factors controlling riffle scale hyporheic exchange flows and their seasonal changes in a gaining stream a three dimensional groundwater flow model 1 subsurface flow within a single riffle of a low gradient gravel bed stream was modeled in three dimensions using modflow a finite difference groundwater flow model model simulations showed that exchange flows can only occur in this low gradient gaining stream because of a zone of alluvial sediment around the stream that has much higher permeability than the surrounding catchment k 10 4 m s 1 compared with k 10 6 to 10 8 m s 1 the key factors controlling exchange flow within the alluvial zone were identified as the hydraulic conductivity of the alluvium the hydraulic gradient between upstream and downstream ends of the riffle and the flux of groundwater entering the alluvium from the sides and beneath in the study riffle each of these factors changes with season causing a reversal of flow paths in the alluvium and a reduction in exchange flows from about 0 2 0 5 m 3 d per meter stream length in summer to about 0 008 0 04 m 3 d 1 per meter stream length during fall to spring the model also revealed that exchange flows are up to twice as strong but more variable at the sides of the stream than near the center and that vertical flow paths beneath the channel are more persistent under the range of conditions modeled than lateral flow paths into the banks

evaluating the reliability of the stream tracer approach to characterize stream subsurface water exchange stream water was locally recharged into shallow groundwater flow paths that returned to the stream hyporheic exchange in st kevin gulch a rocky mountain stream in colorado contaminated by acid mine drainage two approaches were used to characterize hyporheic exchange sub reach scale measurement of hydraulic heads and hydraulic conductivity to compute streambed fluxes hydrometric approach and reachscale modeling of in stream solute tracer injections to determine characteristic length and timescales of exchange with storage zones stream tracer approach subsurface data were the standard of comparison used to evaluate the reliability of the stream tracer approach to characterize hyporheic exchange the reach averaged hyporheic exchange flux 1 5 ml s 1 m 1 determined by hydrometric methods was largest when stream base flow was low 10 l s 1 hyporheic exchange persisted when base flow was 10 fold higher decreasing by approximately 30 reliability of the stream tracer approach to detect hyporheic exchange was assessed using first order uncertainty analysis that considered model parameter sensitivity the stream tracer approach did not reliably characterize hyporheic exchange at high base flow the model was apparently more sensitive to exchange with surface water storage zones than with the hyporheic zone at low base flow the stream tracer approach reliably characterized exchange between the stream and gravel streambed timescale of hours but was relatively insensitive to slower exchange with deeper alluvium timescale of tens of hours that was detected by subsurface measurements the stream tracer approach was therefore not equally sensitive to all timescales of hyporheic exchange we conclude that while the stream tracer approach is an efficient means to characterize surface subsurface exchange future studies will need to more routinely consider decreasing sensitivities of tracer methods at higher base flow and a potential bias toward characterizing only a fast component of hyporheic exchange stream tracer models with multiple rate constants to consider both fast exchange with streambed gravel and slower exchange with deeper alluvium appear to be warranted

dynamic response of surface water groundwater exchange to currents tides and waves in a shallow estuary 1 in shallow fetch limited estuaries variations in current and wave energy promote heterogeneous surface water groundwater mixing benthic exchange which influences biogeochemical activity here we characterize heterogeneity in benthic exchange within the subtidal zone of the delaware inland bays by linking hydrodynamic circulation models with mathematical solutions for benthic exchange forced by current bedform interactions tides and waves benthic fluxes oscillate over tidal cycles as fluctuating water depths alter fluid interactions with the bed maximum current driven fluxes 1 10 cm d occur in channels with strong tidal currents maximum wave driven fluxes 1 10 cm d occur in downwind shoals during high energy storms simulated wave pumping rates increase by orders of magnitude demonstrating the importance of storms in solute transfer through the benthic layer under moderate wind conditions 5 m s integrated benthic exchange rates due to wave current and tidal pumping are each 1 10 m3 s on the order of fluid contributions from runoff and fresh groundwater discharge to the estuary benthic exchange is thus a significant and dynamic component of an estuary s fluid budget that may influence estuarine geochemistry and ecology

