the localized role of base level lowering on channel adjustment of tributary streams in the trinity river basin downstream of livingston dam texas usa the purpose of this study is to describe and explain river channel cross sectional change in tributary streams within the 60 km scour zone of the trinity river texas downstream of livingston dam this study investigates changes within the tributary systems to base level changes triggered by incision associated with scour and channel migration within the dammed trunk stream the dam represents a precise moment and place of a system perturbation and therefore represents an opportunistic geomorphic experiment geomorphological effects of the lower trinity river tributaries were investigated through three different types of data resurveys of bridge cross sections planform change as measured from aerial photographs and field mapping of changes in channel geometry using fallen trees undercut banks and other geomorphic indicators of change while field evidence of channel change was found at sites well upstream of tributary confluences with the trinity these were not consistent with responses to lower base level downcutting and lateral changes in response to lowered base level were confined to lower tributary reaches this suggests distance decay and lag times in transmission of responses upstream and or masking of dam induced effects by other processes and controls the most dynamic locations of geomorphic change are near the confluences of the tributaries with the trinity river where the response to dam induced incision is strongly influenced by local trunk stream morphology indicating a high degree of historical and geographical contingency

development and application of an analytical model of stream aquifer interaction abstract a mathematical model to simulate stream aquifer interactions in an unconfined aquifer subjected to time varying river stage was developed from the linearized boussinesq equation using the principle of superposition and the concept of semigroups the mathematical model requires an estimate of three parameters to simulate ground water elevations transmissivity specific yield and recharge the solution has physical significance and includes terms for the steady state water level the steady state water level as influenced by a change in river stage a transient redistribution of water levels in the aquifer from the previous day and a transient change in water level caused by a change in river stage the mathematical model was tested using observed water table elevations at three locations across a 2 km wide alluvial valley aquifer the average absolute deviation between observed and simulated daily water levels was 0 09 m the difference in river stage over the test year was 4 9 m

seasonal and interannual behaviour of groundwater catchment boundaries in a chalk aquifer groundwater catchment boundaries and their associated groundwater catchment areas are typically assumed to be fixed on a seasonal basis we investigated whether this was true for a highly permeable carbonate aquifer in england the berkshire and marlborough downs chalk aquifer using both borehole hydrograph data and a physics based distributed regional groundwater model borehole hydrograph data time series were used to construct a monthly interpolated water table surface from which was then derived a monthly groundwater catchment boundary results from field data showed that the mean annual variation in groundwater catchment area was about 20 of the mean groundwater catchment area but interannual variation can be very large with the largest estimated catchment size being approximately 80 greater than the smallest the flow in the river was also dependent on the groundwater catchment area model results corroborated those based on field data these findings have significant implications for issues such as definition of source protection zones recharge estimates based on water balance calculations and integrated conceptual modelling of surface water and groundwater systems

river stage tomography a new approach for characterizing groundwater basins received 19 june 2008 revised 16 february 2009 accepted 12 march 2009 published 9 may 2009 1 data from tomographic surveys make an inverse problem better posed in comparison to the data from a single excitation source a tomographic survey provides different coverages and perspectives of subsurface heterogeneity nonfully redundant information of the subsurface fusion of these pieces of information expands and enhances the capability of a conventional survey provides cross validation of inverse solutions and constrains inherently ill posed field scale inverse problems basin scale tomography requires energy sources of great strengths spatially and temporally varying natural stimuli are ideal energy sources for this purpose in this study we explore the possibility of using river stage variations for basin scale subsurface tomographic surveys specifically we use numerical models to simulate groundwater level changes in response to temporal and spatial variations of the river stage in a hypothetical groundwater basin we then exploit the relation between temporal and spatial variations of well hydrographs and river stage to image subsurface heterogeneity of the basin results of the numerical exercises are encouraging and provide insights into the proposed river stage tomography using naturally recurrent stimuli such as river stage variations for characterizing groundwater basins could be the future of geohydrology however it calls for implementation of sensor networks that provide long term and spatially distributed monitoring of excitation as well as response signals on the land surface and in the subsurface

stream floodwave propagation through the great bend alluvial aquifer kansas field measurements and numerical simulations abstract the hypothesis is explored that groundwater level rises in the great bend prairie aquifer of kansas are caused not only by water percolating downward through the soil but also by pressure pulses from stream flooding that propagate in a translatory motion through numerous high hydraulic diffusivity buried channels crossing the great bend prairie aquifer in an approximately west to east direction to validate this hypothesis two transects of wells in a north south and east west orientation crossing and alongside some paleochannels in the area were instrumented with water level recording devices streamflow data from all area streams were obtained from available stream gaging stations a theoretical approach was also developed to conceptualize numerically the stream aquifer processes the field data and numerical simulations provided support for the hypothesis thus observation wells located along the shoulders or in between the inferred paleochannels show little or no fluctuations and no correlations with streamflow whereas wells located along paleochannels show high water level fluctuations and good correlation with the streamflows of the stream connected to the observation site by means of the paleochannels the stream aquifer numerical simulation results demonstrate that the larger the hydraulic diffusivity of the aquifer the larger the extent of pressure pulse propagation and the faster the propagation speed the conceptual simulation results indicate that long distance propagation of stream floodwaves of the order of tens of kilometers through the great bend aquifer is indeed feasible with plausible stream and aquifer parameters the sensitivity analysis results indicate that the extent and speed of pulse propagation is more sensitive to variations of stream roughness manning s coefficient and stream channel slope than to any aquifer parameter

