Hyporheic Research Articles

Introducing “The Integrator”: A novel technique to monitor environmental flow systems

AbstractWe introduce “The Integrator,” a novel technique to quantify transport and reaction metrics commonly used to characterize flow systems. This development consists of two products: (1) The Integrator sampling device and (2) its supporting mathematical framework, which is compatible with semi‐continuous sensor data. The use of The Integrator device simplifies the logistics of sample collection and greatly reduces the number of samples needed, making it ideal to characterize systems that are: (1) difficult to access, (2) large and thus intractable or highly heterogeneous, and (3) highly instrumented otherwise but where a more holistic, mechanistic understanding may be gained by monitoring one or more currently untracked elements. We tested and validated The Integrator technique using experimental data collected from a heart rate monitor (high‐quality, high‐frequency data in response to known excitation events) and solute tracer experiments conducted in two contrasting (fourth and seventh order) rivers. In the Supporting Information, we provide details concerning the design of The Integrator device used in our field case studies and provide insight into potential improvements. Despite our case studies focus on the analysis of conservative and reactive transport of solutes in rivers, the principles behind The Integrator technique can be used to monitor water quality in hyporheic zones, aquifers, wetlands, swamps, karsts, oceans, wastewater treatment plants, pipe networks, and air quality. Furthermore, special arrangements of Integrator devices can be used to gather data at spatial and temporal resolutions that are currently unattainable due to high transportation and/or personnel costs.

Microbial Community Structure and Metabolic Potential of the Hyporheic Zone of a Large Mid-Stream Channel Bar

To develop a greater understanding of hyporheic zone microbial biogeochemistry, we sampled pore fluids from a piezometer array associated with the McCarran Ranch channel bar (MRCB); a partially submerged cobble island in the Truckee River, NV, USA. Flowing surface water and pumped pore fluids were characterized by prokaryotic community structure, metabolic potential, and aqueous physicochemistry. Concentrations of potential respiratory electron acceptors were highest in surface water and riverbed porewater and sequentially depleted in porewaters along the inferred flowpath (O2, then NO3−, then SO42−). Correspondingly, cultivable nitrate reducers/denitrifiers were most abundant in surface water and riverbed porewater, despite oxic conditions. Cultivable sulfate reducers were overall most abundant in surface water. Prokaryotic community reconstruction from 16S rRNA gene sequences indicates that the surface water community was less diverse than that of porewater and supports a shift in metabolic strategy, from aerobic heterotrophy in surface water (e.g., Comamonadaceae and Sporichthyaceae) to chemolithotrophy and anaerobic metabolisms (e.g., Hydrogenophaga spp., Ferribacterium spp., Methanobacterium spp.) along the hyporheic flow path. These data indicate that prokaryotic communities within the MRCB are phylogenetically and metabolically diverse and contribute to biogeochemical cycling in this common yet relatively understudied habitat.

Effect of Surface Water Stage Fluctuation on Mixing‐Dependent Hyporheic Denitrification in Riverbed Dunes

AbstractThe hyporheic zone, where surface water (SW) and groundwater (GW) interact in shallow sediments beneath rivers, is uniquely reactive and attenuates pollutants. Mixing of reactants from SW and GW enables mixing‐dependent (MD) reactions, which may be the last opportunity for GW contaminants to react before entering SW. Yet little is known about hyporheic MD reactions, particularly how they respond to daily or seasonal SW fluctuations or sediment heterogeneity. We used MODFLOW and SEAM3D to simulate non‐mixing‐dependent (NMD) aerobic respiration and MD denitrification in a riverbed dune with nitrate from SW and dissolved organic carbon from GW. We varied SW heads and heterogeneity of sediment hydraulic conductivity. For longer‐term fluctuations (i.e., seasons), increasing SW depth from 0.1 to 1.0 m increased NMD aerobic respiration by 270% and MD denitrification by 78% in homogeneous sediment. MD reactions thus were controlled by mixing zone length or size and would be stronger when SW stage is elevated, for example, during wintertime. Adding sediment heterogeneity to the long‐term scenarios, particularly by increasing hydraulic conductivity correlation length, increased flow focusing and consequently increased MD denitrification by 20–30%. By contrast, the net effect of daily SW fluctuations on MD denitrification in homogeneous sediment was minor. In sum, SW fluctuations are an important control on hyporheic MD reactions, primarily by controlling mixing zone length. The hyporheic zone may attenuate nitrate in upwelling GW plumes, but temporal fluctuations may be considerable as quantified above.