surface subsurface water interactions in an alluviated mountain stream channel data from an instream tracer experiment performed in north fork dry run shenandoah national park virginia illustrate that the transport of water and conservative solute in the stream is greatly influenced by interaction with water present in both the relatively shallow gravel bed and in the relatively deeper alluvial infill material the shape of concentration time curves from the field tracer experiment exhibit storage of tracer in the subsurface short term storage imposed by exchange with the gravel bed and long term storage by exchange with the deeper alluvial infill time series models fit the observed concentration data very well estimates of mixing volumes generated by the time series analysis indicate that a large volume of the gravel bed participates in rapid mixing with the surface stream at one point along the study reach a second order time series model is supported by the data suggesting two independent parallel flow paths observations of tracer in wells more than 10 m from the stream and the very extended recession of the tracer in the stream where it flows over a bedrock ledge support the contention that the deeper alluvium plays an important role in transient storage of solute in north fork dry run the work implies that physically based models to predict transport and storage in alluviated mountain catchments may have to account for interactions between the stream and large portions of its floodplain

hyporheic flow path response to hydraulic jumps at river steps flume and hydrodynamic models 1 the conceptual model of hyporheic exchange below river steps may oversimplify exchange flow paths if it depicts a uniform pattern of downstream directed upwelling this research used nonmobile porous bed flume experiments and hydrodynamic simulation cfd to characterize hyporheic flow paths below a river step with a hydraulic jump bed slope was 1 step height was 4 cm downstream flow depth was 4 cm substrate was 1 cm median diameter gravel and hydraulic jump length was 25 cm in the flume and cfd experiments with the hydraulic jump flow paths changed to include downwelling beneath the water plunging into the pool and upstream directed upwelling at the base of the step and beneath the length of jump failure to represent the influence of static and dynamic pressures associated with hydraulic jumps leads to erroneous prediction of subsurface flow paths in 75 of the streambed beneath the jump a refined conceptual model for hyporheic flow paths below a step with a hydraulic jump includes reversed hyporheic circulation cells in which downwelling water moves upstream and then upwells and flow reversals in which the larger flow net of downstream directed upwelling encounters a nested flow path of upstream directed upwelling heterogeneity in hyporheic flow paths at hydraulic jumps has the potential to explain field observed mosaics in streambed redox patterns and expand structure function relationships used in river management and restoration

the effects of discharge and slope on hyporheic flow in step pool morphologies this paper focuses on surface subsurface water exchange in a steep coarse bedded stream with a step pool morphology we use both flume experiments and numerical modelling to investigate the influence of stream discharge channel slope and sediment hydraulic conductivity on hyporheic exchange the model step pool reach whose topography is scaled from a natural river consists of three step pool units with 0 1 m step heights discharges ranging between base and over bankfull flows scaled values of 0 3 4 5 l s and slopes of 4 and 8 results indicate that the deepest hyporheic flow occurs with the steeper slope and at moderate discharges and that downwelling fluxes at the base of steps are highest at the largest stream discharges in contrast to findings in a pool riffle morphology those in this study show that steep slopes cause deeper surface subsurface exchanges than gentle slopes numerical simulation results show that the portion of the hyporheic zone influenced by surface water temperature increases with sediment hydraulic conductivity these experiments and numerical simulations emphasize the importance of topography sediment permeability and roughness elements along the channel surface in governing the locations and magnitude of downwelling fluxes and hyporheic exchange our results show that hyporheic zones in these steep streams are thicker than previously expected by extending the results from streams with pool riffle bed forms copyright 2014 john wiley sons ltd

