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

spatial patterns of hyporheic exchange and biogeochemical cycling around cross vane restoration structures implications for stream restoration design 1 natural channel design restoration projects in streams often include the construction of cross vanes which are stone dam like structures that span the active channel vertical hyporheic exchange flux hef and redox sensitive solutes were measured in the streambed around four cross vanes with different morphologies observed patterns of hef and redox conditions are not dominated by a single downstream directed hyporheic flow cell beneath cross vanes instead spatial patterns of moderate 0 4 m d 1 upwelling and downwelling are distributed in smaller cells around pool and riffle bed forms upstream and downstream of structures patterns of biogeochemical cycling are controlled by dissolved oxygen concentrations and resulting redox conditions and are also oriented around secondary bed forms strong downwelling into the hyporheic zone 0 5 3 5 m d 1 was observed immediately upstream of structures but was limited to an area 1 2 m from the cross vane these hyporheic flow paths likely rejoin the stream at the base of cross vanes after residence times too short to alter nitrate concentrations or accumulate reaction products total hyporheic exchange volumes are 0 4 of stream discharge in restored reaches of 45 55 m results show that shallow hyporheic flow and associated biogeochemical cycling near cross vanes is primarily controlled by secondary bed forms created or augmented by the cross vane rather than by the cross vane itself this study suggests that cross vane restoration structures benefit the stream ecosystem by creating heterogeneous patches of varying hef and redox conditions in the hyporheic zone rather than by processing large amounts of nutrients to alter in stream water chemistry

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

channel bed steps and pool shapes along soda creek three sisters wilderness oregon abstract field study of bedrock step pool systems along the upper reaches of soda creek in the three sisters wilderness of oregon shows strong correlation between several form variables shape and channel slope although step height and step length showed no regular spacing and variable correlation with channel slope length to height ratios demonstrated strong negative correlations steep slopes 20 to 80 featured greater step height and shorter pool lengths than did flatter channel slopes correlations between step height to length ratios and channel slope varied between three lithologies explained variations ranged from 0 984 for the oldest channel steps developed in basalt to 0 982 for steps of intermediate age developed in andesite to 0 964 for the youngest steps developed in dacite sample size was 57 40 and 33 respectively the frequency of pool shape classes did not vary by lithology but specific shape classes developed under differing slope conditions by rock type all pool classes have adjusted developed their form to maximize resistance to flow h l s and they have done so in remarkably uniform fashion

three dimensional model of modern channel bend deposits 1 we present a three dimensional model of heterogeneous modern channel bend deposits developed through purely structure imitating interpolation kriging of hydraulic properties this model augmented with ground penetrating radar data and directional variograms agrees with detailed observations in similar modern environments and leads to a process based interpretation of the presented hydraulic conductivity structure integration of all available information permitted delineation and characterization of the modern streambed as a distinct hydrostratigraphic unit without coring or outcrop studies our results imply that the modern streambed is commonly oversimplified in available analytical and numerical models of groundwater surface water interactions where it is assumed to be homogeneous and isotropic and characterized by a constant width and thickness this three dimensional approach that integrates concepts and principles developed in sedimentology hydrogeology geophysics and geostatistics has potential implications on model development of stream aquifer systems

a heat pulse technique for the determination of small scale flow directions and flow velocities in the streambed of sand bed streams the hyporheic zone hz has the capability to eliminate and attenuate nutrients and contaminants in riverine systems biogeochemical reactions and the potential elimination of contaminants are strongly controlled by the flow paths and dynamics in the hz nevertheless an easily applicable method for the field determination of flow patterns in the hz is still lacking therefore a heat pulse technique which traces the movement of a short heat pulse in the upper part of the hz and other sand beds was developed five rods are vertically driven into the sediment of the streambed one rod with a heater as point source located in about 10 cm sediment depth and four rods with four temperature sensors in 3 cm distance arranged concentrically with 7 cm diameter around the heating rod subsequently a heat pulse is applied and the resulting breakthrough curves are indicative of flow velocities and flow directions in the streambed a rough data analysis procedure is also suggested in addition laboratory experiments were performed to test the heat pulse technique these experiments were validated based on coupled numerical modelling of flow and heat transport first field tests of the method prove that the method is easily applicable under field conditions these first field tests showed highly complex flow patterns with flow velocities from 1 8 to 4 9 cm min 1 and flow directions from parallel to surface flow to opposite to surface flow this suggests the need for a robust method to quantify hyporheic flow patterns in situ copyright 2011 john wiley sons ltd

the periodic nature of step pool mountain streams rhythmic sequences of steps and pools in steep mountain streams have captured the attention of numerous workers but whether periodicities exist in the occurrence of step pools has not been investigated in a comprehensive way and the implications for such periodicities have not been explored spectral analysis was applied to bed elevation data from streams in the santa monica mountains of southern california to investigate periodicity in the occurrence of step pools results indicate significant step pool periodicities confirmed by independent field measurements that vary from 0 43 channel widths to 2 40 channel widths spectral analysis of channels affected by external influences tends to indicate multiple significant peaks that reflect higher variance in step pool spacing underlying periodicities are nevertheless de tected suggesting that external factors obscure but do not destroy the effects of more general internal mechanisms periodicity in the occurrence of step pools suggests that step pools are bedforms that represent a fundamental mutual adjustment between flow channel morphology and energy expenditure a rhythmic channel thalweg supports the notion that step pools are analogous to meandering in the vertical dimension and implies that because step pool channels are not able to adjust energy expenditure in the plane dimension they instead adjust boundary roughness in the vertical dimension a continuum of rhythmic gravel bedforms that displays the results of such adjustments and that shows how step pools merge into pools and riffles in the stream channel system is discussed and illustrated with empirical data from the santa monica mountains although many differences exist between steep mountain streams and broad alluvial rivers the findings of this study suggest that generalized principles of uniform energy expendi ture may also apply to rhythmic step pool streams these findings are potentially valuable in the management and design of steep channels

