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

hyporheic transport and biogeochemical reactions in pool riffle systems under varying ambient groundwater flow conditions at the interface between stream water groundwater and the hyporheic zone hz important biogeochemical processes that play a crucial role in fluvial ecology occur solutes that infiltrate into the hz can react with each other and possibly also with upwelling solutes from the groundwater in this study we systematically evaluate how variations of gaining and losing conditions stream discharge and pool riffle morphology affect aerobic respiration ar and denitrification dn in the hz for this purpose a computational fluid dynamics model of stream water flow is coupled to a reactive transport model scenarios of variations of the solute concentration in the upwelling groundwater were conducted our results show that solute influx residence time and the size of reactive zones strongly depend on presence magnitude and direction of ambient groundwater flow high magnitudes of ambient groundwater flow lower ar efficiency by up to 4 times and dn by up to 3 orders of magnitude compared to neutral conditions the influence of stream discharge and morphology on the efficiency of ar and dn are minor in comparison to that of ambient groundwater flow different scenarios of o2 and no3 concentrations in the upwelling groundwater reveal that dn efficiency of the hz is highest under low upwelling magnitudes accompanied with low concentrations of o2 and no3 our results demonstrate how ambient groundwater flow influences solute transport ar and dn in the hz neglecting groundwater flow in stream groundwater interactions would lead to a significant overestimation of the efficiency of biogeochemical reactions in fluvial systems

characterisation of hyporheic exchange in a losing stream using radon 222 summary hyporheic and parafluvial flows between streams and the underlying streambed or adjacent alluvium are important drivers of biogeochemical cycling in streams here we present a new method for characterising this exchange in a losing stream based on longitudinal stream radon activities a mass balance approach is used to constrain the radon influx into the stream and estimate exchange parameters flux residence time and exchange zone thickness a net radon flux into the stream of 5 4 10 4 bq m 1 d 1 is required to balance radon losses to groundwater recharge gas transfer and radioactive decay given the radon production rate of the sediments 1 3 0 7 bq l 1 d 1 the minimum volume of alluvium flushed by either hyporheic or parafluvial exchange is 168 m 3 per m length of stream based on the stream width depth of alluvial sediments and porosity this implies that the exchange zone extends beneath the stream and an additional 11 m either side the results of this new method are compared to two existing methods streambed radon disequilibrium and transient storage modelling of breakthrough curves of an injected tracer the stream radon mass balance provides a relatively simple means of estimating hyporheic and parafluvial exchange over tens to hundreds of kilometres of stream concurrent application of the stream radon method transient storage modelling of injected tracer breakthrough curves and hydraulic methods is recommended to capture the full spectrum of hyporheic exchange in losing streams

effect of anisotropy and groundwater system geometry on seepage through lakebeds 1 analog and dimensional analysis abstract distribution of seepage through lakebeds is controlled partly by geometric configuration of the lake and of the groundwater system interacting with the lake to evaluate the effect of these factors conductive paper electric analog models were used to analyze a number of lake and groundwater settings having different geometric configurations most settings analyzed are of lakes that do not penetrate the groundwater system the width ratio the ratio of half the lake width to thickness of the groundwater system is the principal geometric characteristic used in this study because the distribution of groundwater seepage into a lake is not uniform across the lakebed the concept of a streamlinecrowding factor is developed and is used to determine seepage patterns from geometric characteristics of the lake and its contiguous groundwater system analysis of fourteen different width ratios of lake and groundwater systems indicates that lakes can be defined by three general groups of seepage patterns which include flow patterns volumes and rates 1 lakes having width ratios less than 0 6 show relatively uniform distribution of seepage across the lakebed 2 lakes having width ratios of 0 6 to 2 0 change in absolute and relative streamline crowding in the near shore region and 3 lakes having width ratios greater than 2 0 show stable flow patterns near shore however with increasing lake width the relative streamline crowding increases relative to that width for deep lakes and those in anisotropic media the crowding effect is decreased resulting in more uniform seepage across the lakebed

quantifying hyporheic exchange in a tidal river using temperature time series 1 an investigation into groundwater surface water interaction gswi beneath a large tidally influenced river was conducted to determine the effect of tides on the development of a hyporheic zone hz and to quantify mixing of river water and groundwater temperature measurements coupled with independent hydraulic head measurements were used to detect groundwater flow within the riverbed gwsi under tidal forcing produced a 1 m deep hz time averaged riverbed temperature profiles displayed a distinct compressed convex pattern clear evidence of net groundwater discharge however the instantaneous time series data indicate that riverbed temperatures were affected by tidal forcing to a depth of 1 m heat transport modeling revealed that instantaneous velocities within the shallow sediments of the riverbed are rather high creating a zone of vigorous exchange during either a flooding or ebbing tide furthermore the magnitude of the tidal pressure gradient was found to be significantly greater than the pressure gradient expected across 0 8 m high dunes evidence that bed form driven exchange under these conditions and this scale of observation did not contribute to the development of the hz conditions for exchange induced by shear and current bed form are favorable during ebbing tidal conditions only flow paths are therefore limited in depth exchange flow paths in an estuary setting are complex they are limited in duration and space and dominated by tidal pumping

imaging and quantifying salt tracer transport in a riparian groundwater system by means of 3d ert monitoring abstractdetermining groundwater flow paths of infiltrated river water is necessary for studying biochemical processes in the riparian zone but their characterization is complicated by strong temporal and spatial heterogeneity we investigated to what extent repeat 3d surface electrical resistance tomography ert can be used to monitor transport of a salt tracer plume under close to natural gradient conditions the aim is to estimate groundwater flow velocities and pathways at a site located within a riparian groundwater system adjacent to the perialpine thur river in northeastern switzerland our ert time lapse images provide constraints on the plume s shape flow direction and velocity these images allow the movement of the plume to be followed for 35 m although the hydraulic gradient is only 1 43 the ert time lapse images demonstrate that the plume s center of mass and its front propagate with velocities of 2 10 4 m s and 5 10 4 m s respectively these velocities are compatible with groundwate