geostatistical inverse modeling for the characterization of aquitard heterogeneity using long term multi extensometer data abstract the classical aquitard drainage model compac was used to simulate the deformation of a one dimensional vertically heterogeneous aquitard i e heter compac by coupling heter compac with the quasi linear geostatistical inversion approach the vertical heterogeneities of hydraulic conductivity k v elastic specific storage s ske and inelastic specific storage s skp fields could be characterized with the availability of long term multi extensometer data the methodology was first tested through a group of ten synthetic cases with randomly generated k v s ske and s skp fields results of the synthetic cases revealed that a geostatistical model could characterize the variability of aquitard hydraulic parameter fields and yields improved fits of simulated versus observed zone deformation than a geological model that treats each zone to be homogeneous subsequently the methodology was applied to a research site situated in changzhou city jiangsu province china where long term deformation behaviors of the strata within the aquifer aquitard system have been monitored vertical heterogeneities of k v s ske and s skp fields of the investigated aquitard at the site were characterized using the quasi linear geostatistical approach the correlation length of the three fields that yielded the best quality fit was assessed to be approximately 50 m and the fitting quality was slightly improved in comparison with that produced from calibrating a geological or zonation model through monte carlo analysis of aquitard deformation utilizing the inversed parameters and corresponding uncertainties the investigated aquitard is estimated to have compacted 914 8 42 6 mm before the rise in groundwater levels in adjoining aquifers

a new triangular multiple flow direction algorithm for computing upslope areas from gridded digital elevation models 1 gridded digital elevation data often referred to as dems are one of the most widely available forms of environmental data topographic analysis of dems can take many forms but in hydrologic and geomorphologic applications it is typically used as a surrogate for the spatial variation of hydrological conditions topographic indices and flow routing here we report on a new flow routing algorithm and compare it to three common classes of algorithms currently in widespread use the advantage of the new algorithm is that unrealistic dispersion on planar or concave hillslopes is avoided whereas multiple flow directions are allowed on convex hillslopes we suggest that this new triangular multiple flow direction algorithm md1 is more appropriate for a range of flow routing and topographic index applications

temporal and spatial response of hyporheic zone geochemistry to a storm event although there has been recent focus on understanding spatial variability in hyporheic zone geochemistry across different morphological units under baseflow conditions less attention has been paid to temporal responses of hyporheic zone geochemistry to non steady state conditions we documented spatial and temporal variability of hyporheic zone geochemistry in response to a large scale storm event tropical storm irene august 2011 across a pool riffle pool sequence along chittenango creek in chittenango ny usa we sampled stream water as well as pore water at 15 cm depth in the streambed at 14 locations across a 30 m reach sampling occurred seven times at daily intervals once during baseflow conditions once during the rising limb of the storm hydrograph and five times during the receding limb principal component analysis was used to interpret temporal and spatial changes and dominant drivers in stream and pore water geochemistry n 111 results show the majority of spatial variance in hyporheic geochemistry 62 is driven by differential mixing of stream and ground water in the hyporheic zone the second largest driver 17 of hyporheic geochemistry was temporal dilution and enrichment of infiltrating stream water during the storm hyporheic sites minimally influenced by discharging groundwater connected sites showed temporal changes in water chemistry in response to the storm event connected sites within and upstream of the riffle reflected stream geochemistry throughout the storm whereas downstream sites showed temporally lagged responses in some conservative and biogeochemically reactive solutes this suggests temporal changes in hyporheic geochemistry at these locations reflect a combination of changes in infiltrating stream chemistry and hyporheic flowpath length and residence time the portion of the study area strongly influenced by groundwater discharge increased in size throughout the storm producing elevated ca2 and concentrations in the streambed suggesting zones of localized groundwater inputs expand in response to storms copyright 2013 john wiley sons ltd