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

new estimates for io eruption temperatures implications for the interior abstract the initial interpretation of galileo data from jupiter s moon io suggested eruption temperatures 1600 c tidal heating models have difficulties explaining io s prodigious heat flow if the mantle is 1300 c although we suggest that temperatures up to 1450 c may be possible in general io eruption temperatures have been overestimated because the incorrect thermal model has been applied much of the thermal emission from high temperature hot spots comes from lava fountains but lava flow models were utilized we apply a new lava fountain model to the highest reported eruption temperature the ssi observation of the 1997 eruption at pillan this resets the lower temperature limit for the eruption from 1600 to 1340 c additionally viscous heating of the magma may have increased eruption temperature by 50 100 c as a result of the strong compressive stresses in the ionian lithosphere while further work is needed it appears that the discrepancy between observations and interior models is largely resolved

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

observing lingering hyporheic storage using electrical resistivity variations around stream restoration structures crabby creek pa time lapse geophysical surveys can map lingering hyporheic storage by detecting changes in response to saline tracer tracer tests were conducted in crabby creek an urban stream outside philadelphia to examine the influence of stream restoration structures and variable sediment thickness we compared electrical resistivity surveys with extensive well sampling 57 wells in two 13 5 m long reaches each with a step drop created by a j hook the two step drops varied in tracer behaviour based on both the well data and the geophysical data the well data showed more variation in arrival time where the streambed sediment was thick and was more uniform where sediment was thin the resistivity in the reach with thin sediment showed lingering tracer in the hyporheic zone both upstream and downstream from the j hook in the second reach where the sediment was thicker the lingering tracer in the hyporheic zone was more extensive downstream from the j hook the contrasting results between the two reaches from both methods suggested that sediments influenced hyporheic exchange more than the step at this location resistivity inversion differed from well data in both reaches in that it showed evidence for tracer after well samples had returned to background mapping lingering tracer either upstream or downstream of a step we conclude that resistivity surveys may become an important tool for hyporheic zone characterization because they provide information on the extent of slow moving fluids in the hyporheic zone which have the potential to enhance chemical reactions copyright 2012 john wiley sons ltd

macropores as preferential flow paths in meander bends macropores are subsurface connected void spaces caused by processes such as fracture of soils micro erosion and fauna burrows they are common near streams e g hyporheic and riparian zones and may act as preferential flow paths between surface and groundwaters affecting hydrologic and biogeochemical processes we tested the hydrologic function of macropores by constructing an artificial macropore within the saturated zone of a meander bend open macropore om and later filling its upstream end partially filled macropore pfm for each treatment we injected saline tracer at an upgradient monitoring well within the meander and monitored downgradient hydraulics and tracer transport pressure transducers in monitoring wells indicated hydraulic gradients within the meander were 32 higher perpendicular to and 6 higher parallel to the macropore for the om than for the pfm additionally hydraulic conductivities measured via falling head tests were 29 to 550 times higher along the macropore than in nearby sediment we used electrical conductivity probes in wells and electrical resistivity imaging to track solute transport transport velocities through the meander were on average 9 and 21 higher per temporal moment analysis and observed tracer peak respectively for the om than for the pfm furthermore temporal moments of tracer breakthrough analysis indicated downgradient longitudinal dispersion and breakthrough tracer curve tailing were on average 234 and 182 higher for the om respectively this suggests the om enabled solute transport at overall shorter timescales than the matrix but also increased tailing our results demonstrate the importance of macropores to meander bend hydrology and solute transport copyright 2012 john wiley sons ltd

electrical characterization of non fickian transport in groundwater and hyporheic systems 1 recent work indicates that processes controlling solute mass transfer between mobile and less mobile domains in porous media may be quantified by combining electrical geophysical methods and electrically conductive tracers whereas direct geochemical measurements of solute preferentially sample the mobile domain electrical geophysical methods are sensitive to changes in bulk electrical conductivity bulk ec and therefore sample ec in both the mobile and immobile domains consequently the conductivity difference between direct geochemical samples and remotely sensed electrical geophysical measurements may provide an indication of mass transfer rates and mobile and immobile porosities in situ here we present 1 an overview of a theoretical framework for determining parameters controlling mass transfer with electrical resistivity in situ 2 a review of a case study estimating mass transfer processes in a pilot scale aquifer storage recovery test and 3 an example application of this method for estimating mass transfer in watershed settings between streams and the hyporheic corridor we demonstrate that numerical simulations of electrical resistivity studies of the stream hyporheic boundary can help constrain volumes and rates of mobile immobile mass transfer we conclude with directions for future research applying electrical geophysics to understand field scale transport in aquifer and fluvial systems subject to rate limited mass transfer

