local scale variability of seepage and hydraulic conductivity in a shallow gravel bed river seepage rate and direction measured with a seepage metre modified for use in flowing water were greatly variable along a 300 m reach of a shallow gravel bed river and depended primarily on the local scale bed topography the median value of seepage measured at 24 locations was 24 cm day but seepage measured at specific sites ranged from 340 to 237 cm day seepage also varied substantially over periods of hours to days and occasionally reversed direction in response to evolution of the sediment bed vertical hydraulic conductivity was related to seepage direction and was larger during upward seepage than during downward seepage with differences ranging from 4 to 40 in areas of active sediment transport to more than an order of magnitude in areas where current was too slow to mobilize bed sediment seepage was poorly related to hydraulic gradient measured over vertical distances of 0 3 m and appeared to be opposite the hydraulic gradient at 18 of the locations where both parameters were measured results demonstrate the scale dependence of these measurements in coarse grained hyporheic settings and indicate that hydraulic gradients should be determined over a much shorter vertical increment if used to indicate exchange across the sediment water interface published in 2009 by john wiley sons ltd

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

the relative contribution of near bed vs intragravel horizontal transport to fine sediment accumulation processes in river gravel beds understanding flow sediment interactions is important for comprehending river functioning fine sediment accumulation processes in particular have key implications for ecosystem health however the amount of fines generated by intragravel flows and later accumulated in gravel streambeds may have been underestimated as the hydraulic related driving transport mechanisms in play are not clearly identified specifically the relative contribution of fines from upper vs lower sediment layers in gravel beds is not well understood by recreating flooded and dewatered conditions in an experimental flume filled with natural sediment we estimated such contributions by observing and collecting intragravel transported fines that were later accumulated into a void in the middle of the sediment matrix near bed transport in the upper sediment layers named brinkman load during flooded conditions accounted for most 90 of the accumulated fines intragravel transport in the lower sediment layers named interstitial load was the sole source of transport and accumulation during dewatered conditions with steeper hydraulic gradients interstitial load accounted for 10 of the total transport during flooded conditions although small such estimations demonstrate that hydraulic gradient transport in the lower sediment layers occurs in spite of the contradicting analytical assessments we provide a case study to challenge the traditional approaches of assessing intragravel transport and a useful framework to understand the origin and relative contribution of fine sediment accumulation in gravel beds such knowledge will be highly useful for the design of monitoring programs aiding river management particularly in regulated rivers

on the east australian current variability encroachment and upwelling 1 observations from an intensive oceanographic field program which took place in 1998 1999 about the separation point of the east australian current eac show significant spatial and temporal variability of the eac upstream of the separation point southward flowing currents are strong with subinertial velocities of up to 130 cm s 1 in the near surface waters whereas downstream currents are highly variable in both strength 1 70 cm s 1 and direction upwelling is observed to occur through both wind driven and current driven processes with wind effects playing a lesser role by contrast the encroachment of the eac upon the coast has a profound effect on the coastal waters accelerating the southward alongshore currents and decreasing the temperature in the bottom boundary layer bbl by up to 5 degreesc as the axis of the jet moves onshore negative vorticity increases in association with an increase in nonlinear acceleration during this time bottom friction is increased the burger number is reduced and the bbl shut down time lengthens the observed upwelling is attributed to enhanced onshore ekman pumping through the bbl resulting from increased bottom stress as the southerly flow accelerates when the eac encroaches across the continental shelf