Biofilm-specific uptake does not explain differences in whole-stream DOC tracer uptake between a forest and an agricultural stream

Benthic biofilms are often assumed to control terrestrially-derived dissolved organic carbon (tDOC) uptake in streams. We tested this by comparing 13C-enriched ryegrass leachate uptake in an agricultural and a forest stream, hypothesizing that a greater abundance of autotrophic biofilms in the agricultural stream would cause its whole-stream tDOC uptake to be comparatively low. We measured whole-stream and biofilm tDOC tracer uptake, metabolism, bacterial and algal diversity, and nutrient status of benthic epilithic biofilms, and assessed whole-stream hydromorphology. Whole-stream uptake of tDOC was six times lower in the agricultural (3.0 mg m−2 day−1) than in the forest (19.0 mg m−2 day−1) stream, and tDOC uptake velocity indicated lower tDOC demand in the agricultural (1.2 mm min−1) than in the forest (1.9 mm min−1) stream. The agricultural stream differed from the forest stream by slightly lower transient storage capacity and higher benthic biofilm bacterial abundance and production, lower biofilm biomass and lower biofilm molar C:N, C:P, and N:P ratios. Changes in epilithic biofilms contributed little to the differences in whole-stream tDOC tracer uptake between streams, as biofilm tDOC uptake only amounted to 4% and 13% of whole-stream uptake in the forest and agricultural stream, respectively. This comparison of a forest and an agricultural stream suggests that agricultural stressors have the potential to diminish both whole-stream tDOC uptake and uptake efficiency. Furthermore, the weak link between biofilm and whole-stream tDOC uptake implies that benthic biofilms characteristics are poor predictors for human impacts on tDOC uptake in agricultural streams and that hot spots of tDOC uptake are likely situated in the hyporheic zone or in the stream water column.

Revealing chlorinated ethene transformation hotspots in a nitrate-impacted hyporheic zone

Hyporheic zones are increasingly thought of as natural bioreactors, capable of transforming and attenuating groundwater pollutants present in diffuse baseflow. An underappreciated scenario in the understanding of contaminant fate in hyporheic zones is the interaction between point-source trichloroethene (TCE) plumes and ubiquitous, non-point source pollutants such as nitrate. This study aims to conceptualise critical biogeochemical gradients in the hyporheic zone which govern the export potential of these redox-sensitive pollutants from carbon-poor, oxic aquifers. Within the TCE plume discharge zone, discrete vertical profiling of the upper 100 cm of sediment pore water chemistry revealed an 80% increase in dissolved organic carbon (DOC) concentrations and 20–60 cm thick hypoxic zones (<2 mg O2 L−1) within which most reactive transport was observed. A 33% reduction of nitrate concentrations coincided with elevated pore water nitrous oxide concentrations as well as the appearance of manganese and the TCE metabolite cis-1,2-dichloroethene (cDCE). Elevated groundwater nitrate concentrations (>50 mg L−1) create a large stoichiometric demand for bioavailable DOC in discharging groundwater. With the benefit of a high-resolution grid of pore water samplers investigating the shallowest 30 cm of hypoxic groundwater flow paths, we identified DOC-rich hotspots associated with submerged vegetation (Ranunculus spp.), where low-energy metabolic processes such as mineral dissolution/reduction, methanogenesis and ammonification dominate. Using a chlorine index metric, we show that enhanced TCE to cDCE transformation takes place within these biogeochemical hotspots, highlighting their relevance for natural plume attenuation.

