Hyporheic Research Articles

Phosphorus and nitrogen storage, partitioning, and export in a large gravel bed river

Nutrient dynamics in a 25 km long treated wastewater effluent impacted reach of a large, gravel-bed river were evaluated in five river compartments: surficial sediment, surface water, hyporheic zone water, and aquatic biomass (including epilithic algae and macrophytes). Nutrient storage within, and export from, the river reach, was quantified to assess the impact of WWTP effluent on nutrient dynamics.More than 98% of N and P storage was found in the surficial river bed sediment, where it is available to support epilithic algal and macrophyte growth. Nutrient export from the river reach by sediment, hyporheic water, and biomass were small compared to water column transport. The N:P ratios for the five different compartments suggested that the water column was severely P limited, whereas sediment, hyporheic water, and aquatic biomass tended towards co-limitation and N limitation.Within the river reach, the majority of P was stored immediately downstream of the WWTP effluent outfall, whereas N was retained at a higher rate relative to P in the remainder of the reach. Correlation analysis of nutrient exchange between different compartments suggested that multiple nutrient compartments should be considered when establishing nutrient loading criteria. Nutrient analysis in multiple compartments in the river can add valuable insight into nutrient dynamics and nutrient limitation.

Resolving Changes to Freshwater Lens Systems in a “Sea of Salinity” using Multi-date Airborne EM

Saline aquifers in the Murray River or SE Australia are traversed by freshwater rivers, with adjoining riparian and floodplain regions containing freshwater lenses. Bore data and more recent Airborne electromagnetic (AEM) surveys have determined that these lenses are spatially extensive, but have widely varying geometries. The maintenance of these lens systems is important as they support ecologically significant riparian vegetation communities such as Red Gum and Black Box. A more complete understanding of their hydrogeology is required to ascertain how they develop and degrade. Limited ground investigations including 14C geochemistry have determined that the lens systems contain recent water, indicating that they are dynamic systems with their development defined by the relative rates of recharge from the river and mixing with groundwater. Changes in groundwater gradients and depth, floodplain extent, and topography are believed to control their initial location. The same controls also govern their stability.The potential of AEM systems for defining the geometry of these lens systems in 3D is considered along with an assessment of their value for monitoring variations associated with these ecosystems. The advent of “calibrated” AEM systems and robust inversion tools have given added impetus to their use for environmental monitoring. Spatio-temporal variations are observed in the near surface (top 20m) from a multi-temporal assessment of Clark’s Floodplain, adjacent to the Bookpurnong irrigation area, with co-incident AEM surveys acquired between 2008 and 2015. Spatial changes in ground conductivity, attributed to changing groundwater quality have been observed. The freshwater lens systems appear to have contracted significantly over the last decade. This is attributed, in part, to land use patterns and the development of an irrigation-related groundwater mound on the highlands adjacent to the floodplain, and an increased hydraulic gradient towards the river. The results indicate the geometry of the hyporheic zone may have also changed along the river.

Impact of Dynamically Changing Discharge on Hyporheic Exchange Processes Under Gaining and Losing Groundwater Conditions

AbstractChannel discharge, geomorphological setting, and regional groundwater flow determine the spatiotemporal variability of bedform‐induced hyporheic exchange and the emergence of biogeochemical hot spots and hot moments that it drives. Of particular interest, and significantly understudied, is the role that dynamically changing discharge has on the hyporheic exchange process and how regional groundwater flow modulates the effects of transience. In this study, we use a reduced‐complexity model to systematically explore the bedform‐induced hyporheic responses to dynamically changing discharge events in systems with different bedform geometries exposed to varying degrees of groundwater flow (under both upwelling and downwelling conditions). With this in mind, we define metrics to quantify the effects of transience: spatial extent of the hyporheic zone, net hyporheic flux, mean residence time, and denitrification efficiency. We find that regional groundwater flow and geomorphological settings greatly modulate the temporal evolution of bedform‐induced hyporheic responses driven by a single‐peak discharge event. Effects of transience diminish with increasing groundwater upwelling or downwelling fluxes, decreasing bedform aspect ratios, and decreasing channel slopes. Additionally, we notice that increasing discharge intensities can reduce the modulating impacts of regional groundwater flow on the effects of transience but hardly overcomes the geomorphological controls. These findings highlight the necessities of evaluating hyporheic exchange processes in a more comprehensive framework.