orogenic tectonism on io 1 we catalog 143 ionian mountains montes and mountain like features mensae tholi plana and small peaks in order to investigate orogenic tectonism on io from this comprehensive list we select 96 mountains for which there are sufficient coverage and resolution to discern spatial relationships with surrounding geologic features three of the 96 mountains are probably volcanoes 92 appear to be tectonic massifs and 1 is ambiguous of the 92 tectonic mountains 38 abut paterae volcanic or volcano tectonic craters with irregular or scalloped margins this juxtaposition is unlikely to be a coincidence as the probability of it occurring by chance is 0 1 we propose instead that orogenic faults may act as conduits for magma ascent thus fueling patera formation near mountains as resurfacing buries a shell of material from io s surface to the base of the lithosphere its effective radius is reduced and it heats up we calculate the lithospheric volume change due to subsidence and thermal expansion as a function of lithospheric thickness conservation of volume dictates that this material must be uplifted at io s surface by estimating the total volume of the mountains we are able to place a lower limit of 12 km on io s lithospheric thickness we hypothesize that in some cases mountain formation may be facilitated by asthenospheric diapirs impinging on the base of the lithosphere the resulting lithospheric swell could focus the compressive stresses that drive orogenic tectonism this model is one of several possible mechanisms for uplifting isolated mountains such as are observed on io

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 3d analysis algorithm to improve interpretation of heat pulse sensor results for the determination of small scale flow directions and velocities in the hyporheic zone abstract the hyporheic zone is strongly influenced by the adjacent surface water and groundwater systems it is subject to hydraulic head and pressure fluctuations at different space and time scales causing dynamic and heterogeneous flow patterns these patterns are crucial for many biogeochemical processes in the shallow sediment and need to be considered in investigations of this hydraulically dynamic and biogeochemical active interface for this purpose a device employing heat as an artificial tracer and a data analysis routine were developed the method aims at measuring hyporheic flow direction and velocity in three dimensions at a scale of a few centimeters a short heat pulse is injected into the sediment by a point source and its propagation is detected by up to 24 temperature sensors arranged cylindrically around the heater the resulting breakthrough curves are analyzed using an analytical solution of the heat transport equation the device was tested in two laboratory flow through tanks with defined flow velocities and directions using different flow situations and sensor arrays the sensitivity of the method was evaluated after operational reliability was demonstrated in the laboratory its applicability in the field was tested in the hyporheic zone of a low gradient stream with sandy streambed in ne germany median and maximum flow velocity in the hyporheic zone at the site were determined as 0 9 10 4 and 2 1 10 4 m s 1 respectively horizontal flow components were found to be spatially very heterogeneous while vertical flow component appear to be predominantly driven by the streambed morphology

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 flow path response to hydraulic jumps at river steps hydrostatic model simulations 1 this research examined hydrostatic groundwater model modflow predictive adequacy and sensitivity in simulating hyporheic flow paths across a river step with a hydraulic jump in a companion paper we used flume and hydrodynamic model analysis to develop a refined conceptual model depicting these flow paths with zones of downwelling and upstream directed flux below the step the previous coarse conceptual model predicted uniform downstream directed upwelling below the step the hydrostatic model accurately predicted the downwelling and upstream directed fluxes beneath the wave and jump but failed to predict the plunge pool downwelling which is driven by dynamic pressures sensitivity tests varied riverbed topography and water surface profile geometry for a river with 1 slopes 10 cm flow depths and 50 150 cm long jets and jumps the flow paths below the jet jump region were driven by hydrostatic pressures and were highly sensitive to water surface profile and riverbed topography parameters failure to simulate the hydraulic jump caused errors in hyporheic flow path predictions beneath the jump region 1 m long by 0 5 m deep if the jump was poorly parameterized several meters of riverbed flow paths could be erroneously modeled as pointing upstream the hyporheic zone may contain a spatial mosaic of aerobic and anaerobic waters regulating nutrient transformations and biologic productivity accurate parameterization of hydraulic jumps in hyporheic simulation has the potential to improve predictions and explain heterogeneous subsurface flow paths and associated nutrient patterns and ecosystem functions

io evidence for silicate volcanism in 1986 infrared observations of io during the 1986 apparition of jupiter indicate that a large eruptive event occurred on the leading side of io on 7 august 1986 universal time measurements made at 4 8 8 7 and 20 micrometers suggest that the source of the event was about 15 kilometers in radius with a model temperature of 900 kelvin together with previously reported events these measurements indicate that high temperature volcanic activity on the leading side of io may be more frequent than previously thought the inferred temperature is significantly above the boiling point of sulfur in a vacuum 715 kelvin and thus constitutes strong evidence for active silicate volcanism on the surface of io

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

volcanic eruptions on io heat flow resurfacing and lava composition abstract we model an infrared outburst on io as being due to a large erupting lava flow which increased its area at a rate of 1 5 10 5 m 2 sec 1 and cooled from 1225 to 555 k over the 2 583 hr period of observation the inferred effusion rate of 3 10 5 m 3 sec 1 for this eruption is very high but is not unprecedented on the earth and is similar to the high eruption rates suggested for early lunar volcanism eruptions occur 6 of the time on io these eruptions provide ample resurfacing to explain io s lack of impact craters we suggest that the large total radiometric heat flow 10 14 w and the size and temperature distribution of the thermal anomalies mcewen et al 1992 bull am astron soc 24 935 veeder et al 1994 j geophys res 99 17 095 17 162 can be accounted for by a series of silicate lava flows in various stages of cooling we propose that the whole suite of io s currently observed thermal anomalies was produced by multiple high eruptive rate silicate flows within the past century