equilibrium or indeterminate where sediment budgets fail sediment mass balance and adjustment of channel form green river downstream from flaming gorge dam utah and colorado this study examines bed and bank adjustment in the 105 km reach of the green river immediately downstream from flaming gorge dam by the use of historical aerial and oblique photographs analysis of current and abandoned stream gaging records and field observations although this segment has been previously characterized as sediment deficient these data show that sediment is accumulating in all reaches and that the bed has not degraded at any location where historical data are available adjustment is occurring through a combination of deposition of post dam sediment and stabilization of pre dam deposits resulting in a 10 30 reduction in average width of the channel all post dam surfaces are colonized by woody riparian vegetation the style of channel adjustment varies between geomorphically defined reaches in canyons dominated by debris fans and gravel bedded restricted meandering reaches gravel bars have become inactive and accumulated fine sediment in the sand bedded meandering reaches existing islands have increased in size and new mid channel islands have formed in all of these types of reaches post dam deposits line the banks and sediment has accumulated in side channels that previously separated islands from the bank these findings demonstrate that sediment budgets that show a balance between inputs and outputs cannot necessarily be interpreted to indicate channel equilibrium a sediment mass balance for 150 km reach between the dam and the first long term gage indicates approximate balance of inputs and outputs for the pre and post dam periods when uncertainty in budget components is considered the mass balance is indeterminate although the green river may have been in approximate equilibrium in the pre dam period we have shown that channel width is decreasing in the post dam period the post dam deposits constitute a small but a significant component of the sediment budget upstream from the first major tributary sediment is supplied to this reach by small tributaries and to a lesser extent erosion of pre dam alluvium downstream from the study area the volume of the post dam deposits is tiny relative to the volume of sediment input from the first major tributary d 2005 elsevier b v all rights reserved

a stratigraphic case history using three dimensional seismic data in the gulf of thailand a three dimensional marine seismic survey was conducted in the gulf of thailand to aid in the development of a gas field indicated by three wildcat wells the results and interpretation reported previously demonstrated improved fault resolution and better structural definition five successful appraisal wells have now been drilled and these show that most of the sands have limited extent widespread character changes in the seismic data also support stratigraphic variations in many of the sands several new methods of 3d stratigraphic interpretation have been developed while investigating the depositional history of this area anomalous seismic amplitudes tied to sands penetrated by wells and mapped from seiscroptm horizontal sections in time and depth have indicated the distribution of bars and channels horizon seiscrop sections each sliced through a single bed have been used to delineate these depositional features directly g logtm sections displaying seismic logs derived by rigorous wave equation inversion confirm the existence of these features sands greater than 10 m thick have proved mappable

benzene dispersion and natural attenuation in an alluvial aquifer with strong interactions with surface water field and laboratory investigations have been conducted at a former coke plant discharging to an adjacent river in order to assess pollutant attenuation in a contaminated alluvial aquifer various organic btex pahs mineral oils and inorganic as zn cd compounds were found in the aquifer and in concentrations exceeding the obligatory limit values due to redox conditions of the aquifer heavy metals were almost immobile thus not posing a major risk of dispersion off site the brownfield field and laboratory investigations demonstrated that benzene among organic pollutants presented the major worry for off site dispersion mainly due to its mobility and high concentration i e up to 750 mg l 1 in the source zone however benzene could never be detected near the river which is about 160 m downgradient the main source redox conditions together with benzene concentrations determined in the aquifer have suggested that degradation mainly occurred within 100 m distance from the contaminant source under anoxic conditions and most probably with sulphate as main oxidant a numerical groundwater flow and transport model calibrated under transient conditions was used to simulate benzene attenuation in the alluvial aquifer towards the meuse river the mean benzene degradation rate used in the model was quantified in situ along the groundwater flow path using compound specific carbon isotope analysis csia the results of the solute transport simulations confirmed that benzene concentrations decreased almost 5 orders of magnitude 70 m downgradient the source simulated concentrations have been found to be below the detection limit in the zone adjacent to the river and agreed well with the absence of benzene downgradient river close piezometers reported in groundwater sampling campaigns in a transient model scenario including groundwater surface water dynamics benzene concentrations were observed to be inversely correlated to the river water levels leading to the hypothesis that benzene dispersion is mainly controlled by natural attenuation

buffer effect on identifying transient streambed hydraulic conductivity with inversion of flood wave responses abstract determination of streambed hydraulic conductivity ksb and its variations over time are critical for quantifying exchange fluxes between surface water and groundwater previous studies have used flood wave response techniques frt to determine ksb or its transience excluding the streambed storage effect thus the estimated ksb may not be reliable in this study after improving a previous frt by substituting the gradient descent algorithm with an advanced and robust optimization method trust region reflective algorithm we have analyzed the influence of streambed storage effect on the estimation of time dependent ksb through successive inversion of flood wave responses the study shows that the streambed storage effect is not important for identifying the transience in the hydraulic properties of a relatively thin streambed e g several or dozens of centimeters but significant under estimation 1 2 orders of magnitude can be seen because of neglecting the streambed storage effect it is particularly true for a stream confined aquifer system if the streambed is composed of fine deposits e g silt clay with a thicker thickness greater than 1 m corresponding to low permeability but high storativity and the aquifer is characterized by relatively low permeability e g roughly 0 1 10 m d 1 and relatively low storativity roughly 1e 5 m 1 e g dense fine sands the new method in which streambed storage effect is incorporated can provide more reliable estimations of time dependent ksb for a stream confined aquifer system and it has been successfully applied to estimate the time dependent ksb of a reach of the arkansas river kansas usa