Lithologic Control of the Hydrochemistry of a Point-Bar Alluvial Aquifer at the Low Reach of the Nakdong River, South Korea: Implications for the Evaluation of Riverbank Filtration Potential

To assess the groundwater−river water interaction in a point-bar alluvial aquifer as a crucial step in site assessment for riverbank filtration, hydrochemical and hydrogeologic investigations were performed on a riverine island at the low reach of the Nakdong River, South Korea. The site was evaluated for the application of large-scale bank filtration. Unconsolidated sediments (~40 m thick) of the island comprise fine- to medium-grained sand (upper aquifer), silty sand with clay intercalations, and sandy gravel (lower aquifer) in descending order. The intermediate layer represents an impermeable aquitard and extends below the river bottom. A total of 66 water samples were collected for this study; groundwater (n = 57) was sampled from both preexisting irrigation wells, and three multi-level monitoring wells (each 35 m deep). Groundwater chemistry is highly variable, but it shows a distinct hydrochemical change with depth: shallow groundwater (<25 m deep) from the upper aquifer is characteristically enriched in NO3− and SO42−, due to agricultural contamination from the land surface, while deeper groundwater (>25 m deep) from the lower aquifer is generally free of NO3− and relatively rich in F. The lower aquifer groundwater is also higher in pH, and concentrations of K+, Mg2+, and HCO3−, indicating that the aquifer is likely fed by regional groundwater flow. Such separation of groundwater into two water bodies is the result of the existence of an impermeable layer at intermediate depth. In addition, the hyporheic flow of river water is locally recognized at the upstream part of the upper aquifer as the zone of low TDS (Total Dissolved Solids) values (<200 mg/L). This study shows that the study site does not seem to be promising for large-scale riverbank filtration because 1) the productive, lower aquifer is not directly connected to the bottom of the river channel, and 2) the upper aquifer is severely influenced by agricultural contamination. This study implies that the subsurface hydrogeologic environment should be carefully investigated for site assessment for riverbank filtration, which can be aided by a detailed survey of groundwater chemistry.

Toward a conceptual framework of hyporheic exchange across spatial scales

Abstract. Rivers are not isolated systems but interact continuously with groundwater from their confined headwaters to their wide lowland floodplains. In the last few decades, research on the hyporheic zone (HZ) has increased appreciation of the hydrological importance and ecological significance of connected river and groundwater systems. While recent studies have investigated hydrological, biogeochemical and ecohydrological processes in the HZ at bedform and reach scales, a comprehensive understanding of process-based interactions between factors operating at different spatial and temporal scales driving hyporheic exchange flows (HEFs) at reach scale and larger is still missing. Therefore, this review summarizes the factors and processes at catchment, valley, and reach scales that interact to control spatial and temporal variations in hyporheic exchange flows. By using a multi-scale perspective, this review connects field observations and modelling studies to identify the process driving patterns and dynamics of HEF. Finally, the influence of process interactions over multiple spatial scales is illustrated in a case study, supported by new GIS analyses, which highlights the importance of valley-scale factors to the expression of HEF at the reach scale. This conceptual framework will aid the development of approaches to interpret hyporheic exchange across scales, infer scaling relationships, and inform catchment management decisions.

Assessment of Microbial Community Dynamics in River Bank Filtrate Using High-Throughput Sequencing and Flow Cytometry.

Surface-groundwater interactions play an important role in microbial community compositions of river bank filtrates. Surface water contaminations deriving from environmental influences are attenuated by biogeochemical processes in the hyporheic zone, which are essential for providing clean and high-quality drinking water in abstraction wells. Characterizing the flow regime of surface water into the groundwater body can provide substantial information on water quality, but complex hydraulic dynamics make predictions difficult. Thus, a bottom up approach using microbial community shifting patterns as an overall outcome of dynamic water characteristics could provide more detailed information on the influences that affect groundwater quality. The combination of high-throughput sequencing data together with flow cytometric measurements of total cell counts reveals absolute abundances among taxa, thus enhancing interpretation of bacterial dynamics. 16S rRNA high-throughput sequencing of 55 samples among six wells in a well field in Austria that is influenced by river bank filtrate within a time period of 3 months has revealed both, clear differences as well as strong similarity in microbiome compositions between wells and dates. A significant community shift from April to May occurred in four of six wells, suggesting that surface water flow regimes do affect these wells stronger than others. Triplicate sampling and subsequent sequencing of wells at different dates proved the method to be reproducible. Flow cytometric measurements of total cells indicate microbial shifts due to increased cell counts and emphasize the rise of allochthonous microorganisms. Typical freshwater bacterial lineages (Verrucomicrobia, Bacteroidetes, Actinobacteria, Cyanobacteria, Armatimonadetes) were identified as most increasing phyla during community shifts. The changes are most likely a result of increased water abstraction in the wells together with constant river water levels rather than rain events. The results provide important knowledge for future implementations of well utilization in dependency of the nearby Danube River water levels and can help drawing conclusions about the influence of surface water in the groundwater such that hygienically save and clean drinking water with a stable microbial community can be provided.

