Search

This study assesses the settling dynamics of suspended sediments along the hyper‐turbid Gironde Garonne fluvial‐estuarine system, with an innovative optical SCAF instrument (System of Characterization of Aggregates and Flocs). Two fields campaigns were carried out to determine the settling velocity and properties of suspended sediments during a semi‐diurnal tidal cycle, as well as hydrodynamic conditions and water properties. The two sampling stations were representative of two regions: a tidal river dominated by fresh water and an estuary affected by salty or brackish waters. A high spatial variability of the settling velocity was observed along the fluvial‐estuarine system and vertically along the water column. Settling velocities ranged from 0.02 to 0.4 mm/s. This study confirms that in hyper‐turbid systems, the suspended sediment concentration (SSC) is predominantly driving the settling dynamics of suspended sediment. Threshold concentrations have been defined for the flocculation and hindered regimes where the settling velocity may vary by one order of magnitude. Although in natural environments it is difficult to distinguish between the effects of SSC and turbulence (as they are correlated), in the Gironde‐Garonne system the turbulent shear G seems to affect the settling of suspended sediment to a lower extent. Settling velocity variations cannot be directly correlated to salinity or organic matter content. Despite differences in hydrodynamic and environmental conditions in fluvial and estuarine regions, a common prediction law has been found to estimate settling velocity of suspended sediment as a function of suspended sediment concentration. Plain Language Summary: Estuaries and rivers are biotically rich environments strongly impacted by human activities. Mud trapping capacity of such systems has a major influence on water quality by reducing light availability, promoting oxygen depletion and by trapping adsorbed contaminants, bacteria and nutrients. A key dynamical parameter impacting the trapping of mud is the sediment settling velocity. Sediment settling is influenced by a wide range of environmental factors such as salinity, sediment concentration, turbulence of the flow and organic matter. This manuscript presents sediment settling data from field surveys carried out along the Garonne River—Gironde Estuary system (France), where large quantities of mud are trapped during the dry season. It highlights the driving role of sediment concentration on the settling dynamics along the entire system, despite the hydrodynamics and water properties of riverine waters differing from the estuarine waters. The turbulence of the flow appeared to be of secondary importance. An unique empirical prediction law has been established for the whole system contrary to other systems around the world. An improved understanding of sediment fluxes contributes to effective waterways management and the preservation of essential ecological environments. Key Points: In such hyper‐turbid system, the suspended sediment concentration predominantly drives the settling dynamics of suspended sedimentThe turbulent shear appeared to be a control parameter of secondary importanceA common prediction law can be found for the whole system [ABSTRACT FROM AUTHOR]

Salt marshes are being lost worldwide even though they provide essential ecosystem services. Habitat classification of salt marshes based on fauna, which is an effective indicator of their ecosystems, is expected to be useful to effectively assess the baseline for conservation and restoration; however, it has not been well studied. This study aimed to identify the diversity of salt marsh sub-habitats for fauna, where biota was collected and the physicochemical environments were measured in 101 small patches of salt marsh (i.e., survey sites) across 32 temperate tidal rivers and inlets in Fukuoka Prefecture, Kyushu, Japan. A total of 78 aquatic animal species (decapod crustaceans, gastropod mollusks, bivalves, nereidid polychaetes, and fish) were observed, 63 of which were used as indicators for sub-habitat identification. Although reed plants covered 90 of the 101 survey sites, the survey sites were classified into six clusters based on fauna. Representative aquatic animals were identified in each cluster, and these six clusters had different physicochemical environments, such as salinity and sediment conditions. The results indicated that the six sub-habitats had different biotic and abiotic characteristics. On the other hand, the results of the ordination analysis indicated that the fauna of the six sub-habitats do not occur independently of each other but are composed of continuous and gradually changing fauna along environmental gradients. Although no significant differences in the areas of salt marsh patches among the clusters, species richness differed significantly, suggesting that physicochemical environments, such as salinity and sediment conditions, rather than spatial factors, may affect species diversity. The findings emphasize that each sub-habitat supports species diversity in temperate salt marshes and are expected to contribute to planning targets for future salt marsh conservation/restoration and further improvements in artificial marshes.

Against the backdrop of rapid global climate change, the frequency and severity of storm surges in coastal areas are increasing, particularly in tidal river segments that are affected by storm surges and upstream river flooding. Although existing storm surge models have introduced a variety of different boundary settings, the boundary conditions provided are limited and cannot meet the current generalization needs of complex hydraulic engineering projects in China. This study considered the Feiyun River Basin as the research subject and coupled the upstream hydrodynamic model IFMS with the oceanic storm surge model ADCIRC. By utilizing the strengths of both models, a flood evolution model for the estuarine tidal river segment was established, enabling the spatiotemporal simulation of tidal levels in the Feiyun River tidal segment. The model not only effectively considers the impact of storm surge propagation at the estuary on flood evolution in the tidal river segment, but also the effect of upstream river flooding on the area. The study first validated the model with Typhoon Meranti in 2016, where the simulation results showed a high degree of agreement with the observed data series and errors were within acceptable limits. Flood processes at the Ruian, Mayu, Bishan Liqiao, and Dongtou tidal stations during Typhoons Doksuri and Khanun were simulated. The results show that the peak flood errors at all four stations were below 0.30 m, with Nash coefficients >0.80, indicating the model's capability to accurately reflect tidal level fluctuations and effectively contribute to disaster prevention and mitigation efforts in estuarine tidal segments. Finally, the study analyzed the impact of the driving forces of the upstream and downstream boundaries on tidal level predictions at three stations (Ruian, Mayu, and Bishan Liqiao). It was concluded that, compared to Mayu and Bishan Liqiao stations, the influence of the upstream boundary on Ruian can essentially be ignored, suggesting that the error from the upstream boundary under the influence of Typhoon Khanun is negligible for predicting errors at Ruian. The degree of the impact of the downstream boundary fluctuations on the three stations, from largest to smallest, was Ruian, Bishan Liqiao, and Mayu. Compared to the changes in the upstream boundary, the downstream boundary had a greater overall impact on all three stations. Additionally, when the downstream boundary changed by the same magnitude, the variation in low tide levels showed a decreasing trend from downstream to upstream, whereas the variation in high tide levels, although following the same trend, did not show a significant difference between the three. In summary, compared to the upstream boundary, the downstream boundary had a greater impact on tidal-level predictions at the three stations. The result shows that the lower boundary has a greater impact on the tidal level forecasts at three stations compared to the upper boundary. The study not only provides a new method for tidal river flood simulation in coastal urbanized areas but also offers directions for improving model simulation accuracy through analysis.