Level and distribution of nutrients in the hyporheic zone of Lake Taihu (China) and potential drivers.

To better understand the nutrient distributions in the hyporheic zone of a large shallow eutrophic lake (Lake Taihu) and the potential drivers, the total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP) were investigated in March and December 2016. Spatial differences in the TN and TP contents existed in the hyporheic zone, particularly between the eastern and southern zones and the western and northern zones. The TOC/TN ratios in the western, northern, and central zones were mostly <8 and even reached 4, indicating that organic matter originated from aquatic organisms and algae, whereas those in the southern and eastern zones exhibited wide ranges, indicating complex pollution sources. The chloride depth profiles suggested that upwelling hyporheic flow potentially occurred in the southern, western, and northern zones, while alternating flow directions occurred in the eastern zone and no flow or weak flow occurred in the central zone. Compared to the 1st investigation, the TOC, TN, and TP in sediments in the 2nd investigation increased by 13%, 41%, and 87%, respectively, and these changes were mostly due to large hyporheic fluxes with high nutrient concentrations from shallow groundwater. The behavior of the hyporheic zone as an active pollution source/sink due to hyporheic flow should be considered in the comprehensive management of Lake Taihu. PRACTITIONER POINTS: Depth profiles of nutrients differed between sampling sites and between zones of Lake Taihu due to different pollution sources. Nutrients in sediment increased largely during winter compared to spring due to potential groundwater pollution through upwelling flow. No significant difference in sediment total organic carbon and ratios of C/N indicates a similar internal pollution source lake wide.

Comparing biotic drivers of litter breakdown across stream compartments

Litter breakdown in the streambed is an important pathway in organic carbon cycling and energy transfer in the biosphere that is mediated by a wide range of streambed organisms. However, most research on litter breakdown to date has focused on a small fraction of the taxa that drive it (e.g. microbial vs. macroinvertebrate‐mediated breakdown) and has been limited to the benthic zone (BZ). Despite the importance of the hyporheic zone (HZ) as a bioreactor, little is known about what, or who, mediates litter breakdown in this compartment and whether breakdown rates differ between the BZ and HZ.Here, we explore the relationship between litter breakdown and the variation in community structure of benthic and hyporheic communities by deploying two standardized bioassays (cotton strips and two types of commercially available tea bags) in 30 UK streams that encompass a range of environmental conditions. Then, we modelled these assays as a response of the streambed compartment and the biological features of the streambed assemblage (Prokaryota, Protozoa and Eumetazoa invertebrates) to understand the generality and efficiency of litter processing across communities.Litter breakdown was much faster in the BZ compared with the HZ (around 5 times higher for cotton strips and 1.5 times faster for the tea leaves). However, differences in litter breakdown between the BZ and the HZ were mediated by the biological features of the benthos and the hyporheos. Biomass of all the studied biotic groups, α‐diversity of Eumetazoa invertebrates and metabolic diversity of Prokaryota were important predictors that were positively related to breakdown coefficients demonstrating their importance in the functioning of the streambed ecosystem.Our study uses a novel multimetric bioassay that is able to disentangle the contribution by Prokaryota, Protozoa and Eumetazoa invertebrates to litter breakdown. In doing so, our study reveals new insights into how organic matter decomposition is partitioned across biota and streambed compartments.

