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The tidal reach of the Tanjiang River Basin, influenced by both runoff and tidal forces, exhibits complex and variable water levels and river flow conditions. Traditional hydrodynamic models struggle to meet the timeliness requirements for flood forecasting and early warning. Moreover, existing machine learning studies have limitations in integrating hydrological principles and explaining model applicability. This study focuses on this region, constructing a one-dimensional hydrodynamic model based on Saint-Venant’s equations to simulate flood-tide evolution. Using the model’s results, three typical machine learning surrogate models—Long Short-Term Memory (LSTM), Random Forest (RF), and Support Vector Machine (SVM)—are developed to predict the water level at the key control section of Changsha Station. The results show that the surrogate models offer advantages in both timeliness and convenience. Among them, the RF model has the highest prediction accuracy and can accurately identify key hydrological influencing factors. The LSTM model effectively captures the temporal dependencies of hydrological factors, considering the influence of historical moments on current water levels. However, the SVM model performs poorly under extreme flood-tide conditions. SHAP analysis reveals that the feature contribution values in both LSTM and RF models better align with the actual hydrology of the basin. Incorporating hydrological principles into the selection of input features helps improve model accuracy, while employing Stacking model to combine the strengths of different models further enhances prediction precision. This research provides a scientific basis and effective reference for the application of machine learning models in flood forecasting for tidal reaches, with important implications for advancing smart water conservancy construction.

Conventional methods of measuring water discharge and suspended sediment concentration (e.g., water sampling and moving acoustic Doppler current profiler [ADCP]) present challenges in large tidal rivers due to temporal and spatial constraints. This study introduces a novel approach to monitor water discharge and suspended sediment discharge (SSD) in large tidal rivers. Total water discharge and SSD exhibit notable variability in tidal rivers due to the river–tidal interactions; understanding this variability and its causes is essential for effective tidal river management. From June to November 2023, a field study was conducted at Nanjing (NJ) to continuously monitor water discharge, suspended sediment concentration (SSC), and SSD in the tidal reaches of the Yangtze River using coastal acoustic tomography (CAT). Total water discharge ranged from 8,765 to 43,356 m3/s, with a mean of 27,825 m3/s, while tidal discharge varied between −11,998 and 9,983 m3/s, with a mean of 69 m3/s. SSC ranged from 0.02 to 0.09 kg/m3, and SSD ranged from 110 to 3,823 kg/s. Tidal variations in SSC and SSD were within ±0.04 kg/m3 and −1,252 to 1,410 kg/s, respectively. Over short timescales, tides caused instantaneous fluctuations in velocity, water discharge, and SSD, with tides contributing −40% to instantaneous water discharge and SSD at NJ. Over seasonal timescales, no significant wet/dry variations were observed in water discharge, SSC, or SSD during a few months of 2023. Long‐term CAT application (e.g., decades) is required to reveal trends in tidal river dynamics. Plain Language Summary Due to temporal and spatial limitations, traditional methods for measuring suspended sediment concentration (SSC) and discharge, such as moving acoustic Doppler current profilers (ADCP), fail to directly measure transect variations in water discharge, SSC, and SSD in tidal reaches of the Yangtze River. This study developed a new method using coastal acoustic tomography (CAT). Two CAT systems were utilized to continuously measure water discharge, SSC, and SSD at the Nanjing Tidal Station. The CAT results were highly consistent with traditional methods, showing a correlation coefficient greater than 0.9. This study demonstrates the potential of CAT for continuous, real‐time monitoring of water discharge, SSC, and SSD in large tidal rivers. The results showed that mean water discharge, SSC, and SSD are primarily driven by river flow at Nanjing, while tides induce instantaneous variations in water discharge and sediment transport. Key Points Coastal acoustic tomography enabled water discharge and suspended sediment discharge (SSD) monitoring in Nanjing tidal reach of Yangtze River Total water discharge and SSD at Nanjing varied from 8,765 to 43,356 m3/s and 110–3,823 kg/s from June to November 2023, respectively Tides can directly trigger instantaneous variations in sediment discharge, while average sediment discharge is river‐dominated at Nanjing

