To assess the impact of the DWP on estuarine stratification and sediment transport, we analyzed field measurements collected from the North Passage of the Changjiang Estuary before (June 1999) and after (August 2012) the project. These measurements, conducted during spring tides, were obtained from campaigns conducted by the Changjiang Estuary Waterway Administration Bureau. The pre-DWP dataset (1999) includes data from four monitoring stations, whereas the post-DWP dataset (2012) comprises an expanded network of nine stations ( Figure 1 ; Table 1 ).

Hydrological conditions during the two surveys were comparable, facilitating a meaningful analysis of DWP impacts. During the observations, the mean river discharge at the Datong hydrological station was 54,300 m³/s in 1999 and 52,500 m³/s in 2012, differing by less than 3%. Similarly, tidal ranges were closely matched, with observed ranges of approximately 3.3 m in 1999 and 3.7 m in 2012. Despite this consistency in hydrological and tidal conditions, the fluvial sediment load declined significantly between these two periods, from approximately 340 Mt/yr in 1999 to 132 Mt/yr in 2012, as a direct consequence of extensive sediment retention in upstream reservoirs and improved soil conservation practices in the watershed (Guo et al., 2018, 2019; Zhu et al., 2019).

At each monitoring station, hydrodynamic and sedimentary parameters were measured at six relative-depth layers, specifically at normalized depths (z/H) of 0.05 (near-bed), 0.2, 0.4, 0.6, 0.8, and 0.95 (near-surface), where H represents the total water depth and z is the vertical distance above the seabed. Flow velocities and directions were measured every 30 minutes using rotor current meters, with accuracies of ±0.01 m/s for velocity and ±1°for direction. Water samples were collected every 30 minutes at each depth layer to determine salinity and SSC. Salinity was measured using a calibrated salimeter with a precision of ±0.01 PSU. SSC was determined by filtering water samples through pre-weighed membrane filters, followed by drying at 60°C for eight hours and weighing to quantify the sediment mass. These measurements were tidally averaged directly from the observed data for subsequent analyses, without applying harmonic analysis or additional preprocessing.

In this study, we applied a sediment flux decomposition approach to understand changes in sediment transport mechanisms induced by the DWP. The instantaneous along-estuary velocity (u) and SSC (c) at any depth can be divided into tide-averaged and oscillatory components, and the tide-averaged term can be further decomposed into a depth-averaged component and its deviation (Dyer, 1974; Su and Wang, 1986; Zhu et al., 2022), as Equations 1 and 2:

where u 0 ¯ and c 0 ¯ Represent tidally depth-averaged values, u 0 ‘ and c 0 ‘ denote vertical deviations from tidally averaged values, and u t and c t denote tidal oscillation. The residual (net) sediment flux is obtained as Equation 3:

where the brackets represent tidal averaging, the first term denotes advection by mean flow (i.e., barotropic component or non-tidal drift), the second term accounts for transport driven by estuarine circulation, and the third term represents tidal pumping. These components were analyzed to assess the relative contributions of different sediment transport mechanisms before and after the DWP. The last two terms produce no residual sediment fluxes, as their integrals over the water column equal zero.

The influence of the DWP on vertical density stratification was evaluated using the potential energy anomaly ( φ ), a metric representing the energy required to mix the water column into a uniform density profile (Simpson et al., 1990). The potential energy anomaly is defined as Equation 4:

where ρ is the water density, ρ ¯ is the depth-averaged density, g is gravitational acceleration, H is water depth, and z is the vertical coordinate.

Water density was determined from salinity, temperature, and SSC using Equation 5:

where ρ w ( T ,   S ) is the density of freshwater, ρ s is the density of suspended sediments (estimated as 2,570 kg/m³ for the Changjiang Estuary; Guo et al., 2017), and C is SSC. Since temperature variations were minimal (<1°C) during observations, temperature effects were neglected.

To examine the temporal evolution of stratification, we evaluated the rate of change of φ (Simpson et al., 1990), using Equation 6:

where t denotes time. Accounting for only the along-estuary direction ( x ), Simpson et al. (1990) proposed the classical tidal straining, as Equation 7:

where u is along-estuary velocity, and u ¯ is its vertically averaged value.

