottom is significantly higher than the suspended sediment concentration (Wang et al., 2020). At the same time, there is a large amount of waterway dredging (V_wd ), sand mining (V_sm ), and land reclamation (V_lr ) in the river channels, as well as riverbed scouring or deposition. The suspended sediment transport balance and the amount of sediment entering the representative section can be estimated using the following calculation formula:

Where V_{riverbed-sand} is the amount of sediment replenished riverbed scouring; V_Total is the total amount of riverbed scouring.

The cross-sectional areas are calculated from the topographic cross-sections along the river ( Figure 3A ), and the watercourses upstream and downstream of the river channels, according to Eq. (7):

Where i is the number of points at which the elevation of a transverse of the river is measured; j is the number of rivers profiles; A_i is the cross-sectional area (m²); h i + 1 h i and h i + 1 are the water depths at two consecutive points of a section (m); and b_i is the width at two consecutive points (m).

The volume of the rivers channel V_j ( Figure 3B ) between the up and downstream sections at the corresponding water level is calculated according to Eq. (8). Subsequently, the total river channel volume is obtained via Eq. (8):

Where V_j is the volume of the channel between adjacent sections (m³), A_i,j and A_{i,j +1} are the areas of adjacent sections (m²), and L_j is the distance between adjacent sections (m).

Sand mining in the Yangtze River, which increases riverbed scouring, is mainly used to scour for construction materials. The sediment from waterway dredging of the Yangtze River channel is mainly used for reclamation of shoals or land, and will also increase riverbed scouring after waterway dredging. To facilitate the comparison between the amount of riverbed scouring or deposition, the measurement unit of the suspended sediment load from the river basin, the amount of riverbed scouring or deposition, the amount of sand mining, and the amount of waterway dredging are all converted to t/y (tons/year).

According to the operation time of Gezhouba Dam, Three Gorges Dam, and cascade reservoirs in the upper reaches of the Yangtze River, the study period is divided into 1980–2002, 2003–2011, and 2012–2020.

Due to the diversion of Dongting Lake, the runoff at Yichang, Zhicheng, Shashi, and Jianli hydrographic stations decreased, and the rate of decrease slowed down from upstream to downstream. Affected by the inflow of Dongting Lake, Hanjiang River, and Poyang Lake, the runoff at Luoshan, Hankou, and Datong hydrographic stations increased. In general, the operation of the TGR and changes in the relationship between rivers and lakes in the lower reaches of the Yangtze River mainstream has not changed the characteristics of the runoff and had little impact on the total amount of runoff. Around 47% of the runoff entering the Yangtze Estuary from 1980–2020 came from the area above Yichang hydrographic stations ( Figure 5A ; Table 2 ). From 1980–2002, the amount of sediment at each hydrographic station first decreased and then increased, and from 2003–2020, it increased along the way ( Figure 5B ; Table 2 ). From 1980 until 2002, 2003–2011, and 2012–2020, the runoff and sediment of the three mouths of Dongting Lake all decreased, and the decrease of the sediment was significantly higher than that of the runoff ( Figures 5C; D , Table 3 ). Both the runoff and sediments of the Dongting Lake discharged into the Yangtze River decreased first before increasing, and the amount was smaller after the TGR went into operation. The runoff of Poyang Lake discharged into the Yangtze River first decreased and then increased, the amount of sediment first increased and then decreased. Conversely, the amount of runoff and sediment from the Hanjiang River discharged into the Yangtze River decreased throughout this time.