A novel fiber optic system to map dissolved oxygen concentrations continuously within submerged sediments

Dissolved oxygen (DO) concentration is a primary indicator of redox and biogeochemical activity in the hyporheic zone (HZ) of fluvial systems. Due to the inherent difficulty of measuring chemical concentrations in hyporheic sediments, field measurements are typically spatially and temporally sparse. Consequently, conceptualizations of biogeochemical processes within streambed sediment have not been validated but only supported by temporally and spatially sparse observations. To overcome these limitations and provide spatially and temporally high-resolution measurements, we developed a multi-point, in situ DO measurement method based on a multiplexed optical network. This system was deployed in a large-scale flume and a cobble-bed headwater stream. In both settings, pore-water DO concentrations were measured at unprecedented spatial and temporal resolution. Our findings demonstrate the value of high-density DO measurements. These measurements are used to illuminate some shortcomings in the current conceptualization of reactive solute transport in the HZ.

A laboratory method for the visualization and quantification of hyporheic flow paths and velocities

Hyporheic flow, the flow of water through the permeable material immediately surrounding a river, is important for nutrient cycling, dissolved oxygen transport, and contaminant transport. In addition, there is recent concern regarding the role of hyporheic flow on the contamination of rivers following oil spills. To better understand hyporheic flow paths and velocities, it is important to measure hyporheic flow at high spatial and temporal resolution. A practical method to measure hyporheic flow in a laboratory flume based on dye injection, digital images, and moment analysis was developed. An experiment conducted using a single gravel bar demonstrated good agreement between observations and estimates based on image processing. The measured hyporheic flow field showed upstream and downstream flow that discharged downstream of the bar top, the presence of a flow divide and flow stagnation, and hyporheic flow velocities indicative of turbulent flow for which Darcy’s law is not applicable.

Modeling Influence of Sediment Heterogeneity on Nutrient Cycling in Streambeds

AbstractRivers and their hyporheic zones play an important role in nutrient cycling. The fate of dissolved inorganic nitrogen is governed by reactions that occur in the water column and streambed sediments. Sediments are heterogeneous both in term of physical (e.g., hydraulic conductivity) and chemical (e.g., organic carbon content) properties, which influence water residence times and biogeochemical reactions. Yet few modeling studies have explored the effects of both physical and chemical heterogeneity on nutrient transport in the hyporheic zone. In this study, we simulated hyporheic exchange in physically and chemically heterogeneous sediments with binary distributions of sand and silt in a low‐gradient meandering river. We analyzed the impact of different silt/sand patterns on dissolved organic carbon, oxygen, nitrate, and ammonium. Our results show that streambeds with a higher volume proportion of silt exhibit lower hyporheic exchange rates but more efficient nitrate removal along flow paths compared to predominantly sandy streambeds. The implication is that hyporheic zones with a mixture of inorganic sands and organic silts have a high capacity to remove nitrate, despite their moderate permeabilities.

Effects of Turbulent Hyporheic Mixing on Reach‐Scale Transport

AbstractTurbulence causes rapid mixing of solutes and fine particles between open channel flow and coarse‐grained streambeds. Turbulent mixing is known to control hyporheic exchange fluxes and the distribution of vertical mixing rates in the streambed, but it is unclear how turbulent mixing ultimately influences mass transport at the reach scale. We used a particle‐tracking model to simulate local‐ and reach‐scale solute transport for a stream with coarse‐grained sediments. Simulations were first used to determine profiles of vertical mixing rates that best described solute concentration profiles measured within a coarse granular bed in flume experiments. These vertical mixing profiles were then used to simulate a pulse solute injection to show the effects of turbulent hyporheic exchange on reach‐scale solute transport. Experimentally measured concentrations were best described by simulations with a nonmonotonic mixing profile, with highest mixing at the sediment–water interface and exponential decay into the bed. Reach‐scale simulations show that this enhanced interfacial mixing couples in‐stream and hyporheic solute transport. Coupling produces an interval of exponential decay in breakthrough curves and delays the onset of power law tailing. High streamwise velocities in the hyporheic zone reduce mass recovery in the water column and cause breakthrough curves to exhibit steeper power law slopes than predictions from mobile‐immobile modeling theory. These results demonstrate that transport models must consider the spatial variability of streamwise velocity and vertical mixing for both the stream and the hyporheic zone, and new analytical theory is needed to describe reach‐scale transport when high streamwise velocities are present in the hyporheic zone.