evaluation of alternative groundwater flow models for simulating hyporheic exchange in a small mountain stream summary groundwater flow models have been used to estimate the amount of exchange flow and the residence time distribution of stream water in the hyporheic zone however reliability of these predictions have not been tested we ask the questions 1 how reliable are hyporheic groundwater models in typical applications examining hyporheic exchange flows and 2 how does the reliability change with increased data availability and model sophistication we developed groundwater flow models of the hyporheic zone for a mountain stream in the hj andrews experimental forest oregon the models are based on surveyed topography and hydraulic conductivity k measurements from both slug tests and a well to well tracer test we developed several models using different methods to estimate two of the most uncertain parameters k and the depth and shape of the bedrock boundary we first tested the goodness of fit of each model to the water levels observed in a network of wells and piezometers results showed that differences among models in predicted heads were quite small whereas differences among estimated hyporheic fluxes varied by a factor of two we then tested the model predictions of tracer arrival times to each well in the network during a stream tracer injection comparison of simulated and observed travel times showed that increased model sophistication did not lead to improved model reliability because travel time predictions from the homogeneous model were equal to or better than the predictions from the heterogeneous models while general trends in solute breakthrough were correct in the models k data from even 37 wells in a 15 m by 50 m model domain were insufficient to characterize detailed arrival times accurately this suggests that geomorphic data may be sufficient to predict water fluxes through the subsurface and approximate travel times however for detailed analysis of solute transport pathways and breakthroughs intensive sampling of the subsurface may be necessary published by elsevier b v

the hydrogen and oxygen isotope geochemistry of pedogenic clay minerals principles and theoretical background abstract the 18 o 16 o and d h ratios of the structural oxygen and hydrogen of naturally occurring clay minerals carry information about the conditions under which the minerals formed clays generally form in isotopic equilibrium with the environment and hence have isotopic compositions that reflect the temperature of formation and the 18 o 16 o and d h ratios of water in the micro environment the isotopic signature of water in the weathering environment is a reflection of the climate once formed clays are resistant to isotopic alteration under surface and near surface conditions except when they undergo subsequent mineralogical alteration in some instances this alteration may be subtle as in the case of conversion of kaolinite with a high degree of crystalline order as measured by the hinckley index to kaolinite of low crystalline order in a pedogenic profile isotopic studies of pedogenic clays may be expected to yield more information about pedogenesis and paleoclimate when both 18 o 16 o and d h ratios of the clays are measured however few studies have incorporated the use of the two isotope ratios

effects of stream discharge alluvial depth and bar amplitude on hyporheic flow in pool riffle channels 1 hyporheic flow results from the interaction between streamflow and channel morphology and is an important component of stream ecosystems because it enhances water and solute exchange between the river and its bed hyporheic flow in pool riffle channels is particularly complex because of three dimensional topography that spans a range of partially to fully submerged conditions inducing both static and dynamic head variations hence these channels exhibit transitional conditions of streambed pressure and hyporheic flow compared to previous studies of fully submerged two dimensional bed forms here we conduct a series of three dimensional simulations to investigate the effects of bed topography depth of alluvium and stream discharge on hyporheic flow in pool riffle reaches with variable bed form submergence and we propose three empirical formulae to predict the mean depth of hyporheic exchange and characteristic values of the residence time distribution mean and standard deviation hyporheic exchange is predicted with a three dimensional pumping model and hyporheic flow is modeled as a darcy flow we find that the hyporheic residence time is well approximated by a lognormal distribution for both partially and entirely submerged pool riffle topography with the parameters of the distribution defined by the mean and variance of the log transformed residence time depth of alluvium has a substantial effect on hyporheic flow when alluvial depth is less than a third of the bed form wavelength for the conditions examined

