impact of bedrock surface topography on spatial distribution of quaternary sediments and on the flow pattern of late weichselian glaciers on the east european craton russian plain this paper presents the first high quality maps of the bedrock surface topography and the quaternary sediment distribution for the late weichselian glaciation area on the east european craton eec and the reasoning behind the estimated sediment distribution geographic information system gis based three dimensional model of the bedrock surface topography the current digital terrain model dtm and the ice flow pattern of the last glaciation provided the main source data for the analyses and interpretations in the current continental area of the western eec which was covered by the last scandinavian ice sheet sis the bedrock lies overwhelmingly 85 above the contemporary sea level with an average altitude of 54 m above sea level the topography of the bedrock surface is very well reflected in the current topography and an 80 m difference in the ice bed surface altitudes results in the topographic focusing of the ice flow and thus all late weichselian ice streams were accommodated to the depressions in the subglacial surface such depressions are characterised by a thin 10 m layer of sediment with one till bed just 50 m upslope from the bottom of the depressions the thickness of the sediment increases two to threefold and comprises five or more till layers indicating relief enhancement through subsequent glaciations the average calculated thickness of the quaternary deposits in the study area was 49 m and the total volume of sediments was 20 280 km3

the infiltration of fine matrices into coarse grained alluvial sediments and its implications for stratigraphical interpretation traps designed to simulate the framework gravel of natural streams and installed in a coarse grained channel allow the identification and differentiation of the processes governing the infiltration of fines into bed interstices vertical patterns of matrix distribution reflect the relative importance of variables acting at both the surface of the channel bed and within the bed sediments differences in grain size and therefore pore size of the framework exercise a strong controlling influence an armour layer of coarse particles superimposed on comparatively finer sub surface bed material common to many coarse grained rivers is shown to encourage the clogging of near surface pores with matrix fines below the obstruction interstices may be protected from incursion and the gravel framework is often left open this is the reason for patchy concentrations of matrices often reported in the stratigraphical literature in contrast a uniform size grading of the bed sediment or an upwards fining leads to a more uniform packing of the interstices with matrix flood history is shown to be an important determinant of both the size distribution and the spatial distribution of matrices and is useful in providing a dynamic explanation of the variability of matrices observed from place to place in ancient deposits

stabilization of a gravel channel by large streamside obstructions and bedrock bends jacoby creek northwestern california jacoby creek bed width 12 m bankfull discharge 32 6 m 3 s contains stationary gravel bars that have forms and positions controlled by numerous large streamside obstructions bedrock outcrops large woody debris and rooted bank projections and bedrock bends bank projection width and bar volume measured in 104 channel segments 1 bed width long are significantly cross correlated at lags of 1 3 and 4 indicating the tendency for large obstructions and bends to form bars 3 to 4 bed widths downstream and 1 bed width upstream all of the 18 bars downstream of large obstructions or bends in the study reach were along the obstruction side of the channel or outside bank of the bend most of the pools 85 were next to large obstructions or in bends conversely 92 of large obstructions or bends had pools comparison of the volume of four bars with volumetric bar changes and volume of bedload transported during four high flow events suggests that rates of sediment transport were sufficient to cause major changes in bars during bankfull events the only important channel changes observed in 4 yr however have been associated with the movement of large woody debris and with changes in the angle at which the flow approaches an obstruction a general model is proposed that large obstructions and nonalluvial bends stabilize the form and location of gravel bars bars are stabilized by two related mechanisms 1 large obstructions and bends cause intense quasi steady secondary circulation in scour holes that terminate upstream bars at fixed locations obstruction width channel deflection scour hole width and bed width were measured at 26 obstructions these data show that obstructions wider than approximately one third of the bed form pools spanning the entire channel and thus terminating bars smaller obstructions form scour holes contained within a single bar 2 bars are deposited upstream of large obstructions and sharp bends because of backwater reductions in stream power bars are deposited downstream because flow energy is expended around obstructions and bends and because the flow expands downstream of constrictions that result from large obstructions the formation of bars and pools inherent in many gravel channels can thus be enhanced and fixed in position by flow structures set up around large obstructions and bends formed of resistant materials

stress and velocity fields in glaciers part ii sliding and basal stress distribution numerical methods are used to examine the interaction between the spatial distribution of the basal shear traction and the corresponding basal velocity for an inclined slab geometry in our improved treatment we reject the common assumption that basal velocity is a simple function of local variables in favour of a non local treatment that includes normal deviatoric stress and takes basal velocity to be an integrated response to spatially varying influences computationally one must either iterate the basal velocity with a friction parameterization that relates basal shear traction to basal velocity or alternatively prescribe the basal shear traction that results from bed decoupling and substrate deformation the average of basal shear traction over the entire bed of the ice mass is invariant under changes in sliding distribution and thus constitutes a useful reference any local relative reduction of traction leads to basal movement either sliding over the bed or moving with a deforming subglacial layer the local stress reduction is accompanied by a concentration of traction up and down glacier of the moving base growth decay and possible migration ofbasal stress concentrations may be closely related to short lived sliding events and to surges

