surface based transport model for mixed size sediment we present a transport model for mixed sand gravel sediments fractional transport rates are referenced to the size distribution of the bed surface rather than subsurface making the model completely explicit and capable of predicting transient conditions the model is developed using a new data set of 48 coupled observations of flow transport and bed surface grain size using five different sediments the model incorporates a hiding function that resolves discrepancies observed among earlier hiding functions the model uses the full size distribution of the bed surface including sand and incorporates a nonlinear effect of sand content on gravel transport rate not included in previous models the model shares some common elements with two previous surface based transport models but differs in using the full surface size distribution and in that it is directly developed from a relatively comprehensive data set with unambiguous measurement of surface grain size over a range of flow transport rate and sediments

using sediment impact sensors to improve the morphological sediment budget approach for estimating bedload transport rates constructing a sediment budget from morphological channel change typically measured using repeat cross sectional surveys has proven a useful technique for estimating mean bedload transport rates in gravel bed rivers we apply and suggest improvements to the approach using new data sources from an intensely monitored 5 6 km reach of an upland gravel bed river in northern england sediment impact sensors were placed in the bed to record the instantaneous passage of grains during transport events these sensors provide valuable information for the budget including 1 a downstream budget limit from which the rest of the budget can be calculated 2 estimates of sediment input from tributaries and 3 a better indication of active transport time used in the calculation of average transport rates further information collected from field studies include estimates of lateral inputs from the banks using field surveys and bank erosion pins and estimates of the variability of sediment porosity from bed grain size distributions budget calculations for different locations and time periods demonstrate large spatial and temporal variabilities in bedload transport rates over a short distance 3 km whilst some uncertainty in the new data sources remains this work highlights some limitations of the traditional budget approach and some potential solutions in particular the value of sediment impact sensors for sediment transport studies

sampling techniques for gravel sized sediments the methods commonly used for sampling the coarser surface layers of gravel bed streams are reviewed it is found that while an areal sample is biased toward the coarser grains compared with a volumetric sample of the same material the conversion formula suggested by a voidless cube model overcompensates for this effect a modified version of the cube model that accounts for porosity is proposed the modified cube model and tests conducted with areal sampling using wax indicate that area by weight analyses can be converted successfully to the equivalent bulk sieve analyses the average depth of the wax samples ranged from d65 d91 increasing with median grain size the equivalence of the grid by number and bulk sieve analyses is reaffirmed

a hybrid method for size characterization of coarse subsurface fluvial sediments this paper introduces and evaluates a novel method for ascertaining the grain size distribution of subsurface sediments that involves profoundly less sampling effort than standard methods it is based on hybrid sampling principles previously applied to the construction of synthetic surface grain size distributions the method is developed from an empirical demonstration of the approximate similarity of surface and subsurface grain size distributions when compared over a common range of sizes subsurface hybrid models are found to provide good facsimiles of grain size distributions de ned using standard criteria and to yield distribution percentiles with millimetre accuracy the technique is presented as an expedient alternative to standard methods for large perennial gravel bed rivers as this is a new technique prudent application is advised in lieu of further investigation copyright 2004 john wiley sons ltd

a 1 d morphodynamic model of postglacial valley incision chilliwack river is typical of many cordilleran valley river systems that have undergone dramatic holocene degradation of valley fills that built up over the course of pleistocene glaciation downstream controls on base level mainly blockage of valleys by glaciers led to aggradation of significant glaciofluvial and glaciolacustrine valley fills and fan deposits subsequently incised by fluvial action models of such large scale long term degradation present a number of important challenges since the evolution of model parameters such as the rate of bedload transport and grain size characteristics are governed by the nature of the deposit sediment sampling in the chilliwack valley reveals a complex sequence of very coarse to fine textural modes we present a 1 d numerical morphodynamic model for the river floodplain system tailored to conditions in the valley the model is adapted to dynamically adjust channel width to optimize sediment transporting capacity and to integrate relict valley fill material as the channel incises through valley deposits sensitivity to model parameters is studied using four principal criteria profile concavity rate of downstream grain size fining bed surface sand content and the timescale to equilibrium model results indicate that rates of abrasion and coarsening of the grain size distributions exert the strongest controls on all of the interrelated model performance criteria while there are a number of difficulties in satisfying all model criteria simultaneously results indicate that 1 d models of valley bottom sedimentary systems can provide a suitable framework for integrating results from sediment budget studies and chronologies of sediment evacuation established from dating

operator bias characterizing stream substrates using wolman pebble counts with a standard measurement template this study characterizes the variability associated with using a standard measurement template to perform wolman pebble counts of coarse stream substrates four 10 count and two 12 count replicate experiments were conducted by five operators at three sample sites within gravel bed streams using a gravel template results show significant differences between different operators sampling the same location as observed in previous studies conducted without a standard sampling template as well as significant variability within each operator s replicate samples variability was highest in clast estimates larger than d50 suggesting that these metrics cannot be consistently derived from wolman pebble count samples variability was also highest in the low order study site suggesting that the sedimentary characteristics of low order streams increase the chance for operator bias in light of our findings we suggest that traditional 100 particle wolman counts be utilized with more caution than is the current norm