Abiotic filters that interact with wetland plant communities along tidal–fluvial gradients are highly dynamic, and understanding their quantitative thresholds and relationships to interspecific competition is important during an era of sea‐level rise and watershed hydrologic change. Yet, landscape‐scale studies of major coastal rivers from the river mouth to the head of tide, such as this study, remain rare. Here, we develop a new predictive framework for estuarine–tidal river research and management using a river‐specific low‐water datum and the wetland inundation indicator SEVg, the growing‐season sum exceedance value of hourly surface‐water depth. The distribution and variability of the wetland species pool (n = 203) on the 234 river kilometer (rkm) lower Columbia River and estuary floodplain are described for the first time. 4,940 quadrats at 50 marshes were surveyed (2005–2016). Throughout the estuarine–tidal river system, SEVg was well suited to describe the wetland inundation regime and its variability based on the combination of longitudinal river position and elevation. SEVg increased significantly landward. Two primary wetland inundation regimes were identified: the seaward‐tidal, usually greatest during the winter months, and landward‐fluvial, greatest during the growing season. Nearest the ocean, salinity is the abiotic factor limiting species richness and non‐native species. Farther upriver, the daily, seasonal, and interannual variability of the wetting and drying cycle encourage disturbance‐tolerant species and non‐natives and limit the number of hydrophytes and total vegetative cover. Hence, the average between‐year similarity of site‐scale areal cover significantly decreased landward. Hierarchical cluster analysis indicated five vegetative groups and five ecohydrologic zones between rkm 0 and 234 were discriminated with 76 significant species–zone associations. All zones had unique indicator species. Species with high indicator values were Carex lyngbyei throughout the estuarine zones, and Eleocharis palustris, Sagittaria latifolia, and the invasive non‐native Phalaris arundinacea in the upper estuarine and lower, middle, and upper tidal river zones (IV > 0.90). Competition from C. lyngbyei nearest the ocean and P. arundinacea in the tidal river was associated with reduced species richness when total cover was >65%. This framework of filters informs the design and prediction of future wetland plant communities on coastal river floodplains.

River junctions act as critical nodes in river networks because they can affect flows, sediment transports, and morphological and ecological patterns. River junctions subject to the unidirectional flow have been widely studied in the last decades; in contrast, the efforts are limited regarding the understanding of flow behaviors and morphological changes around tidal river junctions. In this study, a numerical model coupling two- and three-dimensional (2D-3D) domains is established to study the flow patterns and sediment motion of the tidal reach of Rongjiang River (RR), which has a typical tidal river junction. The simulation results show that the continuous process of alternating merging and separating streams leads to the swing of flow dynamic axes in the planar field, and results in the periodic inversion of secondary flows (helical flows) around the junction. These features are unique and different from the hydrodynamics of fluvial junctions. Moreover, a simulation of the particle moving indicates that the periodic 3D circulation around the junction can make the suspended sediment tend to gather in the north branch, which leads to the net input of the sediment into the north branch. Additionally, the long-term morphological evolutions and potential changes are analyzed by historical data and a numerical experiment. The numerical experiment results illustrate the significant sedimentation at river banks and deepening at mid-channels under the effects of tidal currents, which also demonstrates that the net sediment input to the tributary is a potential cause and mechanism of the distinct bed discordance in tidal river junctions. Furthermore, these findings emphasize the importance of periodical flow behaviors and local hydraulics on the dynamics around the tidal river junctions, which can expand the understanding of physics both in tidal and non-tidal river reaches and provide a reference for tidal river management.

The Ganges-Brahmaputra (GB) delta is one of the most disaster-prone areas in the world due to a combination of high population density and exposure to tropical cyclones, floods, salinity intrusion and other hazards. Due to the complexity of natural deltaic processes and human influence on these processes, structural solutions like embankments are inadequate on their own for effective hazard mitigation. This article examines nature-based solutions (NbSs) as a complementary or alternative approach to managing hazards in the GB delta. We investigate the potential of NbS as a complementary and sustainable method for mitigating the impacts of coastal disaster risks, mainly cyclones and flooding. Using the emerging framework of NbS principles, we evaluate three existing approaches: tidal river management, mangrove afforestation, and oyster reef cultivation, all of which are actively being used to help reduce the impacts of coastal hazards. We also identify major challenges (socioeconomic, biophysical, governance and policy) that need to be overcome to allow broader application of the existing approaches by incorporating the NbS principles. In addition to addressing GB delta-specific challenges, our findings provide more widely applicable insights into the challenges of implementing NbS in deltaic environments globally.

Spatially varying water-level regimes are a factor controlling estuarine and tidal-fluvial wetland vegetation patterns. As described in Part I, water levels in the Lower Columbia River and estuary (LCRE) are influenced by tides, river flow, hydropower operations, and coastal processes. In Part II, regression models based on tidal theory are used to quantify the role of these processes in determining water levels in the mainstem river and floodplain wetlands, and to provide 21-year inundation hindcasts. Analyses are conducted at 19 LCRE mainstem channel stations and 23 tidally exposed floodplain wetland stations. Sum exceedance values (SEVs) are used to compare wetland hydrologie regimes at different locations on the river floodplain. A new predictive tool is introduced and validated, the potential SEV (pSEV), which can reduce the need for extensive new data collection in wetland restoration planning. Models of water levels and inundation frequency distinguish four zones encompassing eight reaches. The system zones are the wave- and current-dominated Entrance to river kilometer (rkm) 5; the Estuary (rkm-5 to 87), comprised of a lower reach with salinity, the energy minimum (where the turbidity maximum normally occurs), and an upper estuary reach without salinity; the Tidal River (rkm-87 to 229), with lower, middle, and upper reaches in which river flow becomes increasingly dominant over tides in determining water levels; and the steep and weakly tidal Cascade (rkm-229 to 234) immediately downstream from Bonneville Dam. The same zonation is seen in the water levels of floodplain stations, with considerable modification of tidal properties. The system zones and reaches defined here reflect geological features and their boundaries are congruent with five wetland vegetation zones.