Evaluating emerging organic contaminant removal in an engineered hyporheic zone using high resolution mass spectrometry

The hyporheic zone (HZ), located at the interface of surface and groundwater, is a natural bioreactor for attenuation of chemical contaminants. Engineered HZs can be incorporated into stream restoration projects to enhance hyporheic exchange, with flowpaths optimized to promote biological habitat, water quantity, and water quality improvements. Designing HZs for in-stream treatment of stormwater, a significant source of flow and contaminant loads to urban creeks, requires assessment of both the hydrology and biogeochemical capacity for water quality improvement. Here, we applied tracer tests and high resolution mass spectrometry (HRMS) to characterize an engineered hyporheic zone unit process, called a hyporheic design element (HDE), in the Thornton Creek Watershed in Seattle, WA. Dye, NaCl, and bromide were used to hydrologically link downwelling and upwelling zones and estimate the hydraulic retention time (HRT) of hyporheic flowpaths. We then compared water quality improvements across hydrologically-linked surface and hyporheic flowpaths (3–5 m length; ∼30 min to >3 h) during baseflow and stormflow conditions. We evaluated fate outcomes for 83 identified contaminants during stormflow, including those correlated with an urban runoff mortality syndrome in coho salmon. Non-target HRMS analysis was used to assess holistic water quality improvements and evaluate attenuation mechanisms. The data indicated substantial water quality improvement in hyporheic flowpaths relative to surface flow and improved contaminant removal with longer hyporheic HRT (for ∼1900 non-target compounds detected during stormflow, <17% were attenuated >50% via surface flow vs. 59% and 78% via short and long hyporheic residence times, respectively), and strong contributions of hydrophobic sorption towards observed contaminant attenuation.

Natural attenuation of naphthalene along the river-bank infiltration zone of the Liao River, Shenyang, China

In this study, the natural attenuation of naphthalene during riverbank infiltration was examined using batch experiments. The results indicated that, as the grain size and the permeability coefficient decreased, the natural attenuation rate of naphthalene increased, and it was highest in loam (62%) and lowest in coarse sand (20%). The half-life of naphthalene was longest in coarse sand (700 d) and shortest in mild clay (250 d). Facultative anaerobes such as Methylophilaceae accounted for about 70% of the total bacteria and played a major role in naphthalene degradation. A high total organic carbon concentration and large specific surface area can promote natural attenuation of naphthalene. Moreover, the adsorption to riverbank sediment in the hyporheic zone and bioremediation by indigenous microorganisms can effectively eliminate naphthalene during river water infiltration to the riverbank aquifer.

Assessing the effects of forest-to-bog restoration in the hyporheic zone at known Atlantic salmon (Salmo salar) spawning sites

Assessing the effects of forest-to-bog restoration in the hyporheic zone at known Atlantic salmon (Salmo salar) spawning sites

Shear Flows Accelerate Mixing Dynamics in Hyporheic Zones and Hillslopes

AbstractTopographic relief and river bedforms generate nested streamline patterns, which drive the propagation and mixing at depth of changes in surface water concentration or temperature. While concentration distributions and biogeochemical reactions in such flow cells are often studied under steady state transport conditions, there is increasing evidence that transient mixing processes may have a significant contribution to effective mixing and reaction rates. Here we show that these streamline patterns act as shear flows, which significantly accelerate mixing dynamics within flow cells and can lead to the formation of transient mixing hot spots at depth. We provide analytical solutions that quantify the dynamics of mixing in a flow cell for a pulse and a front initial solute distribution, which represent two idealized end‐members of more complex solute distributions in natural systems. These results provide new insights into the patterns and dynamics of mixing at hyporheic zone, hillslope, and catchment scales.