stabilizing self organized structures in gravel bed stream channels field and experimental observations stable reticulate structures which we call stone cells have been observed in cobble gravel channel beds with low bed material transport rates experiments show that such structures develop simultaneously with the armor layer during an extended period when flows do not exceed the shields threshold by more than 2 times so that bed material transport is low they are constructed by particles moving from less stable positions into more stable configurations against each other intermediate developments include clusters and stone lines they reduce sediment transport by orders of magnitude and are evidently a major stability promoting mechanism in gravel channels the timescale for their development suggests that the boundaries of many gravel bed channels are not in equilibrium with recent competent flows but reflect the history of recent dominant flows

age relationship of basaltic and andesitic surface compositions on mars analysis of high resolution tes observations of the northern hemisphere 1 previous mars global surveyor thermal emission spectrometer tes results have identified two principal surface compositions on mars basaltic surfaces are found primarily in the southern highlands while andesitic surfaces are found in the northern lowlands and in smaller concentrations distributed throughout the southern highlands in 1 pixel per degree composition maps the exact boundary between the basaltic and andesitic compositions is obscured by large amounts of equatorial surface dust in this work high resolution tes surface spectral data are used to identify and characterize isolated regions of basalt within the equatorial dust regions and northern lowlands generally between 20 s 60 n 0 360 w in an effort to better understand the nature of the boundary between the two compositions the purpose of refining this boundary is to more accurately determine the relationship of the compositional dichotomy with global surface morphology and derived crustal thickness results show that the basaltic surface composition is closely associated with the ancient cratered terrain morphology but is not well correlated with crustal thickness the northern extent of the tes basalt composition is also established with four exceptions there are no significant concentrations 30 surface cover of basalt greater than 400 km2 in area within the northern plains three are located within or adjacent to knobby terrain which has been suggested to be erosional remnants of the cratered highlands the fourth exception is in milankovic crater and may represent a local basalt flow or basaltic material excavated during crater formation results from this work suggest that there is a stratigraphic relationship between the basaltic and andesitic surface compositions with younger andesitic materials overlying older basaltic materials throughout the equatorial and northern plains regions

experimental study of the coarse surface development effect on the bimodal bed load transport under unsteady flow conditions abstractgravel bed rivers display a surface layer that is coarser than the substrate lining below this armored surface the armored layer that is developed under certain flow conditions may be attributed as water worked sediment bed both the threshold of incipient motion and the bed load transport are affected by the presence of this layer in this study the coarse surface development and its effect on the sediment transport were investigated experimentally in a rectangular flume four antecedent flow rates were combined with an input hydrograph without sediment feeding the reference shear stress of individual fractions and the grain size distribution were determined after the development of the coarse surface due to the antecedent flow then a triangular shaped hydrograph was generated for both the intact surface and the coarse surface cases it was seen that the maximum bed load transport values obtained in unsteady flow experiments were highly dependent on the coarse surface formed by the anteceden

taking the river inside fundamental advances from laboratory experiments in measuring and understanding bedload transport processes bedload transport in gravel bed rivers impacts channel stability the lifespan of hydraulic structures physical components of aquatic habitat and long term channel evolution field measurements of bedload transport are notoriously difficult which precludes understanding many of the processes and mechanics associated with grain motion such uncertainties are exacerbated when using bedload transport equations most of which were derived using data from a single river or set of laboratory flume experiments recently laboratory experiments have focused on better quantifying the processes that impact bedload fluxes which can then be used to improve sediment transport predictions we highlight recent advances in laboratory instrumentation that can be used in bedload transport studies in particular more accurate ways to measure bedload fluxes near bed turbulence bed grain sizes and topography hold great promise laboratory experiments have also fundamentally improved our understanding of the influence of sediment supply and armoring processes on bedload fluxes and channel conditions the importance of flow hydrographs in controlling total bedload transport rates and bedload hysteresis has also been demonstrated using flume experiments finally many details about the mechanics of grain motion including flow turbulence bed arrangement and particle transport statistics are only possible through laboratory investigations and we feature key knowledge gaps that can be improved with further study