This formulation can be extended to three dimensions (Burchard and Hofmeister, 2008; de Boer et al., 2008), as Equation 8:

where x , y , and z represent along-estuary, cross-estuary, and vertical directions, respectively. The above horizontal and wavy lines denote depth-averaged value and deviation, respectively. This formulation allows for the identification of processes such as advection ( A x and A y ), straining ( S x and S y ), which are essential for understanding estuarine circulation and sediment transport dynamics. Since advection and straining are primary contributors to water stratification, and the remaining ( R ) contribution is negligible (de Boer et al., 2008), we focus on advection and straining processes in this work.

Vertical mixing was assessed using eddy diffusivity ( k t ), linked to eddy viscosity ( ν t ) with the turbulent Prandtl-Schmidt number. Eddy viscosity quantifies momentum transfer due to turbulent eddies. Following Munk and Anderson (1948), ν t was estimated as Equation 9:

where κ is von Kármán’s constant (0.41), and u * is the friction velocity, estimated by the regression of von Kármán-Prandtl velocity profile, as Equation 10:

where z 0 is hydraulic roughness. Both u * and z 0 were estimated with a regression coefficient R²>0.80. The gradient Richardson number ( R i ) is defined as Equation 11:

Eddy viscosity was vertically and tidally averaged to assess its role in estuarine circulation following Hansen and Rattray (1965) and MacCready and Geyer (2010). The relationship between stratification and estuarine circulation intensity ( U e ) was evaluated using Equation 12:

where β s is the saline contraction coefficient, H is water depth, K M is vertically averaged eddy viscosity, and ∂ S ∂ x   is the along-channel salinity gradient. By applying these calculations to pre- and post-DWP datasets, we assessed the influence of deepening and narrowing on estuarine circulation.

Due to the DWP, the North Passage underwent significant morphological alterations, including channel deepening and narrowing. To evaluate these changes, we compared the estuarine topography before (1997) and after (2013) the completion of the DWP ( Figure 2 ). The modifications primarily affected channel depth, estuary width, and cross-sectional area along the North Passage.

Bathymetric surveys indicate that the navigation channel deepened substantially throughout the estuary, with the most pronounced changes occurring in the upper reaches (between CS2 and CS3), where the thalweg depth increased by over 70% ( Figure 2c ). Further downstream, between CS6 and CS9, depth increases ranged from 40% to 60%. Correspondingly, the mean depth of cross-sections doubled at CS2 and increased by 10%~60% at other locations ( Figure 2b ). These changes reflect natural sediment redistribution and anthropogenic dredging efforts to maintain navigability.

Despite the increased depth, the total cross-sectional area of the North Passage decreased due to channel narrowing. The width of the estuary contracted by 10%~70% along different segments, with the most significant reductions observed in the downstream sections ( Figure 2d ). As a result, cross-sectional areas decreased progressively seaward, with reductions ranging from approximately 10% at CS1 to nearly 50% at CS8 ( Figure 2a ).

Overall, the DWP led to a deeper and narrower estuarine channel, fundamentally altering the hydrodynamic conditions. These topographic changes have implications for estuarine circulation, salinity stratification, and sediment transport, which are analyzed in subsequent sections.

The hydrodynamic conditions in the North Passage changed notably after the DWP, primarily due to channel deepening and narrowing. These modifications influenced flow velocity, salinity intrusion, and SSC, altering the estuarine circulation and stratification.

The DWP significantly altered the residual flow patterns in the North Passage, particularly affecting estuarine circulation. A comparison of conditions before and after the DWP reveals notable changes in flow structure, including an increase in residual currents and the development of enhanced two-layer circulation.

The DWP significantly altered the stratification dynamics of the North Passage, intensifying vertical density gradients. To quantify these changes, we analyzed the potential energy anomaly ( φ ), a measure of the energy required to mix the water column thoroughly. The results indicate that both salinity- and sediment-induced stratification increased after the DWP, with profound implications for estuarine circulation and sediment transport.

The DWP substantially modified sediment transport dynamics in the North Passage, leading to increased sediment import, intensified ETM formation, and altered transport mechanisms. The primary changes include enhanced residual sediment flux, greater near-bed sediment convergence, and a shift in the relative contributions of different transport processes.

The observed increase in estuarine circulation post-DWP is primarily attributed to channel deepening and narrowing. Theoretical models suggest that estuarine circulation intensity ( U e ) is governed by water depth (H), vertical mixing (represented by eddy viscosity, K_M ), and the along-channel salinity gradient ( ∂ S ∂ x ) (Hansen and Rattray, 1965; MacCready and Geyer, 2010; Ralston and Geyer, 2019). This section discusses their roles in the enhanced estuarine circulation due to the DWP.