Modelling the effects of multiple stressors on respiration and microbial biomass in the hyporheic zone using decision trees

Integrity of freshwater surface- and groundwater ecosystems and their ecological and qualitative status greatly depends on ecological processes taking place in streambed sediments overgrown by biofilm, in the hyporheic zone (HZ). Little is known about the interactions and effects of multiple stressors on biologically driven processes in the HZ. In this study, machine learning (ML) tools were used to provide evidence-based information on how stressors and ecologically important environmental factors interact and drive ecological processes and microbial biomass. The ML technique of decision trees using the J48 algorithm was applied to build models from a data set of 342 samples collected over three seasons at 24 sites within the catchments of five gravel-bed rivers in north-central Slovenia. Catchment-scale land use data and reach-scale environmental features indicating the HZ morphology and physical and chemical characteristics of water were used as predictive variables, while respiration (R) and microbial respiratory electron transport system activity (ETSA) were used as response variables indicating ecological processes and total protein content (TPC) indicating microbial biomass. Separate models were built for two HZ depths: 5–15 cm and 20–40 cm. The models with R as a response variable have the highest predictive performance (67–89%) showing that R is a good indicator of complex environmental gradients. The ETSA and TPC models were less accurate (42–67%) but still provide valuable ecological information. The best model show that temperature when combined with selected water quality elements is an important predictor of R at depth of 5–15 cm. The ETSA and TPC models show the combined effects of temperature, catchment land use and selected water quality elements on both response variables. Overall, this study provides new knowledge on how ecological processes occurring in the HZ respond to catchment and reach-scale variables, and provides evidence-based information about complex interactions between temperature, catchment land use and water quality. These interactions are highly dependent on the selection of the response variable, i.e., each response variable is influenced by a specific combination of predictive environmental variables.

A review on using heat as a tool for studying groundwater–surface water interactions

In terms of the research on groundwater–surface water heat-tracing methods, investigation of the interactions within the compound system of the groundwater–surface water–hyporheic zone can effectively reveal the relevant physicochemical processes and microbial properties. The evaluation of these properties represents a key component in qualitative and quantitative research on groundwater–surface water interactions. Therefore, this paper reviews the research results on groundwater–surface water interactions achieved by related researchers using heat as a natural tracer over the last decade. In connection with the application of heat-tracing theory to the basic principles of hyporheic exchange between groundwater and surface water, research on groundwater–surface water interaction through one-dimensional steady-state and transient-state heat transport analytical models, techniques to collect and analyze temperature time series data, and numerical simulation technology is reviewed. In addition, directions for future research using groundwater–surface water heat-tracing methods are suggested. First, hypothetical, difficult temperature boundary and hydrogeological conditions require further research. Second, hydrodynamic exchange capacity and the processes of heat exchange and solute concentration exchange in the hyporheic zone alongside riverbeds should be appropriately and accurately measured under multi-scale influences. Third, the overall study of the heat transport process inside the hyporheic zone induced by complex riverbed forms should be performed, and the response mechanism of riverbed hyporheic exchanges driven by riverbed form, the hydrodynamic force of surface water, and sediment permeability should be revealed. The objectives and goals of this paper are to encourage scholars interested in analyzing groundwater–surface water interactions using heat as a tracer to creatively solve practical problems and to improve the ecological functions of river aquatic habitats through new research results.

Environmental filtering and community delineation in the streambed ecotone

A current controversy in ecology is whether biological communities are discrete biological entities or simply study units created for convenience; a debate that becomes even more heated when delimiting communities along ecotones. Here, we report an interdisciplinary study designed to address the interplay between environmental drivers and community ecology in a typical ecotone ecosystem: the streambed. Environmental filtering at a micro-scale determined how diversity, productivity and composition of the whole streambed assemblage varied with depth and with the direction of vertical water exchange. Biomass and production decreased with increasing depth, and were lower under upwelling than downwelling conditions. However, the rate at which biomass and production decreased with increasing depth differed significantly for different taxonomic groups. Using quantitative biocenosis analysis, we also showed that benthic and hyporheic zone assemblages (assemblages in close juxtaposition) could be clearly distinguished as discrete communities with individual integrity. Vertical hydrodynamic conditions also influenced the demarcation between both communities; the benthic community reached greater depths in downwelling than in upwelling zones.