first in situ investigation of a dark wind streak on mars 1 prominent low albedo wind streaks issue from embayments at the north end of victoria crater in meridiani planum the site of surface investigations by the mars exploration rover opportunity opportunity was sent to examine the darkest of these wind streaks and compare it to the adjacent soil in the brighter corridor between the streaks two nearby sites inside and outside of the streak were selected for visible and infrared remote sensing and for elemental abundance measurements and images of the microscopic soil morphology the rover was next sent to study the interaction of the wind streak with an obstacle a small rock near the source of the streak opportunity s observations suggest that the victoria wind streaks are deposits of basaltic sand blown out of the crater from the dark dunes nestled below the crater rim particularly at the base of the alcove leading up to the darkest streak no local sources of sand have been identified within the victoria crater wall rock suggesting that the sand is supplied from outside the crater and is presently escaping its temporary topographic trap this process presents a possible explanation for the serrated margin of victoria crater through abrasion of the soft rock as trapped sands are blown out of the crater and carve alcoves under various seasonal winds some indication of the rate of activity of these wind streaks will be provided by continued monitoring of the erasure of the rover s tracks

the sedimentology and dynamics of crater affiliated wind streaks in western arabia terra mars and patagonia argentina abstract wind streaks comprise recent aeolian deposits that have been extensively documented on venus earth and mars martian wind streaks are among the most abundant surface features on the planet and commonly extend from the downwind margins of impact craters previous studies of wind streaks emerging from crater interior deposits suggested that the mode of emplacement was primarily related to the deposition of silt sized particles as these settled from plumes we have performed geologic investigations of two wind streaks clusters one situated in western arabia terra a region in the northern hemisphere of mars and another in an analogous terrestrial site located in southern patagonia argentina where occurrences of wind streaks emanate from playas within maar craters in both these regions we have identified bedforms in sedimentary deposits on crater floors along wind facing interior crater margins and along wind streaks these observations indicate that these deposits contain sand sized particles and that sediment migration has occurred via saltation from crater interior deposits to wind streaks in arabia terra and in patagonia wind streaks initiate from crater floors that contain lithic and evaporitic sedimentary deposits suggesting that the composition of wind streak source materials has played an important role in development spatial and topographic analyses suggest that regional clustering of wind streaks in the studied regions directly correlates to the areal density of craters with interior deposits the degree of proximity of these deposits and the craters rim to floor depths in addition some but not all wind streaks within the studied clusters have propagated at comparable yearly earth years rates extensive saltation is inferred to have been involved in its propagation based on the studied terrestrial wind streak that shows ripples and dunes on its surface and the martian counterpart changes orientation toward the downslope direction where it extends into an impact crater

new views of mars eolian activity materials and surface properties three vignettes from the mars global surveyor mars orbiter camera prior to the mars global surveyor mgs mission a very general view had emerged in which martian surface materials were seen as consisting of a mixture of bright dust dark sand and rocks the configuration of these materials and the winds that transport and deposit sand and dust have been thought to be directly linked to the albedo patterns that have been observed on mars for centuries high spatial resolution images 1 4 20 m pixel obtained by the mgs mars orbiter camera moc between september 15 1997 and july 4 1999 provide new information about the physical nature of the windblown material on the martian surface and show that the pre mgs view was much too simple in addition to bright dust and dark sand moc images show evidence of bright sediment that can be transported by saltation e g sand and dark material that can be transported in suspension e g silt new views of eolian wind streaks in daedalia planum show that part of this region thought to be mantled by bright dust based upon viking and mariner observations is instead covered by a thin veneer of bright windblown sand moc images of sinus sabaeus and parts of syrtis major two regions thought to be sandy based upon viking era observations exhibit thick mantles that are inferred to consist of fine grained sediment deposited from eolian suspension low albedo wind streaks in western arabia terra are also dark mantles and their association with eroded crater floors and megaripples dunes on these floors suggest that these particular wind streaks are deposits of silt sized material that was only briefly suspended before settling to the surface moc images also show evidence that some eolian dunes are active on mars today i e in 1998 and 1999 the evidence for activity is largely based upon wind and avalanche induced streaks superposed on or eroded into seasonal frost on high latitude dune fields moc images show that some other dunes are inactive but the albedo of dunes relative to surrounding terrain is not a good indicator of dune activity because some inactive dunes are not mantled by dust inactive dunes are best identified by superposed features such as impact craters landslide deposits and yardangs