infiltration of fine sediment into a coarse mobile bed a phenomenological study experiments were undertaken to study the nature of granular interaction in running water by examining the influence of fine grain inputs to a coarser sediment bed with a mobile surface video recordings of grain sorting by both kinetic sieving and spontaneous percolation are used to diagnose the critical processes controlling the overall bed response kinetic sieving takes place in the mobile bed surface with the finer sediment moving to the bottom of the bedload transport layer at the interface with the underlying quasi static coarse bed we show that the behavior at this interface dictates how a channel responds to a fine sediment input if by spontaneous percolation the fine sediment is able to infiltrate into the underlying quasi static bed the total transport increases and the channel degrades however if the fine sediment input rate exceeds the transport capacity or is geometrically unable to infiltrate into the underlying bed it forms a quasi static layer underneath the transport layer that inhibits entrainment from the underlying bed resulting in aggradation and an increase in bed slope this article is protected by copyright all rights reserved

changes in hydrologic regime by dams dams have major impacts on river hydrology primarily through changes in the timing magnitude and frequency of low and high flows ultimately producing a hydrologic regime differing significantly from the pre impoundment natural flow regime this paper presents the analysis of pre and post dam hydrologic changes from dams that cover the spectrum of hydrologic and climatic regimes across the united states our overall goals are to document the type magnitude and direction of hydrologic shifts because of impoundment using the entire database for the national inventory of dams nid for dams possessing longstanding u s geological survey usgs gages downstream we identified 21 gage stations that met length of record criteria encompassing an array of types of dams and spanning four orders of magnitude in contributing watershed area to assess hydrologic changes associated with dams we applied a hydrologic model the indicators of hydrologic alteration iha supplemented with orientation statistics for certain hydrograph parameters dams had significant impacts on the entire range of hydrologic characteristics measured by iha for many characteristics the direction and significance of effects were highly consistent across the 21 sites the most significant changes across these sites occurred in minimum and maximum flows over different durations for low flows the 1 day through 90 day minimum flows increased significantly following impoundment the 1 day through 7 day maximum flows decreased significantly across the sites at monthly scales mean flows in april and may tend to decline while mean flows in august and september increase other significant adjustments included changes in annual hydrograph conditions primarily in the number of hydrograph reversals that has generally increased for almost all sites following impoundment the number of high pulses has increased following impoundment but the average length declines the mean rate of hydrograph rise and fall has declined significantly these results indicate that the major pulse of dam construction during the previous century has modified hydrologic regimes on a nationwide scale for large and small rivers

model analyses of the contribution of in channel processes to sediment concentration hysteresis loops summary sediment concentration sc water discharge q relations in rivers are typically governed by multiple and relatively complex processes due to hysteresis effects sediment discharges can differ for similar or equivalent water discharges which causes scatter in empirical datasets and may decrease the predictive power of sc rating curves such hysteresis effects must therefore be understood and accounted for to make dependable predictions for river system management the overall objectives of this study are to develop modelling approaches suitable for reproducing and predicting hysteresis effects at larger scales and to investigate the possible contribution of in channel processes erosion and deposition to sediment concentration hysteresis loops to investigate relevant field scale conditions we develop a one dimensional dynamic sediment transport model of the downstream tuul river northern mongolia investigating in channel processes along a 141 km stretch during a hydrological year the results show that the present modelling approach can reproduce both anti clockwise and clockwise hysteresis effects importantly in channel processes alone can cause considerable anti clockwise hysteresis effects without being reinforced by catchment processes such as hillslope erosion such specific contributions from in channel processes introduced data scatter into the sediment rating curves decreasing their r 2 values from unity to approximately 0 5 to 0 6 more generally possible changes in the number or magnitude of high flow events caused by climatic or other anthropogenic factors could influence total sediment deposition which was primarily found to occur during relatively short high flow events such potential changes also have important implications for the possible spreading of polluted sediments

influence of bed patchiness slope grain hiding and form drag on gravel mobilization in very steep streams steep streams are a major portion of channel networks and provide a link to transport sediment from hillslopes to lower gradient rivers despite their importance key unknowns remain perhaps foremost of which is evaluating in steep streams empirical laws for fluvial sediment transport developed for low gradient rivers to address this knowledge gap we painted sediment in situ over 3 years to monitor incipient sediment motion and sediment patch development in five small drainage areas of 0 04 2 km 2 and steep slopes of 5 37 tributaries of elder creek california united states we found that channel beds organized into size sorted sediment patches which displayed active fluvial transport of gravel annually consistent year to year patch median grain sizes partial transport of bed material and significantly higher values of critical shields stress for incipient sediment motion compared to that observed for lower gradient rivers the high critical shields stresses up to 0 5 for the median grain size agree within a factor of 3 to theoretical predictions which account for slope dependent hydraulics grain hiding and sediment patches for grains of approximately the same size as the roughness length scale slope dependent hydraulics and bed patchiness are the dominant controls on critical shields stress values while grain hiding is important for grains larger or smaller than the roughness length scale form drag exists in our monitored tributaries but has a smaller influence than the above effects our field observations show fluvial processes contribute to sediment mobilization in steep channels which are often considered to be dominated by debris flows