In the periods of 1980–2002, 2003–2011, and 2012–2020, the runoff at the Datong hydrographic station came from Yichang hydrographic station, Dongting Lake, Hanjiang River, and Poyang Lake; the confluence proportion was 47–48%, 21–24%, 3–5%, 16–18%, and 7–10% respectively, indicating that the composition of the runoff into the Yangtze Estuary before and after the operation of the TGR was stable ( Figure 6A ). From 1980–2002, the sediment amount at Yichang hydrographic station was higher than that at Datong hydrographic station; the riverbed at Dongting Lake and from Yichang–Datong reaches were all deposition, only 4% of the sediments at Datong hydrographic station came from Poyang Lake and Hanjiang River. From 2002–2011, the sediments at Datong hydrographic station came from Yichang, Dongting Lake, Hanjiang River, and Poyang Lake, with proportions of 34%, 4%, 12%, 8%, and 42%, respectively. These proportions changed in the period from 2012–2020 to 17%, 10%, 4%, 7%, and 62%. From 2012–2020 compared with 2003–2011, the proportion of suspended sediment in Datong hydrographic station in Yichang hydrographic station decreased by 17%, and the proportion of deposition recharge by riverbed scouring in Datong hydrographic station increased by 20% ( Figure 6B ).

The reach from Yichang–Dabujie (120 km) has a sandy pebble riverbed, transitioning to a purely sandy riverbed below Dabujie, with a great riverbed anti-scouring ability. Under the background of reduced sediment discharge from the TGR, the sediment scouring of the alluvial riverbed shows good exchange and sorting characteristics. The main characteristic is that the fine sediment in the riverbed first compensates for the suspended sediment and thus causes the riverbed to be in a scouring state, while the relatively coarse sediment will remain in the riverbed and only start to transport in a flood period. In the scouring process of the sandy riverbed in the middle-lower reaches of the Yangtze River, the fine sediment (d ≤ 0.125 mm) washed by the riverbed surface fed the suspended sediment, causing an increase in the amount of fine sediment in the suspended sediment ( Figure 7A ). Coarse sediment (d>0.125 mm) was recharged from the reaches between Yichang–Jianli reaches. The coarse sediment showed an increasing trend along the rivers from 2003–2021 ( Figure 7B ), which is also the area with the greatest scouring intensity in the riverbed downstream of the TGR (Yang et al., 2022a; Yang et al., 2023a; Yang et al., 2023b; Yang et al., 2023c). In the reach of the sandy and cobble-riverbed, due to the small thickness of the sandy overburden of the riverbed, the scouring of fine sediment in the riverbed is complete at present ( Figure 7C ), and the anti-scouring of the riverbed is in a quasi-equilibrium state due to the significant coarsening of the riverbed. The median particle size of suspended sediment at the Yichang hydrographic station during 1987–2002, 2003–2012, and 2013–2021 was 0.022 mm, 0.005 mm, and 0.009 mm, respectively, and that at Hankou hydrographic station was 0.018 mm, 0.014 mm, and 0.015mm. At Datong hydrographic station, the values are 0.017 mm, 0.010 mm, and 0.015 mm. The median particle size of suspended sediment decreases first and then increases ( Figure 7D ), that is, the coarse sediment scoured by the riverbed gradually recharges the suspended sediment. Therefore, under the background of riverbed scouring, the sediment on the riverbed surface near the dam section is significantly coarsened, while the sediment coarsening phenomenon is not significant when it enters the sandy riverbed.