Measurement and Analysis of Nitrous Oxide Emissions over Time around a Dune in the Experimental Flume

In this study, the amount of nitrous oxide (N2O) emissions from the vicinity of a dune was measured to investigate the effects of the bedform on N2O emissions. N2O emissions over time around a dune were analyzed and the correlation between N2O emissions and water quality were evaluated. As a result, N2O emissions at the dune crest were 2.8 times higher than that in the flat bed and N2O emissions at the hill and slopes were 2.2 times and 3.8 times higher than those in the flat bed, respectively. Also, it was confirmed that N2O emissions at the different locations of the dune increased or decreased over time after the supply of the discharge. N2O emissions measured at the hill of the dune were the largest, which were increased from 2.97 to 13.9 μg N/m²/h. N2O emissions have tended to be increased in the hills, crests and slopes of the dune and be decreased in the flat bed. This study can be used as a fundamental data to evaluate the potential greenhouse gas emissions in the river system indirectly. Key words: Hyporheic Zone, Greenhouse Gas, Nitrous Oxide, Bedform, Dune

Brook lamprey survival in the dry riverbed of an intermittent stream

Intermittent streams account for approximately one half of the global river network, and are especially common in arid and semi-arid regions. These systems are inhabited by several fish species that are adapted to their natural flow intermittency. While many fish species rely on permanent pools or perennial reaches to survive during the dry season, our knowledge regarding fish survival strategies in dry reaches is incomplete. Our observations revealed that ammocoetes (i.e. lamprey larvae) of a non-parasitic brook lamprey (Lampetra sp.) survived in the hyporheic zone of an intermittent stream in the absence of surface water for at least 22 days. This is the first reported observation of ammocoetes persisting in the hyporheic water after loss of surface water, a behavior that has been previously reported for just a few fish species.

Interplay of hyporheic exchange and fine particle deposition in a riverbed

Hyporheic flow transports fine particles into the riverbed, which can lead to clogging of the bed and in turn affect hyporheic flow and exchange processes. Field measurements and numerical simulations show the formation of a low-permeability layer (LPL) near the bed surface due to fine particle clogging, and consequently reduction of exchange fluxes between the bed and river water. A characteristic porosity (ε*) and time scale were derived to quantify the clogging process and effects on transport. Both the exchange flux and mean solute residence time were found to follow a power law relationship with ε*. For the normalized particle exchange flux, the exponent is close to unity, i.e., a linear relationship with ε*. The results also showed significant effects of the fine particle concentration, pressure difference, sediment collision efficiency and fine particle diameter on the bed clogging. Large values of these parameters led to intensified clogging, with the formation of different types of LPL.

Analyzing and Designing an Arduino Controlled System to Study the Effect of Changing Water Levels on Water Flow Through Sediments

The hyporheic zone is the region of sediment under a stream where water from the stream flows before returning to the stream itself. Many studies focus on steady water flow through this region, however, in natural systems, water levels and water flow rates change due to storms, tides, dams, or melting snow. To investigate flow under unsteady conditions, we built a system that allows us to control the water level and thus the flow rates. We used a pressure sensor that is connected to an Arduino board to measure the water level. The Arduino board uses the measured pressure value to control a water pump. When the water level is lower than desired, the pump will turn on and when it is higher than desired, it will turn off. This allowed us to hold the water level constant or tell it to oscillate. We then evaluated our system by comparing our desired water level functions to those measured with our pressure sensor, those measured by a pressure transducer connected to a separate Arduino, and those we extracted from videos of our system. Faculty Sponsor: Susa H. Stonedahl

The influence of abiotic incubation conditions on the winter mortality of wild salmonid embryos

