the hellenic subduction beneath the peloponnesus first results of a microearthquake study abstract a preliminary examination of the 1070 earthquake locations determined from 6 weeks of recording in 1986 by 46 stations show that the seismicity is spread over a wide area of the peloponnesus and the western hellenic arc and throughout the whole crust no clear individual faults can be identified from the seismicity but clusters of activity are observed in some places seismicity is concentrated above 40 km and deeper earthquakes were not numerous only 28 of the 466 events with uncertainties in depth less than 5 km occurred deeper than 40 km seismicity deeper than 30 km defines a flat zone at a depth between 40 km and 70 km starting from the trench to about 200 km towards the northeast further northeast the dip of the seismic zone abruptly changes to 45 fault plane solutions for the deeper events generally indicate t axes plunging northeast within the subducted slab therefore we interpret the seismicity deeper than 30 km as due to the superposition of two different causes 1 the steep zone is due to the subduction of the african litospheric plate beneath the aegean and 2 the shallow flat zone located between the trench and the argolide is partly due to the loading of the overriding aegean plate which is deforming above the african plate

how do bedform patterns arise new views on the role of bedform interactions within a set of boundary conditions one explanation for bedform patterns is self organization in which the pattern emerges because of interactions among the bedforms themselves models remote images field studies and lab experiments have identified bedform interactions that involve whole bedforms only bedform defects or that are remote interactions between bedforms it is proposed that bedform interactions form a spectrum from constructive to regenerative in pattern development constructive interactions including merging lateral linking cannibalization and remote transfer of sediment push the system toward fewer larger more widely spaced bedforms regenerative interactions including bedform splitting defect creation and calving push the system back toward a more initial state other interactions including off center collision defect migration and bedform and defect repulsion cause pattern change but may not be strongly constructive or regenerative although bedform interactions are ubiquitous to any field of bedforms their dynamics flow field modification and impact upon measurable pattern parameters are yet poorly understood most bedform interactions span bedform types and fluids supporting the hypothesis that pattern emerges from dynamics at the bedform level in a hierarchy that includes lower levels of bedform flow and grain fluid interactions bedform interactions alone however cannot account for the rich diversity of bedform patterns in nature it is proposed that field diversity arises because of boundary conditions which are the environmental variables within which a field evolves conceptually boundary conditions modify the shape of the attractor toward which a field evolves possibly by altering the type and frequency of bedform interactions boundary conditions are broadly similar within system types but are unique for each bedform field so that no two are ever exactly alike although aeolian and fluvial systems share some types of boundary conditions flow depth is a unique boundary condition in shallow fluvial systems copyright 2009 john wiley sons ltd

bed forms in bimodal sand gravel sediments laboratory and field analysis bed forms were studied in goodwin creek and a laboratory flume channel the bed sediment of the field site and flume had median diameters of 8ae3 modes of 0ae4 22ae6 mm and 1ae82 mm modes of 0ae5 5ae6 mm respectively the laboratory andfieldchannelshadsimilarvaluesofbimodalparameters ratiosofflowdepth to median bed material diameter and ratios of shear stress to critical shear stress and were judged to be comparable in the transport of bed load sediment and the resulting bed forms three groupings of bed forms from the laboratory flume experiments ripple like bed forms bed load sheets low relief bed waves were identified using the height and period of the bed forms for the range of flow depths and discharges investigated in the flume bed forms became higher and longer with increasing bed shear stress bed forms from goodwin creek were similar to those from theflume with comparable ratios between bed form length height and flow depth the bed forms in the flume provide a positive link between rate and size fluctuations measured in the field and the bed forms the smaller bed forms identified were sediment starved and are not considered to be dunes while the largest bed forms in which all of the bed material sizes were mobilized in the field and laboratory were judged to be dunes

distribution of erosion across bedrock channels lateral erosion in bedrock rivers is an important control on the shape of channel crosssections and the coupling of channels and hillslopes recent observations link lateral erosionto the variability of flow we propose two mechanisms to explain this one is based on changing shear stress distributions within the channel with varying flood level the other on the competition between cover and tool effects in fluvial bedrock erosion we assess these processes for the liwu river taiwan and conclude that cover and tool effects dominate the partitioning of lateral and vertical erosion in this case

a unified model for subaqueous bed form dynamics 1 bed form evolution remains dynamic even in the special case of steady uniform flow data from the sandy braided north loup river nebraska show that roughness features on the channel bottom display a statistical steady state and robust scaling that are maintained through the collective interactions of transient short lived bed forms motivated by such field data and laboratory observations of bed form growth we develop a nonlinear stochastic surface evolution model for the topography of bed load dominated sandy rivers in which instantaneous sediment flux explicitly depends on local elevation and slope this model quantitatively reproduces laboratory observations of initial growth and saturation of bed forms from a flat surface and also generates long term dynamical behavior characteristic of natural systems we argue that the variability in geometry and kinematics of bed forms in steady flow and the existence of roughness at all wavelengths up to the largest dunes are a consequence of the nonlinear relationship between sediment flux and topography subject to noise