Transverse hyporheic flow in the cross-section of a compound river system

Previous studies of bedform-induced hyporheic flow focused on processes driven by longitudinal hydraulic head variations at the sediment-water interface (SWI) due to river flow and bedform interactions. By way of numerical simulations, we examined the transverse hyporheic flow in a river system with compound sections, comprising a main river channel and a floodplain connected by a bank. The results show that head fluctuations across the compound beds associated with the transverse secondary flow can cause significant lateral hyporheic flow through two pore water circulating cells. Such a 2-D compound bedfrom have a higher flux across the SWI than the 2-D longitudinal dunes since the 2-D compound cross-section form induces more drag. The circulating flow cells originate from the bank surface and terminate at the main channel and floodplain bed surfaces, respectively. The hyporheic exchange concentrates on the bank region below the SWI where an intensive solute transfer concurrently occurred. The head gradient along the SWI is sensitive to the bank slope angle, which consequently determines transverse hyporheic flow dynamics and solute transport processes. Larger bank slope angles induce higher hyporheic fluxes and smaller hyporheic areas and residence time. The effects of increasing flux on solute transport in the compound section are partly influenced by a sharply decreasing hyporheic area. Thus the solute consequently transfers downward within the hyporheic zone with a lower rate at larger bank slope angle. Larger river discharges may promote both the transverse hyporheic flow and solute transformation across the bed surface. These physical processes may have implications for the biogeochemical processing within the hyporheic zone across a compound river system.

How Hydrologic Connectivity Regulates Water Quality in River Corridors.

Downstream flow in rivers is repeatedly delayed by hydrologic exchange with off-channel storage zones where biogeochemical processing occurs. We present a dimensionless metric that quantifies river connectivity as the balance between downstream flow and the exchange of water with the bed, banks, and floodplains. The degree of connectivity directly influences downstream water quality - too little connectivity limits the amount of river water exchanged and leads to biogeochemically inactive water storage, while too much connectivity limits the contact time with sediments for reactions to proceed. Using a metric of reaction significance based on river connectivity, we provide evidence that intermediate levels of connectivity, rather than the highest or lowest levels, are the most efficient in removing nitrogen from Northeastern United States' rivers. Intermediate connectivity balances the frequency, residence time, and contact volume with reactive sediments, which can maximize the reactive processing of dissolved contaminants and the protection of downstream water quality. Our simulations suggest denitrification dominantly occurs in riverbed hyporheic zones of streams and small rivers, whereas vertical turbulent mixing in contact with sediments dominates in mid-size to large rivers. The metrics of connectivity and reaction significance presented here can facilitate scientifically based prioritizations of river management strategies to protect the values and functions of river corridors.

Hyporheic Exchange Controls Fate of Trace Organic Compounds in an Urban Stream.

First-order half-lives for 26 trace organic compounds (TrOCs) were determined in the hyporheic zone (HZ) and along a 3 km reach of a first-order stream in South Australia during both dry and wet seasons. Two salt tracer experiments were conducted and evaluated using a transient storage model to characterize seasonal differences in stream residence time and transient storage. Lagrangian and time-integrated surface water sampling were conducted to calculate half-lives in the surface water. Half-lives in the HZ were calculated using porewater samples obtained from a modified mini-point sampler and hyporheic residence times measured via active heat-pulse sensing. Half of the investigated TrOCs (e.g., oxazepam, olmesartan, candesartan) were not significantly removed along both the investigated river stretch and the sampled hyporheic flow paths. The remaining TrOCs (e.g., metformin, guanylurea, valsartan) were found to be significantly removed in the HZ and along the river stretch with relative removals in the HZ correlating to reach-scale relative removals. Using the modeled transport parameters, it was estimated that wet season reach-scale removal of TrOCs was predominately caused by removal in the HZ when the intensity of hyporheic exchange was also higher. Factors that increase HZ exchange are thus likely to promote in-stream reactivity of TrOCs.