Since estuarine circulation scales with H 3 (Equation 12; Hansen and Rattray, 1965; MacCready and Geyer, 2010), the deepening of the navigation channel, with a 50% increase in H, led to a threefold increase in U e ( Figure 11 ). Increased water depth with a fixed estuary width lowers flow velocity and friction velocity, thereby decreasing eddy viscosity and enhancing estuarine circulation intensity. This was the case for the lower reaches at CS6 and CS8, while the eddy viscosity increased, and estuarine circulation weakened in the upper reaches at CS2 and CS3 ( Figure 11 ). Additionally, Channel deepening is expected to enhance salt intrusion and thereby return a lower along-channel salinity gradient (Hansen and Rattray, 1965; MacCready and Geyer, 2010; Ralston and Geyer, 2019). In the North Passage, however, ∂ S ∂ x decreased at CS2 and CS3, but increased in the lower reaches (e.g., CS6 and CS8; Figure 5e ). Correspondingly, U e declined in the upper reaches but increased in the lower reaches when including changes in ∂ S ∂ x ( Figure 11 ). These deviations from expected deepening consequences highlight the impacts of narrowing on estuarine circulation.

Channel narrowing has three primary impacts on estuarine hydrodynamics: (1) increasing flow velocity by reducing cross-section area, (2) enhancing stratification by increasing freshwater flushing near the surface, and (3) raising along-channel salinity gradient by suppressing salt intrusion. In the upper reaches (CS2 and CS3) of the North Passage, where water columns were well-mixed and thereby stratification effect is negligible, increased eddy viscosity (K_M ) is attributed to increasing flow (friction) velocity ( Figures 4k , 5c ), leading to a weakened estuarine circulation when including changes in K_M ( Figure 11 ). Narrowing could enhance estuarine circulation by increasing stratification and the along-channel salinity gradient. In the downstream reaches, for example, the reduction in eddy viscosity is attributed to intensified stratification induced by both deepening and narrowing. Narrowing increases seaward freshwater flushing near the surface, whereas deepening facilitates saltwater intrusion in the bottom layer, thereby producing stronger stratification. Increased stratification ( R i ) reduces eddy viscosity, and thus gives stronger circulation ( Figure 11 ).

Narrowing of the North Passage, primarily due to the construction of dikes and deposition in groin fields, increases ∂ S ∂ x in two ways. First, the reduction in the cross-sectional area intensified freshwater flushing in the upper reaches, effectively shifting the saltwater seaward. Second, local hydraulic structures, such as dikes and groins, increased form drag and hydrodynamic resistance, constraining saltwater intrusion (Song and Wang, 2013; Zhu et al., 2021a). This restriction maintained the overall length of salt intrusion while increasing ∂ S ∂ x . Consequently, estuarine circulation was enhanced in the lower reaches, where the strengthened salinity gradient promoted landward baroclinic pressure gradients and intensified exchange flows.

In addition to along-channel effects, estuarine narrowing also enhances stratification through lateral processes. One key mechanism involves groin fields, which trap saltwater during flood tides. During ebb tides, the faster-moving water in the main channel creates pronounced lateral salinity gradients between the deep channel and the adjacent shoals (Zhu et al., 2018b, 2020, 2021b; Zhou et al., 2019; Chen et al., 2020). This lateral salinity gradient drives a secondary circulation pattern, transporting saltier water from the groin fields toward the main channel near the bed. This process, known as lateral straining, strengthens density stratification within the estuary (Zhu et al., 2018b; Chen et al., 2020). Overall, the combined effects of deepening and narrowing played a crucial role in intensifying estuarine circulation, with narrowing influencing both mixing processes and salinity distribution.

The enhancement of estuarine circulation following the DWP was critical in increasing sediment import and retention within the North Passage. The strengthened two-layer flow structure, characterized by intensified seaward transport in the surface layer and landward transport near the bed, facilitated the accumulation of suspended sediment at the saltwater intrusion limit. This section discusses how estuarine circulation, modulated by increased stratification, contributed to enhanced sediment trapping.

The decomposition of sediment flux components ( Figure 7 ) reveals that estuarine circulation became a dominant driver of sediment transport after the DWP. Before the DWP, barotropic forces primarily controlled sediment fluxes, with relatively weak contributions from circulation-driven transport. After the DWP, however, landward sediment transport associated with estuarine circulation intensified, particularly in the middle reaches of the estuary. The increase in near-bed inflows transported fine sediments upstream, leading to sediment convergence at the saltwater intrusion limit and contributing to the expansion of ETM.