The complexity of river-tide interaction poses a significant challenge in predicting discharge in tidal rivers. Long short-term memory (LSTM) networks excel in processing and predicting crucial events with extended intervals and time delays in time series data. Additionally, the sequence-to-sequence (Seq2Seq) model, known for handling temporal relationships, adapting to variable-length sequences, effectively capturing historical information, and accommodating various influencing factors, emerges as a robust and flexible tool in discharge forecasting. In this study, we introduce the application of LSTM-based Seq2Seq models for the first time in forecasting the discharge of a tidal reach of the Changjiang River (Yangtze River) Estuary. This study focuses on discharge forecasting using three key input characteristics: flow velocity, water level, and discharge, which means the structure of multiple input and single output is adopted. The experiment used the discharge data of the whole year of 2020, of which the first 80% is used as the training set, and the last 20% is used as the test set. This means that the data covers different tidal cycles, which helps to test the forecasting effect of different models in different tidal cycles and different runoff. The experimental results indicate that the proposed models demonstrate advantages in long-term, mid-term, and short-term discharge forecasting. The Seq2Seq models improved by 6%–60% and 5%–20% of the relative standard deviation compared to the harmonic analysis models and improved back propagation neural network models in discharge prediction, respectively. In addition, the relative accuracy of the Seq2Seq model is 1% to 3% higher than that of the LSTM model. Analytical assessment of the prediction errors shows that the Seq2Seq models are insensitive to the forecast lead time and they can capture characteristic values such as maximum flood tide flow and maximum ebb tide flow in the tidal cycle well. This indicates the significance of the Seq2Seq models. [ABSTRACT FROM AUTHOR]

Flood disasters are destructive especially in prosperous and urbanized estuarine regions, where the flood regime is much more complex due to multiple fluvial–estuarine impacts. The Shenzhen River (SZR), located in one of the most prosperous regions of southern China, is vulnerable to increasing flood risk. Unravelling the influential factors is of particular significance to flood hazards prevention and urban safety for the SZR. Based on the field-measured floods on June 13, 2008 ("2008.06") and August 29, 2018 ("2018.08") with roughly equal magnitudes of rainfall and tide, the changing flood risk in the SZR basin was assessed. Considering the substantial development of tidal flat plants in the past two decades, a physical model of the SZR was built to quantify the impacts of changing river regime on the flood stage. The model covers the whole mainstream of tidal reach and half of the Shenzhen Bay (SZB), which was well calibrated and validated by in situ flow process. Several situations with different ranges of riverine vegetation and estuarine mangrove, including all vegetation (actual situation), half vegetation, no vegetation, mangrove in 2002 and 2018, were modelled to explore flood stage variations in 2-yr and 50-yr return period. The results found that the "2018.08" flood stage was about 1.4 m higher than "2008.06" flood. Moreover, the rainfall–runoff duration in "2018.08" was significantly decreased by 1 h less than that of "2008.06" flood, indicating increased flood risk in the SZR. The flood stage in the middle reach increases by more than 0.6 m driven by the riverine vegetation during the 50-yr return period flood, while the flood stage rises less than 0.1 m for the flood with 2-yr return period. Moreover, the extended estuarine mangrove forest resulted in about 0.2 m flood stage increment in the lower reach. The effects of sea level rise and sediment deposition after channel dredging on the flood risk in the SZR were further discussed. The effects of sea level rise and sediment deposition after channel dredging on flood risk were further discussed in the SZR. Channel infilling probably causes a flood stage increase of approximately 0.5 m for all reaches, while the influence of sea level is relatively slight but cumulative. Sufficient river management and planning, such as seasonal removal of riverine vegetation, mangrove management and regular topography surveys, should be taken into consideration in the near future. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Study region: Chao Phraya River Basin—a major river with unique characteristics located in Thailand. Study focus: This study sought to simulate the flow rates in the Chao Phraya River Basin, which is a tidal river that poses challenges to traditional modeling approaches. The soil and water assessment tool (SWAT) is a hydrological model extensively employed for simulating flow rates. However, limitations arise in applying the SWAT model to the Chao Phraya River Basin due to its tidal nature, resulting in an unsatisfactory model performance. To address this, a long short-term memory (LSTM) model, i.e., the SWAT–LSTM model, was introduced to complement the SWAT model. New hydrological insights for the Region: The collaborative coupling of hydrological information derived from the SWAT and LSTM notably enhanced the model performance. This improvement was assessed using the Nash-Sutcliffe efficiency (NSE), demonstrating an increase from 0.13 to 0.72. The incorporation of topographic static data in the coupling model was also investigated to provide the basic characteristics of the basin to the model. The results yielded an NSE exceeding 0.79. The shoreline water level was identified as a crucial input feature for indicating tidal patterns. The findings highlight the effectiveness of coupling the SWAT with LSTM for predicting tidal river flow rates, implying their applicability in similar scenarios across different basins.

Decreases in shallow-water habitat area (SWHA) in the Lower Columbia River and Estuary (LCRE) have adversely affected salmonid populations. We investigate the causes by hindcasting SWHA from 1928 to 2004, system-wide, based on daily higher high water (HHW) and system hypsometry. Physics-based regression models are used to represent HHW along the system as a function of river inflow, tides, and coastal processes, and hypsometry is used to estimate the associated SWHA. Scenario modeling is employed to attribute SWHA losses to levees, flow regulation, diversion, navigational development, and climate-induced hydrologic change, for subsidence scenarios of up to 2 m, and for 0.5 m fill. For zero subsidence, the system-wide annual-average loss of SWHA is 55 ± 5%, or 51 × 105 ha/year; levees have caused the largest decrease ( 54 - 14 + 5 %, or ~ 50 × 105 ha/year). The loss in SWHA due to operation of the hydropower system is small, but spatially and seasonally variable. During the spring freshet critical to juvenile salmonids, the total SWHA loss was 63 - 3 + 2 %, with the hydropower system causing losses of 5–16% (depending on subsidence). Climate change and navigation have caused SWHA losses of 5 - 5 + 16 % and 4 - 6 + 14 %, respectively, but with high spatial variability; irrigation impacts have been small. Uncertain subsidence causes most of the uncertainty in estimates; the sum of the individual factors exceeds the total loss, because factors interact. Any factor that reduces mean or peak flows (reservoirs, diversion, and climate change) or alters tides and along-channel slope (navigation) becomes more impactful as assumed historical elevations are increased to account for subsidence, while levees matter less. [ABSTRACT FROM AUTHOR]