biogeochemical controls on palaeoceanographic environmental proxies an introduction the current volume samples a selection of papers presented at the geological society of london meeting on biogeochemical controls on palaeoceanographic proxies held at burlington house london uk on 3 4 october 2005 the aim of the meeting was to bring together palaeontologists geochemists and palaeoceanographers who could contribute evidence that when considered together would better constrain the proxies that are used for palaeoclimate reconstruction an improved understanding and quantification of past climate change and the processes that force climate to change has a fundamental role to play in constraining model projections of future climate e g hegerl et al 2006 but it remains a huge challenge this is because key climate variables such as temperature and ocean salinity cannot be observed in a world which no longer exists but must instead be teased from proxies in the geological and ice records there are numerous proxy archives but one of the most important currently lying at the forefront of palaeoceanographic research is the biogeochemical composition of sediment records this publication consists of 11 papers which deal with various aspects of biogeochemical proxies and their interpretation in terms of past climate seven of these specifically focus on the foraminifera what are proxies primarily these are biogenic components which have a close relationship to environmental parameters and may be identified as so called proxy variables wefer et al 1999 providing measurable descriptors of key climatic and environmental variables the methods commonly employed in palaeoceanography have their origins in the biological chemical and physical sciences palaeoceanography therefore represents a relatively young and truly crossdisciplinary field of research at the time of writing an excellent new book entitled proxies in late cenozoic paleoceanography has been published hillaire marcel de vernal 2007 providing a comprehensive review of the subject this volume begins with an overview by james austin highlighting some of the most important biological and geochemical proxies and outlining their contribution to our understanding of the ocean climate system we anticipate that this review article will provide an accessible introduction to the topic as well as an insight into a wide range of analytical methods the second paper by williams highlights some of the fundamental features of the biomineralization process initially contrasting these with inorganic mineral formation before selecting examples from corals foraminifera and coccoliths the paper by zeebe et al develops a modellingbased approach to improve our understanding of stable carbon oxygen and boron isotopes as well as magnesium incorporation into foraminiferal calcite shells the approach adopted by these authors is based on an integrated understanding of the inorganic chemistry inorganic isotope fractionation and biological controls that determine palaeo tracer signals in organisms used in climate reconstructions they argue very convincingly that the integration of laboratory experiments field and culture studies theoretical considerations and numerical modelling holds the key to the method s success these authors demonstrate that a mechanism based understanding is often required before primary climate signals can be extracted from the geological record because these signals can be heavily overprinted by secondary nonclimate related phenomena one such phenomenon is foraminifer test diagenesis which pearson burgess highlight in their study of high latitude eocene sites these authors illustrate the textures using a combination of reflected light microscopy and high resolution scanning electron microscopy highlighting the fact that foraminifer tests are prone to diagenetic recrystallization on a micron scale which can affect their geochemical composition for example diagenetic calcite added on the sea floor or in shallow burial at low temperature may result in the severe underestimation of the apparent tropical sea surface temperatures obtained from calcite foraminifera e g pearson et al 2001 next two papers consider the growth rates and growth patterns in biogenic calcite the first of these by schmidt et al provides an overview of the effect of growth rate on the trace element

surface sampling in gravel streams the accuracy of studies concerning the characteristics of gravel bed waterways is often dependent upon the techniques used to sample and quantify the material found on the channel boundary due to the vertical stratification present in the riverbed the use of sampling techniques that remove only the particles within a thin surface layer is often necessary attributes of clay and grid sampling are considered in the present work criteria for determining the minimum sample size for a desirable level of accuracy are also presented many of the current surface sampling techniques truncate either the upper or lower size range of particles truncation of part of a size distribution not only limits the available information but can also bias the rest of the distribution the present work rectifies this problem by proposing the use of a hybrid technique that can sample the entire size range of either dry or submerged bed material results of field tests that utilize this new method are described in detail

a new sampler for extracting bed material sediment from sand and gravel beds in navigable rivers grain size distributions of bed material sediment in large alluvial rivers are required in various scientific and management applications but characterizing gravel beds in navigable rivers is hampered by difficulties in sediment extraction the newly developed and preliminarily tested sampler reported here can extract sediment from a range of riverbeds the 36 23 28 cm stainless steel toothed sampler is deployed from and dragged downstream by the weight of a jet boat and it improves upon previous samplers that are unable to penetrate gravel bed surfaces have small apertures and or cannot retain fine sediment the presented sampler was used to extract 167 bed material sediment samples of up to 16 kg dry weight with an average sample size of 6 kg from 67 cross sections spanning 160 river kilometres along the sacramento river it was also tested at three sites on a subaerial bar to compare surface subsurface and sampler distributions sampler penetration is 5 cm the device collects individual samples that satisfy the criterion for bed material sediment whereby the largest particle comprises no more than 5 of the total sample mass in gravel and sand beds except where the degree of surface armouring is large e g armor ratios 2 and where more than 10 of bed material sediment is composed of grains larger than 64 mm when aggregated samples exceed 15 kg all satisfy the criterion whereby the largest particle comprises no more than 1 of the total sample mass samples closely resemble surface size distributions except where armouring is strong the sampler should be subject to more rigorous field testing but many of its current limitations are expected to become negligible with the advent a larger heavier version of the sampling device copyright 2008 john wiley sons ltd