estimation of seepage rates in a losing stream by means of fiber optic high resolution vertical temperature profiling summary vertical temperature profiling in the river beds of losing streams has been shown to be useful in obtaining seepage rates we present a method for high resolution vertical temperature profiling in surface water sediments for detailed quantification of seepage flux over depth and time the method is based on fiber optic distributed temperature sensing in which temperature profiles along an optical fiber are obtained by making use of raman scattering an optical fiber was wrapped around a 2 in pvc tube and installed vertically within the streambed sediment the wrapping transfers the spatial resolution along the fiber of 1 m to a vertical resolution of about 5 mm the high resolution temperature profiler was tested at a losing reach of the swiss prealpine river thur resulting in a 20 day long temperature time series with a temporal resolution of 10 min the time series are analyzed by means of dynamic harmonic regression to obtain the diurnal contributions of the measured time series at all depths and time points the time for the diurnal temperature signal to reach the observation depth and the associated attenuation of the signal are calculated from the phase angles and amplitudes of the diurnal contributions the time shift results in an apparent celerity of diurnal temperature propagation which is converted into an apparent seepage rate by fitting the data to the analytical solution for convective conductive heat transfer in a semi infinite uniform one dimensional domain with a sinusoidal surface temperature the high spatial resolution allows the location of discontinuities in the river bed which would have remained undetected if temperature had been measured only at a few individual depths to be identified this is a particular strength of the fiber optic high resolution temperature profiler the time series also give evidence of sporadic high infiltration rates at times of high water tables

defining the hyporheic zone in a large tidally influenced river summary an investigation was conducted to characterize the spatial and temporal distribution of the hyporheic zone of a large tidally influenced river the field site is located on the fraser river in british columbia canada approximately 30 km from its outlet to the ocean the physical attributes of the riverbed were mapped using geophysical techniques coupled with sediment sampling the spatial and temporal distribution of groundwater composition beneath the riverbed was determined through detailed profiling contaminated fresh groundwater discharges through a narrow band of the riverbed at a distance approximately 88 105 m from the shoreline coinciding with the termination of a massive silty unit saline groundwater as part of a regional flow system dominates the riverbed sediments from 105 m beyond the shoreline towards the centre of channel three water types occur within the upper 2 m of the riverbed sediments a result of both mixing of river water contaminated fresh groundwater and saline groundwater and modification by cation exchange reactions the interaction of these waters produced distinct zones of ca hco 3 na cl and ca cl type waters the distribution of groundwater solutes indicates that during a single tidal cycle river water penetrates the riverbed to a depth of approximately 15 cm but the long term effects of tidal pumping of river water into the riverbed is observed to a depth of approximately 1 m below the river bed

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

a model for lateral hyporheic flow based on valley slope and channel sinuosity 1 the meandering of rivers results in head gradients across point bar deposits leading to hyporheic exchange the flow of groundwater from and back to the stream two models for predicting horizontal fluid flux through the hyporheic zone one based solely on valley slope and another that uses both valley slope and channel sinuosity are presented in this article both models require knowledge of aquifer hydraulic conductivity the models are developed via a suite of groundwater flow simulations with boundary conditions taking a variety of channel geometries idealized as sinusoidal and valley slopes that we assume are similar to that of the water table functional predictive models are developed by fitting a line or surface to discrete model output which covers the spectrum of most natural meandering rivers since the fitted models only require channel slope and or sinuosity plus hydraulic conductivity they are amenable for regional estimation of hyporheic flux using digital terrain data

semianalytical analysis of hyporheic flow induced by alternate bars 1 we investigate the effects of alternate bar morphology on the hyporheic flow in gravel bed rivers our goal is to investigate the relations between residence time distribution of a conservative tracer and the parameters controlling bed form morphology we assume homogeneous isotropic or anisotropic hydraulic properties of the streambed sediment and constant flow regime in equilibrium with the bed form which is considered fixed because its formation timescale is much longer than that of the subsurface flow under these assumptions we solve the in stream and hyporheic flow fields analytically in a three dimensional domain we approximate the former with the shallow water equations and model the latter as a darcian flow the two systems are linked through the hydraulic head distribution which is predicted at the streambed by the surface model and applied as a boundary condition to the hyporheic flow model we solve the solute transport equation in the hyporheic zone for a conservative tracer by means of particle tracking our model predicts that the mean value and variance of the hyporheic residence time depend on the alternate bar amplitude at equilibrium this result is found to be applicable also to discharges that are lower 70 in our simulations than the formative and submerge the bars entirely moreover our analysis shows that 95 of the hyporheic flow is confined in a near bed layer whose depth is about the width of the channel and shallows from low to steep gradient streams this causes the hyporheic mean residence time to reach a threshold when the alluvial depth is deeper than the channel width our results also show that as the stream slope increases the streamlines compact near to the streambed thereby reducing the mean residence time and its variance finally we observe that the hyporheic residence time of pulse injections of passive solutes is lognormally distributed with the mean value depending in a simple manner on the amplitude of the alternate bars

a coupled stream flow and depth integrated subsurface flow model for catchment hydrology summary few hydrological models that couple stream flow and subsurface flow in shallow aquifers are based on a compromise between simple and complex depiction of the system although this compromise could result in tractable tools for various applications we present a depth integrated approach in which flows in the vadose and saturated zones are assumed to be parallel to the bottom of the aquifer and thus are integrated in the direction normal to the bottom of the aquifer the hydrodynamic parameters are also integrated in this direction and gravity effects are preserved stream flow is handled by a diffusive wave equation that is calculated over a network of one dimensional bonds the first order coupling between the stream and subsurface flows exchanges water fluxes between the stream network and the subsurface compartment according to the hydraulic head differences between the systems three synthetic test cases one including a comparison with a three dimensional code are used to evaluate the general behavior of the coupled model it is shown that the approach reproduces the main hydrological features at the catchment scale including the generation of runoff infiltration exfiltration into from the vadose zone and smooth transient head variations in the aquifer

the effect of streambed topography on surface subsurface water exchange in mountain catchments a numerical hydrological simulation suggested that water exchange between stream channels and adjacent aquifers is enhanced by convexities and concavities in streambed topography at st kevin gulch an effluent stream in the rocky mountains of colorado subsurface hydraulic gradients and movement of ionic tracers indicated that stream water was locally recharged into well defined flow paths through the alluvium stream water filled flow paths in the alluvium referred to as substream flow paths returned to the stream a short distance downstream 1 to 10 m recharge to the substream flow paths occurred where stream water slope increased at the transition from pools 1 to steeper channel units 5 20 return of substream flow paths to the stream occurred where stream water slope decreased at the transition from steeper channel units to pools a net water flux calculation is typically used to characterize water and solute fluxes between surface and subsurface zones of catchments along our study reach at st kevin gulch the net inflow of water from subsurface to stream 1 6 ml s 1 m 1 underestimated the gross inflow 2 7 ml s 1 m 1 by 40 the influence of streambed topography is to enhance hydrological fluxes between stream water and subsurface zones and to prolong water sediment contact times these effects could have important consequences for solute transport retention and transformation in catchments