Abstract Embryos of many valued salmonid species incubate in the hyporheic zone of boreal streams over winter. Influence of individual winter‐related environmental variables on salmonid embryo success has been previously investigated. However, how multiple variables act together to influence embryo incubation remains poorly understood. Using a naturally spawning population of Atlantic salmon (Salmo salar) in the Miramichi River basin (New Brunswick, Canada), we related variation in the abiotic embryo incubation habitat in different streams (spatial) and over the course of two winters (temporal) to embryo mortality between fertilisation and hatch. Over two years (2013–2014 and 2014–2015), we introduced fertilised eggs to six simulated salmon redds in each of three riffles in each of five active spawning reaches (nredds = 90) with a range of hyporheic conditions. Embryo mortality was quantified at an early sampling event (March; pre‐freshet and during late embryonic development) and a late sampling event (May; post‐freshet and post‐hatch). We extracted 22 abiotic predictor variables for statistical analyses from continuous records of hyporheic environmental conditions, collected for the duration of the incubation period in each study reach. Through partial least squares regression analyses, 37.6% of the total variation in mortality was explained by the predictor variables. Each group of predictor variables explained similar proportions of variation (water temperature: 8.4%, water level: 7.4%, dissolved oxygen: 7.1%, ice conditions: 7.2%, and substrate characteristics: 7.5%), which suggests that mortality is influenced by multiple interacting abiotic conditions, rather than a single variable in isolation, and that the factors contributing to ideal salmonid incubation habitats are complex and interconnected. Our research highlights the value of a multi‐faceted research perspective and provides a baseline from which future changes in threatened salmonid populations can be measured and compared in an effort to identify relevant species‐ or population‐specific differences.

An Analysis of the Factors Affecting Hyporheic Exchange based on Numerical Modeling

The hyporheic zone is a transition zone for the exchange of matter and energy between surface water and subsurface water. The study of trends and sensitivities of bed hyporheic exchanges to the various influencing factors is of great significance. The surface−groundwater flow process was simulated using a multiphysics computational fluid dynamics (CFD) method and compared to previous flume experiments. Based on that, the single-factor effects of flow velocity (u), water depth (H), dune wave height (h), and bed substrate permeability (κ) on hyporheic exchange in the bed hyporheic zone were investigated. The sensitivity analysis of various factors (H, u, dune wavelength (L), h, bed substrate porosity (θ), κ, and the diffusion coefficient of solute molecules (Dm)) in the surface−subsurface water coupling model was done using orthogonal tests. The results indicated that u, h, and κ were positively related, whereas H was negatively related to hyporheic exchange. H and u showed large effects, whereas κ, Dm, and θ had moderate effects, and L and h showed small effects on hyporheic exchange. This study provides valuable references for the protection and recovery of river ecology.

Fate of Trace Organic Compounds in the Hyporheic Zone: Influence of Retardation, the Benthic Biolayer, and Organic Carbon.

The fate of 28 trace organic compounds (TrOCs) was investigated in the hyporheic zone (HZ) of an urban lowland river in Berlin, Germany. Water samples were collected hourly over 17 h in the river and in three depths in the HZ using minipoint samplers. The four relatively variable time series were subsequently used to calculate first-order removal rates and retardation coefficients via a one-dimensional reactive transport model. Reversible sorption processes led to substantial retardation of many TrOCs along the investigated hyporheic flow path. Some TrOCs, such as dihydroxy-carbamazepine, O-desmethylvenlafaxine, and venlafaxine, were found to be stable in the HZ. Others were readily removed with half-lives in the first 10 cm of the HZ ranging from 0.1 ± 0.01 h for iopromide to 3.3 ± 0.3 h for tramadol. Removal rate constants of the majority of reactive TrOCs were highest in the first 10 cm of the HZ, where removal of biodegradable dissolved organic matter was also the highest. Because conditions were oxic along the top 30 cm of the investigated flow path, we attribute this finding to the high microbial activity typically associated with the shallow HZ. Frequent and short vertical hyporheic exchange flows could therefore be more important for reach-scale TrOC removal than long, lateral hyporheic flow paths.