biases of submerged bulk and freeze core samples freeze coring and bulk sampling are routine methods used to sample subsurface spawning gravel under shallow water both methods have limitations freeze coring is not believed to representatively sample coarse grain sizes and the sample volumes are relatively small conversely when bulk sampling even within an enclosure some fine sediment is suspended and washed away from the sample this paper assesses the biases in sampling performance between the two methods and determines whether the loss of fines that occurs when bulk sampling could be predicted and thus corrected for at six riffles the spawning substrate was sampled under approximately 50 cm of water with a bulk sample and three adjacent freeze cores for each riffle data from the two samples were combined using the method of fripp and diplas 1993 and the resultant composite sample was compared with the original freeze core and bulk samples to assess the relative precision and biases of the two techniques on average the d50 of the bulk samples was 4 mm larger and a one third loss of the 16 mm compared with composite samples based on six samples taken from six riffles the amount of sediment finer than 0 5 mm lost using our bulk sampling technique with an enclosure appears to be predictable and correctable copyright 2005 john wiley sons ltd

step pool and cascade morphology mosquito creek british columbia a test of four analytical techniques the identification and geometric definition of individual cascade and step pool bedforms are investigated in a steep coarse grained mountain stream mosquito creek by testing four analytical techniques visual identification zero crossing bedform differencing and power spectral analysis the test is the first use of these techniques in a headwater stream and the analysis of two bed profiles showed that visual identification was able to i identify ii determine the geometry of and iii classify the type of individual bedforms better than the other methods the other techniques were not able to differentiate step pools from cascades and the large range of grain sizes and bedform heights hampered their ability to consistently identify stepped bedforms the step pool pronounced channel spanning steps that alternate with channel spanning pools and cascade multi tiered partially channel spanning structures morphology in mosquito creek has formed in the last 20 years as fluvial action has restructured its previously engineered revetment lined planar bed the channel bed exhibits a morphologic regularity that power spectral analysis captured as periodic fluctuations in the bed profiles with mean wavelengths slightly greater than those identified by the other methods further the active reorganization of revetment has formed stepped structures with geometries similar i e height to wavelength ratios to stepped features found in natural mountain streams channel slope partially controlled bedform geometry wavelength and height and bedform height weakly controlled individual step spacing but there was no relation between wavelength and grain size d 90

characteristics of gravel bars in ephemeral streams abstract bars are major sediment storage places within channels and much is known about their character from proglacial and temperate regions little is know however about bar characteristics from ephemeral streams this study describes the morphology and composition of simple and compound gravel bars in nahal zin israel an ephemeral desert stream a hierarchy of microscale mesoscale and macroscale structures and bed forms was identified in nahal zin that is similar to the hierarchy described for perennial streams while the hierarchy is similar between arid and perennial streams bar composition patterns are distinct compound bars and unit bars in humid areas are known to have armored surfaces composed of structured gravels whereas the bar surfaces of both unit bars and compound bars in nahal zin were found to lack surface structures and to contain a large amount of sand in ephemeral streams both accreted and dissected compound bars do not display any downstream fining in contrast downstream fining along bars in humid areas is well documented the flashy nature of floods high sediment supply high sediment transport rates and large amount of sand associated with the bars are likely reasons the surfaces were not armored in nahal zin erosion and accretion dominate the bar edges and accretion alone dominates the platform part of the bars in summary sediments in ephemeral streams of the nahal zin type are well mixed unsorted and unlikely to leave a well defined stratigraphic record these characteristics are distinct from their counterparts in proglacial and temperate regions and may be useful when investigating paleoenvironmental conditions

calculation of bed changes in mountain streams simulation of flow and sediment transport in mountain streams is complicated by the presence of high gradients abrupt changes in geometry variations in regime of flow and large roughness elements most of the numerical models to predict aggradation and degradation in alluvial channels have been developed for low gradient rivers this paper is devoted to the development of a numerical model to calculate bed elevation and grain size distribution changes in mountain streams where the maximum bed material size is in the range of boulders an attempt is made to validate the model by using observed field data collected upstream from a small retention dam in a venezuelan stream after calibration of the sediment transport equation reasonable agreement is obtained for the variations in the grain size distribution of the bed surface material an additional application is presented in the cocorotico river a small mountain stream located in the northwest region of venezuela which illustrates the adaptability of the model to handle a case of coarsest bed material removal from the active channel and to simulate the armoring process

geomechanical and geochemical changes during early stages of weathering of karamu basalt new zealand abstract variations in geochemical and geomechanical characteristics were measured through a basalt weathering sequence exposed at an abandoned quarry at karamu near hamilton new zealand geochemical measurements show that at the early stages of weathering fresh to slightly weathered reductions in the concentrations cao and mgo are rapid although they are not accompanied by major mineralogical change during later stages of weathering secondary clay minerals form following well described mineralogical changes fresh basalt is strong uniaxial compressive strength 262 mn m 2 point load index 5 59 with well developed closely spaced columnar jointing slightly weathered material is distinguished in the field only by slight discolouration along joint planes but laboratory tests show a dramatic loss of strength in slightly weathered rock point load index 0 41 cbr 43 a causal relationship between the loss of cations and loss of strength in the early stages of weathering is suggested this stage of weathering is diffusion controlled whereby cations are lost from the constituent primary minerals and replaced by h this process disrupts the lattice structure and causes a marked loss of strength implications are that slightly weathered rock which appears competent in the field is likely to have much lower strength and durability than assumed from field evidence