After the operation of the TGR, the riverbeds between Yichang–Zhicheng, Jingjiang (Zhicheng–Chenglingji), Chenglingji–Hankou, Hankou–Hukou, Hukou–Datong reaches all experienced cumulative scouring. Since 2012, this scouring intensity has increased ( Figure 8A ). The cumulative deposition of the Yichang–Datong reaches from 1981–2002 was 3.13×10⁸ m³, respectively ( Figure 8B ). The cumulative scouring of the Yichang–Datong reaches from 2002–2018 was 28.52×10⁸ m³. In the periods of 1981–2002, 2003–2011, and 2012–2018, the amounts of sediments passing through the Datong section of the Yangtze River Basin decreased. Based on the topographic difference calculation, the amount of riverbed scouring increased (switching from deposition to scouring). The amount of scouring after sand mining and waterway dredging deducted also increased, indicating a switch from deposition to scouring ( Figure 8C ). In the 1981–2002, 2003–2011, and 2012–2018 periods, the amount of suspended sediment passing through the Datong section were 3.73×10⁸ t/y, 1.43×10⁸ t/y, and 1.22×10⁸ t/y, respectively. From 1981–2002, 2003–2011 and 2012–2018, the riverbed scouring or deposition of the Yichang–Datong reaches were –0.25 × 10⁸ t/y, 1.98 × 10⁸ t/y, and 3.45 × 10⁸ t/y, the amount of sand mining was 0.50 × 10⁸ t/y, 0.40 × 10⁸ t/y and 0.40 × 10⁸ t/y, the waterway dredging amount was 0, 40 ×10⁴ t/y and 0.13×10⁸ t/y, respectively. The amount that entered the bottom sediment of the tidal reach of the Yangtze River through the Datong section was –0.20×10⁸ t/y, (suspended sediments deposition in the riverbed and no bottom sand migrated downstream) 0.98×10⁸ t/y, and 2.14×10⁸ t/y, respectively ( Figure 8D ). The total amount of suspended and bottom sand that entered the Datong section was 3.56×10⁸ t/y, 2.41×10⁸ t/y, and 3.36×10⁸ t/y, respectively, which first decreased and then increased, in the periods 1981–2002, 2003–2011, and 2012–2018, respectively ( Figure 8D ).

Whether the suspended sediment concentration in the Yangtze Estuary will decrease or not is still under debate (Yang et al., 2014; Yang et al., 2015b). Most studies believe that the estuarine suspended sediment concentration will decrease (Li et al., 2012; Zou et al., 2012; Jing et al., 2013; Yang et al., 2014; Yang et al., 2015b), and some even overestimated the reduction in suspended sediment (Yao et al., 2015), as it provides direct evidence of delta scouring (Luan et al., 2016; Zhu et al., 2016; Yang et al., 2017; Dai et al., 2018; Luan et al., 2021);, front tidal flat scouring (Wei et al., 2015) and riverbed scouring (Pan et al., 2012; Dai et al., 2013; Wei et al., 2017). The suspended sediment concentrations at Xuliujing, Sheshan, and Hengsha stations did not reduce between 1998 and 2001, which can be attributed to the resuspension effect (Jin et al., 2006). The suspended sediment concentration in the flood and dry seasons of 2003 and 2004 did not decrease when compared with their corresponding annual averages (between 1950 and 2002 at the Datong hydrographic station) (Zhai, 2006). From 1980–2020, the runoff at the Xuliujing hydrographic section was equal to that at Datong hydrographic station ( Figure 9A ) and was related to the quasi-balanced runoff at the interval branch (Zhang et al., 2011). The sediment concentration in the Yangtze Estuary from 1980–2020 was similar to the value of the Datong hydrographic station, with a consistent decreasing trend ( Figure 9B ). In 2002–2011 and 2012–2018, the amount of sediment deposition on the inner edge was equivalent to the amount of sediment compensation in the riverbed along the way, indicating in a quasi-balanced state. Due to the decrease in the amount of sediment from the Yangtze River mainstream, the degree of unsaturation in the stream flow in the Datong–Xuliujing reaches increased, increasing the transport intensity of bottom sediments. Therefore, the sediment entering the Yangtze Estuary through the Xuliujing section was composed of suspended sediment in the drainage basin and bottom sand scouring from the riverbed in the hydrographic section between Datong and Xuliujing. Comparing 2003–2020 with 1980–2002, the vertical suspended sediment concentration from outside the Yangtze Estuary decreased by 21.4% ( Figure 9C ), which is consistent with existing research findings (Jing et al., 2013; Yang et al., 2014; Yang et al., 2015b). The peak area of horizontal suspended sediment has been gradually moving to the north and expanding to the deep-water waterway of the North Passage ( Figure 9D ). This is one of the reasons why the North Passage has required waterway dredging and maintenance in recent years (Dai et al., 2013; Luan et al., 2016; Luan et al., 2018).

