Hydropower dams induce downstream sediment starvation, influencing fluvial morphology. With the focus commonly on morphological changes, an aspect of sediment starvation that has received much less attention is the impact of changes in the sediment grain size distribution (GSD) on these morphological changes. In this study, we investigate the effects of the Three Gorges Dam (TGD) on the multi-fraction sediment transport and bed recovery in the middle-lower Yangtze River. Based on long-term field data (1987–2021), we evaluate how fine (d < 0.031 mm), medium (0.031–0.125 mm), and coarse (d > 0.125 mm) fractions differentially respond to dam regulation. Our findings reveal a progressive coarsening of suspended sediment and identify three distinct dam-induced sediment regimes: static armored gravel bed, active bed armoring, and strong erosion. Within the first ~350 km downstream of the TGD, erodible sediments, especially fine and medium fractions, have been almost entirely depleted. In contrast, the subsequent 750 km reach has emerged as the dominant sediment source, increasingly characterized by medium and coarse fractions over time. In addition, tributaries now supply fine-grained sediment during the wet season, whereas lakes, acting as long-term sediment storage zones, release previously deposited material during the dry season. Both sources are playing an increasingly important role in modulating the GSD of the middle-lower Yangtze. These findings shed lights on the dam-induced multi-fraction sediment recovery, offering valuable guidance for the sustainable management of river systems influenced by upstream dams.

Tidal flats are critical coastal ecosystems, with their geomorphic characteristics traditionally understood to be primarily influenced by tidal, wave, and storm forces. This study investigates the impact of rainfall on the morphodynamics of upper tidal flats by combining hydrodynamic-sediment data, meteorological rainfall records, and video monitoring at the Chongming Dongtan tidal flat in the Yangtze River Estuary, China. We show that rainfall significantly increases suspended sediment transport and accelerates tidal channel elongation. Notably, rainfall events—though occurring during only 25 % of observed tidal inundation periods—accounted for 62 % of cumulative net sediment transport. This disproportionate efficiency compared to tidal forcing stems from the rainfall-induced hydraulic connectivity between expansive supratidal areas and tidal channels, where concentrated runoff convergence intensifies scour dynamics. These findings challenge the traditional view of tidal flat dynamics, suggesting that rainfall is a more influential driver of morphodynamic change than previously recognized.

Human interventions influence sediment dynamics, and understanding these mechanisms is essential for predicting short-term and long-term estuarine development. The Deep Channel Navigation Project (DCNP) in the Yangtze Estuary is such a large infrastructural intervention that substantially alters sediment exchanges between channels and shoals and may thereby influence this estuarine development. However, the effect of these constructions on channel-shoal sediment exchange is up to now poorly known. In this study, we use an extensive dataset collected both in channels and on shoals and a numerical model to clarify the exchange mechanisms driving sediment transport patterns in a strongly anthropogenically modified environment. The results indicate that the stepwise construction of hydraulic structures leads to gradual changes in sediment exchange. The first phase was characterized by partially blocked sediment exchange with northward sediment transport towards the main channel and to the northern flats (2002–2010). Next, a transition period was characterized by weaker horizontal sediment exchange and reduced sediment supply (2010–2016). Since 2016, more efficient structures blocking sediment exchange further hinder northward transport and promote deposition on the southern flats. These processes point to the important role of engineering works in strengthening the southward growth of the delta. Moreover, data analyses suggest that northward over-jetty flow during high water induces a net sediment flux towards the channel due to water level gradients. The residual flow controls the net sediment transport both in the longitudinal and lateral direction over the tidal flats. Therefore, a clockwise residual circulation cell forms in the channel-shoal system, contributing to the channel siltation. These findings shed important insights into the role of sediment exchange in channel siltation and large-scale hydrodynamic and delta development. Such knowledge is crucial for sustainable future management of delta distributaries.