grain size along two gravel bed rivers statistical variation spatial pattern and sedimentary links a new set of field data facilitates a detailed analysis of variations in bed material grain size within two confluent gravel bed rivers in northeastern british columbia canada a preliminary assessment of grain size variability establishes a basis for examination of the spatial pattern of grain size change standard anova techniques are inappropriate because individual samples have unequal variances and are not normally distributed alternative tests for homoscedasticity and comparison of means are therefore utilized within site between sample variability is not significant the grain size distributions that were obtained at individual sites are therefore representative of the depositional environments that were sampled in both rivers mean grain size does vary significantly between sites and there is therefore a basis for examining the data for spatial patterns such as downstream fining textural variations along the two rivers studied here are complex and show negligible overall fining in over 100 km this is the consequence of a large number of tributary inputs and non alluvial sediment sources which are the legacy of late pleistocene glaciation the identification of lateral sources like these is fundamental for understanding textural changes within rivers the sedimentary link a channel reach between significant lateral sediment inputs provides a means of isolating fluvial maturation processes abrasion and sorting from contingent lateral inputs strong fining trends are apparent in most links and classification of grain size measurements according to their location within particular links greatly improves the statistical explanation of textural variation identification of sedimentary links provides a means of applying models of fluvial fining processes so isolation of link networks will aid the development of basin scale models of textural variation 1998 john wiley sons ltd

identifying the controls over downstream fining of river gravels abstract a wide range of rates of downstream fining in gravel bed rivers has been reported as a consequence explanations of the phenomenon range from those that consider only abrasion to others that consider only selective sediment transport this apparent confusion results in part from inconsistent definitions of downstream fining analysis of results from a small gravel bed stream is used to show how the method of sediment sampling and the percentile used to characterize fining rates can affect the results when fining is adequately defined a theoretical relationship between fining rate and drainage basin area is derived from established alluvial fan relations this relationship is remarkably consistent with a range of studies by numerous authors using often different methods such results suggest that fining is achieved through the mechanism of selective transport but that the overall control on fining patterns is the volume of sediment supplied from within the drainage basin as such the rate of size decline in sedimentary deposits such as river terraces has the potential to be used to infer past changes in climate tectonics and or relative base level models of this are presented and these are shown to be broadly consistent with observations made in an ancient sandstone body full use of changing rates of fining as paleo environmental indicators requires further work on the rate of adjustment of fining profiles with respect to the rates of change in the forcing environmental conditions

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

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

experiments on surface structure and partial sediment transport on a gravel bed eight flume experiments were conducted to study the development of bed surface texture and structure in the presence of partial bed material transport the experiments have two phases a no feed degradational phase followed by a feeding phase a surface structure of irregular reticulate stone nets and clusters was developed before sediment feeding commenced bed load transport equaled or slightly exceeded the fed supply except at the highest feed rate the bed structure was maintained but bed surface texture fined with increasing sediment load the two phenomena may coexist because the largest grains on the bed moved only very sporadically the actual sediment transport rates were much less than the expected rate calculated from the ratio of bed surface grain size to transported grain size the difference reflects the increase in bed stability introduced by the bed structure between 17 and 47 of the bed shear stress is estimated to be carried by the structure 4 being absorbed by the load while the bed grains carried the balance of the stress bed material transport is exceedingly sensitive to bed surface structure and grain size which raises concerns about the realizable precision of grain size measurements and characterization of the structure

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

influence of the sediment supply texture on morphological adjustments in gravel bed rivers the role played by the texture of the sediment supply on channel bed adjustments in gravel bed rivers is poorly understood to address this issue an experimental campaign has been designed flume experiments lasting 96 h in a 9 m long 0 60 m wide have been performed with different sand gravel mixtures as feed textures the response of the surface texture has been found to be highly dependent on the grain size distribution of the feed when the feed texture included gravel the finest fractions of the sediment supply infiltrate beneath the surface conversely sand remains on the surface when the feed texture lacks gravel this different textural response becomes obscured when water discharge increases further the sediment transport rate approaches the feed rate differently depending on the content of gravel in the feed texture when a small proportion of gravel is part of the feed texture bed load transport rate asymptotically approaches the feed rate however when a significant fraction of gravel is part of the feed grain size distribution bed load transport rate approaches the feed rate by following an oscillatory path these findings have been verified in terms of a one dimensional numerical model this modeling reveals that the higher the differences in mobility among the grain sizes contained in the feed texture the more evident is the nonasymptotic transient trend toward equilibrium