recent developments in centrifuge modelling of tectonic processes equipment model construction techniques and rheology of model materials abstract the experimental tectonics laboratory at queen s university is equipped with a large capacity centrifuge that is capable of subjecting tectonic models measuring 127 76 mm in plan and up to 51 mm in depth to accelerations as high as 20 000 g this high capacity greatly extends the range of potential model materials and permits the use of relatively stiff and or brittle substances a number of new techniques of model construction have been devised that permit internal and surface strain patterns and kinematic evolution to be monitored in detail one particularly useful technique which will find application in non centrifuged experiments as well allows the preparation of highly uniform anisotropic multilayers composed of alternating layers of plasticine and silicone putty with individual layer thicknesses as low as 20 m and with controllable ratio between thicknesses of the relatively competent and incompetent units examples of models constructed using these new techniques are illustrated one particular type of the commonly used model material silicone putty has been subjected to a series of rheological test the results indicate that at strain rates in the range 10 6 10 3 s 1 applicable to the centrifuge experiments the silicone putty exhibits power law rheology with n 7 2 at higher strain rates the material appears to tend towards linear behaviour available rheological data and dimensional analysis using standard scaling laws and appropriate model ratios suggest that the microlaminated plasticine silicone putty multilayer is a suitable analogue in centrifuged experiments for interbedded sequences of indurate limestone and incompetent shale the excellent degree of dynamic similitude attained is demonstrated by the realistic form of fold and fault structures developed in models constructed of this material

the stream function and gauss principle of least constraint two useful concepts for structural geology abstract to the extent that rock deformation can be approximated by a two dimensional newtonian model a powerful stream function simulation method is applicable the significance of stream functions is that velocity strain stress and energy derived from the same stream function satisfy automatically three basic conditions of dynamics 1 1 the condition of continuity 2 2 the navier stokes equations 3 3 conservation of energy hence we state with jaeger if a stream function can be found which satisfies the boundary conditions of a dynamic model the complete solution follows all pertinent bits of dynamic information are implied in the stream function from which they can be directly derived guaranteed so to speak not to violate the basic conditions of dynamics stream functions useful in structural geology are solutions of 4 4 x 4 2 4 x 2 y 2 4 y 4 0 a double polynomial solution of max degree 14 is developed in which the coefficients are related controlled by the 4 0 constraint and their absolute values are determined by the boundary conditions of specific models and by the condition of maximum rate of energy dissipation or maximum rate of decline of potential energy the polynomial stream function is applied to a collapsing viscous nappe consisting of a thin basal layer with low viscosity on which a thicker layer with high viscosity slides due to gravitational spreading the velocity of forward movement depends upon absolute and relative values of the following parameters viscosity thickness the aspect ratio and density the velocity of a variety of nappes with different thicknesses aspect ratios viscosities and densities is determined

dating ice flow change near the flow divide at roosevelt island antarctica by using a thermomechanical model to predict radar stratigraphy 1 radar detected internal layering contained in some ice divides shows upwarped arches termed raymond bumps the distribution of their amplitude with height can date the onset of divide flow reflecting changes in the basin structure of the ice sheet the distribution depends on rheology surface geometry accumulation rate and temperature conway et al 1999 used an isothermal parameterized ice flow model to estimate a date of 3200 years b p with no error estimate for the onset of divide flow in roosevelt island ross ice shelf which they associated with grounding line retreat of the west antarctic ice sheet no other retreat dating exists for an area of the ross ice shelf distant from geological exposures employing a full thermomechanically coupled transient model we use a direct search to determine which ice rheology best fits the observed bump distribution and surface profile estimating the sensitivity of the dating to model parameters and exploring possible reasons for the observed asymmetry of the surface profile our main results are as follows 1 a best estimate of the date of grounding line retreat near roosevelt island is 3000 years b p bounded by 2300 and 4200 years b p 2 standard rheology power n 3 can only match the observed bump distribution for unrealistic isothermal models 3 for thermomechanically coupled models a high power n 4 in glen s law is required 4 low power rheologies and wind scouring cannot produce correct bump amplitude distributions 5 asymmetric accumulation cannot explain the observed asymmetry of the surface profile 6 modeling isochrones in flanking regions also indicates changes in flow around 3000 years b p

structure composition and bulk properties of upland stream gravels upland gravel bedded streams in the u k have received only scant attention from both hydrologists and sedimentologists but are worthy of further investigation the sedimentology of three small streams in teesdale in the pennines has been examined in detail grain size characteristics bedforms structure composition and packing characteristics of these deposits are described and compared where appropriate with published information it is argued that a fuller appreciation of gravel bed composition and morphology should eventually contribute to an improved understanding of sediment transport and deposition mechanisms and hence to improved accuracy in sediment transport and deposition estimates

the shallow ice approximation for anisotropic ice formulation and limits ice sheet flow modeling is generally based on the shallow ice approximation sia developed for isotropic ice we extend this approximation to anisotropic ice and check its validity for both the isotropic and anisotropic cases by comparing the results of up to second order sia with those obtained from a model solving the full set of mechanical equations the theory is developed for a constitutive relation describing newtonian and non newtonian behavior but numerical results are shown only for a newtonian behavior the results are compared for plane flow of isothermal ice under steady state conditions sia gives an excellent representation of the newtonian flow of an isotropic ice sheet and a good description for the anisotropic case the zero order approximation is sufficient to describe ice flow precisely over flat bedrock for the anisotropic and isotropic cases even close to an ice divide for uneven bedrock the second order sia gives excellent results for isotropic ice and acceptable results for anisotropic ice finally we quantify the error introduced by using an enhancement factor to represent the anisotropy when longitudinal stresses are taken into account