how can subsurface modifications to hydraulic conductivity be designed as stream restoration structures analysis of vaux s conceptual models to enhance hyporheic exchange 1 despite the growing interest in hyporheic exchange and the associated stream ecosystem processes few studies consider restoration of hyporheic exchange as a design goal here we study the design of three types of subsurface structures for hyporheic restoration after conceptual designs published over 40 years ago vaux s designs involve modifying the subsurface with low or high hydraulic conductivity material placed at the streambed or adjacent to a confining layer below the stream in this preliminary analysis of subsurface structure design we use two dimensional groundwater flow modeling of structures to simulate structure performance in plane bed streams for ranges of structure geometric design and hydraulic conductivities structure performance is evaluated on the basis of total streambed flux physical extent of hyporheic flow paths created and residence time distributions along flow paths modified by the structures high hydraulic conductivity structures bend flow paths toward and through the structures themselves performance is controlled by the structure hydraulic conductivity results show low hydraulic conductivity structure performance is insensitive to the structure material hyporheic exchange is created by deflecting flow paths away from the structure itself time scales of simulated exchange are great enough to promote nitrification denitrification respiration and thermal buffering in the subsurface though these processes will also be controlled by site specific chemical and biological factors general design recommendations for specific restoration objectives are presented results of this study can be extrapolated to further understand the interaction of natural subsurface heterogeneities e g clay and gravel deposits and bedrock knickpoints and flow fields in creating hyporheic flow paths

hydrologic controls on hyporheic exchange in a headwater mountain stream the spatial and temporal scales of hyporheic exchange within the stream corridor are controlled by stream discharge and groundwater inflow interacting with streambed morphology while decades of study have resulted in a clear understanding of how morphologic form controls hyporheic exchange at the feature scale we lack comparable predictive power related to stream discharge and the spatial structure of groundwater inflows at the reach scale where spatial heterogeneity in both geomorphic setting and hydrologic forcing are present in this study we simulated vertical hyporheic exchange along a 600 m mountain stream reach under high medium and low stream discharge while considering groundwater inflow as negligible spatially uniform or proportional to upslope accumulated area most changes to hyporheic flow path residence time or length in response to stream discharge were small 5 suggesting that discharge is a secondary control relative to morphologically driven hyporheic exchange groundwater inflow was a primary control and mostly caused decreases in hyporheic flow path residence time and length this finding generally agrees with expectations from the literature however flow path response was not consistent across the study reach instead we found that flow paths driven by large hydraulic gradients coinciding with large morphologic features were less sensitive to changes in groundwater inflow than those driven by hydraulic gradients similar to the valley gradient our results indicate that consideration of heterogeneous arrangement of morphologic features is necessary to differentiate between hyporheic flow paths that persist in time and those that are sensitive to changing hydrologic conditions

reduction of the hyporheic zone volume due to the stream aquifer interaction 1 pore water in stream sediments is continuously exchanged with the surface water from the overlying stream this exchange of water and solutes that occurs across the stream sediment interface plays an important role for fluvial ecology because of the unique biochemical conditions rich biodiversity and high rates of metabolism while many studies have observed the extent of the hyporheic zone to be modified by changes in the level of the groundwater table the actual importance of this interaction is still difficult to quantify here we focus on the case of bedform induced hyporheic exchange to show how the the volume of hyporheic sediments that receive water from the stream is significantly reduced by the upwelling of subsurface water a simple scaling relationship for the assessment of maximum depth of the hyporheic zone is proposed by relating hyporheic flow to the groundwater discharge in an aquifer with given hydraulic properties and head difference between the stream and the aquifer