The post-DWP increase in density stratification directly impacted sediment transport mechanisms. Enhanced salinity and sediment-induced stratification increased the gradient Richardson number ( R i ) ( Figure 4j ), which in turn reduced vertical turbulent mixing (Toorman et al., 2002; Winterwerp, 2002; Xu, 2009; Ge et al., 2018). Stratification-induced suppression of vertical mixing facilitated the formation of high-concentration near-bed sediment layers. With SSCs exceeding 80 kg/m³ in certain regions, these concentrated benthic suspensions played a crucial role in sediment trapping. The development of concentrated benthic suspensions can be attributed to two interconnected processes. First, vertical mixing was further inhibited as density stratification increased, preventing sediment resuspension and favoring near-bed accumulation (Winterwerp, 2002; Dijkstra et al., 2018; Ge et al., 2018; Lin et al., 2021). This positive feedback loop reinforced sediment trapping, sustaining high SSCs near the bottom. Second, the strengthened estuarine circulation transported concentrated benthic suspensions landward, leading to significant sediment import and deposition in the mid-estuary. This transport pattern differed from tide-dominated estuaries, where tidal pumping is the primary mechanism for sediment import (Winterwerp, 2011; van Maren et al., 2015b; Dijkstra et al., 2019b; Dijkstra and de Goede, 2024).

The intensified estuarine circulation and the formation of concentrated benthic suspensions collectively contributed to sediment trapping at the saltwater intrusion limit. This process played a central role in reshaping sediment transport pathways and sustaining the ETM in the deepened and narrowed North Passage.

The ETM in the North Passage expanded significantly following the DWP, showing increases in both spatial extent and SSC. Tidal pumping played a crucial role by redistributing fine sediments and enhancing sediment trapping. Combined with intensified stratification and stronger estuarine circulation, the enhanced tidal asymmetry post-DWP facilitated both a landward shift and a seaward extension of the ETM.

Tidal pumping mechanisms, in addition to periodic stratification, are influenced by velocity asymmetry, temporal and spatial lags, and dispersive fluxes, collectively contributing to sediment redistribution (Simpson et al., 1990; Gatto et al., 2017; Wünsche et al., 2024). Following the DWP, tidal pumping intensified significantly, particularly in the middle reaches of the North Passage ( Figure 9 ). This enhancement facilitated the redistribution of sediment trapped around the saltwater intrusion limit, promoting both upstream and downstream sediment transport. Spatial lag effects and dispersive flux emerged as critical mechanisms facilitating this sediment divergence.

Hydrodynamic changes induced by the DWP produced pronounced along-estuary velocity gradients. The greatest flow velocities and hydrodynamic energy were observed in the middle reaches (between stations CS4 and CS6), while velocities diminished upstream and downstream ( Figure 4g ). This uneven velocity distribution generated spatial lag effects, where sediment transport preferentially occurred from regions of higher energy to lower energy zones, resulting in sediment redistribution both seaward and landward (Friedrichs, 2011; Gatto et al., 2017). Consequently, this process actively contributed to ETM elongation and persistence.

In parallel, sediment trapping driven by estuarine circulation enhanced SSC in the mid-estuarine regions, producing significant along-channel SSC gradients ( Figure 10b ). These gradients reached values as high as 1.0 kg/m⁴, elevating the significance of horizontal diffusion, often considered negligible in simpler tidal basins (Pritchard, 2005; Gatto et al., 2017). Horizontal diffusion, driven by strong SSC gradients, induced net dispersive sediment fluxes from high-concentration zones towards lower-concentration areas, contributing further to the bidirectional ETM extension.

Quantifying the precise contribution of horizontal diffusion to sediment transport with field data alone remains challenging due to spatial and temporal resolution limitations. Common analytical methods for estimating horizontal diffusion include the gradient-flux approach, involving detailed field measurements of velocity and concentration gradients (Geyer and Nepf, 1996; Ralston and Stacey, 2007), or numerical modeling techniques capable of directly simulating advective and diffusive fluxes under controlled conditions (Burchard et al., 2018; Ralston and Geyer, 2019). Such numerical modeling approaches, integrating hydrodynamics with sediment transport modules, have been successfully applied in other estuaries (Ralston et al., 2012; Jalón-Rojas et al., 2021) and are highly recommended for future research on the Changjiang Estuary. These models could explicitly quantify the horizontal diffusion term, distinguishing its relative importance alongside advective and gravitational circulation-driven fluxes, thus offering a more comprehensive understanding of sediment dynamics within engineered estuaries.