the distribution and residence time of suspended sediment stored within the channel margins of a gravel bed bedrock river previously undocumented deposits are described that store suspended sediment in gravel bedded rivers termed fine grained channel margin fgcm deposits fgcm deposits consist of sand silt clay and organic matter that accumulate behind large woody debris lwd along the margins of the wetted perimeter of the single thread gravel bed south river in virginia these deposits store a total mass equivalent to 17 to 43 of the annual suspended sediment load radiocarbon 210pb and 137c dating indicate that sediment in fgcm deposits ranges in age from 1 to more than 60 years reservoir theory suggests an average turnover time of 1 75 years and an annual exchange with the water column of a mass of sediment equivalent to 10 to 25 of the annual sediment load the distribution of ages in the deposits can be fitted by a power function suggesting that sediment stored in the deposits has a wide variety of transit times most sediment in storage is reworked quickly but a small portion may remain in place for many decades the presence of fgcm deposits indicates that suspended sediment is not simply transported downstream in gravel bed rivers in agricultural watersheds significant storage can occur over decadal timescales south river has a history of mercury contamination and identifying sediment sources and sinks is critical for documenting the extent of contamination and for developing remediation plans fgcm deposits should be considered in future sediment budget and sediment transport modeling studies of gravel bed rivers in agricultural watersheds copyright 2010 john wiley sons ltd

storage filters upland suspended sediment signals delivered from watersheds climate change tectonics and humans create long and short term temporal variations in the supply of suspended sediment to rivers these signals generated in upland erosional areas are filtered by alluvial storage before reaching the basin outlet we quantified this filter using a random walk model driven by sediment budget data a power law distributed probability density function pdf to determine how long sediment remains stored and a constant downstream drift velocity during transport of 157 km yr for 25 km of transport few particles are stored and the median travel time is 0 2 yr for 1000 km of transport nearly all particles are stored and the median travel time is 2 5 m y both travel time distributions are power laws the 1000 km travel time distribution was then used to filter sinusoidal input signals with periods of 10 yr and 10 4 yr the 10 yr signal is delayed by 12 5 times its input period damped by a factor of 380 and is output as a power law the 10 4 yr signal is delayed by 0 15 times its input period damped by a factor of 3 and the output signal retains its sinusoidal input form but with a power law tail delivery time scales for these two signals are controlled by storage in channel transport time is insignificant and low frequency signals are transmitted with greater fidelity than high frequency signals these signal modifications are essential to consider when evaluating watershed restoration schemes designed to control sediment loading and where source area geomorphic processes are inferred from the geologic record

a quantitative three dimensional depositional model of gravelly braided rivers a quantitative three dimensional depositional model of gravelly braided rivers has been developed based largely on the deposits of the sagavanirktok river in northern alaska these deposits were described using cores wireline logs trenches and ground penetrating radar profiles the origin of the deposits was inferred from observations of 1 channel and bar formation and migration and channel filling interpreted from aerial photographs 2 water flow during floods and 3 the topography and texture of the river bed at low flow stage this depositional model quantitatively represents the geometry of the different scales of strataset the spatial relationships among them and their sediment texture distribution porosity and permeability in the model are related to sediment texture the geometry of a particular type and scale of strataset is related to the geometry and migration of the bedform type e g ripples dunes bedload sheets bars associated with deposition of the strataset in particular the length to thickness ratio of stratasets is similar to the wavelength to height ratio of associated bedforms furthermore the wavelength and height of bedforms such as dunes and bars are related to channel depth and width therefore the thickness of a particular scale of strataset i e medium scale cross sets and large scale sets of inclined strata will vary with river dimensions these relationships between the dimensions of stratasets bedforms and channels mean that this depositional model can be applied to other gravelly fluvial deposits the depositional model can be used to interpret the origin of ancient gravelly fluvial deposits and to aid in the characterization of gravelly fluvial aquifers and hydrocarbon reservoirs

optically stimulated luminescence dating of sediments over the past 200 000 years optical dating of sediment using optically stimulated luminescence has become important for studying earth surface processes and this technique continues to develop rapidly a group of closely linked luminescence methods can be used to estimate the time since grains of quartz and feldspar were last exposed to daylight by detecting their subsequent response to environmental ionizing radiation exposure the technique is well suited to the dating of deposits as young as one year to several hundred thousand years recent technical developments have established a dating protocol with improved precision a high degree of reliability and an in built means to detect incomplete signal removal during deposition this approach has been extended to age estimation for single grains opening up a wider range of potential environments and new possibilities for understanding postdepositional grain movement ongoing research offers the possibility of significant age range extension and novel applications including low t