a modelling study of hyporheic exchange pattern and the sequence size and spacing of stream bedforms in mountain stream networks oregon usa studies of hyporheic exchange flows have identified physical features of channels that control exchange flow at the channel unit scale namely slope breaks in the longitudinal profile of streams that generate subsurface head distributions we recently completed a field study that suggested channel unit spacing in stream longitudinal profiles can be used to predict the spacing between zones of upwelling flux of hyporheic water into the stream and downwelling flux of stream water into the hyporheic zone in the beds of mountain streams here we use two dimensional groundwater flow and particle tracking models to simulate vertical and longitudinal hyporheic exchange along the longitudinal axis of stream flow in second third and fourth order mountain stream reaches modelling allowed us to 1 represent visually the effect that the shape of the longitudinal profile has on the flow net beneath streambeds 2 isolate channel unit sequence and spacing as individual factors controlling the depth that stream water penetrates the hyporheic zone and the length of upwelling and downwelling zones 3 evaluate the degree to which the effects of regular patterns in bedform size and sequence are masked by irregularities in real streams we simulated hyporheic exchange in two sets of idealized stream reaches and one set of observed stream reaches idealized profiles were constructed using regression equations relating channel form to basin area the size and length of channel units step size pool length etc increased with increasing stream order simulations of hyporheic exchange flows in these reaches suggested that upwelling lengths increased from 2 7mt o 7 6 m and downwelling lengths increased from 2 9mt o 6 0 m with increase in stream order from second to fourth order step spacing in the idealized reaches increased from 5 3mt o 13 7 m as stream size increased from second to fourth order simulated downwelling lengths increased from 4 3m in second order streams to 9 7 m in fourth order streams with a pool riffle step channel unit sequence and increased from 2 5m to 6 1 m from second to fourth order streams with a pool step riffle channel unit sequence upwelling lengths also increased with stream order in these idealized channels our results suggest that channel unit spacing size and sequence are all important in determining hyporheic exchange patterns of upwelling and downwelling though irregularities in the size and spacing of bedforms caused flow nets to be much more complex in surveyed stream reaches than in idealized stream reaches similar trends emerged relating the average geomorphic wavelength to the average hyporheic wavelength in both surveyed and idealized reaches this article replaces a previously published version hydrological processes 19 17 2915 2929 2005 doi 10 1002 hyp 5790 copyright 2006 john wiley sons ltd

dynamic discontinuous stream networks hydrologically driven variations in active drainage density flowing channels and stream order despite decades of research on the ecological consequences of stream network expansion contraction and fragmentation surprisingly little is known about the hydrological mechanisms that shape these processes here we present field surveys of the active drainage networks of four california headwater streams 4 27 km2 spanning diverse topographic geologic and climatic settings we show that these stream networks dynamically expand contract disconnect and reconnect across all the sites we studied stream networks at all four sites contract and disconnect during seasonal flow recessions with their total active network length and thus their active drainage densities decreasing by factors of two to three across the range of flows captured in our field surveys the total flowing lengths of the active stream networks are approximate power law functions of unit discharge with scaling exponents averaging 0 27 0 04 range 0 18 0 40 the number of points where surface flow originates obey similar power law relationships as do the lengths and origination points of flowing networks that are continuously connected to the outlet with scaling exponents averaging 0 36 0 48 even stream order shifts seasonally by up to two strahler orders in our study catchments broadly similar stream length scaling has been observed in catchments spanning widely varying geologic topographic and climatic settings and spanning more than two orders of magnitude in size suggesting that network extension contraction is a general phenomenon that may have a general explanation points of emergence or disappearance of surface flow represent the balance between subsurface transmissivity in the hyporheic zone and the delivery of water from upstream thus the dynamics of stream network expansion and contraction and connection and disconnection may offer important clues to the spatial structure of the hyporheic zone and to patterns and processes of runoff generation copyright 2014 john wiley sons ltd

a hydrogeomorphic river network model predicts where and why hyporheic exchange is important in large basins hyporheic exchange has been hypothesized to have basin scale consequences however predictions throughout river networks are limited by available geomorphic and hydrogeologic data and by models that can analyze and aggregate hyporheic exchange flows across large spatial scales we developed a parsimonious but physically based model of hyporheic flow for application in large river basins networks with exchange and subsurface storage nexss we applied nexss across a broad range of geomorphic diversity in river reaches and synthetic river networks nexss demonstrates that vertical exchange beneath submerged bed forms rather than lateral exchange through meanders dominates hyporheic fluxes and turnover rates along river corridors per kilometer low order streams have a biogeochemical potential at least 2 orders of magnitude larger than higher order streams however when biogeochemical potential is examined per average length of each stream order low and high order streams were often found to be comparable as a result the hyporheic zone s intrinsic potential for biogeochemical transformations is comparable across different stream orders but the greater river miles and larger total streambed area of lower order streams result in the highest cumulative impact from low order streams lateral exchange through meander banks may be important in some cases but generally only in large rivers