Water temperature is a critical ecological indicator; however, few studies have statistically modeled century‐scale trends in riverine or estuarine water temperature, or their cause. Here, we recover, digitize, and analyze archival temperature measurements from the 1850s onward to investigate how and why water temperatures in the lower Columbia River are changing. To infill data gaps and explore changes, we develop regression models of daily historical Columbia River water temperature using time‐lagged river flow and air temperature as the independent variables. Models were developed for three time periods (mid‐19th, mid‐20th, and early 21st century), using archival and modern measurements (1854–1876; 1938–present). Daily and monthly averaged root‐mean‐square errors overall are 0.89°C and 0.77°C, respectively for the 1938–2018 period. Results suggest that annual averaged water temperature increased by 2.2°C ± 0.2°C since the 1850s, a rate of 1.3°C ± 0.1°C/century. Increased water temperatures are seasonally dependent. An increase of approximately 2.0°C ± 0.2°C/century occurs in the July–Dec time‐frame, while springtime trends are statistically insignificant. Rising temperatures change the probability of exceeding ecologically important thresholds; since the 1850s, the number of days with water temperatures over 20°C increased from ~5 to 60 per year, while the number below 2°C decreased from ~10 to 0 days/per year. Overall, the modern system is warmer, but exhibits less temperature variability. The reservoir system reduces sensitivity to short‐term atmospheric forcing. Statistical experiments within our modeling framework suggest that increased water temperature is driven by warming air temperatures (~29%), altered river flow (~14%), and water resources management (~57%). [ABSTRACT FROM AUTHOR]

Objective: The Fraser River in British Columbia, Canada, historically supported productive Indigenous, recreational, and commercial fisheries for Eulachon Thaleichthys pacificus. Although Eulachon spend most of their lives in the marine environment, they spawn in freshwater and it is hypothesized that habitat degradation in the Fraser River has contributed to population decline. A greater understanding of Eulachon life history strategies is necessary to develop species recovery strategies. The purpose of this study was to provide insight into population demographics, migration timing, residence time, travel speed and distance, and movement patterns of Eulachon traveling in the Fraser River by conducting index netting and acoustic telemetry. Methods: In total, 244 Eulachon were acoustically tagged at the mouth of the Fraser River, and their movements were monitored using acoustic receivers in the lowermost 70 km of the Fraser River and in the Pitt River (a tributary to the Fraser River) over a 2‐year period. Result: Tagged Eulachon displayed heterogeneous movement behaviors and a protracted migration period between February and May. Eulachon index gill netting revealed three discrete migration groups matching traditional ecological knowledge from Indigenous knowledge holders. There were shifts in population demographics, with sex ratios changing from predominately males to females throughout the migration. Residence time within the estuarine salt wedge and river was short. Upstream travel speeds varied significantly between sexes and slowed as fish traveled upstream. Conclusion: Overall, Eulachon travel speeds, travel distances, and residence times, along with size and sex effects, provided insights into movement patterns. These results will increase our understanding of sex‐specific risks related to instream disturbance, harvest, and predation and will inform management decisions regarding protection of critical habitats. The observed patterns highlight the need for stock assessment methods to consider all discrete migration groups and will assist in prioritizing species recovery efforts. Impact statementThis research increased our understanding of Eulachon ecology by providing insight into migratory movement patterns and population dynamics in the Fraser River, which will assist habitat management decisions and aid future recovery efforts. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present a new hydrological dataset collected during a field campaign in the Saigon-Dong Nai estuary system, Vietnam. These data include water level and water temperature measurements at five locations along the Saigon river and 2 locations along the Dong Nai river as well as discharge measurements from four 24-hour Acoustic Doppler Current Profiler (ADCP) campaigns at 2 locations in the Saigon river and 1 location in the Dong Nai river. Additionally, water level was barometrically compensated using air pressure measurements. Data were sampled between October 21st, 2022 and December 16th, 2022 and are provided in three processing stages namely, direct measurements as provided by the sensors (raw), barometricaly compensated measurements (pre-processed) and corrected measurements (post-processed). Even though of short duration (about 2 months), this dataset provides water level measurements at unprecedented spatial and temporal resolution in a region where data is scarce and not freely available. The synchronous logging of multiple water level sensors along river provides an opportunity to study profiles of water surface slope and upstream tidal propagation. Furthermore, the concurrent discharge measurements can be used to calibrate hydrological and/or hydraulic models of this estuary system. Additionally, the spatial resolution of this dataset is similar to the prospective measurements that the novel Surface Water and Ocean Topography (SWOT) satellite will provide. Thus, it enables the study of synthetic SWOT measurements to evaluate the future potential of the SWOT satellite over this estuary system.

In this study, a novel model that performs ensemble empirical mode decomposition (EEMD) and stepwise regression was developed to forecast the water level of a tidal river. Unlike more complex hydrological models, the main advantage of the proposed model is that the only required data are water level data. EEMD is used to decompose water level signals from a tidal river into several intrinsic mode functions (IMFs). These IMFs are then used to reconstruct the ocean and stream components that represent the tide and river flow, respectively. The forecasting model is obtained through stepwise regression on these components. The ocean component at a location 1 h ahead can be forecast using the observed ocean components at the downstream gauging stations, and the corresponding stream component can be forecast using the water stages at the upstream gauging stations. Summing these two forecasted components enables the forecasting of the water level at a location in the tidal river. The proposed model is conceptually simple and highly accurate. Water level data collected from gauging stations in the Tanshui River in Taiwan during typhoons were used to assess the feasibility of the proposed model. The water level forecasting model accurately and reliably predicted the water level at the Taipei Bridge gauging station. [ABSTRACT FROM AUTHOR]

At present, urban rail transit in China is developing rapidly, and there are many risks in crossing tidal rivers under subway excavation. In order to study the stratum deformation law of shallow overburden shield crossing tidal river, based on the shield method of Qingdao Metro Line 4 crossing a shallow overburden tidal river, Considered the tidal variation of the river, combined the engineering using the construction parameters such as grouting pressure, face pressure, a more realistic undulating stratum was established by means of numerical simulation, and then the deformation law of stratum was studied. It is found that there is a positive correlation between river bed deformation and river water level depth. The shape of lateral settlement curve is different between rising tide and ebb tide and the difference of maximum settlement is 50%. The time effect of tunnel excavation on surface settlement can be divided into four stages. The monitoring frequency of shield excavation and reinforcement should be strengthened. [ABSTRACT FROM AUTHOR]