the frontier mountain meteorite trap antarctica the frontier mountain blue ice field is an important antarctic meteorite trap which has yielded 472 meteorite specimens since its discovery in 1984 remote sensing analyses and field campaigns from 1993 to 1999 have furnished new glaciological data on ice flow ice thickness bedrock topography ice ablation and surface mass transport by wind along with detailed descriptions of the field situation at the trap this solid set of data combined with an updated meteorite distribution map and terrestrial ages available from literature allows us to better describe the nature of the concentration mechanism in particular we observe that the meteorite trap forms in a blue ice field 1 located upstream of an absolute and a shallow sub ice barriers 2 characterized by compressive ice flow with horizontal velocities decreasing from 100 to 10 cm year on approaching the obstacle 3 undergoing mean ablation rates of 6 5 cm year 4 nourished by a limited snow accumulation zone extending 20 km upstream of the blue ice area we also draw the following conclusions 1 the origin of the meteorite trap can be explained according to the present day glaciological situation 2 the meteorite concentration develops according to the general principles of the ice flow model 3 the accumulation model can be described as stagnant ice or slow moving ice against an absolute and submerged barriers according to the descriptive schemes present in literature 4 the frontier mountain ice field is an effective trap for meteorites weighing more than 200 g for smaller masses the combination of wind and glacial drift may remove meteorites in less than a few tens of thousands of years 5 although the activation age of the frontier mountain trap is not yet constrained we infer that one of the most important findsites may be as old as 50 ka predating the last glacial maximum

bed topography and surges in ice streams 1 extensive bed topography at the scale of the ice thickness occurs frequently in the form of drumlins by analogy with hard bed sliding ice flow over this type of topography leads to the generation of drag on the ice particularly when ice flow is rapid a crucial difference with classical nye kamb sliding is that the upper free surface of the ice has a significant effect on the sliding process through the formation of a standing wave using a theoretical model we demonstrate that the presence of this wave introduces a non linearity into the sliding motion which can lead to multiple sliding velocities for the same large scale ice stream geometry and that switches between these velocities can cause surging behavior

ice flow dynamics and meteorite collection in antarctica abstract typical targets for meteorite searches in antarctica are blue ice fields in front of major bedrock obstacles where the meteorite bearing ice slows down is uplifted and exhumed by ablation however in some areas collection occurs in the lee of topographic barriers as documented in the downstream side of the frontier mountain a major emerged bedrock barrier in this study we present an analogue modelling study of ice flow at frontier mountain which has been chosen as example study area because an exceptionally complete set of glaciological data is available to investigate the causes leading to meteorite concentration results of modelling show how the boundary conditions mainly the bedrock topography control ice flow and how this flow coupled to high ablation may give rise to a stable meteorite trap downstream of major topographic obstacles modelling results support the contention that as erosion is not a mountain builder ablation does not represent a driving force in ice flow dynamics although it plays a significant role in shaving stranding surfaces and exhuming meteorites

formation and evolution of an extensive blue ice moraine in central transantarctic mountains antarctica mount achernar moraine is a terrestrial sediment archive that preserves a record of ice sheet dynamics and climate over multiple glacial cycles similar records exist in other blue ice moraines els

catch a falling star meteorites and old ice a model for the process of meteorite concentration in blue ice regions of the antarctic ice sheet is proposed based on data from near the allan hills and the assumptions that both meteorite influx and glacial flow have been constant the meteorite influx is calculated to be 60 x 10 6 kilogram per square kilometer per year and the age of the exposed ice to be 0 to 600 000 years varying with distance from the allan hills these results are in line with other estimates of influx rate and with measurements of the terrestrial ages of the meteorites providing support for the assumption of steady flow and meteorite influx this may be the oldest sequence of ice in stratigraphic order yet discovered and the results imply that this part of the east antarctic ice sheet has been approximately steady during this time interval

a new fully three dimensional numerical model for ice dynamics the problem of describing ice dynamics has been faced by many researchers in this paper a fully three dimensional model for ice dynamics is presented and tested using an approach followed by other researchers ice is considered a non linear incompressible viscous fluid so that a fluid dynamic approach can be used the model is based on the full three dimensional stokes equations for the description of pressure and velocity fields on the saint venant equation for the description of the free surface time evolution and on a constitutive law derived from glen s law for the description of ice viscosity the model computes the complete pressure field by considering both the hydrostatic and hydrodynamic pressure components it is time evolutive and uses high order numerical approximation for equations and boundary conditions moreover it can deal with both constant and variable viscosity three theoretical tests and two applications to priestley glacier antarctica are presented in order to evaluate the performance of the model and to investigate important phenomena of ice dynamics such as the influence of viscosity on pressure and velocity fields basal sliding and flow over perturbed bedrocks all these applications demonstrate the importance of treating the complete pressure and stress fields