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

weathering reactions and hyporheic exchange controls on stream water chemistry in a glacial meltwater stream in the mcmurdo dry valleys 1 in the mcmurdo dry valleys antarctica dilute glacial meltwater flows down wellestablished streambeds to closed basin lakes during the austral summer during the 6 12 week flow season a hyporheic zone develops in the saturated sediment adjacent to the streams longer dry valley streams have higher concentrations of major ions than shorter streams the longitudinal increases in si and k suggest that primary weathering contributes to the downstream solute increase the hypothesis that weathering reactions in the hyporheiczonecontrolstreamchemistrywastestedbymodelingthedownstreamincreasein solute concentration in von guerard stream in taylor valley the average rates of solute supplied from these sources over the 5 2 km length of the stream were 6 1 10 9 mol si l 1 m 1 and 3 7 10 9 mol k l 1 m 1 yielding annual dissolved si loads of 0 02 1 30 mol si m 2 of watershed land surface silicate minerals in streambed sediment were analyzed to determine the representative surface area of minerals in the hyporheic zone subject to primary weathering two strategies were evaluated to compute sediment surface area normalized weathering rates the first applies a best linear fit to synoptic data in order to calculate a constant downstream solute concentration gradient dc dx constant weathering rate contribution crc method the second uses a transient storage model to simulate dc dx representing both hyporheic exchange and chemical weathering hydrologic exchange he method geometric surface area normalized dissolution rates of the silicate minerals in the stream ranged from 0 6 10 12 mol si m 2 s 1 to 4 5 10 12 mol si m 2 s 1 and 0 4 10 12 mol k m 2 s 1 to 1 9 10 12 mol k m 2 s 1 these values are an order of magnitude lower than geometric surface area normalized weathering rates determined in laboratory studies and are an order of magnitude greater than geometric surface area normalized weathering rates determined in a warmer wetter setting in temperate basins despite the cold temperatures lack of precipitation and lack of organic material these results suggest that the continuous saturation and rapid flushing of the sediment due to hyporheic exchange facilitates weathering in dry valley streams index terms 1045 geochemistry low temperature geochemistry 1806 hydrology chemistry of fresh water 1625 global change geomorphology and weathering 1824 1886 1890 hydrology wetlands keywords hyporheic zone chemical weathering flow path antarctica stream

relative influences of the river channel floodplain surface and alluvial aquifer on simulated hydrologic residence time in a montane river floodplain abstract current methods for characterizing the influence of geomorphic structure on river processes are not well suited for study of large rivers with extensive hydrologic exchanges between the channel floodplain surface and alluvial aquifer here we applied a spatially explicit three dimensional hydrologic model to simulate surface and subsurface flow within the 16 km 2 nyack floodplain on the middle fork flathead river in northwest montana usa we ran the model for four years and simulated nine conservative particle releases across a range of river discharges our objective was to evaluate the interactions of hydrologic dynamics and floodplain structure in determining the residence time of water in the system particle tracking simulations revealed when river discharge was below bankfull mean residence time of surface water decreased with river discharge during overbank flows however mean residence time increased with discharge because of surface water storage on the floodplain when surface and subsurface water were considered mean hydrologic residence time was more than an order of magnitude greater than the residence time of surface water alone because of storage in the alluvial aquifer residence time also decreased exponentially with river discharge because a smaller percentage of surface water entered the subsurface at high discharges our analyses illustrate the importance of considering in channel flow floodwater storage and surface subsurface exchange in determining the overall residence time of water within a river segment especially in floodplain environments

channel reach morphology in mountain drainage basins a classification of channel reach morphology in mountain drainage basins synthesizes stream morphologies into seven distinct reach types colluvial bedrock and five alluvial channel types cascade step pool plane bed pool rime and dune ripple coupling reach level channel processes with the spatial arrangement of reach morphologies their links to hillslope processes and external forcing by confinement ripar ian vegetation and woody debris defines a process based framework within which to assess channel condition and response potential in mountain drainage basins field investigations demonstrate character istic slope grain size shear stress and roughness ranges for different reach types observations consistent with our hypothesis that alluvial channel morphologies reflect specific roughness configurations ad justed to the relative magnitudes of sediment supply and transport ca pacity steep alluvial channels cascade and step pool have high ratios of transport capacity to sediment supply and are resilient to changes in discharge and sediment supply whereas low gradient alluvial channels pool rime and dune ripple have lower transport capacity to supply ra tios and thus exhibit significant and prolonged response to changes in sediment supply and discharge general differences in the ratio of transport capacity to supply between channel types allow aggregation of reaches into source transport and response segments the spatial distribution of which provides a watershed level conceptual model linking reach morphology and channel processes these two scales of channel network classification define a framework within which to in vestigate spatial and temporal patterns of channel response in moun tain drainage basins