statistical characterization of bed roughness due to bed forms a field study in the elbe river at aken germany 1 bed form geometry and dynamics in a straight section of the elbe river in germany is analyzed considering the measured bed surfaces as two dimensional random fields of bed elevations statistically derived roughness parameters are evaluated from high resolution digital elevation models which were available for a range of flow rates from low flows to floods the key results relate to the identification of characteristic scaling regions in the bed surface spectra and to observed relationships between water discharge and both the standard deviation and a factor of the 3 spectral law of bed elevations two dimensional second order structure functions of bed elevations are also analyzed to gain further insight into the spatial structure of sand wave beds in addition the interrelations between flow rate hysteresis and the statistical structure of bed forms as well as effects of channel modification by groynes are highlighted and discussed the reported results demonstrate that statistical parameters of bed forms may be used for characterization and prediction of flow dependent sand bed roughness

steady state reach scale theory for radioactive tracer concentration in a simple channel floodplain system 1 a steady state analytical model is presented for reach scale variation in the concentration of a decaying radioactive tracer associated with sediment particles that regularly pass through an off channel floodplain the floodplain is represented as a series of well mixed sediment reservoirs that continually exchange sediment with the channel the model allows for tributary input and valley wide aggradation or degradation tracer concentration depends on the upstream boundary concentration the tracer and sediment load floodplain geometry and the rates of in floodplain tracer production and or decay the theory predicts relatively modest down channel change in the concentration of long lived isotopes but implies that significant change may occur for 1 tracers with a short enough half life such as 14c or 2 floodplains with sediment residence times that are large enough for cosmogenic production or meteoric fallout to increase tracer concentration in the down valley direction the profiles are shown to be strongly dependent on the grain size distributions of both the sediment load and the floodplain the results imply that down channel 14c profiles have the potential to constrain holocene bed material loads in systems with sufficient storage the theory concisely describes the general importance of a floodplain for modifying in situ produced cosmogenic tracer concentration and can also characterize floodplain importance for fallout radioisotopes i e 10be 210pb or 7be or organic 14c

which geomorphic processes can be informed by luminescence measurements abstract luminescence signals from quartz and feldspar contain a wealth of information about exposure to heat and sunlight while many studies have focused on quantifying individual processes such as grain transport and burial cooling during exhumation or bleaching of freshly exposed rock surfaces not much work has yet been done to illustrate how these effects might interact for example how might a geologist geomorphologist interpret the luminescence signals within a riverine cobble or within a recent rockfall from an uplifting terrain this study attempts to address this issue by first presenting a simple kinetic expression for osl signal accumulation in response to background radiation and osl loss by exposure to heat or to direct or attenuated sunlight this model is then used to simulate three example sample histories simulation results demonstrate how much information can be embedded within one sample s luminescence signal and how disparate geomorphic histories can produce strikingly similar signals the temperatures and timescales which can be examined with luminescence signals are presented in comparison to those of various geomorphic processes finally simulation results illustrate the benefit of using signals of different stability to better understand a sample s recent geologic history

experimental study of the response of a gravel streambed to increased sediment supply if increased sediment supply to a river channel exceeds its transport capacity deposition necessarily occurs as the bed adjusts to accommodate the increased supply both the mean and spatial patterns in bed elevation and grain size may change and an ability to understand their relative importance is needed to predict bed response we report on an experiment in a field scale flume in which sediment supply is increased to a gravel bed with alternate bars sediment was recirculated in the experiments but augmented in two steps after which the bed was allowed to reach a new steady state the transport rate at the end of the experiment was three times larger than at the start high resolution sediment flux and topographic measurements grain size derived from photographs and hydrodynamic modeling allow us to document the topographic and textural response of the bed to increased sediment supply the spatial patterns of bed topography and texture were forced by the flume setup and the initial and final steady states included long stationary alternate bars with associated grain size sorting the transient bed contained several scales of shorter wavelength migrating bedforms superimposed on and temporarily replacing the stationary alternate bars bed topography and textural patterns adjusted to increased sediment supply over different timescales bed slope and mean stress increased directly with sediment supply rate to produce a new transport steady state in a time about 2 5 times the minimum needed to deposit the required sediment wedge indicating a trap efficiency of about 40 for the aggrading wedge adjustments in local topography and sorting primarily in the form of smaller migrating bars continued for a period approximately equal to that required to initially reach transport steady state copyright 2013 john wiley sons ltd