transmission of basal variability to a glacier surface 1 transmission of basal variability to a glacier surface is investigated using analytical models for a linearly viscous medium the three dimensional transient response of the surface to both bedrock undulations and spatial variations in basal slipperiness for perturbations of arbitrary wavelengths is determined using perturbation methods both information transfer toward the surface and lateral transmission of horizontal stresses are strongly affected by the slip ratio that is the ratio of basal sliding to deformational velocity for any mean bedrock slope and above a minimum value of slip ratio the amplitude transfer of bedrock undulations toward the surface has a local maximum at undulation span corresponding to about 3 8 times the mean ice thickness the transmission of basal variability to a glacier surface increases quite significantly with increasing slip ratio this explains why the surfaces of fast flowing ice streams are more undulating than the slower moving bordering areas at slip ratios higher than about 100 the flow of glaciers and ice sheets becomes nonlocal in the sense that surface velocities and buildup and propagation of surface undulations cannot be calculated accurately on the basis of local thickness and slope using linearized long wave theories at these slip ratios instead of the more accurate arbitrary wavelength theory gives estimates of decay times that are an order of magnitude too small and phase velocities several times too large the problem of the propagation and decay of small amplitude surface undulations on glaciers in three dimensions is solved small amplitude surface waves on glaciers are strongly diffusive and dispersive redistribution of mass on ice sheets and glaciers is a diffusion process and it is misleading albeit not mathematically incorrect to describe the reaction of glaciers to surface perturbations in terms of a wave propagation

physical modeling of the influence of bedrock topography and ablation on ice flow and meteorite concentration in antarctica 1 three dimensional laboratory physical experiments have been used to investigate the influence of bedrock topography and ablation on ice flow different models were tested in a plexiglas box where a transparent silicone simulating ice in nature was allowed to flow experimental results show how the flow field in terms of both flow lines and velocity and variations in the topography of the free surface and internal layers of the ice are strongly influenced by the presence and height of bedrock obstacles in particular the buttressing effect forces the ice to slow down rise up and avoid the obstacle the higher the bedrock barrier the more pronounced the process only limited uplift of internal layers is observed in these experiments in order to exhume deep material embedded in the ice ablation simulated by physically removing portions of silicone from the model surface to maintain a constant topographic depression must be included in the physical models in this case the analogue ice replenishes the area of material removal thereby allowing deep layers to move vertically to the surface and severely altering the local ice flow pattern this process is analogous to the ice flow model proposed in the literature for the origin of meteorite concentrations in blue ice areas of the antarctic plateau

a 14 year mass balance record of a blue ice area in antarctica accumulation and ablation rates over an antarctic blue ice area spanning the 14 year period 1988 2002are presented data were obtained by direct stakemeasurements large spatial and temporal variations in the net balance were observed without any clear trend over the entire period there are marginally significant increases in snow accumulation and in ablation in the blue ice area farthest from the equilibrium zone both at the 95 confidence level the snow blue ice transition zone shows no change over the entire period of observation andtheblue ice area near the zone showsno change inablation rate over the 14 year period the mass balance gradient in scharffenbergbotnen may have increased during the period 1988 2002 however the changes are small especially when compared with the changes observed elsewhere in antarctica even relatively close to the blue ice area this may indicate that the blue ice areas are relatively stable to changes in accumulation rate and possibly temperature

experiments on the effect of hydrograph characteristics on vertical grain sorting in gravel bed rivers 1 desert ephemeral gravel bed streams typically have bed surfaces that are relatively unarmored compared to the substrate below while gravel bed streams in humid and snowmelt areas typically have well armored surfaces the degree of armoring can be characterized in terms of an armor ratio defined as the ratio of the surface median size to the substrate median size a set of field data shows desert ephemeral gravel bed streams with armor ratios ranging from 0 5 to 2 4 and with an average value of 1 2 the armor ratio of snowmelt fed gravel bed streams in the same set ranges from 2 to 7 with an average value of 3 4 the reason for this difference is sought in terms of differing hydrological characteristics and sediment supply regimes thirteen experiments were conducted to study the formation of armoring under a range of hydrological conditions the experiments have two limiting cases a relatively flat hydrograph that represents conditions produced by continuous snowmelt and a sharply peaked hydrograph that represents conditions associated with flash floods all constant hydrograph experiments developed a well armored structured surface while short asymmetrical hydrographs did not result in substantial vertical sorting all symmetrical hydrographs show some degree of sorting and the sorting tended to become more pronounced with longer duration sediment supply appears to be a first order control on bed surface armoring while the shape of the hydrograph plays a secondary role

on the glaciological meteorological and climatological significance of antarctic blue ice areas this paper presents the current state of our knowledge about antarctic blue ice areas scattered widely over the antarctic continent yet covering only approximately 1 of its surface area blue ice areas have some peculiar characteristics that make them of particular interest for planetary geologists glaciologists meteorologists and climatologists for instance blue ice areas are subject to net ablation mainly by sublimation since any accumulation is easily swept off their smooth surfaces by winds moreover they are significantly warmer than their surroundings and the wind over them is generally more gusty an important feature of blue ice is its relatively low albedo 0 56 compared with that of snow 0 80 blue ice is probably considerably smoother than snow in the aerodynamic sense with roughness lengths as low as 3 10 6 m which indicates that blue ice is probably the only permanent natural surface over which the air flow is mainly in the aerodynamic smooth regime further since it is likely that very old ice is exposed at the surface of some blue ice areas this ice would be of great interest for paleoclimatic studies it has also been suggested that blue ice areas are sensitive to climate change this would make them useful as antarctic climate indicators because they can easily be monitored by satellite finally this paper addresses several gaps in our knowledge about blue ice areas and gives suggestions for future blue ice area research