Investigating subtidal friction and mass transport is pivotal for examining subtidal dynamics in tidal rivers. Although the behavior of subtidal friction and transport has been discussed in recent years, most studies have been conducted on tidal rivers that are affected by high amounts of river runoff. The aim of this study is to offer an initial understanding of the spatial and temporal behaviors of subtidal friction and subtidal flux in a tidal river channel with limited river runoff. This study utilized the frequency domain and theoretical decomposition analyses to determine the dominant tidal and subtidal mechanisms. Frequency domain analysis indicated the dominance of semidiurnal and diurnal tides in the observed tidal river channel. The rate of energy transfer owing to shallow water interaction was found to be stronger for the current velocity than for the water elevation. Decomposition analysis showed that subtidal friction and flux in a low-discharge tidal river channel were largely influenced by subtidal flow-induced subtidal friction and Eulerian return flux, respectively. The key findings of this study are as follows: (i) the limited amount of river runoff (4–20 m3/s) leads to the vertical variability of subtidal friction contributions from subtidal flow and subtidal-tidal interaction, as well as Eulerian return flux, and (ii) the vertical variability of the aforementioned terms can be associated with the existence of influential longitudinal subtidal density gradients along the tidal river. We believe that these findings advance our understanding of subtidal dynamics in tidal river systems, particularly those with limited discharge. [ABSTRACT FROM AUTHOR]

Abstract Significant declines in Atlantic sturgeon (Acipenser oxyrhynchus oxyrhynchus) abundances along the US east coast have spurred major research efforts and management actions over the last 50 years, yet information on spawning stock abundances and habitat use is still lacking for many river systems, including in the Chesapeake Bay, USA. Here, we developed and tested a new quantitative PCR (qPCR) assay to detect Atlantic sturgeon environmental DNA (eDNA) in water samples with the goal of providing an alternative method to monitor presence and relative abundance. We also examined Atlantic sturgeon eDNA shedding rates in laboratory experiments. A qPCR‐probe assay targeting Cytochrome B was developed and showed no amplification of other related and co‐occurring fishes. Pond trials at a density of ~0.2 g/L sturgeon produced relatively strong eDNA detections (~1,000–25,000 copies/L) in all seven water samples assayed. Water samples taken from two river systems in the Chesapeake Bay produced zero eDNA detections in the summer, while fall sampling during sturgeon spawning produced positive eDNA detections in 26% of samples, though at much lower concentrations (400–1,800 copies/L) compared with the pond (mesocosm) detections. Acoustic detections of sturgeon near river sampling sites were positively associated with eDNA detections. However, the eDNA assay failed to detect the presence of sturgeon in some samples when abundances were very low or when fish were in deep water. Finally, Atlantic sturgeon eDNA shedding rates were estimated to be on the order of estimates for other fish species, which suggests that relatively weak detections in the field are not necessarily driven by low rates of eDNA shedding. Overall, eDNA analysis represents a promising new monitoring tool for Atlantic sturgeon. Applying these methods in other rivers along the US east coast is an important next step in documenting Atlantic sturgeon distribution for management and conservation purposes.

[Display omitted] • Surficial lithology affects As mobility in riverbank aquifers. • Sedimentary As was associated with Fe-oxides at sandy riverbank. • Mixing with surface water drives the NRB formation at sandy riverbank. • Arsenic was associated with aluminosilicate clay minerals at silt-capped riverbank. • Aluminosilicate minerals at silt-capped riverbank can act as a potential As source. Arsenic (As) contamination in groundwater persists in South Asia. Precipitated amorphous Fe(III)-oxides regulate the mobilization of aqueous As and iron (Fe) within the hyporheic zone (HZ). Depending on the chemical stability of these Fe(III)-oxides, this so-called Natural Reactive Barrier (NRB) can function as a sink or source of aqueous As and Fe within shallow alluvial aquifers under influences of tidal and seasonal fluctuations of river stage. The extent to which surficial lithology influences the As mobility along a riverine (upstream) to tidal (downstream) continuum is uncertain. To explore this process along a tidally fluctuating river, two new study sites with contrasting surface lithology were characterized along the banks of Hooghly River. The upstream sandy riverbank aquifer experiences robust mixing with oxygen-rich surface water under influences of tidal fluctuations which maintain oxic conditions in the riverbank aquifer. Introduced riverine dissolved oxygen (DO) drives the in-situ precipitation of crystalline Fe(III)-oxides which remove dissolved As and Fe from groundwater before discharging to the river. Although sediment from the downstream silt-capped riverbank contains higher concentrations of sedimentary As and Fe compared to the sandy site, lower proportions of crystalline Fe(III)-oxide minerals were observed. Arsenic was more easily mobilized from the aluminosilicate clay minerals to which the As was primarily bound at the silt-capped riverbank, compared to the As bound to Fe(III)-oxides at the sandy site. Thus, aluminosilicates can be an important source of dissolved As. These findings demonstrate that the surficial lithology of a riverbank along a tidally and seasonally fluctuating river regulates the mobility of As and its mineralogical association within riverbank sediments in shallow riverbank aquifers. [ABSTRACT FROM AUTHOR]

The prediction for tidal level of the tidal river is affected by the upstream runoff and the tidal wave in the outer sea. The aim of this study is to overcome the harmonic constant of tidal level prediction cannot remain changed for tidal reaches. A new idea named harmonic analysis combined with the least square method, Fourier-based corrected interpolation and ridge regression analysis method was proposed. The key step of the idea is as follows: firstly, the new harmonic constant can be determined via harmonic analysis of water level station data over the corresponding period. Secondly, the Fourier-based correction method is used to interpolate the short-term tidal level data and make the data interval equal. Thirdly, the application of the ridge regression analysis method is used to deal with the multicollinearity problem which may occur in the short-term harmonic analysis. Besides, the selection rules of tidal data from data length, tide type and starting point were considered. Finally, based on an exponential function which can describe relationship between the new harmonic constant and the upstream runoff, the tidal level prediction model of Baimao station in the Yangtze River tidal reaches is constructed. The result shows that the average error of model in the water level was smaller than 0.20 m, and all the time points with a total water level error smaller than 0.25 m exceeded 69% of all points. The mean error in the tide time was smaller than 34 min, and the maximum error was smaller than 60 min. The experimental results conformed to the trend of the observed water level changes, which verified the feasibility of the established model. [ABSTRACT FROM AUTHOR]