Estimating cumulative wastewater treatment plant discharge influences on acesulfame and Escherichia coli in a highly impacted watershed with a fully-integrated modelling approach

Wastewater treatment plant (WWTP) discharge is often considered a principal source of surface water contamination. In this study, a three-dimensional fully-integrated groundwater–surface water model was used to simulate the transport characteristics and cumulative loading of an artificial sweetener (acesulfame) and fecal indicator bacteria (Escherichia coli) from WWTPs within a 6800 km2 mixed-use, highly impacted watershed in Ontario, Canada. The model, which employed 3.5 × 106 computational nodes and 15 layers, facilitated a comprehensive assessment of groundwater–surface water interactions under high and low flow conditions; processes typically not accounted for in WWTP cumulative effects models. Simulations demonstrate that the model had significant capacity in reproducing the average and transient multi-year groundwater and surface water flow conditions in the watershed. As a proxy human-specific conservative tracer, acesulfame was useful for model validation and to help inform the representation of watershed-scale transport processes. Using a uniform WWTP acesulfame loading rate of 7.14 mg person−1 day−1, the general spatial trends and magnitudes of the acesulfame concentration profile along the main river reach within the watershed were reproduced; however, model performance was improved by tuning individual WWTP loading rates. Although instream dilution and groundwater–surface water interactions were strongly dependent on flow conditions, the main reach primarily consisted of groundwater discharge zones. For this reason, hydrodynamic dispersion in the hyporheic zone is shown as the predominant mechanism driving acesulfame into near-stream shallow groundwater, while under high flow conditions, the simulations demonstrate the potential for advective flushing of the shallow groundwater. Regarding the cumulative impact of the WWTPs on E. coli concentrations in the surface flow system, simulated transient E. coli levels downstream of WWTPs in the watershed were significantly lower than observed values, thus highlighting the potential importance of other sources of E. coli in the watershed.

Multi-scale preferential flow processes in an urban streambed under variable hydraulic conditions

Spatially preferential flow processes occur at nested scales at the sediment-water interface (SWI), due in part to sediment heterogeneities, which may be enhanced in flashy urban streams with heavy road sand influence. However, several factors, including the flow-rate dependence of preferential hyporheic flow and discrete groundwater discharge zones are commonly overlooked in reach-scale models of groundwater/surface water exchange. Using a series of controlled-head tracer-injection experiments coupled with cm-scale geophysics within the highly reactive upper 30 cm of the hyporheic zone of an urban stream, we quantified the flow dependence of local less-mobile porosity volume, mass-transfer rate coefficient, and the resulting local residence time in the less-mobile pore space at three controlled downward fluid fluxes (0.8, 2, and 3 m/d). Experiments were performed in two adjacent streambed locations, representing different sediment bulk vertical permeability. Less-mobile porosity parameters were generally substantial and similar between the two streambed locations; though a more competent, thin, organic layer at ∼15 cm depth in one location strongly impacted tracer loading, flushing dynamics, and local residence times. Increased downward flux led to (1) a decrease in less-mobile porosity residence time in all experiments, and (2) an increase in less-mobile porosity fraction for most experiments. Additionally, at the larger stream reach-scale, surface electrodes for electrical resistivity measurement were installed along 22 m of the wetted stream channel. These surface electrode measurements were collected during a natural storm flow event, which revealed widespread, short-term, flushing (e.g. <3 h) of the hyporheic zone with stream water, followed by longer-term (e.g. >60 h) flushing of the SWI with riparian zone groundwater. Flow dependence of preferential hyporheic zone flowpaths, like in the controlled tracer experiments, was also observed in these reach-scale electrical resistivity tomography measurements. Our findings reveal that the spatial and temporal dependence of preferential flow processes create highly dynamic SWI conditions that will affect the physical and coupled biogeochemical functions of the SWI in urbanized, sand-impacted streams.