a model of two phase bedload transport in an oregon coast range stream a descriptive model for bed material routing in small sand and gravel bedded channels with sequences of pools and armoured riffles was developed aspects of the model are demonstrated at flynn creek a 202 ha third order drainage in the oregon coast range assuming that channel geometry sediment transport competence and the availability of sediments for transport are all non uniform in the downstream direction the model described bedload transport as occurring in two phases phase i involves the transport of fine predominantly sand sized bed materials over stable gravel armoured riffles phase ii occurs at higher flows that can entrain riffle armour and transport riffle sediments in addition to phase i sediments phase i bedload transport was sampled during three moderate flow events a power function related phase i bedload transport to water discharge the relationship was consistent between storms and between two successive years phase ii bedload transport sampled during a 1 8 yr return period streamflow event in february 1979 proved non uniform in a downstream direction and unsteady over time at a given stream discharge transport rates closely correlated with the rapid scour and redeposition of the riffle at the bedload sampling cross section transport peaks corresponded to scour and transport decreased greatly during deposition the transport of large 12 5 mm diam bed material increased up to 12 fold during riffle scour

bedload sheets in heterogeneous sediment field observations in streams with beds of coarse sand and fine gravel have revealed that bedload moves primarily as thin migrating accumulations of sediment and coarse grains cluster at their leading edge these accumulations are one or two coarse grains high and are much longer 0 2 0 6 m long in sand 0 5 2 0 m in fine gravel than their height the authors propose the term bedload sheet for these features and the authors argue that they result from an instability inherent to bedload movement of moderately and poorly sorted sediment in essence coarse particles in the bedload slow or stop each other trap finer particles in their interstices and thus cause the coarse particles to become mobile again bedload sheets develop on the stoss side of dunes causing the dune to advance incrementally with the arrival of each sheet successive deposition of coarse sediment from the leading edge followed by fine sediment may generate the grain size sorting that distinguishes cross bedding available flume experiments and field observations indicate that bedload sheets are a common but generally unrecognized feature of heterogeneous sediment transport

the relative stability of salmon redds and unspawned streambeds where female salmon build nests redds streambed material is mixed fine sediment is winnowed and bed material is moved into a tailspill mound resembling the shape of a dune completed redd surfaces are coarser and better sorted than unspawned beds which is thought to increase redd stability because larger grains are heavier and harder to move and sorting increases friction angles for mobility however spawning also loosens sediment and creates topography that accelerates flow which can increase particle mobility we address these factors controlling the relative stability of redds and unspawned beds in flume experiments where redds were constructed with a technique that mimics the nesting behavior of female salmon although redds exhibited relatively coarse surfaces measured entrainment forces indicate particle loosening by spawning lowered grain resistance to motion by 12 37 on average compared to unspawned beds in addition for the same discharges boundary shear stress was 13 41 higher on a redd due to flow convergence on the tailspill visual measurements of particle entrainment further indicated redd instability as bed average shear stress was 22 lower at incipient motion and 29 lower at the discharge that mobilized all grain sizes on a redd overall results demonstrate redds are unstable compared to unspawned beds which increases the risk of scour for buried eggs but may facilitate fine sediment flushing and improve the quality of spawning gravels for future generations of spawners therefore managing salmon returns to increase streambed disturbance may be an effective tool for reducing sedimentation impacts on salmon reproduction this article is protected by copyright all rights reserved

on why gravel bed streams are paved bedload transport in poorly sorted gravel bed streams downstream of dams is considered bedload and typical bed material subpavement size distributions are observed to be similar it follows that the coarse half of the subpavement moves through a reach at a rate near that of the fine half since coarser grains are intrinsically less mobile than fine grains it follows that some mechanism must act to nearly equalize mobility it is hypothesized that the pavement seen in gravel bed streams at low flow is in fact in place during typical transport events capable of moving all available sizes this pavement can provide the equalizing mechanism by exposing proportionally more coarse grains to the flow field data are used to quantify this concept and to develop a predictive relation for river pavement the model indicates that pavement should be absent in most sand bed streams in agreement with observation

theory of fine sediment infiltration into immobile gravel bed a theoretical model is developed to describe the process of fine sediment infiltration into immobile coarse sediment deposits the governing equations are derived from mass conservation and the assumption that the amount of fine sediment deposition per unit vertical travel distance into the deposit is either constant or increases with increasing fine sediment fraction model results demonstrate that fine sediment accumulation decreases rapidly with depth into coarse substrate initially void of fine sediment which is consistent with experimental observations that significant fine sediment infiltration occurs to only a shallow depth comparisons of the theory with flume data indicate that the model adequately reproduced the weighted averaged fine sediment fraction values from experiments an early model developed by sakthivadivel and einstein for fine sediment infiltration is in part based on the generally incorrect assumption that intragravel flow remains constant following fine sediment infiltration applying a correction to the sakthivadivel and einstein model based on alternate hypothesis that introgravel flow is driven by a constant head gives similar results as the proposed model