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

mountains on io high resolution galileo observations initial interpretations and formation models during three close flybys in late 1999 and early 2000 the galileo spacecraft acquired new observations of the mountains that tower above io s surface these images have revealed surprising variety in the mountains morphologies they range from jagged peaks several kilometers high to lower rounded structures some are very smooth others are covered by numerous parallel ridges many mountains have margins that are collapsing outward in large landslides or series of slump blocks but a few have steep scalloped scarps from these observations we can gain insight into the structure and material properties of io s crust as well as into the erosional processes acting on io we have also investigated formation mechanisms proposed for these structures using finite element analysis mountain formation might be initiated by global compression due to the high rate of global subsidence associated with io s high resurfacing rate however our models demonstrate that this hypothesis lacks a mechanism for isolating the mountains the large fraction 40 of mountains that are associated with paterae suggests that in some cases these features are tectonically related therefore we have also simulated the stresses induced in io s crust by a combination of a thermal upwelling in the mantle with global lithospheric compression and have shown that this can focus compressional stresses if this mechanism is responsible for some of io s mountains it could also explain the common association of mountains with paterae

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

mountains and calderas on io possible implications for lithosphere structure and magma generation abstract the combination of voyager images and newly acquired galileo images with low illumination and resolutions ranging from 2 to 6 km pixel now allows determination of the global distribution of mountains and volcanic centers on io the mountains generally do not have characteristics typical of terrestrial volcanic landforms they are evenly distributed across the surface and show no obvious correlation with known hot spots or plumes relative elevations determined by shadow measurements and stereoscopy indicate that mountains in the newly imaged area range in elevation up to at least 7 6 km the origin of the mountains remains uncertain some appear to be multitiered volcanic constructs others enclosing the partial remains of large circular depressions appear to be remnants of old volcanoes yet others show extensive tectonic disruption volcanic centers also appear to be distributed evenly across the surface except for an apparently somewhat lower density at high latitudes the low latitudes have one volcanic center per 7 10 4 km 2 and on average the centers are spaced roughly 250 km apart the global distribution of high mountains suggests that the lithosphere over most of io is thick although the thickness cannot be calculated the previously suggested 30 km appears reasonable as a lower limit the high rates of resurfacing combined with the likely dissipation of most of the tidal energy in the asthenosphere and underlying mantle implies a very low temperature gradient in the upper part of the lithosphere and steep gradients in the lower lithosphere the slow rate of separation of melt from host rock in the magma source regions as a consequence of the low gravity on io coupled with the high rate of magma production will likely result in larger melt fractions than is typical for source regions on earth the variety of volcanic landforms suggests that volcanic products with a range of compositions are deposited on the surface this mixture will be carried downward through the lithosphere as a consequence of the 0 5 1 5 cm yr resurfacing rates during descent the more volatile components will tend to be driven off early but complete or near complete melting at the base of the lithosphere may result in rehomogenization of the silicate mixture that remains

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

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

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

hyporheic exchange flow around constructed in channel structures and implications for restoration design in channel rock vane structures are widely used in stream restoration as a way to reduce stream channel erosion and create pool or riffle features when these structures change hydraulic gradients they may affect ecological stream functions such as hyporheic exchange flow hef patterns a study of constructed in channel structure controls on hef was conducted in the third order batavia kill new york using stream and hyporheic temperature amplitude analysis and computational fluid dynamics cfd hydraulic simulations temperature monitors were installed in the water column and channel bed at six locations around each of seven in channel restoration structures three cross vanes and four j hooks at baseflow in 2007 elevation surveys of the structures were then used to simulate hef using cfd the results indicate a pattern of pronounced upwelling in the run section just below the structure upwelling transitioning to downwelling within the pool and pronounced downwelling in the glide out of the pool this pattern is consistent with natural riffle pool sequences the direction of hef inferred from the temperature amplitude analysis agreed with the direction of flow simulated with cfd at 80 of the locations and the few disagreements were expected due to model limitations cfd simulation demonstrated that increasing stream flows result in changes in hef spatial patterns and magnitude at each structure this work illustrates how cfd simulations can guide design of in channel restoration structures for hef function copyright 2009 john wiley sons ltd

the heartbeat of the volcano the discovery of episodic activity at prometheus on io the temporal signature of thermal emission from a volcano is a valuable clue to the processes taking place both at and beneath the surface the galileo near infrared mapping spectrometer nims observed the volcano prometheus on the jovian moon io on multiple occasions between 1996 and 2002 the 5 micron m brightness of this volcano shows considerable variation from orbit to orbit prometheus exhibits increases in thermal emission that indicate episodic though non periodic effusive activity in a manner akin to the current pu u o o kupaianaha afterwards referred to as the pu u o o eruption of kilauea hawai i the volume of material erupted during one prometheus eruption episode defined as the interval from minimum thermal emission to peak and back to minimum from 6 november 1996 to 7 may 1997 is estimated to be 0 8 km3 with a peak instantaneous volumetric flux effusion rate of 140 m3 s 1 and an averaged volumetric flux eruption rate of 49 m3 s 1 these quantities are used to model subsurface structure magma storage and magma supply mechanisms and likely magma chamber depth prometheus appears to be supplied by magma from a relatively shallow magma chamber with a roof at a minimum depth of 2 3 km and a maximum depth of 14 km this is a much shallower depth range than sources of supply proposed for explosive possibly ultramafic eruptions at pillan and tvashtar as prometheus type effusive activity is widespread on io shallow magma chambers containing magma of basaltic or near basaltic composition and density may be common this analysis strengthens the analogy between prometheus and pu u o o at least in terms of eruption style even though the style of eruption appears to be similar effusive emplacement of thin insulated compound pahoehoe flows the scale of activity at prometheus greatly exceeds current activity at pu u o o in terms of volume erupted area covered and magma flux whereas the estimated magma chamber at prometheus dwarfs the pu u o o magma chamber it fits within expectations if the pu u o o chamber were scaled for the greater volumetric flux and lower gravity of io recent volumetric eruption rates derived from galileo data for prometheus were considerably smaller than the rate that produced the extensive flows formed in the 17 years between the voyager and galileo missions these smaller eruption rates coupled with the fact that flows are not expanding laterally may mean that the immediate heat source that generates the prometheus plume is simultaneously running out of available volatiles and the thermal energy that drives mobilization of volatiles this raises the question of whether the current prometheus eruption is in its last throes