mixing of surface and groundwater induced by riverbed dunes implications for hyporheic zone definitions and pollutant reactions 1 the hyporheic zone is often defined as where mixing of surface water and groundwater occurs in shallow sediments beneath and adjacent to rivers this mixing is credited with creating unique biogeochemical conditions that can attenuate contaminants from either upstream surface water or groundwater under gaining conditions however reactions of contaminants upwelling from groundwater may be more dependent on such mixing than contaminants from surface water we numerically modeled mixing between hyporheic flow paths induced by riverbed dunes and flow paths of adjacent upwelling of deeper groundwater results show that only 12 7 or less tracer mass upwelling from deeper groundwater dispersed across into hyporheic flow paths originating in surface water the spatial extent of a mixing defined hyporheic zone was smaller than a hyporheic zone defined as hydrologic flow paths leaving and returning to surface water mixing dependent reactions will therefore be localized within a thin mixing zone yet vary considerably with site conditions for example mixing in homogeneous sediments was controlled most by variation in hydraulic conductivity and upwelling flow rate which primarily affected mixing zone length by contrast introduction of heterogeneity increased mixing primarily by increasing mixing zone thickness consistent with studies of flow focusing in aquifers finally dispersivity is a critical parameter for which data are needed for shallow sediments our results help clarify hyporheic zone definitions and potential for mixing dependent reactions in particular the biogeochemically reactive portion of riverbed sediments from the perspective of upwelling contaminants does not necessarily spatially coincide with more traditional hydrologic conceptions of the hyporheic zone

the effect of river bend morphology on flow and timescales of surface water groundwater exchange across pointbars summary head gradients along meandering rivers induce surface water groundwater exchange across pointbars we analyze this exchange process by finite element modeling of groundwater flow and transport through model generated river planforms taken from previous studies the river is considered as a constant head boundary that decreases linearly along the direction of the channel this leads to an irregular groundwater flowfield in the pointbar that is controlled by channel sinuosity simulations under homogeneous aquifer conditions shows that the variable flowpath lengths coupled with the heterogeneous flow velocities lead to non fickian transport characterized by power law residence time distributions this anomalous transport behavior is enhanced by increased aquifer dispersivity the broad transit time distribution implies that time dependent chemical transformation processes in near stream zones may be equally variable as the flowfield even in the absence of heterogeneity in aquifer hydraulic properties

spatial variations in surface sediment structure in riffle pool sequences a preliminary test of the differential sediment entrainment hypothesis dseh riffle pool sequences are maintained through the preferential entrainment of sediment grains from pools rather than riffles this preferential entrainment has been attributed to a reversal in the magnitude of velocity and shear stress under high flows however the differential sediment entrainment hypothesis dseh postulates that differential entrainment can instead result from spatial sedimentological contrasts here we use a novel suite of in situ grain scale field measurements from a riffle pool sequence to parameterise a physically based model of grain entrainment field measurements include pivoting angles lift forces and high resolution dems acquired using terrestrial laser scanning from which particle exposure protrusion and surface roughness were derived the entrainment model results show that grains in pools have a lower critical entrainment shear stress than grains in either pool exits or riffles this is because pool grains have looser packing hence greater exposure and lower pivoting angles conversely riffle and pool exit grains have denser packing lower exposure and higher pivoting angles a cohesive matrix further stabilises pool exit grains the resulting predictions of critical entrainment shear stress for grains in different subunits are compared with spatial patterns of bed shear stress derived from a 2d computational fluid dynamics cfd model of the reach the cfd model predicts that under bankfull conditions pools experience lower shear stresses than riffles and pool exits however the difference in sediment entrainment shear stress is sufficiently large that sediment in pools is still more likely to be entrained than sediment in pool exits or riffles resulting in differential entrainment under bankfull flows significantly this differential entrainment does not require a reversal in flow velocities or shear stress suggesting that sedimentological contrasts alone may be sufficient for the maintenance of riffle pool sequences this finding has implications for the prediction of sediment transport and the morphological evolution of gravel bed rivers

alluvial characteristics groundwater surface water exchange and hydrological retention in headwater streams conservative solute injections were conducted in three first order montane streams of different geological composition to assess the influence of parent lithology and alluvial characteristics on the hydrological retention of nutrients three study sites were established 1 aspen creek in a sandstone siltstone catchment with a fine grained alluvium of low hydraulic conductivity 1 3 10 4 cm s 2 rio calaveras which flows through volcanic tuff with alluvium of intermediate grain size and hydraulic conductivity 1 2 10 3 cm s and 3 gallina creek located in a granite gneiss catchment of coarse poorly sorted alluvium with high hydraulic conductivity 4 1 10 3 cm s all sites were instrumented with networks of shallow groundwater wells to monitor interstitial solute transport the rate and extent of groundwater surface water exchange determined by the solute response in wells increased with increasing hydraulic conductivity the direction of surface water groundwater interaction within a stream was related to local variation in vertical and horizontal hydraulic gradients experimental tracer responses in the surface stream were simulated with a one dimensional solute transport model with inflow and storage components otis model derived measures of hydrological retention showed a corresponding increase with increasing hydraulic conductivity to assess the temporal variability of hydrological retention solute injection experiments were conducted in gallina creek under four seasonal flow regimes during which surface discharge ranged from baseflow 0 75 l s in october to high 75 l s during spring snowmelt model derived hydrological retention decreased with increasing discharge the results of our intersite comparison suggest that hydrological retention is strongly influenced by the geologic setting and alluvial characteristics of the stream catchment temporal variation in hydrological retention at gallina creek is related to seasonal changes in discharge highlighting the need for temporal resolution in studies of the dynamics of surface water groundwater interactions in stream ecosystems 1997 by john wiley sons ltd