optical resetting in large drainage basins tests of zeroing assumptions using single aliquot procedures abstract optical resetting of previously accumulated charge populations at the time of transportation and burial of sediments remains a critical assumption in luminescence dating this is particularly the case for fluvial sediments we have tested the extent of bleaching in fluvial environments by applying the single aliquot regeneration sar equivalent dose d e determination technique coupled with tests for partial bleaching in the majority of cases samples collected from a point bar complex on the colorado river texas exhibit evidence of incomplete bleaching use of the minimum sar d e in these cases resulted in estimated residual ages of c 100 300 years in a similar examination of sar d e s for bed load samples collected along the loire river we observe a monotonic decrease in effective equivalent dose d e from values of 14 10 2 and 3 2 mean median and minimum gy at source down to 3 3 3 6 and 1 6 gy in the first kilometre of transportation down to levels of 0 3 0 2 and 0 0 gy for transportation distances exceeding 300 km we observed virtually no effect on the d e in the bed load of the introduction of trunk tributary streams and conclude that the likelihood of resetting of osl in large fluvial systems is somewhat more effective that previously considered when the recycling of sediments down the system are borne in mind

increased mid twentieth century riverbank erosion rates related to the demise of mill dams south river virginia a recent hypothesis suggests that fluvial processes in areas of the eastern united states are strongly influenced by the demise of colonial mill dams rather than reflecting a quasi equilibrium adjustment to the current hydrologic and sediment regime we evaluated the control of colonial mill dams on twentieth and twenty first century bank erosion rates on the south river virginia through studies of historical aerial photographs historical documents hydrologic and climatic records and hydraulic modeling historical sources and aerial photographs document eight colonial mill dams along the study reach in the early twentieth century all but one of these dams disappeared in the 1950s and the last was breached by 1976 from initially low values between 1937 and 1957 mean bank erosion rates increased by more than a factor of 2 after 1957 remaining high through 2005 accelerated bank erosion rates cannot be explained by changes in storm intensity the frequency of freeze thaw cycles or by changes in the density of riparian trees hydraulic modeling suggests that mill dams reduced velocities of the 5 yr flood through 80 of our study reach by considering the timing of mill dam loss the spatial extent of backwater influence and the locations of our study sites we find that the loss of mill dams explains the observed trends in bank erosion rates at 9 and possibly 10 of our 14 monitoring sites these results support the hypothesis that the demise of mill dams has been an important influence on fluvial processes in the region

a geochronologic framework for the ziegler reservoir fossil site snowmass village colorado abstract the ziegler reservoir fossil site near snowmass village colorado usa provides a unique opportunity to reconstruct high altitude paleoenvironmental conditions in the rocky mountains during the last interglacial period we used four different techniques to establish a chronological framework for the site radiocarbon dating of lake organics bone collagen and shell carbonate and in situ cosmogenic 10 be and 26 al ages on a boulder on the crest of a moraine that impounded the lake suggest that the ages of the sediments that hosted the fossils are between 140 ka and 45 ka uranium series ages of vertebrate remains generally fall within these bounds but extremely low uranium concentrations and evidence of open system behavior limit their utility optically stimulated luminescence osl ages n 18 obtained from fine grained quartz maintain stratigraphic order were replicable and provide reliable ages for the lake sediments analysis of the equivalent dose d e dispersion of the osl samples showed that the sediments were fully bleached prior to deposition and low scatter suggests that eolian processes were likely the dominant transport mechanism for fine grained sediments into the lake the resulting ages show that the fossil bearing sediments span the latest part of marine oxygen isotope stage mis 6 all of mis 5 and mis 4 and the earliest part of mis 3

modelling transverse dunes transverse dunes appear in regions of mainly unidirectional wind and high sand availability a dune model is extended to two dimensional calculation of the shear stress it is applied to simulate dynamics and morphology of three dimensional transverse dunes in the simulations they seem to reach translational invariance and do not stop growing hence simulations of two dimensional dune elds have been performed characteristic laws were found for the time evolution of transverse dunes bagnold s law of the dune velocity is modi ed and reproduced the interaction between transverse dunes led to the interesting conclusion that small dunes can travel over bigger ones copyright 2004 john wiley sons ltd

a simple universal equation for grain settling velocity abstract a new equation is presented for sediment fall velocity as a function of grain diameter for given values of fluid viscosity and fluid and solid density sediment fall velocity is a fundamental parameter in the modeling and interpretation of fluviatile and coastal deposition the equation applies to the entire range of viscous to turbulent conditions and its simple explicit form makes it easy to use in computer models and other applications in sedimentology geomorphology and engineering the equation is derived from dimensional analysis and converges on stokes law for small grains and a constant drag coefficient for large grains its two physically interpretable parameters are easily adjusted for shape effects or for the use of sieve diameter rather than nominal grain diameter it gives a close fit to published and new experimental data for both quartz sand and low density materials with no more error than previous equations of more complicated form