long term retention of impact crater topography on ganymede previous analyses into viscous relaxation of impact craters on ganymede have predicted short characteristic relaxation times of the order of 100 myr or less for larger craters in disagreement with the apparent great ages of ganymede s terrains by applying improved understanding of rheologic parameters and initial crater shapes to a viscoelastic model we calculate upper limit zero heat flow relaxation times in excess of the age of the solar system eliminating this discrepancy these very long relaxation times are not due to elastic effects but are simply due to the effective viscosity of water ice at the appropriate temperatures and stresses

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

variation in reach scale hydraulic conductivity of streambeds abstract streambed hydraulic conductivity is an important control on flow within the hyporheic zone affecting hydrological ecological and biogeochemical processes essential to river ecosystem function despite many published field measurements few empirical studies examine the drivers of spatial and temporal variations in streambed hydraulic conductivity reach averaged hydraulic conductivity estimated for 119 surveys in 83 stream reaches across continental france even of coarse bed streams are shown to be characteristic of sand and finer sediments this supports a model where processes leading to the accumulation of finer sediments within streambeds largely control hydraulic conductivity rather than the size of the coarse bed sediment fraction after describing a conceptual model of relevant processes we fit an empirical model relating hydraulic conductivity to candidate geomorphic and hydraulic drivers the fitted model explains 72 of the deviance in hydraulic conductivity and 30 using an external cross validation reach hydraulic conductivity increases with the amplitude of bedforms within the reach the bankfull channel width depth ratio stream power and upstream catchment erodibility but reduces with time since the last streambed disturbance the correlation between hydraulic conductivity and time since a streambed mobilisation event is likely a consequence of clogging processes streams with a predominantly suspended load and less frequent streambed disturbances are expected to have a lower streambed hydraulic conductivity and reduced hyporheic fluxes this study suggests a close link between streambed sediment transport dynamics and connectivity between surface water and the hyporheic zone

formation of mountains on io variable volcanism and thermal stresses abstract thermal stresses are potentially important drivers of io s tectonics and mountain building it has been hypothesized that sustained local or regional shut down of heat pipe volcanism on io could lead to deep crustal heating and large compressive stresses mckinnon w b schenk p m dombard a j 2001 geology 29 103 106 such large stresses would then be relieved by thrust faulting and uplifting of crustal blocks producing mountains like those observed on io here we analyze the tectonic consequences of the heat pipe model in detail considering both the initial thermal stress state of a basalt or peridotite crust created by heat pipe volcanism and relative roles of subsidence stresses due to burial of preexisting layers and thermal stresses arising from variable volcanism and changes in crustal lithosphere thickness we limit the magnitude of the potential subsidence stresses in our study because the magnitude of subsidence stresses can be quite large if not dominant results indicate that for a fixed crustal thickness the region of failure and faulting moves closer to the surface as eruption rate decreases and time increases when the crust melts at its base as volcanism decreases as might occur under steady state tidal heating resulting in crustal thinning the region of failure is brought even closer to the surface naturally when compressive subsidence stresses are included the vertical extent of crust in brittle failure thickens to include most of the lithosphere in contrast increases in eruption rate cause the extent of the region in compressional failure to decrease and be driven very deep in the crust in the absence of sufficient subsidence stress therefore regions of declining volcanism are more likely to produce mountains whereas regions of extensive or increasing volcanism are less likely to do so this is consistent with the observation of a global anticorrelation between mountains and volcanic centers on io finally we find that the choice of crustal composition rheology dry basalt vs dry peridotite has little effect on our results implying that basalt peridotite and komatiite are all similarly stiff in the io environment

the morphologic structure of step pools in mountain streams abstract step pool sequences in streams in the santa monica mountains of southern california impart a characteristic morphologic structure that varies with slope but despite the strong association with slope that suggests a direct process control analysis of 464 step pools in 13 study reaches reveals the relationship to reflect correlation but not necessarily causality the direct control on step height is particle size whereas discharge is the probable causal process determining step wavelength because both particle size and discharge generally change with channel slope downstream a slope variation with step wavelength and height is the apparent indirect geometric result a conceptual model of process form linkages is proposed based on these results whereby the step pool morphology is produced by positive relationships between wavelength and discharge and between height and particle size the model offers new insights for explaining the step pool morphology and it is potentially useful in channeling future efforts toward the appropriate scales and variables