hydrodynamic and isotopic investigations for evaluating the mechanisms and amount of groundwater seepage through a rockslide dam this study addresses the influence of landslide dams on surface water drainage and groundwater flow in the study area of scanno lake and sagittario river central italy a limestone rockslide avalanche formed a lake which has an outlet that is occasionally active showing infiltration into the rockslide dam several springs are present at the lake s base and are partly fed by seepage through the rockslide debris piezometric surveys discharge measurements pumping tests and chemical analyses are tools used to build a conceptual model of the groundwater flow and to evaluate the flow through the rockslide debris seasonal water isotopic signatures validate the assumed model showing a mixing of infiltration recharge and groundwater seepage throughout the rockslide debris various recharge areas have been found for springs pointing out those directly fed by the rockslide debris aquifer hypotheses about seasonal groundwater mixing between the regional carbonate aquifer and the rockslide debris aquifer are supported by isotope results seasonal changes in groundwater table level due to recharge and surface losses from seasonal outlet have been correlated with isotopic groundwater composition from the rockslide debris aquifer and the downstream springs this relationship highlights the role of the rockslide dam body on the hydrodynamics of the studied area relationships between surface waters and groundwater in the area have been completely understood on the basis of water isotopic fingerprinting finally obtaining a complete evaluation of groundwater renewable resources and its regimen copyright 2010 john wiley sons ltd

density driven groundwater flow in closed desert basins field investigations and numerical experiments abstract with a numerical model duffy and al hassan demonstrated that free convection of brine could occur under the playa lake in a closed basin since then continued effort has been made to further our understanding of the role of free convection in closed basin hydrology in this paper we present the results of 5 years of field investigation in pilot valley nevada of a literature search for field studies in closed basins around the world and of numerical simulations to examine the effect of heterogeneity on free convection and the effect of climate changes observations along the divide playa transect revealed evidence of forced convective flow and the existence of a hinge line the salinity profile and the inferred flux suggest that diffusion is the dominant solute transport mechanism under the field sites but the stable isotopes o h suggest that free convection of playa brine also occurs we propose a model that explains these observations first free convection could occur farther out on the playa where the density of surface brines is higher second once free convective flow penetrates the surface sediments it could migrate laterally in the more permeable layers of alluvium third free convection could occur during periods of time when brines pond on the surface for extended periods the search for field studies of groundwater processes in other closed basins leads to a few cases where free convection has occurred on a basin scale based on this survey we discuss the geologic and climatic conditions that favor free convection numerical simulations are carried out to examine the effect of basin scale heterogeneity in hydraulic conductivity the results indicate that heterogeneity changes the pattern of flow by creating multiple convection cells finally we address how a closed basin may respond to climate change climate changes alter the salinity of surface brines and the freshwater recharge both acting as boundary conditions for subsurface processes using dimensional analysis and numerical simulations we evaluate the effect of a warming and drying trend on free convection and how the geologic features of a closed basin affect the response 1997 elsevier science b v

dune dammed paleovalleys of the nebraska sand hills intrinsic versus climatic controls on the accumulation of lake and marsh sediments although running water is the dominant geomorphic agent on earth eolian processes can gain ascendancy in regions where the climate is arid vegetation is sparse and abundant sand is available for transport with climate change the boundaries between fluvial dominated and eolian dominated areas may shift although there have been few reports in the north american literature of river systems blocked by dune sand our work in the nebraska sand hills provides evidence of multiple episodes of such blockage events during prolonged arid intervals in latest pleistocene and middle holocene time eolian dune sand blocked two large valley systems in western nebraska these blockages raised the water table of the high plains aquifer as much as 25 m over an area of 7000 km2 and created over one thousand lakes wetlands far removed from the discharge points of the buried paleovalley system are strongly alkaline exceeding 250 000 mg l total dissolved solids tds relatively fresh 280 mg l tds flow through lakes are present at the distal end of the system where the gradient of the water table is steep and the cross section of the buried valley is large anomalously thick marsh and lake sediments accumulated in deep paleovalleys upstream of dune dams near the southern margin of the sand hills our cores and radiocarbon dates from blue and crescent lakes reveal their histories to be quite distinct from adjacent swan lake these differences are best explained by multiple blockage events our work explains why lakes are most abundant in the driest part of the sand hills it also provides another line of evidence for major dune activity in the sand hills region during holocene time and shows that factors other than regional climate specifically location height and hydraulic conductivity of dune dams can control the rise and fall of the ground water table and the chemistry of lakes