field evidence for the influence of weathering on rock erodibility and channel form in bedrock rivers erosion processes in bedrock floored rivers shape channel cross sectional geometry and the broader landscape however the influence of weathering on channel slope and geometry is not well understood weathering can produce variation in rock erodibility within channel cross sections recent numerical modeling results suggest that weathering may preferentially weaken rock on channel banks relative to the thalweg strongly influencing channel form here we present the first quantitative field study of differential weathering across channel cross sections we hypothesize that average cross section erosion rate controls the magnitude of this contrast in weathering between the banks and the thalweg erosion rate in turn is moderated by the extent to which weathering processes increase bedrock erodibility we test these hypotheses on tributaries to the potomac river virginia with inferred erosion rates from 0 1 m kyr to 0 8 m kyr with higher rates in knickpoints spawned by the migratory great falls knickzone we selected nine channel cross sections on three tributaries spanning the full range of erosion rates and at multiple flow heights we measured 1 rock compressive strength using a schmidt hammer 2 rock surface roughness using a contour gage combined with automated photograph analysis and 3 crack density crack length area at three cross sections on one channel all cross sections showed significant p 0 01 for strength p 0 05 for roughness increases in weathering by at least one metric with height above the thalweg these results assuming that the weathered state of rock is a proxy for erodibility indicate that rock erodibility varies inversely with bedrock inundation frequency differences in weathering between the thalweg and the channel margins tend to decrease as inferred erosion rates increase leading to variations in channel form related to the interplay of weathering and erosion rate this observation is consistent with numerical modeling that predicts a strong influence of weathering related erodibility on channel morphology copyright 2017 john wiley sons ltd

variables and potential models for the bleaching of luminescence signals in fluvial environments abstract luminescence dating of fluvial sediments rests on the assumption that sufficient sunlight is available to remove a previously obtained signal in a process deemed bleaching however luminescence signals obtained from sediment in the active channels of rivers often contain residual signals this paper explores and attempts to build theoretical models for the bleaching of luminescence signals in fluvial settings we present two models one for sediment transported in an episodic manner such as flood driven washes in arid environments and one for sediment transported in a continuous manner such as in large continental scale rivers the episodic flow model assumes that the majority of sediment is bleached while exposed to sunlight at the near surface between flood events and predicts a power law decay in luminescence signal with downstream transport distance the continuous flow model is developed by combining the beer lambert law for the attenuation of light through a water column with a general order kinetics equation to produce an equation with the form of a double negative exponential the inflection point of this equation is compared with the sediment concentration from a rouse profile to derive a non dimensional number capable of assessing the likely extent of bleaching for a given set of luminescence and fluvial parameters although these models are theoretically based and not yet necessarily applicable to real world fluvial systems we introduce these ideas to stimulate discussion and encourage the development of comprehensive bleaching models with predictive power

storms channel changes and a sediment budget for an urban suburban stream difficult run virginia usa abstract determining erosion and deposition rates in urban suburban settings and how these processes are affected by large storms is important to understanding geomorphic processes in these landscapes sediment yields in the suburban and urban upper difficult run are among the highest ever recorded in the chesapeake bay watershed ranging from 161 to 376 mg km2 y erosion and deposition of streambanks channel bed and bars and deposition of floodplains were monitored between 1 march 2010 and 18 january 2013 in upper difficult run virginia usa we documented the effects of two large storms tropical storm lee september 2011 a 100 year event and super storm sandy october 2012 a 5 year event on channel erosion and deposition variability in erosion and deposition rates for all geomorphic features temporally and spatially are important conclusions of this study tropical storm lee was an erosive event where erosion occurred on 82 of all streambanks and where 88 of streambanks that were aggrading before tropical storm lee became erosional statistical analysis indicated that drainage area explains linear changes cm y in eroding streambanks and that channel top width explains cross sectional area changes cm2 y in eroding streambanks and floodplain deposition mm y a quasi sediment budget constructed for the study period using the streambanks channel bed channel bars and floodplain measurements underestimated the measured suspended sediment load by 61 2130 mg y underestimation of the sediment load may be caused by measurement errors and to contributions from upland sediment sources which were not measured but estimated at 36 of the gross input of sediment eroding streambanks contributed 42 of the gross input of sediment and accounted for 70 of the measured suspended sediment load similar to other urban watersheds the large percentage of impervious area in difficult run and direct runoff of precipitation leads to increased streamflow and streambank erosion this study emphasizes the importance of streambanks in urban suburban sediment budgets but also suggests that other sediment sources such as upland sources which were not measured in this study can be an important source of sediment