flow law hypotheses for ice sheet modeling ice flow modeling requires a flow law relating strain rates to stresses in situ but a flow law cannot be measured directly in ice sheets microscopic processes such as dislocation glide and boundary diffusion control both the flow law for ice and the development of physical properties such as grain size and c axis fabric these microscopic processes can be inferred from observations of the physical properties and the flow law can then be estimated from the microscopic processes a review of available literature shows that this approach can be imperfectly successful interior regions of large ice sheets probably have depth varying flow law constants with the stress exponent n for power law creep less than 3 in upper regions and equal to 3 only in deep ice n probably equals 3 through most of the thickness of ice shelves and ice streams

dating antarctic blue ice areas using a novel ice flow model 1 we present a new type of flow model suitable for antarctic blue ice areas with application to dating ice for paleoclimate purposes the volume conserving model uses field data for surface velocities mass balance and ice thickness along a flow line with parameterized variation of ice rheology with depth to produce particle trajectories and isochrones the model is tested on the contrasting allan hills near western ice field and the scharffenbergbotnen blue ice fields in antarctica by comparing predicted ages with ages inferred from meteorites and 14c data during the glacial periods ice surface velocities at the allan hills must have been 25 less and accumulation rates 50 less than present day values in order to match meteorite ages in contrast scharffenbergbotnen velocities have probably been fairly constant over time due to the atypical valley where it resides

the impact of biological bedforms on near bed and subsurface flow a laboratory evaluated numerical study of flow in the vicinity of pits and mounds the complex surface topography of river substrates controls near bed hydraulics and drives the exchange of subsurface and surface flow in rivers the topographic structures that are studied are usually formed by the flow but it is known that many animals also create biogenic bedforms such as pits and mounds here a large eddy simulation les model of flow over a pit and a mound is evaluated with flume experiments the model includes actual bedform topography and the topographic complexity of the surrounding bed surface subsurface grains are organized in a body centered cubic packing arrangement model evaluation showed strong agreement between experimental and modelling results for velocity r2 0 8 and good agreement for reynolds stresses r2 0 7 which is comparable to other similar studies simulation of the pit shows that the length of the downwelling region is smaller than the upwelling region and that the velocity magnitude is higher in the downwelling region simulation of the mound reveals that the flow is forced into the bed upstream of the mound and re emerges near the top of the mound the recirculation zone is limited at the leeside of the mound with increasing reynolds number the depth of the upwelling region at the leeside of the mound increases the analysis of shear stress indicates that sediments on the upstream edge of the pit and on the downstream face of the mound are relatively unstable these results demonstrate the effect of biogenic structures on the near bed flow field hyporheic exchange and sediment stability

meteorites constrain the age of antarctic ice at the frontier mountain blue ice field northern victoria land abstract we show that meteorites can provide chronological constraints upon the age of the ice cropping out at the frontier mountain meteorite trap antarctica when their terrestrial age is placed in a glaciological context amongst the over 700 meteorites found so far frontier mountain fro 84001 99028 93005 and 93054 were most likely not wind drifted across the ice field since their masses 772 1665 g are much heavier than the local 200 g wind transport threshold the four meteorites were found along a stretch of ice where a representative section of the frontier mountain blue ice crops out based on the bedding of englacial tephra layers the structure of the ice along the section appears to be essentially an up glacier dipping monocline the 14 c terrestrial age of fro 8401 99028 and 93005 are 13 2 21 3 and 27 2 ky respectively the 41 ca 36 cl age of fro 93054 is 40 10 ky the terrestrial ages of the four meteorites increase from the top to the bottom layers of the monocline this geographic distribution is best explained by delivery of meteorites at the ice surface through the ice flow model i e englacial transport from the snow accumulation zone and exhumation in the blue ice area through ablation rather than direct fall since the effect of ablation in decoupling terrestrial ages of meteorites and the age of the ice on which they sit must have been minor most likely 7 ky based on the local ice dynamics we conclude that the age of the bulk of the ice body currently under ablation at frontier mountain is up to 50 ky old this result has implications on both the meteorite concentrations mechanism at frontier mountain and the regional ice dynamics

unravelling the high altitude nansen blue ice field meteorite trap east antarctica and implications for regional palaeo conditions abstract antarctic blue ice zones the most productive locations for meteorite recovery on earth contain old ice that is easily accessible and available in large quantities however the mechanisms behind these meteorite traps remain a topic of ongoing debate here we propose an interdisciplinary approach to improve our understanding of a meteorite trap in dronning maud land east antarctica on the nansen blue ice field meteorite trap 2600 3100 m above sea level where more than half of the asuka meteorites have been collected based on 185 surface blue ice samples one of the largest observed spatial patterns in oxygen isotopic variation to date is found relying on meteorites for which the terrestrial ages are determined using 14c and 36cl this surface ice is interpreted to date from the last interglacial up to the present day by combining state of the art satellite derived surface velocities surface mass balance modelling and ice flow modelling we estimate that about 75 85 of the meteorites found on the ice field were supplied by ice flow after entering the ice sheet in an accumulation area of a few hundred square kilometres located south upstream of the ice field less than 0 4 new meteorites per year are supplied to the ice field through ice flow suggesting that the hundreds of meteorites found 25 years after the first visit to this ice field mostly represent meteorites that were previously not found rather than newly supplied meteorites by combining these findings the infall rate of meteorites from space is estimated which is in line with values from the literature but situated at the higher end of the range a comparison of the oxygen isotopic variation of the surface blue ice to that of the european project for ice coring in antarctica epica dronning maud land edml ice core located 750 km to the west at the same elevation suggests that the regional changes in topography have been relatively limited since the last interglacial supporting theories of an overall stable east antarctic ice sheet eais over this time period