Cities and towns along the tidal Hudson River are highly vulnerable to flooding through the combination of storm tides and high streamflows, compounded by sea level rise. Here a three-dimensional hydrodynamic model, validated by comparing peak water levels for 76 historical storms, is applied in a probabilistic flood hazard assessment. In simulations, the model merges streamflows and storm tides from tropical cyclones (TCs), offshore extratropical cyclones (ETCs) and inland “wet extratropical” cyclones (WETCs). The climatology of possible ETC and WETC storm events is represented by historical events (1931–2013), and simulations include gauged streamflows and inferred ungauged streamflows (based on watershed area) for the Hudson River and its tributaries. The TC climatology is created using a stochastic statistical model to represent a wider range of storms than is contained in the historical record. TC streamflow hydrographs are simulated for tributaries spaced along the Hudson, modeled as a function of TC attributes (storm track, sea surface temperature, maximum wind speed) using a statistical Bayesian approach. Results show WETCs are important to flood risk in the upper tidal river (e.g., Albany, New York), ETCs are important in the estuary (e.g., New York City) and lower tidal river, and TCs are important at all locations due to their potential for both high surge and extreme rainfall. The raising of floods by sea level rise is shown to be reduced by ~ 30–60% at Albany due to the dominance of streamflow for flood risk. This can be explained with simple channel flow dynamics, in which increased depth throughout the river reduces frictional resistance, thereby reducing the water level slope and the upriver water level.

Soundscape ecology is a relatively new scientific field that uses sound to characterize ecosystems, which can be helpful in tracking species, estimating relative population sizes, and monitoring behavior and overall habitat quality. Estuarine soundscapes are acoustically rich, and sound patterns in these systems are understudied. Therefore, the goal of this study was to understand the soundscape of a deep tidal river estuary, the May River, South Carolina, USA. Acoustic recorders (DSG-Oceans) were deployed to collect sound samples for 2 min every 20 min at 6 stations from February to November 2014. Acoustic data revealed that sound pressure levels (i.e. broadband, low, and high frequency) varied spatially and temporally, exhibiting distinct rhythmic patterns. Acoustic detection rates and diversity of biophonic (e.g. snapping shrimp, fish, and bottlenose dolphins Tursiops truncatus) and anthrophonic sounds (e.g. boat noise) were higher near the river mouth and decreased towards the headwaters. The soundscape exhibited strong temporal patterns of snapping shrimp (genus Alpheus and Synalpheus) snaps, fish calls and choruses (e.g. silver perch Bairdiella chrysoura, black drum Pogonias cromis, oyster toadfish Opsanus tau, spotted seatrout Cynoscion nebulosus, and red drum Sciaenops ocellatus), bottlenose dolphin vocalizations, and vessel noise. Depending upon the species, certain variables (i.e. location, month, day length, lunar phase, day/night, tide, and temperature anomaly) influenced sound production. These data provide new tools and baseline measurements to better understand how soundscapes can be used to gauge habitat quality and impacts of stormwater runoff, climate change, and noise pollution.

The southwest coastal delta of Bangladesh is not only geographically home to a dynamic interplay between land and water, and between fresh surface water and saline tides, but also to contentious debates on flood management policy. It has been argued that dealing with delta floods in this region boils down to adopting either open or closed approaches. This paper longitudinally structures the open-or-closed debate based on a number of emblematic water management projects in the region. Departing from a typical open wetland history, river and polder embankments increasingly started to constrain flood dynamics. Upheaval among rural populations in response to the negative impacts of hydraulic engineering plans and works coalesced in efforts to restore open approaches, synthesized in the Tidal River Management concept. Its resemblance to historic overflow irrigation is often used politically as a yardstick to challenge the dominant hydraulic engineering paradigm. This paper argues that dealing with floods in Bangladesh requires plans, policies and projects formulated against the historic background of complex interactions among social processes, environmental dynamics and technological interventions: a lesson to be incorporated in on-going policy-making processes and long-term delta management plans.

Intense anthropogenic impacts in tidal rivers can cause habitat loss and ecosystem degradation. In addition, changes in water temperature associated with climate change are significantly impacting the distribution area of fish fauna within tidal rivers. In the present study, we used long-term fish fauna data to determine the relationship between climate change-induced increases in water temperature and changes in the distribution of fish species in tidal rivers in the Japanese archipelago. The distribution ranges of many subtropical and tropical fish species were found to move northward in areas affected by warm currents, suggesting further possible distributional dispersal in future. This study is the first to examine the nationwide distributional changes and future projections of fish fauna in tidal rivers. The results suggest that many subtropical and tropical fishes are expanding their distribution areas in tidal rivers and in coastal and estuarine areas.