As tides propagate inland, they become distorted by channel geometry and river discharge. Tidal dynamics in fluvial‐marine transitions are commonly observed in high‐energy tidal environments with relatively steady river conditions, leaving the effects of variable river discharge on tides and longitudinal changes poorly understood. To study the effects of variable river discharge on tide‐river interactions, we studied a low‐energy tidal environment where river discharge ranges several orders of magnitude, the diurnal microtidal Tombigbee River‐Mobile Bay fluvial‐marine transition, using water level and velocity observations from 21 stations. Results showed that diurnal tidal attenuation was reduced by the width convergence in seaward reaches and height convergence of the landward backwater reaches, with the channel convergence change location ∼40–50 km inland of the bayhead and seaward of the largest bifurcation. River events amplified tides in seaward regions and attenuated tides in landward regions. This created a region of river‐induced peak amplitude seaward of the flood limit (i.e., bidirectional‐unidirectional current transition), allowing more tidal energy to propagate inland. Tidal currents were attenuated and delayed more by river discharge than water levels, making the phase lag dynamic. The river impacts on the tides were delineated longitudinally and shifted seaward as river discharge increased, ranging up to ∼180 km. Results indicated the longitudinal shifts of river impacts on tides in alluvial systems can be estimated analytically using the ratio of river discharge to tidal discharge and the geometric convergence of the system. Our simple analytical theory provides a pathway for understanding the tide‐river‐geomorphic equilibrium along increasingly dynamic coasts. Plain Language Summary: Coastal cities and ports are affected by tides and rivers, concentrating industry, infrastructure, and economies in regions where tide‐river interactions affect flooding, erosion, and water quality. Tide‐river interactions are commonly studied in stable rivers with large tides, limiting the variability of tide‐river interactions, leaving the effects of river discharge on tides poorly understood, and the risks for coastal communities during large river discharge events difficult to determine. To study the effects of river discharge on tide‐river interactions, observations in Alabama were investigated, spanning the Tombigbee River through Mobile Bay to the Gulf of Mexico. Results show that channel geometry and river discharge affected the tides. As river discharge increased, flood‐ebb tidal currents became seaward only, causing a region up to ∼180 km (110 miles) long to change from estuarine to riverine. The shift was captured in a simple equation using the river‐tidal energy ratio and generalized geometry, suggesting the affected region increases with the range of river discharge and that the greatest flooding and erosion risks to coastal communities can change between the sea and rivers. This simple equation provides a new tool for managers to understand tide‐river interactions now and under future changes from human development and sea level rise. Key Points: Along the fluvial‐marine transition, river discharge attenuated tides in landward regions and amplified tides in seaward regionsRegions of bidirectional tidal flow became unidirectional as river discharge increased, shifting the tidal river ∼180 km seawardPlanform channel geometry (e.g., width convergence) may not be a robust indicator of the transition in tide‐river hydrodynamics [ABSTRACT FROM AUTHOR]

In this study, a novel model that performs ensemble empirical mode decomposition (EEMD) and stepwise regression was developed to forecast the water level of a tidal river. Unlike more complex hydrological models, the main advantage of the proposed model is that the only required data are water level data. EEMD is used to decompose water level signals from a tidal river into several intrinsic mode functions (IMFs). These IMFs are then used to reconstruct the ocean and stream components that represent the tide and river flow, respectively. The forecasting model is obtained through stepwise regression on these components. The ocean component at a location 1 h ahead can be forecast using the observed ocean components at the downstream gauging stations, and the corresponding stream component can be forecast using the water stages at the upstream gauging stations. Summing these two forecasted components enables the forecasting of the water level at a location in the tidal river. The proposed model is conceptually simple and highly accurate. Water level data collected from gauging stations in the Tanshui River in Taiwan during typhoons were used to assess the feasibility of the proposed model. The water level forecasting model accurately and reliably predicted the water level at the Taipei Bridge gauging station.

The hydrodynamic of river channels in plain cities are generally of poor conditions. Accelerating water renewal and strengthening orderly flow is one of the most important ways to improve the water environment quality of river network. In this paper, a typical plain city in the lower reaches of the Yangtze River, Qidong, was studied as an example. Making full use of the natural tidal dynamic conditions, a hydrodynamic model of the river network was established. Numerical simulation experiment was carried out to quantify the improvement of hydrodynamic characteristics of the river network through water system pattern and overall arrangement optimization and water conservancy project operation. The results show that, the optimized scheme increases the ecological velocity compliance rate of the river network from 52.3% to 94.2% for the whole period under the worst tidal condition; the maximum instantaneous ecological velocity compliance rate is raised from 42.6% to 85.0%;the regional ecological velocity continuity increases to 91.8%. Meanwhile, the urban ecological water consumption reduces 67.5%. Under the optimized scheme, the diversion water volume from external regions can be reduced, and the renewal rate of urban river network significantly increases which shrinks the accumulation time of pollutants. This study provides a novel perspective and method for improving the water environment of tidal river channels in plain cities. [ABSTRACT FROM AUTHOR]

Metals are mainly transported with suspended sediments in tidal rivers, and it is difficult to describe the transformation process of metals. Freely dissolved metals and suspended metals (including acid‐soluble, reducible, oxidizable, and residual metals) were separately analyzed according to a sequential extraction scheme. The regression of the observed suspended metals and total suspended solids (TSS) concentrations allowed us to couple the two pollutant types. TSS is a highly dynamic parameter, and a model can be developed to simulate TSS transport. Thus, a method including the following five steps was proposed to estimate the loadings of metals entering the sea using TSS concentrations and water quality models: (1) collecting and observing basic information on the hydrological conditions, pollution sources, and water quality of the studied river; (2) obtaining a regression model between the suspended metals and TSS and analyzing the potential of establishing a suspended metal flux model; (3) introducing a model describing the fluxes of water‐soluble metals into the sea; (4) establishing a calculation model to determine the fluxes of suspended metals into the sea; and (5) characterizing the fluxes of metals into the sea. The method was programmed, and metal concentrations and fluxes could be characterized quickly when the basic river data were sufficient. In addition, if regional development scenarios could be set, metal loadings in all scenarios could be predicted through the procedure developed herein, and some effective suggestions on regional sustainable development might be proposed for decision makers. Integr Environ Assess Manag 2022;18:198–208. © 2021 SETAC KEY POINTS: A procedure for the metals' flux estimation into the sea from the tidal river using the total suspended solids concentrations and water quality models was proposed.The regressions of the suspended metals and the total suspended solids concentrations observed allowed for coupling of the two pollutants.The freely dissolved metals and suspended ones (including acid‐soluble, reducible, oxidizable, and residue) were separately simulated.The methodology was programmed, and the metals' concentrations and fluxes could be characterized quickly when the basic data on the river was sufficient. [ABSTRACT FROM AUTHOR]