nature s complex flume using a diagnostic state and transition framework to understand post restoration channel adjustment of the clark fork river montana abstract there is an imperfect symmetry between the patterns of channel evolution observed during laboratory flume experiments and those which materialize in rivers exposed to ambient environmental conditions that produce hydrogeomorphic fluxes which are more complex contingent and unpredictable one strategy to improve our understanding of short to medium term channel evolution is to study landscapes that have undergone significant disturbance and have had their biogeomorphic templates reset to a known condition in effect creating a flume in nature this study adopts a diagnostic state and transition framework to narrate and document baseline hypotheses for the potential evolutionary trajectories clark fork river near milltown montana following dam removal and remediation a 5 km stretch of the clark fork river and its adjoining floodplain were reconstructed since flow was introduced to the newly constructed channel in december 2010 complex evolutionary trajectories have been observed on the clark fork s mainstem its secondary channels and floodplain focusing particularly on the river s secondary channels this paper develops a typology of channel states that have been observed and demonstrates that multiple adjustment trajectories have materialized sometimes within the same channel a diagnostic state and transition framework offers a parsimonious strategy to quantitatively or qualitatively anticipate the influence of water sediment and ecological fluxes on channel evolution at the basin reach or segment scale it provides environmental agencies with a robust method to devise spatially explicit scenario based management plans for rivers in a variety of geomorphic settings

interpretative lessons from three dimensional modeling three dimensional 3 d seismic modeling has been accomplished by describing geologic surfaces with triangular plates and then computing the seismic response by kirchhoff wave theory the resulting time sections illustrate many interesting 3 d phenomena which are useful in interpreting geologic structures three dimensional resolution studies relate the concept of fresnel zone reflection to seismic resolution if high resolution is desired both horizontally and vertically then not only is a dense field survey required but also a detailed amplitude study the dense seismic coverage is required to map the focal line of concave boundary edges which are difficult to delineate with conventional seismic data additional studies on complex models such as grabens and 3 d permeability traps associate interpretational pitfalls to a wandering specular reflection path that is side swipe in each geologic model maximum resolution is obtained on a principal plane line dip line if a seismic dip line is not

statistical and analytical study of the propagation of flood induced groundwater rise in an alluvial aquifer the propagation of stage rises of the river danube in an adjoining alluvial aquifer kisalfold nw hungary has been studied by preparing a map which shows the lag times corresponding to maximum correlation values between the hydrographs of groundwater observation wells and the river stages as expected the lag times for the unconfined aquifer generally exceed those for the semi confined part the complex pattern suggests that other rivers in the area also play a role and that transmissivities and resistances of a cover layer are not sufficiently well known despite the presence of a relatively dense network of bore holes a systematic increase of the lag times with the distance from the river was noted at two sections nine flood events have been selected to study how well observed head rises could be predicted using equations for the calculation of the propagation of the head for phreatic and for confined conditions the results suggest that the water levels in the complex flood plain and the river bed conductance of silted up branches during the rise of the flood play an important role

recent morphological channel changes in a deltaic environment the case of the rhone river france abstract channel morphological changes were measured at two sites within the rhone delta along the main channel of the rhone river over one century results show an average erosion rate of 2 8 mm year 1 during the 20th century and decreasing to

experimental studies in natural groundwater recharge dynamics the analysis of observed recharge events abstract the amounts and time distribution of groundwater recharge from precipitation over an approximately 19 month period were investigated at two instrumented sites in south central kansas precipitation and evapotranspiration sequences soil moisture profiles and storage changes water fluxes in the unsaturated zone and hydraulic gradients in the saturated zone at various depths soil temperatures water table hydrographs and water level changes in nearby wells clearly depict the recharge process antecedent moisture conditions and the thickness and nature of the unsaturated zone were found to be the major factors affecting recharge although the two instrumented sites are located in sand dune environments in areas characterized by shallow water table and subhumid continental climate a significant difference was observed in the estimated effective recharge the estimates ranged from less than 2 5 to approximately 154 mm at the two sites from february to june 1983 the main reasons for this large difference in recharge estimates were the greater thickness of the unsaturated zone and the lower moisture content in that zone resulting from lower precipitation and higher potential evapotranspiration for one of the sites effective recharge took place only during late winter and spring no summer or fall recharge was observed at either site during the observation period of this study

capture zone between two streams summary we have investigated stream aquifer interaction with a single pumping well in an aquifer bounded by two parallel nearby streams this study presents steady state semi analytical solutions to calculate the fraction of water withdrawal from two streams potential theory is used to describe the capture zone between two streams when low permeability streambeds are not present steady state flow equations in the aquifer and two streambeds are solved following rigorous mass balance requirements if low permeability streambeds are present when the low permeability streambeds are not present and the regional flow exists between two streams this study finds that the maximal capture size without extracting water from the down gradient stream decreases with the normalized well location in an approximately linear fashion furthermore the normalized flux from the up gradient stream decreases with the normalized well location faster than the linear fashion when the low permeability streambeds exist and the regional flow is neglected the normalized flux across either streambed varies with the normalized well location in a linear function furthermore the magnitude of the slope of that function is nearly unity when the hydraulic conductance ratio of the two streambeds is one and is less than unity when the hydraulic conductance ratio of the two streambeds is either greater or smaller than one when the normalized well location with equal fluxes from two streams versus the hydraulic conductance ratio of the two streambeds are plotted semi logarithmically we observed a segmented curve including a steep segment at the beginning followed by a flat segment and a final steep segment