The survival of salt marshes, especially facing future sea-level rise, requires sediment supply. Sediment can be supplied to salt marshes via two routes: through marsh creeks and over marsh edges. However, the conditions of tides and waves that facilitate sediment import through these two routes remain unclear. To understand when and how sediment is imported into salt marshes, 2-month measurements were conducted to monitor tides, waves, and suspended sediment concentration (SSC) in Paulina Saltmarsh, a meso-macrotidal system. The results show that the marsh creek tends to import sediment during neap tides with waves. A tidal cycle with a small tidal range result in weaker flow in the marsh creek during ebb tides, reducing the export of sediment. Waves enhance sediment supply to the marsh creek by eroding mudflats. However, strong waves can directly resuspend sediment in marsh creeks during spring tides when the water level is above the marsh canopy, enhancing sediment export through creeks. Net sediment import over marsh edges requires the opposite tidal and wave conditions: spring tides with weak waves. Spring tides provide stronger hydrodynamics, facilitating sediment import over the marsh edge. Increased SSC during the ebb phase can occur with strong waves over the marsh edge, resulting in net sediment export. Therefore, the net import or export of sediment, through the creek and over the marsh edge, depends on the combination of tidal and wave conditions. These conditions can vary between estuaries and even individual marshes. Understanding these conditions is crucial for better management of salt marshes.

Marsh creeks are perceived as important conduits for transporting water and sediment between mudflats and marshes. In order to advance the understanding of the transport mechanisms in creeks, the source and ultimate sink of sediment which moves between mudflats and marshes through creek channels need further investigation. Therefore, two field campaigns were conducted in two intertidal systems with varying sediment availability. The water depth, flow velocity, suspended sediment concentration, and bed level change were measured simultaneously in a marsh creek and on the adjacent mudflat in Chongming Island (China) and in Paulina Saltmarsh (the Netherlands). Paulina Saltmarsh is much smaller, more frequently flooded, and has lower sediment concentration than Chongming. These contrasting conditions allow for a comparison of transport mechanisms and functioning of the creek. Both systems first show that the high suspended sediment concentration (SSC) measured in marsh creeks is mainly the consequence of sediment advection rather than local erosion. In addition, erosion in marsh creeks is usually limited during ebb tides, reducing the export of sediment through these creeks. However, differences have been observed between two systems. The measured SSC was highly asymmetric between flood and ebb tides in Chongming. Large peaks in SSC during the flood period can be observed for most tidal cycles. The marsh creeks in Chongming therefore function as conduits for sediment import. Additionally, there are distinct overbank and underbank tides in Chongming. Sediment was trapped and retained in creeks during underbank tides, which can then be eroded and transported to the marsh during subsequent overbank tides. We also observed that mudflats in Chongming quickly recovered after erosion. These mechanisms have not been observed in Paulina Saltmarsh, where net sediment export via the marsh creek was observed due to a lack of abundant sediment in suspension during flood tides. Furthermore, the remaining bed surface of mudflats after an erosion event was stronger than before, limiting further erosion in Paulina Saltmarsh. These findings from the two systems indicate that the role of creeks in sediment import/export depends on the availability of sediment from mudflats, shedding light on nourishment strategies for salt marshes.

Strong hydrodynamic forces generated by storms are key in shaping coastal tidal flats. Most tidal flats achieve equilibrium by adapting to hydrodynamic conditions and sediment inputs. However, high-energy wave activity during storms disrupts this equilibrium, causing rapid and significant changes, particularly in tidal flats, especially in microtidal flats, which are characterized by low tidal ranges. In this study, we conducted an 11-d field campaign on a microtidal flat in the Yellow River Delta (YRD), capturing data during both stormy and calm weather conditions. We measured tidal currents, wave activity, suspended sediment concentrations and sediment grain sizes. The results demonstrated that the tidal flat maintained equilibrium under calm conditions, with minimal fluctuations in bed level (within ±2 mm). Contrastingly, severe erosion and sediment removal during the storm significantly altered the equilibrium of the area. The storm-induced high shear stresses, ranging from 1.02 to 1.48 N/m ², along with alongshore sediment transport, resulted in an elevation change of −10 mm. Furthermore, the subsequent bed level recovery was minimal and insufficient to offset the erosion. Compared to that of the mesotidal and macrotidal flats, post-storm recovery on microtidal flats was limited due to shorter inundation periods and weaker hydrodynamic forces. Therefore, frequent storms may lead to continuous shoreline retreat on microtidal coasts. Conclusively, the present findings underscore the significant impact of storm-induced erosion on the evolutionary processes of microtidal flats and suggest that greater attention should be given to protecting these areas during storms in the Yellow River Delta. The insights can guide the development of more effective coastal protection strategies, highlighting the need for enhanced measures to mitigate erosion and promote resilience in microtidal regions.