laboratory simulation of clast abrasion brewer paul lewin j 2002 laboratory simulation of clast abrasion earth surface processes and landforms 27 pp 145 164 rae2008

the downstream gradation of particle sizes in the squamish river british columbia the exponential downstream decline in particle size predicted by sternberg 1875 is examined within a highly active alpine fluvial environment the gravel bed squamish river in southwestern british columbia transect survey procedures for sampling within the coarse alluvial gravels are described in detail downstream trends for various particle size statistics plotted with distance from a major sediment source are considered according to two scenarios evaluation of overall trends indicates that the downstream expression derived by sternberg 1875 is not the most appropriate rather curves are better described by power functions this reflects a very rapid decline in particle size immediately downstream of the major sediment source secondly evaluation of downstream trends in relation to channel planform determines that exponential functions with different coefficients describe adequately relations within individual planform reaches there is insufficient evidence to suggest which of these two scenarios may be appropriate several lines of evidence suggest that the trends found are more the result of selective sediment transport phenomena than of abrasion a condition attributed to channel competence within a system dominated by sediment supply conditions

longitudinal changes in the size and shape of stream bed material evidence of variable transport conditions summary the pattern of downstream changes in particle size and shape is used to identify dominant transport conditions in a basin with two main lithologies having distinct spatial distributions grain size decreases exponentially and roundness increases geometrically in the sandstone group but shale particles show no such changes largely because shale is supplied discontinuously below headwater reaches where abrasion is no longer at its most effective particle flatness varies more consistently between the lithological groups an initial increase being followed by a geometric decrease discs and blades are the dominant zingg shapes reflecting the influence of bedrock structure and preferential transport three process regimes are identified in which different processes are dominant abrasion and size sorting in the headwater area shape and size sorting in the middle reaches sorting and breakage towards the downstream end of the stream underlying the general tendencies are discontinuities related to tributary inflow the main effects being to increase grain size and flatness below junctions

a mechanistic model for river incision into bedrock by saltating bed load 1 abrasion by bed load is a ubiquitous and sometimes dominant erosional mechanism for fluvial incision into bedrock here we develop a model for bedrock abrasion by saltating bed load wherein the wear rate depends linearly on the flux of impact kinetic energy normal to the bed and on the fraction of the bed that is not armored by transient deposits of alluvium we assume that the extent of alluvial bed cover depends on the ratio of coarse sediment supply to bed load transport capacity particle impact velocity and impact frequency depend on saltation trajectories which can be predicted using empirical functions of excess shear stress the model predicts a nonlinear dependence of bedrock abrasion rate on both sediment supply and transport capacity maximum wear rates occur at moderate relative supply rates due to the tradeoff between the availability of abrasive tools and the partial alluviation of the bedrock bed maximum wear rates also occur at intermediate levels of excess shear stress due to the reduction in impact frequency as grain motion approaches the threshold of suspension measurements of bedrock wear in a laboratory abrasion mill agree well with model predictions and allow calibration of the one free model parameter which relates rock strength to rock resistance to abrasive wear the model results suggest that grain size and sediment supply are fundamental controls on bedrock incision rates not only by bed load abrasion but also by all other mechanisms that require bedrock to be exposed in the channel bed

field evidence for rapid downstream fining of river gravels through selective transport doubts have been expressed about the ability of either abrasion or sorting to explain strong downstream fining of river gravels we describe pronounced fining over a short distance in a scottish river that has no human disturbance or lateral input of water and sediment measured abrasion rates are far too small to explain observed downstream fining but bed load trap measurements and the dispersion of magnetic tracer pebbles in six subreaches both show sorting during transport the observed downstream fining is also simulated well by a numerical sediment routing model using hydraulic and transport laws consistent with our field measurements the geomorphological cause of the strong fining is slope reduction above a local base level control in this common situation the development of downstream fining is part of the river9s tendency to minimize downstream variation in bed load transport rates and can proceed far more rapidly than the major aggradation otherwise required for equilibration