field appraisal with three dimensional seismic surveys offshore trinidad a consortium comprising texaco trinidad inc trinidad and tobago oil company ltd and trinidad tesoro petroleum company ltd commenced exploration in the south east coast consortium block offshore trinidad in 1973 after four years of intensive exploration a gas condensate discovery was announced in early 1977 on the pelican prospect later that year in anticipation of the possible future need to site drilling production platforms a three dimensional 3 d seismic survey was recorded over the prospect this survey resulted in improvements in seismic record quality multiple attenuation and fault resolution a coordinated geologic geophysical interpretation based on the 3 d seismic survey a reevaluation of log correlations and the use of seismic logs differed significantly from earlier interpretations because of this it is anticipated that development of the field will need to be initiated in a different fault block from that previously envisioned a second 3 d survey contiguous to the pelican su

underlying gravel layers in a large sand bed river and their influence on downstream dam channel adjustment due to the changes in environmental factors during a river s historical development underlying gravel layers are found in many large plain rivers when the buried depth of this gravel layer is within the reach of down cutting by clear water scour after reservoir construction it may be exposed and exert a far reaching influence on channel adjustment in hanjiang river the longest tributary of the yangtze river in china down cutting has been greatly reduced and even stopped due to the total change of bed material composition the exposure of a gravel layer results in a dramatic increase in the bed s hydraulic roughness so the channel slope may increase after a decline during the period prior to the exposure of the gravel layer moreover the exposed gravel layer increases the erosion resistance of bed material making a lower relative erosion resistance of bank to bed material where the bank s erosional resistance is weak this may lead to a tendency towards channel widening

structural interpretation from horizontal seismic sections the interpreter of a 3 d survey must utilize a data volume horizontal slices through a data volume called seiscrop sections have unique properties and structural interpretation from them is fast convenient and effective an event on a seiscrop section displays local strike a property which permits direct contouring of a structural surface without any timing and posting the width of an event on a seiscrop section is a composition of the frequency of the data and the structural dip event terminations indicate faults or other discontinuities when they are transverse to structural strike faults parallel to structural strike are much less evident on a single seiscrop section but become apparent with the relative movement of events from section to section in practical mapping the interpreter normally contours one fault block before proceeding to the next with the correlation between them being established from the vertical sections with dual polarity variable area displays the interpreter can perc

impact of hydroelectric power releases on the morphology and sedimentology of the bed of the warche river belgium the butgenbach dam on the warche river was built in 1932 in order to maintain a suf cient supply of water to the robertville reservoir situated 7 km downstream for the production of hydroelectricity during winter months releases are made almost every day from the butgenbach dam from a hydrological point of view this has resulted in signi cantly reducing the number of discharges that are higher than bankfull despite the reduction in peak discharge there is a signi cant increase in the number of ef cient discharges 0 6 bankfull the impacts of these hydrological modi cations on the bed morphology and sedimentology below the butgenbach dam have been studied and the following geomorphological modi cations have been identi ed a doubling of the width of the channel in 45 years a reduction in the number of rif es and pools an increase in the number of gravel bars and islets and an increase in bedrock outcrops in the channel moreover the nest bed particles are mobilized by the almost daily releases inducing a signi cant increase in bed material size sorting the reduction of sinuosity and the disappearance of bed differentiation and rif e pool sequences have produced a diminution of bed roughness and an increase of the competence of the river thus relatively small oods can remove the armoured layer copyright 2003 john wiley sons ltd

analysis of seismic data using horizontal cross sections three dimensional data provide the interpreter with a new dimension for interpretation horizontal cross sections this article suggests formats for the display of the data and techniques for deriving contour maps of horizons strike directions dip angles thicknesses of formations and fault blocks

effect of ground water recharge on configuration of the water table beneath sand dunes and on seepage in lakes in the sandhills of nebraska u s a abstract analysis of water level fluctuations in about 30 observation wells and 5 lakes in the crescent lake national wildlife refuge in the sandhills of nebraska indicates water table configuration beneath sand dunes in this area varies considerably depending on the configuration of the topography of the dunes if the topography of an interlake dunal area is hummocky ground water recharge is focused at topographic lows causing formation of water table mounds these mounds prevent ground water movement from topographically high lakes to adjacent lower lakes if a dune ridge is sharp the opportunity for focused recharge does not exist resulting in water table troughs between lakes lakes aligned in descending altitudes parallel to the principal direction of regional ground water movement generally have seepage from higher lakes toward lower lakes

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

the nature of a dynamic water table in a system of non tidal freshwater coastal wetlands abstract the miller woods wetland system contains over 150 individual minerotrophic fens the fens are established in progradational beach complex of relict beach ridges and parabolic dunes and are in hydraulic connection with lake michigan large scale flow in the wetland system is horizontal to the northwest the position of a ground water divide which forms the southern hydrologic boundary does not coincide with geographical boundaries of the system and migrates in time and space the position of the divide is not solely a function of topography but is dependent on an interplay among evapotranspirative demands antecedent soil moisture and recent recharge small scale local flow systems develop and dissipate on variable temporal scales along the margins of individual wetlands in response to evapotranspirative demands by wetland vegetation and other climatic influences