time and depth scales of fine sediment delivery into gravel stream beds constraints from fallout radionuclides on fine sediment residence time and delivery abstract particles of fine sediment can clog interstitial pore spaces of coarser grained sediment in river beds and thereby impede the exchange of water dissolved constituents and particulate matter with consequent ecological impacts the extent to which fine sediment reduces connectivity between the stream bed and overlying channel is a function in part of sediment residence time short residence times imply frequent exchange of matter and reduced impact of fine sediment on stream bed habitat while long residence times indicate that with respect to fine sediment delivery the stream bed is more isolated from the overlying channel here we present a novel technique to quantify the residence time of river bed sediment at various depths over annual and decadal timescales using the fallout radionuclides frns 7be t1 2 53 days and 210pbex t1 2 22 3 years we placed mesh cylinders filled with 7be free sediment into a stream bed to quantify the capture of 7be tagged particles in the absence of scour and fill we also took cores to the depth of refusal in alluvial sediment in unregulated and regulated rivers in vermont and new hampshire sampled watershed areas ranged from 29 to 410 km2 and core depths ranged from 19 to 77 cm the 210pbex activity profiles of cores show that bed sediment is exchanged to the depth of refusal at decadal timescales in contrast 7be activity profiles indicate that fine sediment infiltrating into the bed had residence times ranging from 4 to 300 days in unregulated rivers cores from a regulated river are notably different subsurface sediment residence times were always longer than in unregulated rivers at comparable depths likely owing to restriction of bed mobilization and clogging of bed material by fine sediment these results suggest that filtration can be an important component of bed material delivery to stream beds but filtration does not deliver material as deeply into the bed as scour and fill we find that fallout radionuclides provide direct measurements of time and depth scales of active exchange of particulate matter which are key controls on hyporheic function

geologic mapping of the amirani gish bar region of io implications for the global geologic mapping of io abstract we produced the first geologic map of the amirani gish bar region of io the last of four regional maps generated from galileo mission data the amirani gish bar region has five primary types of geologic materials plains mountains patera floors flows and diffuse deposits the flows and patera floors are thought to be compositionally similar but are subdivided based on interpretations regarding their emplacement environments and mechanisms our mapping shows that volcanic activity in the amirani gish bar region is dominated by the amirani eruptive center aec now recognized to be part of an extensive combined amirani maui flow field a mappable flow connects amirani and maui suggesting that maui is fed from amirani such that the post voyager designation maui eruptive center should be revised amirani contains at least four hot spots detected by galileo and is the source of widespread bright sulfur flows and active dark silicate flows being emplaced in the promethean style slowly emplaced compound flow fields the floor of gish bar patera has been partially resurfaced by dark lava flows although other parts of its floor are bright and appeared unchanged during the galileo mission this suggests that the floor did not undergo complete resurfacing as a lava lake as proposed for other ionian paterae there are several other hot spots in the region that are the sources of both active dark flows confined within paterae and so2 and s2 rich diffuse deposits mapped diffuse deposits around fractures on mountains and in the plains appear to serve as the source for gas venting without the release of magma an association previously unrecognized in this region the six mountains mapped in this region exhibit various states of degradation in addition to gaining insight into this region of io all four maps are studied to assess the best methodology to use to produce a new global geologic map of io based on the newly released combined galileo voyager global mosaics to convey the complexity of ionian surface geology we find that a new global geologic map of io should include a map sheet displaying the global abundances and types of surface features as well as a complementary gis database as a means to catalog the record of surface changes observed since the voyager flybys and during the galileo mission

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

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

density structure of io and the migration of magma through its lithosphere we model the density structure of the outer several tens of kilometers of io on the basis of the assumption that condensed volatiles mainly so2 exist at shallow depths in a mafic to ultramafic lithosphere which has a finite porosity due to its accumulation as a mixture of pyroclastic and effusive volcanic deposits which include condensed volatiles the porosity decreases with increasing depth due to pressure driven compaction and temperature driven melting of the volatiles and this controls the bulk density structure of the lithosphere we also model the densities of ascending magmas as a function the types and amounts of volatiles that are available in their source regions the relative densities of the accumulated volcanic deposits and of the magmas rising through them control whether the magmas will erupt at the surface or will stall to form intrusions which can grow into magma reservoirs we find that the depths at which magma reservoirs may exist are strongly influenced by the void space fraction in surface deposits and only weakly controlled by the mass fraction of so2 in the deposits and the mass fraction of so2 in the magmas once nucleated a reservoir has the potential to grow to a very large vertical extent 15 km total height before the magma immediately beneath its roof is again buoyant relative to the surrounding rocks the presence of large volume reservoirs may explain the great extent of the larger lava flow fields seen on io and the longevity of some eruptions the presence of very soluble volatiles such as water in magmas would have virtually no effect on these findings because these volatiles would influence the densities of magmas only very near the surface though they would even in very small amounts then have a dramatic effect on the explosivities of eruptions

io s thermal emission from the galileo photopolarimeter radiometer galileo s photopolarimeter radiometer instrument mapped io s thermal emission during the i24 i25 and i27 flybys with a spatial resolution of 2 2 to 300 kilometers mapping of loki in i24 shows uniform temperatures for most of loki patera and high temperatures in the southwest corner probably resulting from an eruption that began 1 month before the observation most of loki patera was resurfaced before i27 pele s caldera floor has a low temperature of 160 kelvin whereas flows at pillan and zamama have temperatures of up to 200 kelvin global maps of nighttime temperatures provide a means for estimating global heat flow