one dimensional numerical modeling of sediment transport and bed deformation in open channels 1 a one dimensional numerical model for simulating unsteady flow and sediment transport in open channels is presented and tested the flow hydrodynamics is represented by the shallow water equations and the bed morphodynamics is represented by the exner equation and an additional equation describing the nonequilibrium sediment transport sediment size distribution is represented by the median grain diameter and the standard deviation instead of the usual modeling with multiple particle size classes various methods for computing bed elevation changes at a cross section due to erosion or deposition of sediment are proposed and tested including an innovative approach that relates the spatial pattern of erosion and deposition rates to boundary shear stress distribution which is calculated by the merged perpendicular method an explicit finite difference scheme is employed for solving the water and sediment governing equations the pertinence of the model is examined for two hypothetical cases the model is then tested on one set of laboratory experiments on bed degradation under steady flow showing excellent model data fit and indicating that incorporating a nonequilibrium sediment transport equation into the model structure is an important element in reproducing the bed degradation process finally the model is applied to simulate the morphological changes taking place in the ha ha river quebec after the failure of the ha ha dyke on july 1996 relevant results can be obtained in terms of changes in longitudinal bed profile and cross sectional geometry as well as water levels although some discrepancies are obtained between the simulated and surveyed cross sectional geometries mainly because bank failure and channel widening are not modeled

the form and function of headwater streams based on field and modeling investigations in the southern appalachian mountains headwater streams drain the majority of most landscapes yet less is known about their morphology and sediment transport processes than for lowland rivers we have studied headwater channel form discharge and erosive power in the humid moderate relief valley and ridge and blue ridge provinces of the appalachian mountains field observations from nine headwater 2k m 2 drainage area mixed bedrock alluvial channels in a variety of boundary conditions demonstrate variation with respect to slope area channel initiation basic morphology slope distribution hydraulic geometry substrate grain size and role of woody debris these channels display only some of the typical downstream trends expected of larger lowland rivers variations are controlled mainly by differences in bedrock resistance from the formation level down to short wavelength outcrop scale variations hydrologic modeling on these ungauged channels estimates the recurrence of channel filling discharge and its ability to erode the channel bed two year recurrence discharge is generally larger and closer to bankfull height in the valley and ridge due to low soil infiltration capacity discharge that fills the channel to its surveyed bankfull form is variable generally exceeding two year flows at small drainage areas 0 5 km 2 and being exceeded by them at greater drainage areas this suggests bankfull is not controlled by the same recurrence storm throughout a channel or physiographic region stream power and relative competence are also variable these heterogeneities contrast relations observed in larger streams and illustrate the sensitivity of headwater channels to local knickpoints of resistant bedrock and armoring of channels by influx of coarse debris from hillslopes the general lack of predictable trends or functional relationships among hydraulic variables and the close coupling of channel form and function with local boundary conditions indicate that headwater streams pose a significant challenge to landscape evolution modeling copyright 2005 john wiley sons ltd

visual and infrared observations of wind streaks on mars abstract estimation of surface properties and physical setting of three common martian wind streak types bright dark and splotch related provides constraints on models of the formation and variability of streaks bright streaks form independently of surface properties other than local topography this is consistent with their formation being due to deposition of atmospheric dust in the lee of topographic features although they are widespread on mars dark streaks are noted as variable only in regions near 30 s latitude and elevations between 3 and 7 km and are associated with dark surfaces that have relatively high thermal inertias splotch related streaks occur at elevations between 0 and 6 km and in regions of relatively high thermal inertia splotch related streaks occur near the boundaries of thermally defined regions such as the south polar cap and other areas of either low or high thermal inertia these thermal conditions are responsible for the production of surface winds which form and modify these streaks the source of sidements which form splotch related streaks varies from dunes to well indurated stratified deposits regional studies of the various types in syrtis major syria planum claritas fossae oxia palus mesogea and pettit craters and noachis confirm that the correlations found at the global level occur at regional scales