an experimental study of complex response in river channel adjustment downstream from a reservoir an experimental study is described which attempts to reveal the complex response process of river channel adjustment downstream from a reservoir a descriptive model has been established which identifies three adjustment stages in stage i following the change of water and sediment input the width depth ratio of river channel w d and the river channel gradient s both decrease but the river channel sinuosity r increases in stage ii because of feedback from the changing boundary conditions w d starts to increase r decreases and the rate of gradient decrease becomes very slow in stage iii equilibrium is reestablished w d r and s tend to constant values and the system recovers its stable state

statistical analysis of lateral migration of the rio grande new mexico abstract the lateral migration rates of alluvial rivers are affected by changes in water and sediment regimes the rio grande downstream from cochiti dam exhibits spatial and temporal variability in lateral movement rates documented since 1918 a tremendous database exists that documents planform bed material size channel geometry and water and sediment regimes a statistical analysis reveals that migration rates primarily decreased with decreasing flow energy r2 0 50 p 60 the findings show that lateral movement increases with increasing flow energy and with degree of braiding

classification of geomorphological effects downstream of dams abstract the effects of dams on downstream geomorphology are reviewed and a typology is devised consisting of nine cases the classification can be seen as a further development of lane s balance between water discharge sediment load grain size and river slope depending on changes in released water flow and changes in released sediment load relative to the transport capacity of the flow it is possible to estimate resulting cross sectional geomorphology the longitudinal extent of changes and their variability with time and the tributary response to altered mainstream cross section changes are also discussed

the rate and pattern of bed incision and bank adjustment on the colorado river in glen canyon downstream from glen canyon dam 1956 2000 closure of glen canyon dam in 1963 transformed the colorado river by reducing the magnitude and duration of spring floods increasing the magnitude of base flows and trapping fine sediment delivered from the upper watershed these changes caused the channel downstream in glen canyon to incise armor and narrow this study synthesizes over 45 yr of channel change measurements and demonstrates that the rate and style of channel adjustment are directly related to both natural processes associated with sediment deficit and human decisions about dam operations although bed lowering in lower glen canyon began when the first cofferdam was installed in 1959 most incision occurred in 1965 in conjunction with 14 pulsed high flows that scoured an average of 2 6 m of sediment from the center of the channel the average grain size of bed material has increased from 0 25 mm in 1956 to over 20 mm in 1999 the magnitude of incision at riffles decreases with distance downstream from the dam while the magnitude of sediment evacuation from pools is spatially variable and extends farther downstream analysis of bed material mobility indicates that the increase in bed material grain size and reduction in reach average gradient are consistent with the transformation of an adjustable bed alluvial river to a channel with a stable bed that is rarely mobilized decreased magnitude of peak discharges in the post dam regime coupled with channel incision and the associated downward shifts of stage discharge relations have caused sandbar and terrace erosion and the transformation of previously active sand bars and gravel bars to abandoned deposits that are no longer inundated erosion has been concentrated in a few pre dam terraces that eroded rapidly for brief periods and have since stabilized the abundance of abandoned deposits decreases downstream in conjunction with decreasing magnitude of shift in the stage discharge relations in the downstream part of the study area where riffles controlling channel elevation have not incised channel narrowing has resulted from decreased magnitude of peak discharges and minor post dam deposition these physical changes to the aquatic and riparian systems have supported the establishment and success of an artifact ecosystem dominated by non native species models for the channel response downstream from large dams typically consider factors such as the degree of sediment deficit the pre dam surface and subsurface grain size and the magnitude of post dam average flows these results suggest that it is also necessary to consider 1 the possibility of variable responses among different channel elements and 2 the potential importance of exceptional flows resulting from management decisions

river channel response to climate and human induced hydrological changes case study on the meandering hernad river hungary abstract the increasing extent of human activities and climate change may have notable effects on river systems therefore a comprehensive knowledge of these effects is essential to mitigate their unfavourable consequences the aim of the present study is to analyse the hydromorphological changes caused by climate change and human impacts on the hungarian section of the hernad river and to evaluate the morphological responses of this river from a flood prevention perspective over the past 60 years considerable morphological changes were measured in the studied river sections caused by the alteration of their hydrology the main morphological change was the evolution of secondary bends on large meanders this process was driven by extrinsic factors as the morphometric changes were determined by an altered water regime caused by precipitation change and engineering works further mean discharge decreases caused the propagation of this process on smaller meanders as well meanwhile the cross sectional area of the channel also decreased primarily as a result of narrowing this altered morphology enhances the flood risk in the short run as the narrow channel is not sufficient to drain the rapid flood waves in the twenty first century climate change may cause decreasing mean discharge while also increasing the frequency of rapid floods further increasing flood risk however the projection of the geomorphological effects of future precipitation changes is very uncertain