spectral behavior of gravel dunes abstract spectral behavior of gravel dunes formed under different flow discharges is analyzed with an attempt to verify the 3 spectral law that has been confirmed extensively for sand dunes a schematic spectrum of gravel dunes is proposed based on the spectral analysis as well as results from the literature the results of spectral analysis show a significant deviation from the 3 spectral law for gravel dunes and the magnitude of deviation correlates with flow discharge possible explanations for the deviation from the 3 spectral law being associated with kinetic and geometrical characteristics have been explored to investigate the kinetic characteristics of gravel dunes a wavelet based method that calculates the celerity of dunes based on a pair of elevational time series is quantitatively tested our results suggest that 1 the kinetic explanation based on the relationship between dune celerity and dune length cannot fully explain the spectral behavior of gravel dunes 2 the geometrical explanation based on the self similarity hypothesis is confirmed by the relationship between dune length and dune height and 3 the development of gravel sheets accounts for the differences in kinetic and geometrical characteristics between gravel dunes and sand dunes

active overbank deposition during the last century south river virginia abstract we quantify rates of overbank deposition over decadal to centennial timescales along the south river in virginia using four independent methods detailed mercury profiles sampled adjacent to the stream channel preserve the peak historic mercury concentration on suspended sediment dating from 1955 to 1961 and suggest sedimentation rates of 8 to 50 cm 100 years sediment accumulation over the roots of trees suggest rates of 0 to 100 cm 100 years with significantly higher values on levees and lower values on floodplains farther from the channel profiles of 137cs and 210pb from two eroding streambanks are fit with an advection diffusion model calibrated at an upland reference site these methods suggest sedimentation rates of 44 to 73 cm 100 years mercury inventories from 107 floodplain cores combined with a previously published reconstruction of the history of mercury concentration on suspended sediment provide spatially comprehensive estimates of floodplain sedimentation median sedimentation rates are 3 8 cm 100 years for the

bed form development based upon the results of a series of 47 bed development experiments the evolution of bed features subsequent to their initial generation from flat bed conditions is examined for each such experiment bed profiles were measured at frequent time intervals as sand wave configurations developed under the action of open channel water flow the two sediments used for these experiments were of geometric mean size 0 20 mm and 0 82 mm respectively in line with the bed form unification model of bed development postulated by saudkivi and witte in 1990 the principle of bed form propagation speed decreasing with increasing bed form height and the mechanisms of bed form coalescence and bed form throughpassing are all found to be central to bed evolution processes a quantitative relation between bed form speed and bed form height is presented bed development predictions based upon bed form unification models can be seen to clearly parallel the present experimental results stability analyses based upon potential

on extracting sediment transport information from measurements of luminescence in river sediment accurately quantifying sediment transport rates in rivers remains an important goal for geomorphologists hydraulic engineers and environmental scientists however current techniques for measuring long timescale 102 106 yr transport rates are laborious and formulae to predict transport are notoriously inaccurate here we attempt to estimate sediment transport rates using luminescence a property of common sedimentary minerals that is used by the geoscience community for geochronology this method is advantageous because of the ease of measurement on ubiquitous quartz and feldspar sand we develop a model from first principles using conservation of energy and sediment mass to explain the downstream pattern of luminescence in river channel sediment we show that the model can accurately reproduce the luminescence observed in previously published field measurements from two rivers with very different sediment transport styles the model demonstrates that the downstream pattern of river sand luminescence should show exponential like decay in the headwaters which asymptotes to a constant value with further downstream distance the parameters from the model can then be used to estimate the time averaged virtual velocity characteristic transport lengthscale storage timescale and floodplain exchange rate of fine sand sized sediment in a fluvial system the sediment transport values predicted from the luminescence method show a broader range than those reported in the literature but the results are nonetheless encouraging and suggest that luminescence demonstrates potential as a sediment transport indicator however caution is warranted when applying the model as the complex nature of sediment transport can sometimes invalidate underlying simplifications

natural streams and the legacy of water powered mills gravel bedded streams are thought to have a characteristic meandering form bordered by a self formed fine grained floodplain this ideal guides a multibillion dollar stream restoration industry we have mapped and dated many of the deposits along mid atlantic streams that formed the basis for this widely accepted model these data as well as historical maps and records show instead that before european settlement the streams were small anabranching channels within extensive vegetated wetlands that accumulated little sediment but stored substantial organic carbon subsequently 1 to 5 meters of slackwater sedimentation behind tens of thousands of 17th to 19th century milldams buried the presettlement wetlands with fine sediment these findings show that most floodplains along mid atlantic streams are actually fill terraces and historically incised channels are not natural archetypes for meandering streams