paterae on io a new type of volcanic caldera paterae defined by the international astronomical union as irregular crater s or complex one s with scalloped edges are some of the most prominent topographic features on io paterae on io are unique yet in some aspects they resemble calderas known and studied on earth mars and venus they have steep walls flat floors and arcuate margins and sometimes exhibit nesting all typical of terrestrial and martian basalt shield calderas however they are much larger many are irregular in shape and they typically lack shields their great sizes some 200 km diameter and lack of associated volcanic edifices beg comparison with terrestrial ash flow calderas however there is no convincing evidence on io for the high silica erupted products or dome resurgence associated with this type of caldera ionian paterae seem to be linked with the eruption of large amounts of mafic to ultramafic lavas and colorful sulfur rich materials that cover the floors and sometimes flow great distances away from patera margins they are often angular in shape or are found adjacent to mountains or plateaus indicating tectonic influences on their formation a database of 417 paterae on io measured from images with 3 2 km pixel 1 resolution 80 of its surface reveals that their mean diameter of 41 0 km is close to that for calderas of mars 47 7 km is smaller than that for venus 68 km but dwarfs those for terrestrial basalt shield calderas 6 6 km and ash flow calderas 18 7 km thirteen percent of all paterae are found adjacent to mountains 42 have straight or irregular margins and 8 are found atop low shields abundant smaller paterae with more continuously active lava eruptions are found between 25 s and 25 n latitude whereas fewer and larger paterae are found poleward of these latitudes patera distribution shows peaks at 330 w and 150 w longitude likely related to the direction of greatest tidal massaging by jupiter ionian patera formation may be explained by portions or combinations of models considered for formation of terrestrial calderas yet their unusual characteristics may require new models with a greater role for tectonic processes

io in the near infrared near infrared mapping spectrometer nims results from the galileo flybys in 1999 and 2000 galileo s near infrared mapping spectrometer nims observed io during the spacecraft s three flybys in october 1999 november 1999 and february 2000 the observations which are summarized here were used to map the detailed thermal structure of active volcanic regions and the surface distribution of so2 and to investigate the origin of a yet unidentified compound showing an absorption feature at 1 m we present a summary of the observations and results focusing on the distribution of thermal emission and of so2 deposits we find high eruption temperatures consistent with ultramafic volcanism at pele such temperatures may be present at other hot spots but the hottest areas may be too small for those temperatures to be detected at the spatial resolution of our observations loki is the site of frequent eruptions and the low thermal emission may represent lavas cooling on the caldera s surface or the cooling crust of a lava lake high resolution spectral observations of emakong caldera show thermal emission and so2 within the same pixels implying that patches of so2 frost and patches of cooling lavas or sulfur flows are present within a few kilometers from one another thermal maps of prometheus and amirani show that these two hot spots are characterized by long lava flows the thermal profiles of flows at both locations are consistent with insulated flows with the amirani flow field having more breakouts of fresh lava along its length prometheus and amirani each show a white ring at visible wavelengths while so2 distribution maps show that the highest concentration of so2 in both ring deposits lies outside the white portion visible measurements at high phase angles show that the white deposit around prometheus extends into the so2 ring this suggests that the deposits are thin and that compositional or grain size variations may occur in the radial direction so2 mapping of the chaac region shows that the interior of a caldera adjacent to chaac has almost pure so2 the deposit appears to be topographically controlled suggesting a possible origin by liquid flow

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

joints in venusian lava flows venusian plains regions as imaged by the magellan spacecraft display many styles of tectonic and volcanic deformation radar images of several areas of the volcanic plains reveal polygonal patterns of bright lineations intersection geometries of the lineations defining the polygonal patterns are typical of those found in tensile networks in addition the polygonal patterns generally exhibit no preferred orientation implying that they are the result of horizontally isotropic stress fields such stress fields usually arise on the earth as a consequence of desiccation freeze thaw cycles or cooling and produce mud cracks ice wedge polygons and columnar joints respectively we propose that the polygonal patterns seen in the magellan images of some of the volcanic plains are the result of thermal stresses we consider two alternative scenarios which would generate sufficient tensile thermal stresses to cause failure the first scenario is that of a cooling lava flow the residual thermal stress which would develop assuming no failure of the rock is tensional and of the order of 400 mpa this is much greater than the strength of unfractured terrestrial basalt 10 mpa so we can expect joints to form during cooling of venusian lava flows however the spacing of the polygonal lineations seen in magellan images is typically 1 2 km much larger than the largest spacings of decimeters for joints in terrestrial lavas the second scenario involves an increased heat flux to the base of the lithosphere the resulting thermal stresses cause the upper lithosphere to be in tension and the lower lithosphere to be in compression brittle tensile failure occurs near the surface due to the finite yield strength of the lithosphere the maximum depth to which failure occurs increases with increasing elevation of the temperature gradient for an initially 25 km thick lithosphere and temperature gradient of 11 km this maximum depth varies from 0 5 km to 2 km as the temperature gradient is increased to 12 km and 22 km respectively both the cooling flow scenario and the heated lithosphere scenario produce isotropic tensile surface stress patterns but the heated lithosphere model is more compatible with the kilometer scale of the polygonal patterns seen in magellan images