spatially distributed characterization of hyporheic solute transport during baseflow recession in a headwater mountain stream using electrical geophysical imaging summary the transport of solutes along hyporheic flowpaths is recognized as central to numerous biogeochemical cycles yet our understanding of how this transport changes with baseflow recession particularly in a spatially distributed manner is limited we conducted four steady state solute tracer injections and collected electrical resistivity data to characterize hyporheic transport during seasonal baseflow recession in the h j andrews experimental forest oregon usa we used temporal moment analysis of pixels generated from inversion of electrical resistivity data to compress time lapse data into descriptive statistics mean arrival time temporal variance and temporal skewness for each pixel a spatial visualization of these temporal moments in the subsurface at each of five 2 d transects perpendicular to the stream was interpreted to inform transport processes as baseflow recession progressed we found increasing first arrival times persistence mean arrival time temporal variance and coefficient of variation and decreasing skewness these trends suggest that changes in hydrologic forcing alter the relative influence of transport phenomena e g advection vs other transport processes such as dispersion along flowpaths spatial coverage obtained from electrical resistivity images allowed for qualitative comparison of spatial patterns in temporal moments both at an individual cross section as well as between cross sections we found that geomorphologic controls e g bedrock confinement vs gravel wedge deposits resulted in different distributions and metrics of hyporheic transport results of this study provide further evidence that hyporheic transport is highly variable both in space and through the baseflow recession period geophysical images differentiate advection dominated flowpaths from those that are more affected by other transport processes e g dispersion mobile immobile exchange

a multiscale model for integrating hyporheic exchange from ripples to meanders 1 it is necessary to improve our understanding of the exchange of dissolved constituents between surface and subsurface waters in river systems in order to better evaluate the fate of water borne contaminants and nutrients and their effects on water quality and aquatic ecosystems here we present a model that can predict hyporheic exchange at the bed form to reach scale using readily measurable system characteristics the objective of this effort was to compare subsurface flow induced at scales ranging from very small scale bed forms up to much larger planform geomorphic features such as meanders in order to compare exchange consistently over this range of scales we employed a spectral scaling approach as the basis for a generalized analysis of topography induced stream subsurface exchange the spectral model involves a first order approximation for local flow boundary interactions but is fully three dimensional and includes the lateral hyporheic zone in addition to the flow directly beneath the streambed the primary model input parameters are stream velocity and slope sediment permeability and porosity and detailed measurements of the stream channel topography the primary outputs are the distribution of water flux across the stream channel boundary the resulting pore water flow paths and the subsurface residence time distribution we tested the bed form exchange component of the model using a highly detailed two dimensional data set for exchange with ripples and dunes and then applied the model to a three dimensional meandering stream in a laboratory flume having spatially explicit information allowed us to evaluate the contributions of both gravitational and current driven hyporheic flow through various classes of stream channel features including ripples dunes bars and meanders the model simulations indicate that all scales of topography between ripples and meanders have a significant effect on pore water flow fields and residence time distributions furthermore complex interactions across the spectrum of topographic features play an important role in controlling the net interfacial flux and spatial distribution of hyporheic exchange for example shallow exchange induced by current driven interactions with small bed forms dominates the interfacial flux but local pore water flows are modified significantly by larger scale surface groundwater interactions as a result simplified representations of the stream topography do not adequately characterize patterns and rates of hyporheic exchange

interactions between hyporheic flow produced by stream meanders bars and dunes 1 stream channel morphology from grain scale roughness to large meanders drives hyporheic exchange flow in practice it is difficult to model hyporheic flow over the wide spectrum of topographic features typically found in rivers as a result many studies only characterize isolated exchange processes at a single spatial scale in this work we simulated hyporheic flows induced by a range of geomorphic features including meanders bars and dunes in sand bed streams twenty cases were examined with 5 degrees of river meandering each meandering river model was run initially without any small topographic features models were run again after superimposing only bars and then only dunes and then run a final time after including all scales of topographic features this allowed us to investigate the relative importance and interactions between flows induced by different scales of topography we found that dunes typically contributed more to hyporheic exchange than bars and meanders furthermore our simulations show that the volume of water exchanged and the distributions of hyporheic residence times resulting from various scales of topographic features are close to but not linearly additive these findings can potentially be used to develop scaling laws for hyporheic flow that can be widely applied in streams and rivers