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

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

transient infiltration from ephemeral streams a field experiment at the reach scale 1 an infiltration experiment at the stream reach scale was performed to estimate infiltration rates beneath an ephemeral losing stream during streamflow events at a time when the stream was dry a 7 m stream section was dammed upstream and downstream using metal sheets during a 5 day period water was pumped into the isolated section of the stream and the surface water level was maintained at three successive increasing stages the infiltration rate at each water level was thereby equal to the pumping rate required to maintain that water level the advantages of the method are that it samples a much greater area than traditional methods and provides information on infiltration through stream banks as well as through the streambed experimental results provide insight into transient infiltration and recharge processes beneath ephemeral streams although the experiment continued for 5 days infiltration during the first hour accounted for 19 of the total infiltration high transient infiltration rates were also observed following each increase in stream stage experimental infiltration rates were used to calibrate a two dimensional model developed within hydrus which was subsequently used to estimate infiltration associated with a natural flow event in the same stream reach during the natural flow event the total infiltration was 33 greater than would have been estimated assuming steady state infiltration rates dry antecedent moisture content controls the transient infiltration rate and hence increases the total infiltrated volume during flow events but it does not increase the aquifer recharge

dynamic streambed fluxes during rainfall runoff events observed 3 d temperature distributions within a streambed were used to analyze the effects on exchange fluxes between groundwater and the stream during rainfall runoff events by combining a dense vertical and lateral monitoring network of streambed temperatures with coupled surface subsurface 3 d flow and heat transport modeling we demonstrate how temperature can be used directly as a calibration target three model setups with different hydraulic conductivity distributions were evaluated in an optimization approach using temperature and hydraulic head data the hydraulic conductivity distributions were based on slug test surveys within the streambed and aquifer a detailed characterization of the hydraulic conductivity of the streambed and aquifer is needed to accurately simulate observed temperatures hence the most sensitive parameters the vertical hydraulic conductivity and the thermal conductivity were calibrated within different conductivity zones of the heterogeneous model simulated exchange fluxes across the streambed showed variations up to a factor of four within just a meter such differences may not have been correctly predicted using 1 d heat transport models due to lateral conduction amongst the different flow paths during the rainfall runoff event fluxes decreased substantially 50 due to a decrease in the hydraulic gradient with increased stream stage although no flow reversals were observed during the studied conditions it is possible that these can occur during larger rainfall runoff events we show that with the current sampling and modeling techniques 3 d temperature data can be used to estimate dynamic exchange in heterogeneous flow fields encountered in the field

numerical verification of hyporheic zone depth estimation using streambed temperature summary the hyporheic zone is the active region of groundwater and surface water mixing and consequently through which the contaminants released to surface water are transferred to groundwater because of the high energy demand and expense associated with the remediation of contaminated groundwater it has become a matter of primary concern to find an efficient and cost effective way to restore the contaminated groundwater therefore delineating the hyporheic zone is of vital importance for the prevention and management of groundwater contamination this paper proposes a method to estimate the depth of the hyporheic zone using heat transfer analysis for a streambed in order to assess the adequacy of the proposed method the sensitivity of the results of heat transfer analyses to the hyporheic flux was evaluated due to the high sensitivity to the hyporheic zone depth the heat transfer analysis was determined to be appropriate to delineate the hyporheic zone depth the depth estimated from the heat transfer analysis was comparable with that from a conventional tracer test the proposed method has an advantage over existing methods of determining the hyporheic zone depth due to the fact that it only requires field temperature measurements

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

evaluating fine sediment mobilization and storage in a gravel bed river using controlled reservoir releases two controlled flow events were generated by releasing water from a reservoir into the olewiger bach located near trier germany this controlled release of near bank full flows allowed an investigation of the fine sediment 63 m mobilized from channel storage both a winter november and a summer june release event were generated each having very different antecedent flow conditions the characteristics of the release hydrographs and the associated sediment transport indicated a reverse hysteresis with more mass but smaller grain sizes moving on the falling limb fine sediment stored to a depth of 10 cm in the gravels decreased following the release events indicating the dynamic nature and importance of channel stored sediments as source materials during high flow events sediment traps filled with clean natural gravel were buried in riffles before the release of the reservoir water and the total mass of fine sediment collected by the traps was measured following the events twice the mass of fine sediment was retained by the gravel traps compared with the natural gravels which may be due to their altered porosity although the amount of fine sediment collected by the traps was not significantly related to measures of gravel structure it was found to be significantly correlated to measures of local flow velocity and froude number a portion of the traps were fitted with lids to restrict surface exchange of water and sediment these collected the highest amounts of event mobilized sediments indicating that inter gravel lateral flows not just surface infiltration of sediments are important in replenishing and redistributing the channel stored fines these findings regarding the magnitude and direction of fine sediment movement in gravel beds are significant in both a geomorphic and a biological context copyright 2006 john wiley sons ltd