late glacial and holocene record of climatic change in the southern rocky mountains from sediments in san luis lake colorado usa abstract large rapid climate changes occurred over the last glacial cycle in the southwestern united states and elsewhere in many regions of the world some of these changes were attributed to alternations between stadial and interstadial conditions in the north atlantic but intense debate exists on how climate anomalies in the north atlantic transmit to the southwest here we report a sediment record from san luis lake in southern colorado through analyses of grain size magnetic susceptibility mg ca total inorganic carbon 18 o and 13 c to indicate climatic and environmental changes in the southern rocky mountains over the last 16 5 ka we found that san luis lake remained hydrologically closed most of the time but overflowed during the second half of the mystery interval the big wet 15 7 14 9 ka and the latter part of the mid holocene the neopluvial 4 3 ka over the course of the last deglaciation san luis lake underwent a series of large millennial scale hydroclimatic changes such as the big dry 16 5 15 7 ka the big wet the bolling allerod dry 14 9 12 7 ka and the younger dryas wet 12 8 11 6 ka corresponding to warm cold phases in the high latitude northern hemisphere the north american monsoon waxed during the pre boreal interval 11 6 10 5 ka and waned through the holocene in phase with northward and southward displacement of the intertropical convergence zone itcz the san luis lake basin was relatively dry in the early holocene 10 5 6 7 ka wet and fluctuating in the mid holocene 6 7 2 6 ka and dry and less variable in the late holocene 2 6 0 ka we found evidence that extreme pluvial episodes of the southern rocky mountains and elsewhere in the american southwest were coeval with cold phases of the north pacific our results highlight the role of the north pacific in modulating atmospheric circulations over the region on millennial timescales

the interaction of lakes with variably saturated porous media numerical simulation of variably saturated porous media indicates that groundwater recharge is variable in time and space depending on the thickness of the unsaturated zone through which infiltrating water must move the resulting complex transient groundwater flow systems have significant impact on contiguous surface water in very permeable media small local closed groundwater flow systems can develop and dissipate within a few weeks to several months after major recharge these have a direct effect on contiguous surface water by alternately causing seepage to and seepage from the surface water the transient nature of these flow systems indicates that reversals of the direction of groundwater flow may be common in less permeable media the same complex flow systems may occur but the time for development and dissipation is much greater for example it is conceivable that small local flow systems may exist for many months or years as a result of major recharge therefore directions of flow in such systems are more stable and the effect on contiguous surface water also is more stable the findings of this study indicate that wells and groundwater quality sampling sites need to be carefully located to define accurately water table configuration groundwater recharge direction of seepage through the beds of surface water bodies and complex geochemical processes related to changing directions of groundwater flow

causes of river bed degradation river bed degradation can proceed downstream as well as upstream depending upon the cause of degradation the causes of downstream progressing degradation are primarily related to changes in independent river channel variables such as increase in water discharge decrease in size of bed material and decrease in bed material discharge the causes of upstream progressing degradation are all related to an imposed increase in river slope which can occur as a result of natural river behavior or by man made changes study of various case histories indicates that river slopes are increased by lowering a base level by decreasing the length of a river or by removal of a control point case histories also show that downstream and upstream progressing degradation can act in combination along a river system downstream progressing degradation along the main stream of a river system can initiate upstream progressing degradation on a tributary

climate driven hydrologic transients in lake sediment records calibration of groundwater conditions using 20th century drought abstract the effect of 20th century drought on groundwater and lakes in a climatically sensitive area of the northern great plains grant county western minnesota was investigated by analysis of lake sediments and historical air photos in conjunction with groundwater flow simulations drought caused an observed 4 0 to 5 1 m lake head decline between 1923 and 1938 all but three either dried completely or declined to 210 pb dated sediment core was analyzed for ostracode species distribution and geochemistry of c rawsoni shells 18 o 13 c mg ca sr ca mg ca and 13 c increased during drought at the same time that salinity tolerant ostracodes thrived suggesting increasing salinity however 18 o decreased during drought anti correlative with mg ca a numerical model for transient response of groundwater flow to drought modeled after that of 1923 38 was constrained by drought end recessional strand lines observed on air photos of one lake with strand line elevations inferred from bathymetry simulated lake water level declines in the first 15 drought years were consistent with strand line elevations the rate of decline was exponential at drought onset slowing as water in lakes and wetlands receded to below land surface and lake evaporation declined a near steady state was attained between 40 and 50 simulation years after onset of drought with the water table from 1 5 m below the level of dry lakebeds and only the two deepest lakes still holding water they were however greatly constricted in area and depth simulated evaporation fluxes were reduced by over 50 within 15 years despite increased et rates due to lake area constriction and water table declines both relate to land surface morphology beneath and around lakes model results suggest that lake bed elevations exert control on the rate and ultimate level to which groundwater is depressed by drought the deepest lakes in any region will tend to become the focus of groundwater flow during sustained drought once lakes around it dry out the unexpected finding of decreasing shell 18 o in response to drought is interpreted as some combination of source and or evaporation reduction effects such as seasonality of recharge precipitation or reduced et flux from wetlands that seep back into lakes

controls of groundwater floodwave propagation in a gravelly floodplain summary interactions between surface water and groundwater can occur over a wide range of spatial and temporal scales within a high hydraulic conductivity gravelly floodplain in this research dynamics of river groundwater interactions in the floodplain of the matane river eastern canada are described on a flood event basis eleven piezometers equipped with pressure sensors were installed to monitor river stage and groundwater levels at a 15 min interval during the summer and fall of 2011 results suggest that the alluvial aquifer of the matane valley is hydraulically connected and primarily controlled by river stage fluctuations flood duration and magnitude the largest flood event recorded affected local groundwater flow orientation by generating an inversion of the hydraulic gradient for 16 h piezometric data show the propagation of a well defined groundwater floodwave for every flood recorded as well as for discharges below bankfull 0 5 qbf a wave propagated through the entire floodplain 250 m for each measured flood while its amplitude and velocity were highly dependent on hydroclimatic conditions the groundwater floodwave which is interpreted as a dynamic wave propagated through the floodplain at 2 3 orders of magnitude faster than groundwater flux velocities it was found that groundwater exfiltration can occur in areas distant from the channel even at stream discharges that are well below bankfull this study supports the idea that a river flood has a much larger effect in time and space than what is occurring within the channel