hyporheic exchange in gravel bed rivers with pool riffle morphology laboratory experiments and three dimensional modeling received 7 june 2005 revised 30 june 2006 accepted 25 august 2006 published 31 january 2007 1 we report the first laboratory simulations of hyporheic exchange in gravel pool riffle channels which are characterized by coarse sediment steep slopes and three dimensional bed forms that strongly influence surface flow these channels are particularly important habitat for salmonids many of which are currently at risk worldwide and which incubate their offspring within the hyporheic zone here we perform a set of laboratory experiments examining the effects of discharge and bed form amplitude on hyporheic exchange with surface subsurface mixing measured directly from the concentration decay of a conservative tracer fluorescein injected into the surface flow near bed pressure measurements were also used to predict hyporheic exchange from a three dimensional pumping transport model comparison of the predicted and observed hyporheic exchange shows good agreement indicating that the major mechanism for exchange is bed form induced advection however the effect of bed forms is modulated by discharge and the degree of topographic submergence we also tested the performance of the hydrostatic pressure as a proxy for the observed near bed pressure in driving hyporheic exchange which would facilitate field measurement and analysis of hyporheic flow in natural rivers we found agreement with measured hyporheic exchange only for low bed form amplitudes and high flows

impact of heterogeneity bed forms and stream curvature on subchannel hyporheic exchange 1 advection through hyporheic zones hz consisting of heterogeneous channel bend streambed deposits and their equivalent homogenous medium was investigated using finite difference groundwater flow and transport simulations and forward particle tracking the top prescribed head boundary was varied in order to mimic various stream channel head distributions resulting from the presence of bed forms and channel curvature flux calculations show that heterogeneity causes significant additional hz flux compared to an equivalent homogenous medium however the major cause of hz flux is a spatially periodic sinusoidal head distribution along the boundary representing the effect of bed forms the additional influence of heterogeneity on the total channel bed exchange and the overall hz geometry are increased when boundary head sinusoidal fluctuation is more subdued we present dimensionless numbers that summarize these relationships heterogeneity s influence is further magnified by considering the effect of channel curvature on boundary heads the simulations illustrate the dynamic influence of heterogeneity on the hyporheic zone since the various head boundaries employed in our modeling efforts are a proxy for different surface water conditions and bed form states that may occur during a single flood furthermore we show that residence times total tracking times of particles originating from the streambed follow a lognormal distribution in the presence of heterogeneity residence times can decrease or they can increase compared to residence times for homogeneous conditions depending on the relative positions of the heterogeneities and the bed forms hence streambed heterogeneity and stream curvature factors often neglected in previous modeling efforts combine with bed form configuration to dynamically determine hz geometry fluxes and residence time distributions

application of heat pulse injections for investigating shallow hyporheic flow in a lowland river 1 hyporheic zone processes can have significant impact on groundwater and surface water resources detailed knowledge of exchange flow patterns is crucial for understanding the ecohydrological and biogeochemical functioning of river corridors in particular small scale hyporheic exchange flow is still poorly understood partially because of the lack of adequate in situ monitoring technology this paper investigates the spatial heterogeneity of hyporheic exchange flow in a lowland river at multiple scales it demonstrates the conjunctive use of active heat pulse tracing at shallow depths 15 cm and vertical hydraulic gradients vhg at 120 150 cm streambed depth for improving the understanding of hyporheic exchange flow processes generally positive vhg indicated a regional dominance of groundwater up welling high and temporally variable vhg were used to identify confined conditions caused by low conductivity layers in the subsurface low connectivity while locations with lower and temporally less variable vhg indicated free groundwater up welling high connectivity in highly conductive sediments a heat pulse sensor hps was applied for identifying shallow hyporheic flow at three locations representative for high versus low streambed connectivity shallow hyporheic flow patterns were found to be spatially heterogeneous subsurface flow could only partially be explained by streambed topography surface water infiltration and horizontal flow coincided with inhibited groundwater up welling whereas locations with high streambed connectivity were characterized by increased up welling the combined information of spatiotemporal vhg variability and flow vector frequency distribution by hps has the potential to improve the understanding of impacts of streambed topography and subsurface stratification on hyporheic flow patterns

hydraulic and thermal effects of in stream structure induced hyporheic exchange across a range of hydraulic conductivities in stream structure induced hyporheic exchange and associated thermal dynamics affect stream ecosystems their importance is controlled by spatial variability of sediment hydraulic conductivity k we calibrated a computational fluid dynamics cfd model of surface and groundwater hydraulics near a channel spanning weir represents log dams boulder weirs to field data and varied k from 10 7 to 10 2 m s silt to gravel surface water stopped cresting the weir for k 10 3 m s non darcy hyporheic flow was also prevalent for k 10 3 m s and velocity errors using non cfd models ranged up to 32 2 we also modeled weir induced heat transport during summer as k increased from 10 7 to 10 3 m s weir induced hyporheic heat advection steadily increased cooling and buffering along hyporheic flow paths decreased with increasing k particularly above k 10 5 and 10 4 m s respectively vertical heat conduction between surface water and groundwater near the weir decreased with increasing k particularly for k 10 5 m s conduction between hyporheic flow paths and adjacent groundwater helped cool hyporheic flow downstream surface water cooling by hyporheic advection increased steadily with k as increases in hyporheic flow overwhelmed decreases in cooling along hyporheic flow paths yet such effects were small 0 016 c even at k 10 3 m s the largest thermal effect of weir induced exchange was therefore spatial expansion of subsurface diel variability particularly for k 10 5 m s which affects benthic habitat and chemical reactions the specific values of k where such trend shifts occur is likely variable among streams based on flow conditions but we expect the presence of such trend shifts to be widespread

retracted and replaced a modelling study of hyporheic exchange pattern and the sequence size and spacing of stream bedforms in mountain stream networks oregon usa this article has been retracted and replaced see retraction and replacement notice doi 10 1002 hyp 6350 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 7 m to 7 6 m and downwelling lengths increased from 2 9 m to 6 0 m with increase in stream order from second to fourth order step spacing in the idealized reaches increased from 5 3 m to 13 7 m as stream size increased from second to fourth order simulated upwelling lengths increased from 4 3 m in second order streams to 9 7 m in fourth order streams with a pool riffle step channel unit sequence and increased from 2 5 m to 6 1 m from second to fourth order streams with a pool step riffle channel unit sequence downwelling 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 copyright 2005 john wiley sons ltd

in stream geomorphic structures as drivers of hyporheic exchange 1 common in stream geomorphic structures such as debris dams and steps can drive hyporheic exchange in streams exchange is important for ecological stream function and restoring function is a goal of many stream restoration projects yet the connection between in stream geomorphic form hydrogeologic setting and hyporheic exchange remains inadequately characterized we used the models hec ras modflow and modpath to simulate coupled surface and subsurface hydraulics in a gaining stream containing a single in stream geomorphic structure and to systematically evaluate the impact of fundamental characteristics of the structure and its hydrogeologic setting on induced exchange we also conducted a field study to support model results model results indicated that structure size background groundwater discharge rate and sediment hydraulic conductivity are the most important factors determining the magnitude of induced hyporheic exchange followed by geomorphic structure type depth to bedrock and channel slope model results indicated channel spanning structures were more effective at driving hyporheic flow than were partially spanning structures and weirs were more effective than were steps across most structure types downwelling flux rate increased linearly with structure size yet hyporheic residence time exhibited nonlinear behavior increasing quickly with size at low structure sizes and declining thereafter important trends in model results were observed at the field site and also interpreted using simple hydraulic theory thereby supporting the modeling approach and clarifying underlying processes