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

a systematic analysis of eight decades of incipient motion studies with special reference to gravel bedded rivers data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size t c 50 calculated t c 50 values were stratified by initial motion definition median grain size type surface subsurface or laboratory mixture relative roughness and flow regime a traditional shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition t c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel shields curves with the latter generally underlying the former values derived from competence functions define a separate but poorly developed field while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous rather than time averaged critical shear stresses the available data indicate that for high critical boundary reynolds numbers and low relative roughnesses typical of gravel bedded rivers reference based and visually based studies have t c50 ranges of 0 052 0 086 and 0 030 0 073 respectively the apparent lack of a universal t 50 for gravel bedded rivers warrants great care in choosing defendable t c50 values for particular applications

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

slope control in western boundary currents an analytic solution is presented for the steady state depth averaged western boundary current flowing over the continental slope by combining three highly idealized models the stommel model the munk model and the arrested topographic wave model the main vorticity balance over the slope is between planetary vorticity advection and the slope induced bottom stress torque which is proportional to ry h21 x where r is the rayleigh friction coefficient h is the water depth and y is the meridional velocity this slope induced torque provides the necessary source of vorticity for poleward flow over the slope its simple interpretation being that vorticity is produced because the bottom stress has to act over the seaward deepening water column the character of the solution depends on the slope a as well as on the assumed bottom drag coefficient and the length scale of the boundary current is it is further shown that if the depth averaged velocity flows along isobaths 2r ba then the stretching of water columns associated with cross isobath geostrophic flow which compensates bottom ekman transport is identical to the slope induced torque by the geostrophic velocities

hyporheic exchange with heterogeneous streambeds laboratory experiments and modeling 1 hyporheic exchange is generally analyzed with the assumption of a homogeneous hyporheic zone in reality streambed sediments have a heterogeneous structure and this natural heterogeneity produces spatially variable interfacial fluxes and complex hyporheic exchange patterns to assess the basic effects of sediment structure on hyporheic exchange we performed salt and dye injection experiments in a recirculating laboratory flume with two heterogeneous sediment beds characterized by negative exponential correlated random hydraulic conductivity fields dye injections showed that the hyporheic flow structure was controlled by the spatial relationship of bed forms to high and low permeability regions of the streambed as no existing model could represent these effects we developed a new finite element model to calculate the pore water flow field resulting from the interaction of the bed form induced boundary head distribution and the heterogeneous sediment structure a numerical particle tracking approach was then used to assess the resulting hyporheic exchange the combined flow transport model did an excellent job of predicting the complex hyporheic flow pathways in the heterogeneous bed and the net hyporheic exchange up to t 30 hours the heterogeneous hydraulic conductivity field caused both greater spatial variability in the water flux through the bed surface and a greater average interfacial flux than would have occurred with a homogeneous bed the layered correlation structure of the streambed produced an effective anisotropy that favored longitudinal pore water flow and caused a relatively rapid decrease of the mean pore water velocity with depth as a result solute penetration into the bed was confined to a more shallow region than would have occurred with a homogeneous bed the combination of faster near surface transport and shallower solute penetration produced a shorter mean hyporheic residence time on the basis of the combination of experimental results and model simulations we conclude that the structural heterogeneity of streambed sediments produces more spatially limited hyporheic exchange that occurs with greater spatial variability and at a higher overall rate