According to Changjiang Sediment Bulletin (Changjiang Water Resources Commission (CWRC), 2004–2022), few stations test riverbed load sediment in the middle-lower reaches of the Yangtze River, and the amount of riverbed load sediment is much smaller than that of suspended sediment. In the middle–lower reaches of the Yangtze River, only the Yichang–Dabujie reaches have a sandy and cobbly bed with a certain amount of bedload. Below the Dabujie, the riverbed load sediment of the sandy riverbed is small (Yao et al., 2017), but the development of sediment wave is significant (Zheng et al., 2022). In July 2021, suspended sediment and bottom sand were monitored by way of sitting bottom-mounted observation ( Figure 10A ), in the straight reach of the Yangtze River tidal reach. The sediment concentration above the 1.0 m of the riverbed surface was significantly higher than that below the 1.0 m of the riverbed surface ( Figure 10B ), with an average ratio of 2.21 ( Figure 10C ).

Under the influence of hydrodynamic environments such as runoff, tidal currents, water waves, and salt water in the Yangtze Estuary, the sediment deposited by saltwater action flocculation in the mouth bar area is fine particles. In the other areas, the surface sediment of the riverbed is the same as that in the tidal reach (Yan et al., 2009; Yan et al., 2011; Luo et al., 2012; Chen et al., 2021), and there is a good relationship between sediment particle changes and riverbed scouring or deposition (Chen et al., 2021). Therefore, the sediment from the Yangtze River Basin into the estuarine includes two parts, fine grained sediment in the river water body and bottom sand in the riverbed surface. The composition of deposition sediment in the estuary is different due to the differences in the hydrodynamic environment.

The high sand-bearing water (bottom sand) in the above 1.0 m of the riverbed surface links the areas of the suspended sediment and the areas of riverbed scouring or deposition, with the exchange of bed sediment between the riverbed and the water body. The grain size of the sediment in this area is significantly coarser than the suspended sediment in the water body ( Figure 10D ). After the operation of the TGR, the suspended sediment particles in the estuary were refined (Yang et al., 2018; Yang et al., 2022b), and the sediment in the riverbed was slightly coarse (Luo et al., 2012). The grain size characteristics of sediments in different areas of the Yangtze Estuary are significantly different. The surface sediment particles in the mouth groin dam area are fine, and the surface sediment particles in the south branch and South channel are the same as those in the tidal reach (Zhang and Zhao, 2016).

After the operation of the TGR, the riverbeds between Datong–Jiangyin and Jiangyin–Xuliujing reaches all experienced cumulative scouring. Since 2012, this scouring intensity has increased ( Figure 11A ). The cumulative deposition of the Datong–Xuliujing reaches from 1981–2002 was 1.60×10⁸ m³, respectively ( Figure 10B ). The cumulative scouring of the Datong–Xuliujing reaches from 2002–2018 was 16.01×10⁸ m³, respectively ( Figure 11B ). The total amount of suspended and bottom sand that entered the Datong section were 3.56×10⁸ t/y, 2.41×10⁸ t/y, and 3.36×10⁸ t/y, respectively, which first decreased and then increased. In the periods 1981–2002, 2003–2011, and 2012–2018, the total sediment passing through the Xuliujing section in the upper reaches of the river basin decreased. Based on topographic difference calculation, the amount of riverbed scouring increased (switching from deposition to scouring). The amount of scouring after sand mining and waterway dredging also increased (switching from deposition to scouring) ( Figure 11C ). Suspended sediment passing through the Xuliujing section in the periods from 1981–2002, 2003–2011, and 2012–2018 were 3.73×10⁸ t/y, 1.43×10⁸ t/y, and 1.22×10⁸ t/y, respectively, showing decreasing trends; the amount of bottom sand entering the Yangtze Estuary through the Xuliujing section were –0.61×10⁸ t/y (suspended sediments deposition in the riverbed and no bottom sand migrated downstream), 0.89×10⁸ t/y, and 1.51×10⁸ t/y ( Figure 11D ). From 1981–2002, 2003–2011, and 2012–2018, the riverbed scouring or deposition of the Datong–Xuliujing reaches were –0.11 × 10⁸ t/y, 1.11 × 10⁸ t/y and 1.98 × 10⁸ t/y, the amount of sand mining was 0.50 × 10⁸ t/y, 0.22 × 10⁸ t/y, and 0.37 × 10⁸ t/y, and the waterway dredging amount was 0, 20 ×10⁴ t/y and 0.11×10⁸ t/y, respectively. The total amount of suspended and bottom sand entering the Yangtze Estuary was 3.12×10⁸t/y, 2.32×10⁸t/y, and 2.73×10⁸t/y, respectively, which first decreased and then increased. This study believes that the total sediment passing through the Datong–Xuliujing reaches from 2003–2020 did not show a continuous decreasing trend due to the reduction in suspended sediment load, while the supply from bottom sand volume exhibited an increasing trend and the “sink” of sediment scouring or deposition turned into the “source” of sediment replenishing.