breakup and reestablishment of the armour layer in a large gravel bed river below dams the lower ebro changes in armour layer during floods under supply limited conditions are little known this paper describes the breakup and the reestablishment of the bed armour layer in the regulated gravel bed ebro river during a flooding period the study was conducted over a 28 km study reach from 2002 to 2004 the surface subsurface and bed load grain size distribution constitute the bases for the analysis of bed armouring dynamics the results indicate that the magnitude of floods controlled the degree of armouring of the river bed the initial mean armouring ratio was 2 3 with maximum values reaching 4 4 floods in the winter of 2002 2003 q8 caused the breakup of the armour layer in several sections this resulted in the erratic bed load pattern observed during the december 2002 flushing flow and in the increase in bed load transport during successive events most grain size classes were entrained and transported causing river bed incision the mean armouring ratio decreased to 1 9 in contrast during low magnitude floods in 2003 2004 q2 the coarsest fractions 64 mm did not take part in the bed load while finer particles were winnowed thus surface deposits coarsened as a result the armour layer was reestablished i e the mean armouring ratio increased to 2 3 and the supply of subsurface sediment decreased the supply and transport of bed material appear to be in balance in the river reach immediately below the dam in contrast the transport of medium and finer size classes in the downstream reaches was higher than their supply from upstream a phenomenon that progressively reduced their availability in the river bed surface hence the armour layer reworking

comparison of methods for quantifying active layer dynamics and bedload discharge in armoured gravel bed rivers several methods were employed in the ardennian rivers belgium to determine the depth of the active layer mobilized during floods and to evaluate the bedload discharge associated with these events the use of scour chains has shown that the depth of the active layer is systematically less than the b axis of the average particle size d50 of the elements which compose the surface layer of the riffles this indicates that only a partial transport exists during low magnitude floods the bedload discharge has been evaluated by combining data obtained using the scour chains technique and the distance covered by tracers quantities of sediment transported during frequent floods are relatively low 0 02 t km 2 due to the armour layer which protects the subsurface material these low values are also related to the fact that the distance calculated for mobilized bedload only applies to tracers fitted with pit passive integrated transponder tags diameter 20 mm whereas part of the bedload discharge is composed of sand and fine gravel transported over greater distances than the pebbles the break up of the armour layer was observed only once for a decennial discharge during this event the bedload discharge increased considerably 2 t km 2 the use of sediment traps data from dredging and a helley smith sampler confirm the low bedload transport in ardennian rivers in comparison to the bedload transport in other geomorphological contexts this difference is explained by the presence of an armoured layer but also by the imbricated structures of flat bed elements which increase the resistance to the flow finally the use of the old iron industry wastes allowed to quantify the thickness of the bed reworked over the past centuries in the lembree river the river bed contains slag elements up to a depth of about 50 cm indicating that exceptional floods may rework the bed to a considerable depth copyright 2012 john wiley sons ltd

areal seismic methods for determining the extent of acoustic discontinuities the collection of areal seismic reflection data is becoming fairly routine it is now generally realized that the solution of three dimensional 3 d structural problems is only possible when the target has been adequately sampled and the data have been correctly migrated to produce an accurate image of the subsurface however many of our exploration prospects are associated with lithological changes or stratigraphic features rather than structural features we show how areal seismic techniques can provide an added dimension in determining the extent of acoustic discontinuities in areas where the strata are generally flat

stream aquifer interactions at great sand dunes national monument colorado influences on interdunal wetland disappearance abstract between 1937 and 1995 a complex of more than 100 interdunal wetlands disappeared from great sand dunes national monument colorado we investigated three hypotheses that could explain wetland disappearance 1 dune movement during a severe drought in the 1950s buried the wetlands 2 agriculture related ground water pumping lowered the regional water table and 3 changes in local hydrologic processes led to wetland loss we used regional stream flow records ground water level measurements natural stable isotope analyses soil stratigraphy buried seed banks and ground water modeling to address these hypotheses hydrologic data and stable isotope analyses illustrated the interaction between sand creek a nearby stream and the unconfined aquifer in the area where wetlands occurred when the intermittent sand creek flows seepage through its bed creates a large ground water mound under the creek the seasonal development and dispersion of this mound propagates pressure waves through the aquifer that influence ground water levels up to 2 km from sand creek our data suggest the primary factors contributing to wetland disappearance were recent climatic fluctuations and incision of the sand creek channel below average stream flow between 1950 and 1980 reduced the duration of sand creek flow across the dune complex minimizing ground water mound development consequently the water table in the unconfined aquifer dropped 1 0 m and interdunal wetlands dried up twentieth century incision of sand creek s channel reduced ground water mound height 2 5 m decreasing seasonal water table fluctuations at interdunal wetlands and contributing to the overall water table decline long term wet and dry cycles affect the water table elevation more than channel incision leading us to conclude that many interdunal wetlands are ephemeral features wetland area is maximized during consecutive years of above average sand creek discharge and minimized as the water table drops during dry periods