temporal responses of groundwater surface water exchange to successive storm events groundwater surface water exchange within the hyporheic zone is widely recognized as a key mechanism controlling the fate of nutrients within catchments in gaining river systems groundwater surface water interactions are constrained by upwelling groundwater but there is increasing evidence that a rapid rise in river stage during storm events can result in a temporary reversal of vertical hydraulic gradients leading to surface water infiltration into the subsurface and supply of surface borne reactive solutes to this biogeochemically active interface at a uk study site using logged hydraulic heads in the surface water riverbed and riverbanks and logged electrical conductivity at multiple depths in the riverbed we show that storm events can lead to a temporary reversal of vertical hydraulic gradient with mixing evident up to 30 cm beneath the riverbed cross channel variability is evident with the center of the channel consistently having shorter reversals of hydraulic gradient compared to the channel margins the direction of shallow subsurface riverbank flow at the site is also reactive to storm events temporarily aligning with the surface flow direction and then reverting back to pre event conditions such a transition of flow paths during events is also likely to lead to expansion of lateral hyporheic exchange this study provides evidence that storm events can be a key driver of enhanced hyporheic exchange in gaining river systems which may support nutrient reactions beyond the duration of event driven change our observations demonstrate the dynamic nature of the hyporheic zone which should be considered when evaluating its biogeochemical function

coupled 3 d stream flow and hyporheic flow model under varying stream and ambient groundwater flow conditions in a pool riffle system 1 exchange of water and solutes across the stream sediment interface is an important control for biogeochemical transformations in the hyporheic zone hz in this paper we investigate the interplay between turbulent stream flow and hz flow in pool riffle streams under various ambient groundwater flow conditions streambed pressures derived from a computational fluid dynamics cfd model are assigned at the top of the groundwater model and fluxes at the bottom of the groundwater model domain represent losing and gaining conditions simulations for different reynolds numbers re and pool riffle morphologies are performed results show increasing hyporheic exchange flows m3 d for larger re and a concurrent decrease in residence time rt losing and gaining conditions were found to significantly affect the hyporheic flow field and diminish its spatial extent as well as rates of hyporheic exchange flow the fraction of stream water circulating through the hyporheic zone is in the range of 1 10 5 to 1 10 6 per meter stream length decreasing with increasing discharge complex distributions of pressure across the streambed cause significant lateral hyporheic flow components with a mean lateral travel distance of 20 of the longitudinal flow paths length we found that the relationship between pool riffle height and hyporheic exchange flow is characterized by a threshold in pool riffle amplitude beyond which hyporheic exchange flow becomes independent of riffle height hyporheic residence time distributions rtd are log normally distributed with medians ranging between 0 7 and 19 h

water table dynamics and groundwater surface water interaction during filling and draining of a large fluvial island due to dam induced river stage fluctuations 1 dam controlled river stage fluctuations alter groundwater surface water interaction between persistent bars and islands and the rivers bounding them by rapidly changing hydraulic gradients and expanding hyporheic zones a 300 m long and 80 m wide sand gravel island with established vegetation located on the colorado river austin texas usa is subjected to 1 m daily river stage variations due to upstream dam operations piezometer nests with probes monitored the evolution of the water table and groundwater flow paths through several cycles of dam induced stage fluctuations results show that hydraulic head and the water table within the island closely track the river stage associated with dam release water table mounds and depressions which overlap in time were mapped through the course of one storage release cycle over which 4 000 m 3 of water moved in and out of the island dam operations have drastically altered groundwater surface water connectivity between the colorado river and the fluvial island aquifer by pumping substantial amounts of water in and out of the aquifer during dam release and storage cycles