The north branch of the Yangtze Estuary was dominated by deposition between 2002–2018, and the south branch, north channel, and south channel mainly scouring. The high shoal area of the front tidal flat mainly experienced deposition, and the front of the delta mainly experienced scouring ( Figures 12A, B ). In the Yangtze River Estuary, the continent body is at the condition of sediment deposition, which is caused by the “sink” of the sediment after entering the Yangtze Estuary. The waterway area is scouring due to the regulation of buildings, and the deltas area scouring ( Figure 12C ). Simultaneously, all the areas of the front tidal flat increased at first and then decreased. During 2002–2011, the areas at the water depths of 0 m, 2 m, and 5 m were 22.12%, 12.67%, and 9.35% respectively, while during 2011–2018 it became –7.13%, –17.58%, and –14.67% respectively ( Figure 12D ). During 2002–2018, the changes of the areas at the water depths of 0 m were –21.09%, +47.63%, –0.57%, +35.91%, and –7.23% for Yiyuansha shoal, Chongming shoal, Hengsha shoal, Jiuduansha shoal, and Nanhui shoal respectively, with an increased total area of 13.41% ( Figure 12D ).

From 1982–2018, both the north and south branches experienced cumulative scouring, but the total amount of riverbed sand in the Yangtze Estuary was relatively small, which means that without considering the impact of riverbed scouring or deposition in engineering areas such as land reclamation, waterway dredging, the Yangtze Estuary was in a quasi-balanced state between scouring or deposition ( Figure 13A ). The cumulative waterway dredging volume of the North Passage of the Yangtze Estuary between 1982 and 2018 was 11.07×10⁸ m³ (ship volume/topographic volume=1.30, sediment wet bulk density/dry bulk density=1.65, converted to a weight of 14.05×10⁸ t) ( Figure 13B ). During this period, the land reclamation area of the Yangtze Estuary was 820.57 km², and the amount of deposition caused by the reclamation was 12.31×10⁸ m³ (sediment wet bulk density/dry bulk density=1.65, and the average thickness of waterway dredging filling was calculated as 1.5 m ( Figure 13B ), and the converted sediment weight was 20.31×10⁸ t) ( Figures 13C, D ). The total amount of suspended and bottom sand entering the estuary area through the Xuliujing section in the 1982–2002, 2003–2011, and 2012–2018 periods was 3.12×10⁸ t/y, 2.32×10⁸ t/y, and 2.73×10⁸ t/y, a decrease of 16.9% in 2003–2018 compared with 1984–2002. In the above periods, after deducting the amount of riverbed scouring or deposition, land reclamation, and waterway dredging, through the Yangtze Estuary, the amount of dispersed sediment was 2.82×10⁸ t/y, 1.81×10⁸ t/y, and 1.28×10⁸ t/y, respectively.