fluvial transport of sand and gravel mixtures with bimodal size distributions abstract initiation of motion and fractional transport rates of sediments with bimodal size distributions were measured in a laboratory channel and in a natural channel transport experiments were conducted with unimodal 100 sand and 100 gravel bed sediments and three bimodal mixtures of the two sediments 10 gravel 90 sand 25 gravel 75 sand and 45 gravel 55 sand in the bimodal sediment beds the critical shear stress for the initiation of motion showed very little change with grain size for the sand sizes but increased with grain size for the gravel sizes for low flow strengths only the sand sizes were in transport while for the highest flow strengths the bed load sediment size distribution approached that of the bed material

determining fluvial sediment virtual velocity on the mojave river using k feldspar irsl initial assessment abstract the mojave river of southern california was chosen as a field site to investigate the applicability of luminescence dating to sediment transport rate problems grains in the active channel of the river are expected to show signs of partial bleaching and this makes it difficult to determine time since deposition accurately a modification of the multiple elevated temperature post ir irsl metpirir procedure buylaert et al 2009 li and li 2011 was used for k feldspar grains 175 200 m at temperature increments of 50 95 140 185 and 230 c in order to provide more information about relative signal bleaching among samples the measurements show an exponential decrease in equivalent dose de with distance down the mojave river higher temperature pirir signals are bleached more slowly than lower temperature ones buylaert et al 2009 the de for samples at 50 c is roughly constant along the river these results suggest cyclical bleaching and burial as grains are transported downriver and higher energy deeper traps are vacated the pattern of de values for the mojave river can be used to constrain the sediment transport rate for this river by building a model of growth and bleach for each temperature increment a bleaching experiment was run with multiple aliquot samples for direct sunlight exposure times of 0 10 30 300 1000 3000 10 000 and 30 000 s the met pirir procedure was applied at each temperature increment for each exposure time aliquot and the results for all exposure times were fit to the general order kinetics equation using a non linear regression the bleaching parameters were used in conjunction with the sar growth curves to build a model of partial bleaching of grains during transport that is fitted with a 2 test to the pirir data from the mojave river this model is not a unique solution but can be used to assess the likelihood of various sediment transport regimes

sediment transit time and floodplain storage dynamics in alluvial rivers revealed by meteoric 10be quantifying the time scales of sediment transport and storage through river systems is fundamental for understanding weathering processes biogeochemical cycling and improving watershed management but measuring sediment transit time is challenging here we provide the first systematic test of measuring cosmogenic meteoric beryllium 10 10bem in the sediment load of a large alluvial river to quantify sediment transit times we take advantage of a natural experiment in the rio bermejo a lowland alluvial river traversing the east andean foreland basin in northern argentina this river has no tributaries along its trunk channel for nearly 1 300 km downstream from the mountain front we sampled suspended sediment depth profiles along the channel and measured the concentrations of 10bem in the chemically extracted grain coatings we calculated depth integrated 10bem concentrations using sediment flux data and found that 10bem concentrations increase 230 from upstream to downstream indicating a mean total sediment transit time of 8 4 2 2 kyr bulk sediment budget based estimates of channel belt and fan storage times suggest that the 10bem tracer records mixing of old and young sediment reservoirs on a reach scale 10bem transit times are shorter where the channel is braided and superelevated above the floodplain and longer where the channel is incised and meandering suggesting that transit time is controlled by channel morphodynamics this is the first systematic application of 10bem as a sediment transit time tracer and highlights the method s potential for inferring sediment routing and storage dynamics in large river systems

on the transition between 2d and 3d dunes sediment transport in sand bedded alluvial channels is strongly conditioned by bedforms the planimetric morphology of which can be either two or threedimensional experiments were undertaken to examine the processes that transform the bed configuration from two dimensional 2d dunes to threedimensional 3d dunes a narrowly graded 500 lm size sand was subjected to a 0ae15 m deep non varying mean flow ranging from 0ae30 to 0ae55 m sec 1 in a 1 m wide flume changes in the planimetric configuration of the bed were monitored using a high resolution video camera that produced a series of 10 sec time lapsed digital images image analysis was used to define a critical value of the non dimensional span sinuosity of the bedform crestlines that divides 2d forms from 3d forms significant variation in the non dimensional span is observed that cannot be linked to properties of the flow or bedforms and thus appears random images also reveal that once 2d bedforms are established minor transient excesses or deficiencies of sand are passed from one bedform to another the bedform field appears capable of absorbing a small number of such defects but as the number grows with time the resulting morphological perturbations produce a transition in bed state to 3d forms that continue to evolve but are pattern stable the 3d pattern is maintained by the constant rearrangement of crestlines through lobe extension and starving downstream bedforms of sediment which leads to bifurcation the experiments demonstrate that 2d bedforms are not stable in this calibre sand and call into question the reliability of bedform phase diagrams that use crestline shape as a discriminator