nature and distribution of surficial deposits in chryse planitia and vicinity mars color images of bright red dust deposits at the mutch memorial station were acquired at variable incidence angles during sol 611 subsolar longitude 70 northern spring season after removing effects due to atmospheric scattering and absorption the data were used to estimate the independent variables in the hapke 1986 photometric function in blue green and red coordinates the vector representing the space radiance factor of the landing site extracted from viking orbiter images acquired on sol 609 is separated by a euclidean distance of only 0 022 units and an angle of only 1 5 from the vector estimated from the station data for the orbiter lighting and viewing geometries this result implies that light reflected from dust exposures dominates the orbiter signal multiplicative and additive atmospheric terms cancel one another and surface roughness is a second order effect in the orbiter data dust radiance factors computed from station data are most like laboratory spectra for fine grained hawaiian palagonite and are indistinguishable from earth based spectra of classical bright areas color composites of orbiter images show that the dust is found immediately south of acidalia planitia and in association with topographic barriers such as craters and cliffs examination of viking infrared thermal mapper data shows that the dust deposits typically do not have distinctive thermal inertia signatures implying that the deposits are optically thick hundreds of micrometers but thinner than the diurnal thermal skin depth centimeters dark gray material with thermal inertia values 8 12 10 3 cal cm 2 s 1 2 k 1 indicative of sand grain sizes 0 5 1 5 mm dominates the acidalia planitia lowlands and parts of kasei vallis this material also occurs as dark streaks extending from craters in xanthe terra and oxia palus space radiance factors of dark gray material are similar to spectra of mafic rock mixed with a minor amount of palagonitelike material material that is darker than but just as red as the dust deposits occurs in lunae planum xanthe terra and oxia palus areas of intermediate elevation multiple phase angle orbiter images suggest that the dark red exposures are a mixture of bright red and dark gray materials with the dark red exposures being relatively rough at a subpixel scale as compared to exposures of the other two materials thus only two types of materials can be detected in the data covering the study area palagonitelike dust and mafic rock fragments thermal inertia values 4 8 10 3 cal cm 2 s 1 2 k 1 for the dark red material are consistent with the presence of fine to medium sand size particles 0 10 0 5 mm such material should be easily moved by winds the lack of aeolian features implies that the dark red deposits are not composed of loose material rather they are probably more eroded versions of blocky soil or duricrust cemented dust and rock fragments exposed at the station both bright red dust aeolian suspension load and dark gray materials saltation traction loads migrate over the dark red substrate the overall distribution of the surficial units is controlled by topography at a variety of length scales lower wind threshold friction velocities associated with higher atmospheric densities in lowlands keep dust from accumulating and duricrust from forming while higher threshold velocities in highlands lead to net accumulation of dust local topographic obstacles craters walls ridges perturb wind flow and lead to local accumulation or erosion at a variety of elevations in addition the bright red dust ubiquitously found between exposures of dark gray and dark red materials may accumulate as the wind velocity gradient decreases at the transition from smoother dark gray exposures to rougher dark red exposures the distribution of the materials must also be modulated by climatic variations induced by quasi periodic oscillations in obliquity eccentricity and spin axis direction constraining these surficial deposits to be 1 ma

wind streaks geological and botanical effects on surface albedo contrast abstract two wind streaks in the eastern mojave desert of california were examined to gain insight into the origin of the surface brightness contrast that makes them visible both on the ground and in remote sensing data the two localities are a 4 km long dark streak oriented s43e from the amboy cinder cone 34 32 n 115 46 w located on a quaternary basalt flow covered with aeolian sand and a 2 km long dark streak oriented s22e from a low hill near the southwestern base of sleeping beauty mountain 34 48 n 116 20 w located on a sand covered alluvial surface in both cases the dark streaks have enhanced rock abundances on the streak surface relative to the surroundings at the amboy streak slope wash likely contributed to the rock concentration on the streak surface shielded from burial under aeolian sand by the cinder cone at the sleeping beauty streak the relative albedo contrast is strongly emphasized by the presence of big galleta grass only outside of the streak the albedo contrast of the sleeping beauty streak can be effectively eliminated by the seasonal presence of annual grass preferentially within the streak some plants may have reflectances that are strongly dependent upon viewing and illumination geometry raising the possibility that certain terrestrial aeolian features may appear variable on a diurnal basis alluvial processes appear to have been important at both localities for redistributing surface materials even given the infrequent rain conditions present in the mojave desert

hydrological controls on the morphogenesis of low energy meanders cher river france summary river restoration is a key issue for european hydrosystems that were modified over centuries by human activities this is particularly true for numerous low energy rivers flowing in the plateaus of western europe because of this limited energy their potential in terms of autogenic restoration a priori appears strongly restricted this study examines the conditions under which the morphogenesis on one of these systems occurs in relation to various hydrologic conditions two complementary approaches are combined on three reaches of the meandering cher river france firstly we examine at a pluri decadal scale the control of duration frequency and intensity of floods on the planimetric erosion bank retreat mainly secondly we estimate the range of effective discharge for bedload transport the results show that the morphogenesis is controlled by low magnitude hydrological events two major controlling factors are suggested low differential of energy between small and large floods peculiar to low energy rivers of mid latitude and low critical discharges for lateral erosion and bedload mobilization for these reasons the ability of the alluvial cher river to self restore its fluvial dynamics seems to be relatively high

properties of various sediment sampling procedures several issues regarding the sampling and analysis of bed material from gravel bed streams are addressed in this paper the need to convert surface samples into volumetric equivalents and methods to do so are explained it is found that most surface samples are unique and thus are not directly comparable particular attention is paid to areal sampling with clay and with wax the areal to volumetric conversion of samples removed from different sediment structures is also considered criteria are proposed for determining the minimum dimensions of unbiased volumetric samples these criteria can be used to determine the minimum area of an areal sample the minimum depth of a volumetric sample is shown to be larger than the depth of most surface layers thus a surface layer can not be sampled volumetrically a technique is presented for sampling a wide size range of underwater surface particles this technique has been tested in the field and found to provide good results