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

buffered lagged or cooled disentangling hyporheic influences on temperature cycles in stream channels 1 we monitored summertime base flow water temperatures of hyporheic discharge to surface water in main side and spring channels located within the bank full scour zone of the gravel and cobble bedded umatilla river oregon usa diel temperature cycles in hyporheic discharge were common but spatially variable relative to the main channel s diel cycle hyporheic discharge locations typically had similar daily mean temperatures but smaller diel ranges compressed by 2 to 6 c and desynchronized phases offset by 0 to 6 h in spring channels which received only hyporheic discharge surface water diel cycles were also compressed by 2 to 6 c and desynchronized by 4 to 6 h relative to the main channel creating diverse daytime and nighttime mosaics of surface water temperatures across main side and spring channels despite only minor differences 1 c in daily mean temperatures among the channels the river s hyporheic zone received and stored heat from the channel yet hyporheic return flows carried heat back to the channel minutes to months after removal associated surface water temperature dynamics were therefore complex hyporheic discharge was not simply cooler or warmer than main channel water instead instantaneous temperature differences between channel water and hyporheic discharge typically arose from diel temperature cycles in hyporheic discharge that were buffered and lagged relative to diel cycles in the main channel

spatio temporal variations of hyporheic flow in a riffle step pool sequence subsurface flow in streambeds can vary at different scales in time and space recognizing this variability is critical for understanding biogeochemical and ecological processes associated with the hyporheic zone the aim of this study was to examine the variability of hydraulic conductivity k vertical hydraulic gradients vhgs and subsurface fluxes over a riffle step pool sequence and at a high spatio temporal resolution a 20 m reach was equipped with a network of piezometers in order to determine the distribution of vhgs and k during a summer month temporal variations of vhgs were regularly surveyed and for a subset of piezometers the water level was automatically recorded at 15 min intervals by logging pressure transducers additionally point dilution tests were carried out on the same subset of piezometers whereas the distribution of vertical fluxes can be derived from k and vhg values point dilution tests allow for the estimation of horizontal fluxes where no vhg is detectable results indicate that spatially vhgs switched from upwelling to downwelling across lateral as well as longitudinal sections of the channel vertical fluxes appeared spatially more homogeneous than vhgs suggesting that the latter can be a poor indicator of the intensity of flow finally during flow events some vhgs showed little or no fluctuations this was interpreted as the result of a pressure wave propagating from upstream through highly diffusive alluvial sediments

nitrate concentration changes at the groundwater surface water interface of a small cumbrian river for an experimental field site at the river leith united kingdom the spatial and temporal distribution of nitrate was observed along the upwelling flow path from groundwater to surface water the study was carried out during baseflow conditions for two successive years for two contrasting stream reaches the physical and chemical characteristics of streambed sediment cores were analysed together with observations of hydraulic head dissolved oxygen redox and nitrogen speciation using an array of nested streambed piezometers pressure head gradients in the streambed piezometers showed that upwelling flows dominated the exchange between groundwater and surface water throughout the observation period infiltration of surface water into the streambed was not evident at depths below 10 cm pore water collected from sediment cores and streambed piezometers showed spatially variable redox conditions and nitrogen speciation within up to 100 cm depth in the streambed in particular nitrate concentrations along upwelling flow paths appeared to follow two opposite trends with both decreasing and increasing nitrate concentrations being observed at different points in the experimental reach the observed changes of nitrate concentrations in the upwelling groundwater are restricted to the loose superficial sediments that overlay the sandstone bedrock and do not appear to coincide with surface water groundwater mixing in the streambed the magnitude of variation in nitrate concentration along the upwelling flow path to the streambed appears to be governed by the sediment structure and characteristics in the two contrasting field sites the results suggest that changes in redox status and pore water nitrate concentrations in the hyporheic may occur at depths greater than surface water infiltration into the streambed and may call for new conceptual understanding of hyporheic nutrient transformations

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

changes in hyporheic exchange flow following experimental wood removal in a small low gradient stream 1 we investigated the response of hyporheic exchange flow hef to wood removal in a small low gradient gravel bed stream in southeast alaska using a series of groundwater models built to simulate hef for the initial conditions immediately after wood removal and 1 month 2 years 4 years and 16 years following wood removal the models were based on topographic surveys of the stream channel and surrounding floodplain and surveyed water surface elevations wses were used to assign stream boundary conditions using the groundwater flow model modflow and the particle tracking model modpath we calculated hyporheic exchange fluxes their residence time distributions and both longitudinal and plan view spatial patterns of downwelling and upwelling zones in the first few years streambed scour and sediment deposition smoothed the streambed and wse profile reducing hef also large contiguous patches of downwelling or upwelling were fragmented nearly doubling the total number of patches present on the streambed as the stream continued to adjust to the loss of wood those trends began to reverse accretion of sediment onto alternating bars resulted in better developed pool riffle morphology enhanced hef and increased residence times and also resulted in downwelling and upwelling zones coalescing into elongated patches along bar margins this study showed that the hyporheic zone is sensitive to changes in wood loading and that initial changes in hef resulting from the direct effects of wood removal were contrary to longer term channel adjustments to changes in wood loading