geologic effects on groundwater salinity and discharge into an estuary summary submarine groundwater discharge sgd can be an important pathway for transport of nutrients and contaminants to estuaries a better understanding of the geologic and hydrologic controls on these fluxes is critical for their estimation and management we examined geologic features porewater salinity and sgd rates and patterns at an estuarine study site seismic data showed the existence of paleovalleys infilled with estuarine mud and peat that extend hundreds of meters offshore a low salinity groundwater plume beneath this low permeability fill was mapped with continuous resistivity profiling extensive direct sgd measurements with seepage meters n 551 showed fresh groundwater discharge patterns that correlated well with shallow porewater salinity and the hydrogeophysical framework small scale variability in fresh and saline discharge indicates influence of meter scale geologic heterogeneity while site scale discharge patterns are evidence of the influence of the paleovalley feature beneath the paleovalley fill fresh groundwater flows offshore and mixes with saltwater before discharging along paleovalley flanks on the adjacent drowned interfluve where low permeability fill is absent fresh groundwater discharge is focused at the shoreline shallow saltwater exchange was greatest across sandy sediments and where fresh sgd was low the geologic control of groundwater flowpaths and discharge salinity demonstrated in this work are likely to affect geochemical reactions and the chemical loads delivered by sgd to coastal surface waters because similar processes are likely to exist in other estuaries where drowned paleovalleys commonly cross modern shorelines the existence and implications of complex hydrogeology are important considerations for studies of groundwater fluxes and related management decisions

lithostratigraphic evolution of the bandamian volcanic cycle in central cote d ivoire insights into the late eburnean magmatic resurgence and its geodynamic implications late to post collisional volcanism in the southern portion of the west african craton swac has been described along the paleo craton margin between the paleoproterozoic baoule mossi domain and archean kenema man nucleus new lithostratigraphic and isotopic data for central cote d ivoire as well as revised regional stratigraphic correlations across the western swac indicate that this volcanic resurgence is not only restricted to the paleo craton margin but extends to most greenstone belts of the western half of the swac this late volcanic event historically referred to as the ca 2117 2080 ma bandamian volcanic cycle has received little attention since its identification in 1996 detailed geological investigation of the bouafle granite greenstone belt in cote d ivoire located 150 km east of paleo craton margin along with newly acquired u pb geochronological data indicate a peak in igneous activity showing evidence for bimodal magmatism affiliated to the bandamian volcanic cycle in central cote d ivoire between 2108 5 ma and 2105 4 ma whole rock geochemistry and lu hf isotopic measurements on magmatic zircons from the bouafle region show that these bandamian rocks are marked by a juvenile signature with little degree of crustal contamination collectively the data collected in this study show that the bandamian volcanic cycle represents an important thermal event that affected the swac during the late stages of the eburnean orogeny spanning the incremental change from folding and thrusting to dominantly transcurrent tectonics along major faults and shear zones this paper discusses the possible geodynamic settings that may account for such a peak in thermal activity during the late stages of the eburnean orogeny

circulation on the northwestern iberian margin vertical structure and seasonality of the alongshore flows abstract we describe the seasonal cycle of the alongshore flows on the northwestern iberian margin and explain what are the important mechanisms forcing the system we used a 20 year model simulation at 2 3 km horizontal resolution with realistic atmospheric forcing and covering the whole western iberian margin the model results are compared with satellite data with data measured at two moored buoys and with a compilation of current meter data available for the study region we show that currents over the slope are divided in three different cores the iberian poleward current ipc occupying the top 350 m a deeper core at mediterranean water levels 600 1200 m and in between the two an equatorward core centered just beneath the ipc core the upper slope countercurrent uscc the ipc is present almost yearlong including in summer months when it is close to the shelf break and topped by the equatorward upwelling jet after september the ipc intensifies and its core surfaces heat and salinity budgets on the shelf and slope are dominated by advection confirming the important role of the ipc in driving the temperature and salinity seasonal cycles analysis of the seasonal cycle of the barotropic vorticity equation on the slope shows that the main forcing mechanism of northward flows is the joint effect of baroclinicity and relief jebar whose contribution is higher in summer than in winter in december and january when the ipc is stronger and surface intensified the main contribution is from southerly winds the cross shore analysis of these terms shows that jebar decreases substantially at the core of the ipc because as advection terms become important the northward density flux diminishes the local meridional density gradients resulting in the decrease of jebar in the months of strong ipc

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