sediment transport and resulting deposition in spawning gravels north coastal california incubating salmonid eggs in streambeds are often threatened by deposition of fine sediment within the gravel to relate sedimentation of spawning gravel beds to sediment transport infiltration of fine sediment 2 mm in diameter into clean gravel beds bed material size distributions scour fill depths and sediment transport during 10 storm flow events were measured in three streams of north coastal california although suspended sediment comprised most 75 94 of the clastic load during storm flows bed load material 0 25 2 mm accounted for most 70 78 of the fine sediment accumulated in experimental gravel implanted in the streambeds sand trapped in the interstices of the top several centimeters formed a seal that impeded deeper deposition of very fine sand and finer material the seal was responsible at least in part for a decrease in the rate of fine sediment accumulation with increasing cumulative bed load transport areas of the streambeds commonly scoured or filled 0 1 m or more during storm flows and thus scour and fill commonly created a sandy layer at least as thick as the seal formed by sediment infiltration scour could erode eggs laid in the bed and expose deeper levels of the bed to infiltration by fine sediment but at the same time could allow fine sediment to be winnowed away great temporal and spatial variation in sedimentation in these streams suggests that individual storms of moderate size pose a threat to eggs in many but not all areas selected by fish for spawning

flow law for ice in polar ice sheets theories of glacier flow are based commonly on the assumption that ice is not a newtonian fluid but has a non linear stress dependent viscosity1 5 here we re examine the spreading of antarctic ice shelves and suggest that the data cannot define a unique flow law tilt measurements in four boreholes in both the arctic and antarctic seem to show that a linear flow law e a1 where a1 10 15 s 1 pa 1 at 28 c where e is the effective strain rate and the effective shear stress may be just as appropriate for describing the flow of polar ice sheets

basal perturbations under ice streams form drag and surface expression classical sliding theories consider ice sliding over obstacles which are much shorter than the thickness of overlying ice here we present a theory which considers form drag generated under ice streams by large obstacles such as subglacial bedforms which may have lengths comparable to ice thickness we also investigate how perturbations at the surface of an ice stream can be generated by such bedforms and develop a mathematical framework for separating the effects of such local kilometre scale variations in ice flow from the bulk flow of the ice stream

the incidence and nature of bedload transport during flood flows in coarse grained alluvial channels a continuous record reveals that the incidence of bedload in a coarse grained river channel changes from flood to flood long periods of inactivity encourage the channel bed to consolidate sufficiently so that bedload is largely confined to the recession limb of the next flood wave but when floods follow each other closely the bed material is comparatively loose and offers less resistance to entrainment in this case substantial amounts of bedload are generated on the rising limb this is confirmed by values of bed shear stress or stream power at the threshold of initial motion which can be up to five times the overall mean in the case of isolated floods or those which are the first of the season this produces a complicated relationship between flow parameters and bedload and explains some of the difficulties in establishing bedload rating curves for coarse grained channels besides this the threshold of initial motion is shown to occur at levels of bed shear stress three times those at the thresholds of final motion this adds further confusion to attempts at developing predictive bedload equations and clearly indicates at least one reason why equations currently in use are unsatisfactory bedload is shown to be characterized by a series of pulses with a mean periodicity of 1 7 hours in the absence of migrating bedforms it is speculated that this well documented pattern reflects the passage of kinematic waves of particles in a slow moving traction carpet the general pattern of bedload including pulsations is shown to occur more or less synchronously at different points across the stream channel

entrainment of gravel from naturally sorted riverbed material the critical dimensionless shear stress ci required to entrain a given size particle di was computed from extensive bedload transport measurements in three self formed rivers that have naturally sorted gravel and cobble bed material the value of ci was determined to be significantly affected by the size distribution of the riverbed material for bed particles between 0 3 to 4 2 times the median diameter of the subsurface bed material d50 the average critical dimensionless shear stress ci is equal to ci 0 0834 di d50 0 872 with a coefficient of determination of 0 980 for bed particles larger than 4 2 times d50 ci appears to approach a constant value of approximately 0 020 in a noneroding channel thus ci varies from 0 25 to 0 020 for a given particle depending on the ratio of di to d50 previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0 25 to 0 020 for a given particle size this analysis indicates that virtually all of the variation is due to differences in the subsurface bed material size distribution this analysis shows that ci varies almost inversely proportional to the particle diameter for a nonuniform bed material therefore bed particles between 0 3 to 4 2 times d50 are entrained at nearly the same discharge an investigation of the hydraulic characteristics and bed material mobility of 24 self formed gravel bed rivers in colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less particles as large as the 90th percentile fraction were entrained by the bankfull discharge in nine of the rivers studied consequently in these rivers a significant fraction of the riverbed was entrained rather frequently over a period of years although the transport rate was very small