Protected areas contribute to managing bird diversity, but few estuarine areas, including tidal rivers, are protected in Japan. We attempted to determine the most important tidal rivers that would conserve estuarine waterbird diversity. We used waterbird data from 105 A-class rivers collected by the National Censuses on River Environments bird survey, and the most important tidal rivers were selected through complementary analysis using the number of individuals of 72 species in summer and 82 species in winter. As a result, 22 rivers in summer and 20 rivers in winter were selected. The number of species and individuals of the target waterbirds were higher in the selected than the non-selected tidal rivers. Thirteen and eight selected tidal rivers in summer and winter, respectively, were not designated as Wildlife Protection Areas, suggesting a gap between the important tidal rivers selected in this study and the present protection areas. Furthermore, the number of individuals of Larus schistisagus, Charadrius alexandrinus, and Anas falcata, which are threatened/near-threatened species, were relatively higher in the selected rivers without protection than the rivers within the Wildlife Protection Areas. These species may not be adequately managed in the present protection areas. Thus, our study reveals the effective conservation areas of tidal rivers for waterbird diversity, and emphasizes the need for priority conservation of the selected tidal rivers without protection. The waterbird diversity in tidal river habitats can be effectively conserved by additionally designating the tidal rivers selected in this study as Wildlife Protection Areas. [ABSTRACT FROM AUTHOR]

Death investigations in aquatic ecosystems are challenging due to abiotic and biotic factors that may influence the estimation of a postmortem submersion interval (PMSI). In this study, we examined bacterial changes throughout the decomposition process on porcine carcasses submerged in a tidal‐influenced river and identified predictors of epinecrotic community succession. Fetal porcine (Sus scrofa) carcasses (N = 6) were submerged with epinecrotic samples collected every 3 days (6 collections) over a period of 19 days (~7415 accumulated degree hours (ADH)). Amplicon sequencing was performed using the Illumina MiSeq platform (16S V4 region, 2 × 250 bp format) to identify changes in bacterial relative abundance and diversity. To match bacterial succession with rough taphonomy, carcasses were visually assessed at each sampling time point to determine the decomposition stage. Notably, the three most abundant families were Moraxellaceae, Burkholderiaceae (Proteobacteria), and Clostridiaceae (Firmicutes), though communities composition varied significantly across decomposition stages. Greater bacterial phylogenetic diversity was observed in in latter decomposition stages (advanced floating decay, sunken remains). Random Forest Models were built to predict ADH and explained 77%–80.8% of variation in ADH with an error rate of +/−1943.2 ADH (Root Mean Square Error) or approx. ±2.7 days at the mean water temperature of this study. This study provided a useful model that could be used to estimate a PMSI in this river system utilizing bacterial community succession, and thus, potentially improve the accuracy of such estimations to be used in the court of law. [ABSTRACT FROM AUTHOR]

Study region: South Korea (KR), Ulsan tidal river. Study focus: Climate change and urbanization pose a serious threat to rising water levels and flooding in the estuary region. In particular, tidal river, which is located in the estuary area, is impacted by many hydrological factors such as tide, wave height, and rainfall. This paper develops a new rating curve that can predict the water level of the tidal river. It also assesses the accuracy of the developed rating curve through the verification event. New hydrological insights: Tidal rivers show non-linear hydrological characteristics that are influenced by various hydrological factors. Thus, no rating curve has been developed for them. This study proposes a method to develop a rating curve that can estimate the water level by using the runoff from rainfall in a tidal river. To develop the rating curve, the water level at Ulsan station, Korea was decomposed into four components, which are tide, wave, noise and runoff component by using wavelet analysis, curve fitting, and a high pass filter. A distributed grid-based rainfall-runoff model (GRM) was applied to simulate the runoff. The simulated runoff and the water level components influenced by rainfall were used to develop a rating curve and estimate the water level from the simulated runoff. The results were then added to the other three components (tide, wave, noise) to calculate an integrated water level, which was compared with the observed water level at Ulsan station. With less than 10 % error during validation, the results are expected to contribute to the accurate prediction of water level of tidal river during flood season.

Streams play a critical role in attenuating the excess reactive nitrogen (N) generated from human activities. Consequently, streams can also emit significant amounts of the potent greenhouse gas N 2 O. Models and manipulative experiments now suggest that hydrology regulates the balance between N removal and N 2 O production, but validating this hypothesis under field conditions has been difficult. We aimed to redress this knowledge gap by measuring changes in the concentration and isotopic composition of NO 3 − (δ18O-NO 3 −, δ15N-NO 3 −) and N 2 O (δ18O-N 2 O, δ15N-N 2 O, 15N-N 2 O site preference) in the sediments and surface water of a 30 m stream reach as discharge dropped from 2.7 to 1.8 m3 s−1. Over the eight-day measurement period the changes in conductivity, δ18O-H 2 O, and 222Rn indicated that hyporheic mixing decreased and net groundwater inputs increased as discharge declined. This coincided with increases in surface water NO 3 − (1–3 mg N l−1) and N 2 O (700–1000% saturation) that were beyond what could be explained by increased groundwater N inputs. Instead, both N 2 O and NO 3 − isotopic composition indicated that concentration increases were caused by increasing within-stream production (nitrification), rather than decreased reduction (denitrification), as hyporheic exchange decreased. This highlights the importance of oxidising processes in regulating N cycling even under strongly heterotrophic conditions (productivity/respiration: 0.005–0.2). Together these findings provide a first empirical confirmation that relatively short term (daily-weekly) stream flow dynamics directly regulate biological cycling of both NO 3 − and N 2 O. [ABSTRACT FROM AUTHOR]

Where rivers meet the sea, tides can exert a physical and chemical influence on the lower reaches of a river. How tidal dynamics in these tidal river reaches interact with upstream hydrological drivers such as storm rainfall, which ultimately determines the quantity and composition of material transferred from watersheds to estuaries, is currently unknown. We monitored a small freshwater tidal river in the Pacific Northwest, United States in high resolution over 1 year to evaluate the relative importance of tides vs. upstream hydrological flows (i.e., base flow and precipitation events) on basic physico-chemical parameters (pH, dissolved oxygen, turbidity, specific conductivity, and temperature), and how these interactions relate to the downstream estuary. Tidal variability and diurnal cycles (i.e., solar radiation) dominated water physico-chemical variability in the summer, but the influence of these drivers was overshadowed by storm-driven sharp pulses in river physico-chemistry during the remainder of the year. Within such events, we found incidences of counterclockwise hysteresis of pH, counterclockwise hysteresis of dissolved oxygen, and clockwise hysteresis of turbidity, although systematic trends were not observed across events. The dominance of storm rainfall in the river’s physico-chemistry dynamics, and similar pulses of decreased pH observed in adjacent estuarine waters, suggest that the linkage between tidal streams and the broader system is variable throughout the year. High-frequency monitoring of tidal river biogeochemistry is therefore crucial to enable the assessment of how the relative strength of these drivers may change with future sea level rise and altered precipitation patterns to modulate biogeochemical dynamics across the land-ocean-atmosphere continuum.