Overall, spatial lag effects by velocity gradients and dispersive flux driven by SSC gradients were critical to the ETM expansion following the DWP. These processes effectively redistributed sediments both landward and seaward, underpinning the elongation and intensification of the ETM. A conceptual overview of these mechanisms and their interactions within the modified North Passage is summarized in Figure 12 .

In addition to the impacts of the DWP, the declined fluvial sediment supply can trigger substantial regime shifts in estuarine hydrodynamics, sediment dynamics and morphology (Luo et al., 2012, 2022; Wang et al., 2015; Guo et al., 2021b; Luan et al., 2021; Zhu et al., 2021a). A disruption in the sediment balance, resulting from reduced riverine input, potentially prompts increased sediment import from marine sources, influencing the ETM evolution. The sediment load at Datong has decreased by ~70% from the 1980s through the 2010s (Guo et al., 2019). Nonetheless, the Changjiang Estuary exhibits a delayed response to this dramatic sediment decline due to sediment replenishment from downstream channel erosion and the estuarine buffering capacity (Zhu et al., 2019; Guo et al., 2021a). Spatial variability in estuarine SSC responses further complicates this picture. For example, at Xuliujing, located near the upstream entrance of the estuary, surface SSC remained relatively stable until 2010, after which it began to decline (Chen et al., 2022). Conversely, in the ETM region, bathymetric changes showed an approximate 30-year lag relative to riverine sediment reductions, excluding the effects of dredging associated with the DWP (Zhu et al., 2019). Surface SSC in the ETM region declined since the early 2000s, corresponding with a roughly 20% reduction in the spatial extent of ETM (Jiang et al., 2013b; Luo et al., 2022). This reduction became particularly pronounced post-DWP, as increased stratification suppressed vertical mixing, limiting sediment transport into upper water layers ( Figure 6 ).

However, despite the significant drop in fluvial sediment load, the near-bed SSC markedly increased in the North Passage, rising from a few kg/m³ in the 1990s to tens of kg/m³ in the 2010s (Wan and Zhao, 2017; Ge et al., 2018; Lin et al., 2020, 2021). This unexpected increase in SSC under reduced riverine sediment supply highlights the critical role of marine-derived sediments, primarily sourced from local and delta-front erosion (Zhu et al., 2016, 2019). The correlation between decreased riverine sediment load and increased siltation since the DWP is evident from the temporal comparison of sediment load at Datong Station and annual siltation volumes within the North Passage ( Figure 13 ). Marine sediment supply, therefore, emerges as a key contributor to the observed hyper-turbidity and ETM expansion in the North Passage post-DWP.

These observations imply that, while the DWP predominantly altered hydrodynamics and sediment transport from 1999 to 2012, the ongoing sediment reductions from upstream must be carefully considered in long-term predictions of hydrodynamic conditions and ETM evolution. Future numerical modeling studies should aim to explicitly differentiate and quantify the individual effects of declining fluvial sediment supply and anthropogenic interventions such as the DWP.

This study sheds light on how channel deepening and narrowing alter estuarine circulation, stratification, and sediment transport. However, our analysis has several limitations. First, parameters such as water depth ( H ), eddy viscosity ( K M ), and along-channel salinity gradient ( ∂ S ∂ x ) employed in our calculations represent cross-sectionally and tidally averaged values. The direct measurements analyzed in this study were collected primarily from the main navigation channel, assuming representativeness for the whole cross-section. Although this assumption is reasonable given the relatively uniform characteristics of the North Passage (~10 m depth and 7 km width, Figure 1 ), a more precise quantification would necessitate numerical models that capture full cross-sectional variability. Recent modeling studies have highlighted the profound effect of deepening on estuarine dynamics (e.g., along-estuary salinity gradient, stratification, estuarine circulation, and sediment transport), emphasizing the need for such detailed simulations (Jalón-Rojas et al., 2021; Grasso and Caillaud, 2023; Dijkstra and de Goede, 2024). Moreover, future modeling efforts should explore differential responses of sediment transport between the main channel and shallow shoals, improving our understanding of the impact of channel modifications.