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

seasonal variation in surface subsurface water exchange and lateral hyporheic area of two stream aquifer systems abstract we used two dimensional unconfined transient groundwater flow models to investigate the interface between stream and groundwater flow systems or hyporheic zone of two first order streams that drain catchments with distinctly different alluvial sediments and bedrock lithology particle tracking showed that lateral hyporheic area planimetric area of flow paths lateral to the stream that are recharged by and return to the stream with travel times of 10 days or less differed between the two study streams and varied with discharge within each system at the rio calaveras welded tuff lateral hyporheic area ranged from 1 7 to 4 m 2 over the annual cycle in the aspen creek system sandstone lateral hyporheic area 1 1 5 m 2 was restricted to roughly half of that observed at rio calaveras the size of the hyporheic zone lateral to the streams at both sites decreased by approximately 50 during high flows sensitivity analyses indicated that changes in the hydraulic conductivity of alluvial and streambed sediments and variation in recharge rates have greatest impact on the magnitude direction and spatial distribution of stream groundwater exchange

patterns and dynamics of river aquifer exchange with variably saturated flow using a fully coupled model summary the parallel physically based surface subsurface model parflow was used to investigate the spatial patterns and temporal dynamics of river aquifer exchange in a heterogeneous alluvial river aquifer system with deep water table aquifer heterogeneity at two scales was incorporated into the model the architecture of the alluvial hydrofacies was represented based on conditioned geostatistical indicator simulations subscale variability of hydraulic conductivities k within hydrofacies bodies was created with a parallel gaussian simulation the effects of subscale heterogeneity were investigated in a monte carlo framework dynamics and patterns of river aquifer exchange were simulated for a 30 day flow event simulation results show the rapid formation of saturated connections between the river channel and the deep water table at preferential flow zones that are characterized by high conductivity hydrofacies where the river intersects low conductivity hydrofacies shallow perched saturated zones immediately below the river form but seepage to the deep water table remains unsaturated and seepage rates are low preferential flow zones although only taking up around 50 of the river channel account for more than 98 of total seepage groundwater recharge is most efficiently realized through these zones subscale variability of k sat slightly increased seepage volumes but did not change the general seepage patterns preferential flow zones versus perched zones overall it is concluded that typical alluvial heterogeneity hydrofacies architecture is an important control of river aquifer exchange in rivers overlying deep water tables simulated patterns and dynamics are in line with field observations and results from previous modeling studies using simpler models alluvial heterogeneity results in distinct patterns and dynamics of river aquifer exchange with implications for groundwater recharge and the management of riparian zones e g river channel floodplain connectivity via saturated zones

seasonal hyporheic temperature dynamics over riffle bedforms there is growing interest in riverbed temperature due to the ecological and biogeochemical significance of the hyporheic zone and its potential to moderate river temperature riffles exhibit complex thermal behaviour hypothesized to be caused by local alteration of groundwater surface water gw sw interactions but hitherto most research has been in upland gravel bed hard rock catchments accordingly this article aims 1 to characterize spatio temporal variability in hyporheic temperature over two riffles r1 and r2 in a lowland river basin tern shropshire uk underlain by sandstone and 2 to explain thermal dynamics by inferring hyporheic processes and the influence of gw sw interactions hyporheic riffle head crest and tail at 0 1 0 2 and 0 4 m water column spring water and air temperature were collected at 15 min resolution over 22 months and used to explore seasonal variations borehole water levels and temperature provide insight into groundwater variability over a hydrological year hyporheic temperature is cooler warmer than water column in summer winter with convergence in spring and autumn riffle heads and r2 crest yield small thermal gradients and r1 tail larger vertical difference r1 crest temperature is similar and attenuated cf water column at all depths r2 tail temperature differs markedly from surface water thus hyporheic temperature varies temporally across and between riffles reflecting 1 hydroclimatological controls on river and groundwater temperature and 2 hydrological local morphological and sedimentary controls on surface water and groundwater flux this research demonstrates the utility of depth related riverbed temperature time series in understanding hyporheic zone processes and gw sw interactions

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

effect of instream wood on vertical water flux in low energy sand bed flume experiments 1 in streams interaction of current with bed forms and solid objects can produce vertical water flux across the streambed instream wood is a major obstacle to flow and alters the bed topography of natural sand bed streams in the present experimental study in large circulating flumes simulating sand bed streams with low constant slope we compared systems with and without natural quantities of wood the introduction of wood resulted in the production of irregular bed forms as a result flow resistance tripled and vertical water flux across the streambed increased by a factor of 1 8 to 2 5 the mixing depth was spatially variable and seemed unrelated to the local bed form after the addition of wood surface water mixed deeper into the sediments and the total sediment pore water volume involved in mixing increased by factors of 1 2 to 1 5 the practice of increasing instream wood should be a valuable tool for hyporheic rehabilitation of degraded sand bed streams

distinct asymmetry in rupture induced inelastic strain across dipping faults an off fault yielding model 1 off fault stresses associated with rupture propagation cause material yielding under a mohr coulomb condition leaving irrecoverable deformation in the fault zone 2d dynamic rupture simulations for a 30 reverse fault and a 60 normal fault in a depth dependent stress environment show that the yielding inelastic zone off the fault broadens as it nears the surface with decreasing confining pressure forming a skewed flower like structure bounded at the top by the free surface the inelastic strain in the hanging wall is significantly larger and broader than the footwall for both reverse and normal faults the occurrence of inelastic strain however significantly reduces ground motion especially on the hanging wall and gives rise to a reduced asymmetry in ground motion on the hanging wall and footwall compared to elastic solutions these results provide theoretical predictions for fault zone structure of dipping faults that can be tested by future observational experiments

temporal variability of exchange between groundwater and surface water based on high frequency direct measurements of seepage at the sediment water interface 1 seepage at the sediment water interface in several lakes a large river and an estuary exhibits substantial temporal variability when measured with temporal resolution of 1 min or less already substantial seepage rates changed by 7 and 16 in response to relatively small rain events at two lakes in the northeastern usa but did not change in response to two larger rain events at a lake in minnesota however seepage at that same minnesota lake changed by 10 each day in response to withdrawals from evapotranspiration seepage increased by more than an order of magnitude when a seiche occurred in the great salt lake utah near the head of a fjord in puget sound washington seepage in the intertidal zone varied greatly from 115 to 217 cm d 1 in response to advancing and retreating tides when the time averaged seepage was upward at 43 cm d 1 at all locations seepage variability increased by one to several orders of magnitude in response to wind and associated waves net seepage remained unchanged by wind unless wind also induced a lake seiche these examples from sites distributed across a broad geographic region indicate that temporal variability in seepage in response to common hydrological events is much larger than previously realized at most locations seepage responded within minutes to changes in surface water stage and within minutes to hours to groundwater recharge associated with rainfall likely implications of this dynamism include effects on water residence time geochemical transformations and ecological conditions at and near the sediment water interface