The blue crab, Callinectes sapidus, has a broad geographic distribution encompassing coastal waters of the eastern USA and Gulf of Mexico, but intraspecific patterns of habitat use and quality are lacking at northern latitudes. This study examined the population structure of blue crabs in the Seekonk and Taunton Rivers (Rhode Island and Massachusetts, USA): two tidally influenced rivers contiguous with the Narragansett Bay Estuary and dominated by shallow-water, unvegetated habitats. Crabs were collected fortnightly from May through August (2012–2016), and abundance- and growth-based metrics were used to assess riverine habitat use and quality. These metrics were also analyzed with respect to crab life history traits and in situ abiotic conditions to elucidate patterns of habitat selection throughout ontogeny. Crabs measuring 8 to 185 mm carapace width (CW; n = 2577) were collected, and two distinct age classes occupied the rivers during the spring and summer (maximum abundance ~ 5 crabs/10 m2). The younger age-0+ cohort was numerically dominant (~ 88% of total catch) and comprised of male and juvenile female crabs (mean ± SD abundance = 0.28 ± 0.26 males/10 m2 and 0.14 ± 0.12 juvenile females/10 m2). Males accounted for the majority of age-1+ crabs (~ 83% of cohort), yet sexually mature females were also observed (9% of cohort; mean ± SD abundance = 0.04 ± 0.06 adult females/100 m2; size at 50% maturity ± 95 CI = 129.0 ± 0.2 mm CW). Crabs were spatially segregated along a salinity gradient with males and juvenile females prevalent in oligohaline waters (upper river salinity ~ 5 ppt) and adult females mainly concentrating in higher salinity areas (mid- and lower-river salinity ~ 11–21 ppt). Seasonal and interannual patterns in crab abundance also differed by sex and ontogeny. Peak catches of males and juvenile females occurred during the spring and mid-summer, and annual abundances were positively related to dissolved oxygen (DO) concentrations. In contrast, mature females were most abundant during August and years with elevated water temperatures. The absolute and relative growth rates of juvenile crabs equaled 0.9 ± 0.3 mm CW/day and 1.5 ± 0.6% CW/day, respectively, and were directly related to DO levels. A synoptic examination of crab abundance and growth across a broad geographic range indicated that shallow-water, unvegetated habitats presently serve as functional nurseries in southern New England tidal rivers. [ABSTRACT FROM AUTHOR]

The largest deltas on Earth are tide‐dominated with multiple distributaries, which can experience different forcings. Yet, most observational studies of sediment dynamics in these systems focus on a single distributary. Comparison of the tidal‐to‐estuarine reaches of three representative distributaries of the Ayeyarwady Delta, Myanmar highlights the variability in sediment retention and export within a megadelta. The Ayeyarwady River has no mainstem dams, providing an opportunity to examine a relatively natural, monsoon‐dominated system. Observations were collected during high flow (September 2017) and low flow (March 2018 and 2019). The eastern Yangon distributary empties into the sheltered Gulf of Mottoma while the central Bogale and western Pathein distributaries are exposed to seasonal waves. During high flow, suspended‐sediment concentration (SSC) was consistent across all three distributaries (0.2–0.4 g/L). The upper delta retained 20%–60% of the mainstem sediment load, while the lower distributaries were tidal rivers exporting sediment. During low flow, the lower distributaries were partially mixed estuaries with little net discharge. However, SSC increased to >1 g/L in the Yangon distributary, remained constant in the Bogale distributary, and decreased in the Pathein distributary compared to high flow. This difference was driven by offshore sediment supply; fine sediment is retained near the mouth of the Yangon and advected away from the Pathein distributary. Consequently, nearshore sediment residence time in monsoon‐influenced megadeltas is controlled by coastal orientation, the timing of discharge with respect to ocean conditions, and tidal amplification. Changes to fluvial sediment delivery can cause non‐uniform coastal impacts due to these differing distributary processes. Key Points: Individual distributaries of tropical megadeltas can exhibit significantly different hydrodynamics and sediment transport processesIn the Ayeyarwady Delta, low‐flow estuarine processes import offshore sediment into distributaries when offshore sediment is available and resuspendedSediment residence time in the nearshore is controlled by tidal amplification and the coastal orientation with respect to monsoon winds [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Increasing salinity levels in the tidal rivers are becoming a significant challenge to inland water quality. Salinization of the adjoining water resources and agricultural lands have prompted the water managers to optimize the solutions for mitigation of adverse effects. Keeping this in view, a hydrodynamic model coupled with a water quality model was used to analyze the propagation of tides from the Arabian Sea into the Indus River and their subsequent effect over longitudinal salinity distribution. Field observations of hydraulic parameters and salinity were used for validation of the model predictions in dry (no river flow) and wet (specific river flow) hydrological conditions. Results suggest that the tidal amplitude dissipates landward; salinity intrudes about 58 km (near Dandari) into the Indus River Estuary (IRE) at spring tide. Salinity in the proximity of Dandari point is sensitive to variations in fresh water supplies. A complete neap-spring tidal cycle was chosen for scenario-based evaluation of the impact of fresh water over the longitudinal salinity distribution. A daily river discharge 250 m3/s was found optimum for limiting the salinity interface (≤1) up to 31 km from the estuary mouth. In addition, tidal inlet modifications based on the theoretical framework of stable channel design combined with the optimum river discharge were tested. The cross-sectional modifications based on the regime/Lacey theory proven better and salinity interface was further pushed back up to 27 km. Thus, it is maintained that synergic impact of the optimized freshwater quantity and the estuary mouth modifications are considered as one of the pragmatic solutions for control of salinity intrusion and achieving the specific environmental conditions along the alluvial estuary. Image 1 [ABSTRACT FROM AUTHOR]