Drought Conditions Maximize the Impact of High‐Frequency Flow Variations on Thermal Regimes and Biogeochemical Function in the Hyporheic Zone

AbstractAnthropogenic activities, especially dam operations, often induce larger and more frequent stage fluctuations than those occurring in natural rivers. However, long‐term impacts of such flow variations on thermal and biogeochemical dynamics of the associated hyporheic zone (HZ) are poorly understood. In this study, we built a heterogeneous, two‐dimensional, thermo‐hydro‐biogeochemical model for an aerobic respiration dominated river system. Our results revealed important interactions between subdaily to weekly flow variations and mean flow conditions controlled by snowpack at the watershed. High‐frequency stage fluctuations had their strongest thermal and biogeochemical impacts when mean river stage was low during fall and winter. In an abnormal drought year (2015) with low river stages during summer and early fall, high‐frequency stage fluctuations caused the HZ to be warmer than average. Furthermore, high‐frequency stage fluctuation enhanced all biogeochemical reactions by increasing nutrient supply and creating more oxygenated conditions. Overall carbon consumption in the HZ increased due to high‐frequency flow variations. Thermal dynamics altered by high‐frequency stage fluctuation impacted biogeochemical reactions in the HZ less than effects imposed by enhanced nutrient and oxygen supply. In addition to these results, we demonstrated that the HZ's hydrogeologic properties control flow paths that influence residence times and nutrient supply and consequently control spatial distribution of biogeochemical reaction hot spots in the HZ. Here we provide scientific basis for assessing potential ecological consequences of high‐frequency flow variations in a regulated river, as well as guidance for maximizing potential benefits—or minimizing drawbacks—of dam regulation to riverine ecosystems.

A MATHEMATICAL MODEL OF THE HYPORHEIC ZONE FLUIDIZATION

In the hyporheic zone, exchange of surface water and groundwater takes place. During the groundwater drainage and the increase in hydraulic gradient, this zone can be fluidized. The paper presents a mathematical model of hyporheic zone fluidization. It has been obtained from a balance of forces related to the water filtration through the hyporheic zone. The mathematical model makes it possible to predict the occurrence of fluidization depending on the increase in water table position. A series of laboratory tests were carried out, which confirmed the predictions according to the model. The hydrological and hydrogeological influences of hyporheic zone fluidization were also analyzed, namely: the decline of diurnal water level fluctuations in headwaters and the variable rate of decline of water level after rainfall flood.

Multiscale Framework for Modeling Multicomponent Reactive Transport in Stream Corridors

AbstractTravel time‐based representations of transport, a highly successful strategy for modeling conservative tracer transport in stream corridors, are extended to accommodate multicomponent reactive transport. Specifically, convolution representations used to model exchange of solute with the hyporheic zone are shown to be equivalent to solving one‐dimensional subgrid models in Lagrangian form coupled to the advection dispersion equation for the stream channel. Unlike the convolution‐based representations of previous travel time‐based stream transport models, the subgrid model generalizes to include multicomponent reactive transport with general nonlinear reactions. An example involving biomass growth, the establishment of redox zonation, and the resulting impact on denitrification rates demonstrate reach‐scale application of the new approach. Although simplified, those example simulations show some of the key phenomena associated with hyporheic zone denitrification that are not represented with conventional first‐order estimates.

Geochemical Exports to River From the Intrameander Hyporheic Zone Under Transient Hydrologic Conditions: East River Mountainous Watershed, Colorado

AbstractTo understand how redox processes influence carbon, nitrogen, and iron cycling within the intrameander hyporheic zone, we developed a biotic and abiotic reaction network and incorporated it into the reactive transport simulator PFLOTRAN. Two‐dimensional reactive flow and transport simulations were performed (1) to evaluate how transient hydrological conditions control the lateral redox zonation within an intrameander region of the East River in Colorado and (2) to quantify the impact of a single meander on subsurface exports of carbon and other geochemical species to the river. The meander's overall contribution to the river was quantified by integrating geochemical outfluxes along the outside of the meander bend. The model was able to capture the field‐observed trends of dissolved oxygen, nitrate, iron, pH, and total inorganic carbon along a 2‐D transect. Consistent with field observations, simulated dissolved oxygen and nitrate decreased along the intrameander flow paths while iron (Fe2+) concentration increased. The simulation results further demonstrated that the reductive potential of the lateral redox zonation was controlled by groundwater velocities resulting from river stage fluctuations, with low‐water conditions promoting reducing conditions. The sensitivity analysis results showed that permeability had a more significant impact on biogeochemical zonation compared to the reaction pathways under transient hydrologic conditions. The simulation results further indicated that the meander acted as a sink for organic and inorganic carbon as well as iron during the extended baseflow and high‐water conditions; however, these geochemical species were released into the river during the falling limb of the hydrograph.