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

hyporheic zone hydrologic science a historical account of its emergence and a prospectus the hyporheic zone defined by shallow subsurface pathways through river beds and banks beginning and ending at the river is an integral and unique component of fluvial systems it hosts myriad hydrologically controlled processes that are potentially coupled in complex ways understanding these processes and the connections between them is critical since these processes are not only important locally but integrate to impact increasingly larger scale biogeochemical functioning of the river corridor up to the river network scale thus the hyporheic zone continues to be a growing research focus for many hydrologists for more than half the history of water resources research this manuscript partly summarizes the historical development of hyporheic zone hydrologic science as gleaned from papers published in water resources research from the birth of the concept of the hyporheic zone as a hydrologic black box sometimes referred to as transient storage zone to its adolescent years of being torn between occasionally competing research perspectives of interrogating the hyporheic zone from a surface or subsurface view to its mature emergence as an interdisciplinary research field that employs the wide array of state of the art tools available to the modern hydrologist the field is vibrant and moving in the right direction of addressing critical fundamental and applied questions with no clear end in sight in its growth there are exciting opportunities for scientists that are able to tightly link the allied fields of geology geomorphology hydrology geochemistry and ecology to tackle the many open problems in hyporheic zone science

surface subsurface flow modeling with path based runoff routing boundary condition based coupling and assimilation of multisource observation data received 21 october 2008 revised 2 september 2009 accepted 16 september 2009 published 13 february 2010 1 a distributed physically based model incorporating novel approaches for the representation of surface subsurface processes and interactions is presented a path based description of surface flow across the drainage basin is used with several options for identifying flow directions for separating channel cells from hillslope cells and for representing stream channel hydraulic geometry lakes and other topographic depressions are identified and specially treated as part of the preprocessing procedures applied to the digital elevation data for the catchment threshold based boundary condition switching is used to partition potential atmospheric fluxes into actual fluxes across the land surface and changes in surface storage thus resolving the exchange fluxes or coupling between the surface and subsurface modules nested time stepping allows smaller steps to be taken for typically faster and explicitly solved surface runoff routing while a mesh coarsening option allows larger grid elements to be used for typically slower and more compute intensive subsurface flow sequential data assimilation schemes allow the model predictions to be updated with spatiotemporal observation data of surface and subsurface variables these approaches are discussed in detail and the physical and numerical behavior of the model is illustrated over catchment scales ranging from 0 0027 to 356 km 2 addressing different hydrological processes and highlighting the importance of describing coupled surfacesubsurface flow

simulating unsteady flow anabranching and hyporheic dynamics in a glacial meltwater stream using a coupled surface water routing and groundwater flow model 1 flooding affects ecosystems by transporting water and solutes across aquatic terrestrial interfaces removing nutrient and organic substrate limitations and spurring biogeochemical activity few studies have considered the influence of flooding on surface water groundwater interactions this research examines the temporally variable water storage and exchange in a stream in the mcmurdo dry valleys mdv of antarctica where diel flood pulses occur due to glacial melt several mdv streams display truncated discharge peaks suggesting water storage between the source glacier and the gauging station we tested the hypothesis that stream braids and subsurface water storage contribute to the difference between glacial melt and stream outflow hydrographs by constructing a coupled surface water routing and subsurface water flow model this model routes water into stream braids at high flows and allows this water to infiltrate and return to the stream via subsurface flow paths as flows recede our simulation demonstrates the importance of surface subsurface water interactions in controlling the hydrograph shape maximum simulated discharge was sensitive to storage parameters including aquifer depth and the flooding threshold while minimum discharge was sensitive to hydraulic conductivity subsurface storage volume varied by 38 over a diel cycle and stream subsurface exchange rates varied from 0 to 0 19 m3 h 1 m 1 with exchange from the stream to the subsurface during high flows and vice versa at low flows these results underscore how unsteady flow can increase hyporheic interactions and ecosystem productivity and provide support for maintaining natural stream morphology and flow regimes

subsurface flow in lowland river gravel bars geomorphic and hydraulic processes which form gravel bars in large lowland rivers have distinctive characteristics that control the magnitude and spatial patterns of infiltration and exfiltration between rivers and their immediate subsurface environments we present a bedform infiltration relation together with a set of field measurements along two reaches of the san joaquin river ca to illustrate the conditions required for infiltration and exfiltration of flow between a stream and its undulating bed and a numerical model to investigate the factors that affect paths and residence times of flow through barforms at different discharges it is shown that asymmetry of bar morphology is a first order control on the extent and location of infiltration which would otherwise produce equal areas of infiltration and exfiltration under the assumption of sinusoidal bedforms hydraulic conductivity varies by orders of magnitude due to fine sediment accumulation and downstream coarsening related to the process of bar evolution this systematic variability not only controls the magnitude of infiltration but also the residence time of flow through the bed the lowest hydraulic conductivity along the reach occurred where the difference between the topographic gradient and the water surface gradient is at a maximum and thus where infiltration would be greatest into a homogeneous bar indicating the importance of managing sand supply to maintain the ventilation and flow through salmon spawning riffles numerical simulations corroborate our interpretation that infiltration patterns and rates are controlled by distinctive features of bar morphology

a numerical investigation into the importance of bed permeability on determining flow structures over river dunes although permeable sediments dominate the majority of natural environments past work concerning bedform dynamics has considered the bed to be impermeable and has generally neglected flow between the hyporheic zone and boundary layer herein we present results detailing numerically modelled flow which allow the effects of bed permeability on bedform dynamics to be assessed simulation of an isolated impermeable bedform over a permeable bed shows that flow is forced into the bed upstream of the dune and returns to the boundary layer at the leeside in the form of returning jets that generate horseshoe shaped vortices the returning flow significantly influences the leeside flow modifying the separation zone lifting the shear layer adjoining the separation zone away from the bed simulation of a permeable dune on a permeable bed reveals even greater modifications as the flow through the dune negates the formation of any flow separation in the leeside with two dunes placed in series the flow over the downstream dune is influenced by the developing boundary layer on the leeside of the upstream dune for the permeable bed case the upwelling flow lifts the separated flow from the bed modifies the shear layer through the coalescence with vortices generated and causes the shear layer to undulate rather than be parallel to the bed these results demonstrate the significant effect that bed permeability has on the flow over bedforms that may be critical in affecting the flux of water and nutrients