A tidal river is a special zone where the basic hydrodynamic process is the interaction between riverine flow and tides. Existing studies focus mainly on the water level fluctuations, while the effort is limited regarding the flow regime between uni- and bi-directional currents. Along a tidal river, the current direction reverses where the landward flow meets with seaward flow. So far, the occurrence and movement of current reversal is not fully investigated, and the number and location of reversal points are not well understood. In this study, the water flow in the Yangtze tidal river was simulated using a hydrodynamic model, and the phenomenon of current reversals in the tidal river were investigated. It is found that in the dry season up to three reversal points may occur simultaneously, including two convergence points and one divergence point, separating the entire tidal river into two seaward-current reaches and two landward-current reaches. These multiple reversal points result from the co-existence of the preceding and present flood tidal waves. During a spring tide the peak landward flux of the preceding wave can reach up to 10,000 m3/s, about the same magnitude as the riverine discharge and the length of the landward-current occupation is as large as 120 km. It was also found that a current reversal point always coexists with a zero-gradient point of water level, and the location of this reversal is downstream of the zero-gradient point. The reversals shift landward from the river mouth consecutively, and could finally disappear farther at the upper reach of the tidal river. The bidirectional flow could extend up to 550 and 200 km upstream from the river mouth in the dry and wet seasons, respectively. The current reversals are influenced by the discharge regulation of the Three Gorges Dam. These movements of current reversals in the flow regime could furtherly have significant impact on local hydrodynamic process and material transportation. Image 1 • Transition between uni- and bi-directional flows in a large tidal river is studied. • Up to three current reversals emerge simultaneously in dry season. • Preceding landward flow occupies as long as 100 km with the max-flux of 10000 m3/s. • A reversal is always located downstream of a zero-gradient-water-level point. • Movement of reversals is multi-timescale due to tides and river flow. [ABSTRACT FROM AUTHOR]

Mainstem–floodplain material exchange in the tidal freshwater reach of major rivers may lead to significant sequestration of riverine sediment, but this zone remains understudied compared to adjacent fluvial and marine environments. This knowledge gap prompts investigation of floodplain‐incising tidal channels found along the banks of tidal rivers and their role in facilitating water and suspended‐sediment fluxes between mainstem and floodplain. To evaluate this role, and how it evolves along the tidal river and with time, we measured water level, flow velocity, temperature, and suspended‐sediment concentration (SSC) in four tidal channels along the tidal Amazon River, Brazil. Eleven deployments were made during low, rising, high, and falling seasonal Amazon discharge. Generally, channels export high‐SSC water from the mainstem to the tidal floodplain on flood tides and transfer low‐SSC water back to the mainstem on ebbs. Along the length of the tidal river, the interaction between tidal and seasonal water‐level variations and channel–floodplain morphology is a primary control on tidal‐channel sediment dynamics. Close to the river mouth, where tides are large, this interaction produces transient flow features and current‐induced sediment resuspension, but the importance of these processes decreases with distance upstream. Although the magnitude of the exchange of water and sediment between mainstem and floodplain via tidal channels is a small percentage of the total mainstem discharge in this large tidal‐river system, tidal channels are important conduits for material flux between these two environments. This flux is critical to resisting floodplain submergence during times of rising sea level. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Organisms that live in coastal estuaries often experience significant seasonal and annual fluctuations in salinity that they must either endure or move to avoid. Largemouth Bass Micropterus salmoides is a freshwater fish that lives in coastal estuaries and does not migrate/move to avoid seasonal salinity increases. Additionally, estuarine Largemouth Bass exhibit growth rates, condition factors, and life history strategies that differ from their inland counterparts. These differences suggest the potential for physiological adaptations to tolerate and even thrive in estuarine environments. We compared swimming performance (quantified as critical swimming speed, Ucrit) of Largemouth Bass (280–404 mm total length) from an Alabama estuarine population versus an inland population at 0, 4, 8, and 12 ppt salinities to test for physiological performance-based adaptation to tolerate elevated salinities. Ucrit values did not differ between inland and estuarine Largemouth Bass nor were there any salinity effects. Although inland and estuarine Largemouth Bass may possess different physiological mechanisms for tolerating salinity, those mechanisms did not affect swimming performance. [ABSTRACT FROM AUTHOR]

Abstract Saigon-Dongnai Rivers in Southern Vietnam is a complex lowland hydrological network of tributaries that is strongly influenced by the tidal cycles. The increasing economic, industrial and domestic developments in and around Ho Chi Minh City (HCMC) have led to serious impacts on water quality due to lack of appropriate wastewaters treatment. Drinking water production is impacted and the large aquaculture production areas may also be affected. We analyzed spatial and seasonal variability of nutrient concentrations (Phosphorus, Nitrogen and Silica) and eutrophication indicators (Organic Carbon, Chlorophyll- a and Dissolved Oxygen) based on bi-monthly monitoring during two hydrological cycles (July 2015–December 2017). Four monitoring sites were selected to assess the impact of HCMC: two upstream stations on the Saigon River and Dongnai River branches to provide the reference water quality status before reaching the urbanized area of HCMC; one monitoring station in the city center to highlight Saigon River water quality within the heart of the megacity; the fourth station downstream of the confluence to evaluate the impact of HCMC on the estuarine waters. This study points to excess nutrients in HCMC's water body with concentrations of NH 4 + and PO 4 3− averaging to 0.7 ± 0.6 mgN L−1 and 0.07 ± 0.06 mgP L−1, respectively in mean over the monitored period and rising up to 3 mgN L−1 and 0.2 mgP L−1, in extreme conditions. During the dry season, we evidenced that untreated domestic discharges leads to degradation of the Saigon River's water quality with extreme values of algal biomass (up 150 μChl-a L−1) and hypoxic conditions occurring episodically (DO < 2 mg L−1) in the heart of the megacity. Until now, eutrophication in the urban center has had no clear effect downstream because eutrophic water mass from the Saigon River is efficiently mixed with the Dongnai River and sea water masses during the successive semi-diurnal tidal cycles. Graphical abstract Unlabelled Image Highlights • Rapid economic and population growth in Ho Chi Minh City. • Lack of wastewater treatment plant in HCMC. • High level of phytoplankton and nutrients in the heart of HCMC. • Phosphorus is the limiting factor of the algal development in the Saigon River. • Extreme algal blooms are confined in the densely populated area due to the diurnal tides [ABSTRACT FROM AUTHOR]