measuring gravel transport and dispersion in a mountain river using passive radio tracers random walk models of fluvial sediment transport recognize that grains move intermittently with short duration steps separated by rests that are comparatively long these models are built upon the probability distributions of the step length and the resting time motivated by these models tracer experiments have attempted to measure directly the steps and rests of sediment grains in natural streams this paper describes results from a large tracer experiment designed to test stochastic transport models we used passive integrated transponder pit tags to label 893 coarse gravel clasts and placed them in halfmoon creek a small alpine stream near leadville colorado usa the pit tags allow us to locate and identify tracers without picking them up or digging them out of the streambed they also enable us to find a very high percentage of our rocks 98 after three years and 96 after the fourth year we use the annual tracer displacement to test two stochastic transport models the einstein hubbell sayre ehs model and the yang sayre gamma exponential model gem we find that the gem is a better fit to the observations particularly for slower moving tracers and suggest that the strength of the gem is that the gamma distribution of step lengths approximates a compound poisson distribution published in 2012 this article is a us government work and is in the public domain in the usa

recent and historic sediment dynamics along difficult run a suburban virginia piedmont stream abstract suspended sediment is one of the major concerns regarding the quality of water entering the chesapeake bay some of the highest suspended sediment concentrations occur on piedmont streams including difficult run a tributary of the potomac river draining urban and suburban parts of northern virginia accurate information on catchment level sediment budgets is rare and difficult to determine further the sediment trapping portion of sediment budget represents an important ecosystem service that profoundly affects downstream water quality our objectives with special reference to human alterations to the landscape include the documentation and estimation of floodplain sediment trapping present and historic and bank erosion along an urbanized piedmont stream the construction of a preliminary sediment balance and the estimation of legacy sediment and recent development impacts we used white feldspar markers to measure floodplain sedimentation rates and steel pins to measure erosion rates on floodplains and banks respectively additional data were collected for from legacy sediment thickness and characteristics mill pond impacts stream gaging station records topographic surveying and sediment density texture and organic content data were analyzed using gis and various statistical programs results are interpreted relative to stream equilibrium affected by both post colonial bottomland sedimentation legacy and modern watershed hardening associated with urbanization six floodplain channel sites from high to low in the watershed were selected for intensive study bank erosion ranges from 0 to 470 kg m y and floodplain sedimentation ranges from 18 to 1369 kg m y m refers to meters of stream reach upstream reaches are net erosional while downstream reaches have a distinctly net depositional flux providing a watershed sediment balance of 2184 kg m y trapped within the system the amounts of both deposition and erosion are large and suggest nonequilibrium channel conditions both peak discharge and number of peaks above base have substantially increased since the mid 1960s when urbanization of the watershed began deposition patterns are most closely correlated with channel gradient sinuosity and channel width floodplain width for recent and historic periods the substantial amounts of fine grained sediment deposited on the floodplain over the past two centuries or so do not appear to be closely related to historic mill pond presence or location the floodplain continues to provide the critical ecosystem service of sediment trapping in the face of multiple human alterations trends in sediment deposition erosion may react rapidly to land use practices within the watershed and offer a valuable barometer of the effects of management actions

particle trajectories on hillslopes implications for particle age and 10be structure many geomorphic systems act as conveyor belts onto which material is loaded at a particular rate and is transported in one direction toward another system that serves as a sink as the material travels it ages it changes in grain size it accumulates cosmogenic radionuclides it adsorbs or releases nutrients and it weathers here i address the hillslope conveyor as many geochemical processes are depth dependent the depth history of a particle becomes important to know i calculate soil particle trajectories in the horizontal depth plane and address three cases one in which horizontal speeds decline exponentially with depth a second in which they are uniform with depth and a third in which horizontal speeds are also uniform but all profile values are vertically well mixed vertical speeds are governed by continuity in an incompressible medium and by the boundary condition of zero vertical particle speed at the soil surface particle trajectories must therefore become surface parallel at the surface knowledge of soil particle trajectories allows calculation of residence times and concentration profiles of 10be in the soil the results inform strategies for interpretation of nuclide concentrations in soils and stream sediments and for inference of transport rate profiles in all steady cases the particle age and 10be structure is uniform with distance from the divide when significant vertical gradients in horizontal speed occur the patterns of particle age and of 10be concentration are dominated by the depth scale of the transport process in unmixed cases the particle age and 10be concentration in near surface samples can greatly exceed the vertically averaged values reflecting the fact that the vertical speeds of particles slow dramatically as they near the surface in cases in which horizontal speed varies significantly with depth the vertically averaged concentration of 10be within the soil can significantly under predict the mean 10be concentration of sediment delivered to the channel proper averaging requires weighting by particle speed these results provide the backdrop for addressing chemical evolution of soil on a hillslope