comparing modflow simulation options for predicting intra meander flux during the evolution of meander bends the intra meander groundwater head gradients steepen and generate zones of accelerated water and nutrient intra meander fluxes important for ecosystem processes this paper compares and contrasts three modflow groundwater model packages based on their simulation of intra meander flux for two stages of meander evolution observed in a sandbox river table and one level of river bed clogging where the hydraulic conductivity in the river bed is lower than in the adjacent aquifer these packages are the time variant specified head package constant head chd river package riv and streamflow routing package sfr2 each controlling the groundwater or river head bounding the intra meander region the riv and sfr2 packages fix river stage and allow for variation in groundwater head below the river which is suggested for simulating intra meander flux for all sinuosities with and without river bed clogging whenever river bed parameters are available the chd package fixes below river groundwater head and fails to simulate intra meander head loss and flux in meanders with high sinuosity or river bed clogging in low sinuosity meanders and in cases without river bed clogging there were no significant differences between modflow packages for simulating river intra meander head loss and flux this research demonstrates why modflow users need to consider the limitations of each package when simulating intra meander flux in reaches with river bed clogging high sinuosity or similarly steep hydraulic gradients copyright 2014 john wiley sons ltd

rates of erosion and landscape change along the blue ridge escarpment southern appalachian mountains estimated from in situ cosmogenic 10be the blue ridge escarpment located within the southern appalachian mountains of virginia and north carolina forms a distinct steep boundary between the lower elevation piedmont and higher elevation blue ridge physiographic provinces to understand better the rate at which this landform and the adjacent landscape are changing we measured cosmogenic beryllium 10 10be in quartz separated from sediment samples n 50 collected in 32 streams and from three exposed bedrock outcrops along four transects normal to the escarpment allowing us to calculate erosion rates integrated over 104 105 years these basin averaged erosion rates 5 4 49 m myr 1 are consistent with those measured elsewhere in the southern appalachain mountains and show a positive relationship between erosion rate and average basin slope erosion rates show no relationship with basin size or relative position of the brevard fault zone a fundamental structural element of the region the cosmogenic isotopic data when considered along with the distribution of average basin slopes in each physiographic province suggest that the escarpment is eroding on average more rapidly than the blue ridge uplands which are eroding more rapidly than the piedmont lowlands this difference in erosion rates by geomorphic setting suggests that the elevation difference between the uplands and lowlands adjacent to the escarpment is being reduced but at extremely slow rates copyright 2016 john wiley sons ltd

hyporheic temperature dynamics and heat exchange near channel spanning logs 1 the flow of river water around large woody debris lwd creates pressure gradients along the riverbed that drive river groundwater mixing or hyporheic exchange and heat transport within the hyporheic zone we quantify hyporheic fluid and heat exchange induced by current interaction with channel spanning logs using two approaches laboratory flume experiments and numerical simulations that link turbulent open channel fluid flow porous fluid flow and heat transport flume and numerical experiments show that logs produce a characteristic diel temperature pattern within sediment that shifts with log blockage ratio a fraction of the channel depth blocked by the log channel froude number and sediment permeability upstream of a log downwelling water transports the river s diel thermal signal deep into the sediment downstream upwelling water from shallow flow paths has a thermal signal similar to the surface water while upwelling water from deep flow paths forms a wedge of buffered low amplitude temperatures since most hyporheic water emerges from shallow flow paths upwelling water has limited potential to buffer surface water temperature because of historic channel clearing practices modern rivers have unnaturally low densities of lwd a key implication is that lwd removal has contributed to thermal homogenization and potential degradation of hyporheic habitats lwd reintroduction is a promising strategy to improve vertical connectivity in rivers and increase thermal patchiness within the hyporheic zone however hyporheic exchange near lwd may not impact diel surface water temperatures at the reach scale

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

in situ quantification of spatial and temporal variability of hyporheic exchange in static and mobile gravel bed rivers seepage meters modified for use in flowing water were used to directly measure rates of exchange between surface and subsurface water in a gravel and cobble bed river in western pennsylvania usa allegheny river qmean 190 m3 s and a sand and gravel bed river in colorado usa south platte river qmean 9 7 m3 s study reaches at the allegheny river were located downstream from a dam the bed was stable with moss algae and river grass present in many locations median seepage was 0 28 m d and seepage was highly variable among measurement locations upward and downward seepage greatly exceeded the median seepage rate ranging from 2 26 upward to 3 76 downward m d at the south platte river site substantial local scale bed topography as well as mobile bedforms resulted in spatial and temporal variability in seepage greatly in exceedence of the median groundwater discharge rate of 0 24 m d both upward and downward seepage were recorded along every transect across the river with rates ranging from 2 37 to 3 40 m d despite a stable bed which commonly facilitates clogging by fine grained or organic sediments seepage rates at the allegheny river were not reduced relative to those at the south platte river seepage rate and direction depended primarily on measurement position relative to local and meso scale bed topography at both rivers hydraulic gradients were small at nearly all seepage measurement locations and commonly were not a good indicator of seepage rate or direction therefore measuring hydraulic gradient and hydraulic conductivity at in stream piezometers may be misleading if used to determine seepage flux across the sediment water interface such a method assumes that flow between the well screen and sediment water interface is vertical which appears to be a poor assumption in coarse grained hyporheic settings copyright 2011 john wiley sons ltd

new empirical relationship between grain size distribution and hydraulic conductivity for ephemeral streambed sediments grain size distribution porosity and hydraulic conductivity were determined for 39 sediment samples collected from ephemeral streams wadis in western saudi arabia the measured hydraulic conductivity values were then compared to values calculated using 20 different empirical equations commonly used to estimate hydraulic conductivity from grain size analyses it was found that most of the hydraulic conductivity values estimated from the empirical equations correlated very poorly with the measured hydraulic conductivity values modifications of the empirical equations including changes to special coefficients and statistical offsets were made to produce modified equations that considerably improved the hydraulic conductivity estimates from grain size data for wadi sediments the chapuis hazen kozeny slichter terzaghi and barr equations produced the best correlations but still had relatively high predictive errors the chapius equation was modified for wadi sediments by incorporating mud percentage and the standard deviation in phi units into a new equation that reduced the predicted hydraulic conductivity error to 14 1 m day the equation is best applied to ephemeral stream samples that have hydraulic conductive values greater than 2 m day

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