Second, the friction velocity ( u * ), a key parameter in eddy viscosity estimation, was obtained by fitting the von Kármán-Prandtl velocity profile based on flow velocities measured at six relative depth layers. This approach likely overestimates friction velocities under significant sediment-induced stratification (Wright et al., 1999; Kim et al., 2000). The pronounced sediment-induced stratification at CS6 and CS8 suggests our eddy viscosity estimates may be inflated, consequently underestimating estuarine circulation. Future work should employ advanced turbulence methods, such as the turbulent kinetic energy method (Kim et al., 2000; Lin et al., 2021; Zhou et al., 2025), requiring high-frequency near-bed velocity measurements to enhance accuracy.

Additionally, a critical limitation arises from the absence of bottom sediment data during the survey periods. Observations from other studies indicate a shift towards finer sediment after the DWP, despite declining fluvial sediment supply. Specifically, bottom sediment grain sizes decreased from 69 μm in 1982 to 44~60 μm in 2003, and further to 10~25 μm in 2015 (Lou, 2005; Liu et al., 2010; Qiao et al., 2020); concurrently, suspended sediment grain sizes declined from 6.8 μm (pre-DWP) to 5.3 μm (post-DWP) (Chen et al., 2019; Yu et al., 2021). These grain size changes suggest shifts in sediment sources and increased marine sediment input. The feedback between bottom sediment properties and hydrodynamic variations, particularly under varying width-depth conditions, remains underexplored and represents an important research direction. Numerical modeling studies are particularly recommended to investigate these feedback mechanisms, further elucidating sediment transport processes.

This study demonstrates that enhanced estuarine circulation is the dominant driver of sediment import from the ocean following channel deepening and narrowing in a relatively river-dominated estuary. Intensified circulation significantly promotes sediment trapping at the saltwater intrusion limit, forming a positive feedback loop involving intensified flow velocity, increased stratification, turbulence damping, and hindered settling. This feedback loop results in extremely high near-bed SSCs, reducing benthic light penetration and contributing to hypoxia (low oxygen levels) conditions detrimental to estuarine ecology (Talke et al., 2009; Winterwerp et al., 2017; Schmidt et al., 2019). Furthermore, elevated SSCs increase channel siltation, necessitating frequent maintenance dredging with substantial economic implications (Liu et al., 2011a, 2019; de Nijs and Pietrzak, 2012). Thus, integrated management strategies balancing ecological and navigational requirements are critically important in engineered estuaries.

Finally, our findings underscore the critical influence of channel width on estuarine stratification, circulation, and sediment transport. Contrary to initial expectations of enhanced sediment flushing through narrowing (as commonly observed in river systems), narrowing by the DWP in the North Passage increased sediment import via intensified estuarine circulation and stratification. Strengthened stratification promotes near-bed sediment trapping and the formation of concentrated benthic suspensions, which are transported landward by intensified near-bed inflows. Although narrowing enhanced sediment export driven by barotropic flows, the stronger estuarine circulation-induced sediment import ultimately led to net sediment convergence at the saltwater intrusion limit. As a result, post-DWP siltation rates (120 Mt) far exceeded initial predictions (30 Mt) (Wan, 2015; Liu et al., 2019). Similar width-controlled sediment transport dynamics, albeit predominantly via tidal pumping, have been observed in tide-dominated estuaries such as the Ems Estuary (van Maren et al., 2016) and the North Branch of the Changjiang Estuary (Guo et al., 2022). Thus, estuary width emerges as a universal critical parameter controlling sediment transport processes, albeit through differing mechanisms depending on the dominance of river discharge or tidal forces.

The sediment import mechanisms in engineered estuaries fundamentally depend on the relative influence of river discharge versus tidal forcing, which can be systematically characterized using the dimensionless Canter-Cremers number (N). In tide-dominated estuaries with small N values (e.g., Ems Estuary, N ≈ 0.005), tidal pumping predominantly drives sediment import, supported by strong tidal asymmetries and sediment-induced drag reduction (Winterwerp, 2011; van Maren et al., 2015b; Dijkstra et al., 2019b). Conversely, in river-dominated estuaries such as the Changjiang Estuary (N ≈ 0.5), sediment import primarily occurs through enhanced estuarine circulation triggered by deepening and narrowing. In these systems, gravitational circulation significantly outweighs tidal pumping in transporting sediment landward. These contrasting mechanisms underscore the complexity and diversity of estuarine responses to engineering interventions. Future global analyses are needed to comprehensively categorize estuaries across a spectrum of N values, systematically comparing sediment dynamics and regime shifts to support better-informed management strategies.