do gravel bed river size distributions record channel network structure 1 bed load sediment particles supplied to channels by hillslopes are reduced in size by abrasion during downstream transport the branching structure of the channel network creates a distribution of downstream travel distances to a given reach of river and thus may strongly influence the grain size distribution of the long term bed load flux through that reach here we investigate this hypothesis using mass conservation and the sternberg exponential decay equation for particle abrasion to predict bed material variability at multiple scales for both natural and artificial drainage networks we assume that over a sufficiently long timescale no net deposition occurs and that grains less than 2 mm are swept away in suspension we find that abrasion during fluvial transport has a surprisingly small effect on the bed load sediment grain size distribution for the simple case of spatially uniform supply of poorly sorted hillslope sediments this occurs because at any point in the channel network local resupply offsets the size reduction of material transported from upstream thus river bed material may essentially mirror the coarse component of the size distribution of hillslope sediment supply furthermore there is a predictable distance downstream at which the bed load grain size distribution reaches a steady state in the absence of net deposition due to selective transport large scale variability in bed material such as downstream fining must then be due primarily to spatial gradients in hillslope sediment production and transport characteristics a second key finding is that average bed load flux will tend to stabilize at a constant value independent of upstream drainage area once the rate of silt production by bed load abrasion per unit travel distance is equal to the rate of coarse sediment supply per unit channel length q bed load flux equilibrates over a distance that scales with the inverse of the fining coefficient in the abrasion rate law and can be approximated simply as q 3 thus the efficiency of particle abrasion sets a fundamental length scale shorter for weaker rocks and longer for harder rocks which controls the expression in the river bed of variability in sediment supply we explore the role of the abrasion length scale in modulating the influence of sediment supply variability in a number of channel network contexts including individual tributary junctions a sequence of tributary inputs along a main stem channel and variable basin shapes and network architecture as expressed by the width function these findings highlight the need for both data and theory that can be used to predict the grain size distributions supplied to channels by hillslopes

new facility to study river abrasion processes this technical note presents a new facility which was constructed in order to study sediment and bedrock abrasion processes during fluvial transport these processes exert an important control on long term landscape evolution but they are still poorly understood and inadequately quantified the proposed experimental device is an annular flume with four fluid injections coupled to a close water circuit and a powerful pump in order to reach hydrodynamic conditions up to whose prevailing in mountain streams fluid surface geometry and visualization experiments with a high speed camera allow us to monitor hydrodynamic variables and sediment motion during the experiments despite the circular geometry of the flume pebble trajectories are found to closely mimic the bedload behavior in straight flume based also on a direct comparison with pebble abrasion rates along a large himalayan river we hypothesize that our device simulates in a realistic way transport processes and consequently abrasion processes in mountain rivers

experimental study of abrasion and its role in producing downstream fining in gravel bed rivers abstract previous experimental studies on abrasion of gravel showed diminution rates much lower than those typical of gravel bed rivers this might be one reason many studies of downstream fining in rivers stress hydraulic sorting as the dominant fining mechanism previous experiments however did not replicate well the particle to particle impact between the bed load gravels and bed gravels in rivers on alluvial fans during floods the purpose of this study was to reexamine the effect of abrasion on downstream fining to determine if abrasion is responsible for longitudinal changes in gravel size in rivers on alluvial fans a rotating drum with three interior vanes was used to produce repeated collisions among test gravel particles in water the drum was only partially filled with water in order to simulate the impact velocity of particle collisions during a flood the test particles were slightly weathered andesite and chert taken from the bed of the watarase river japan experiments were conducted using three unisize grains l 7 0 128 mm to 6 5 91 mm m 5 5 45 mm to 5 0 32 mm s 4 0 16 mm to 3 5 11 mm and mixtures of two different sizes from the three size classes there are six principal results from the experiments 1 test gravel particles break frequently and decrease in weight rapidly 2 abrasion of chert produces mostly gravel while andesite produces mostly sand and silt 3 l size chert cobbles decrease in weight rapidly as a result of being broken into smaller pieces while andesite cobbles break so rarely that their weight decreases very slowly 4 s size andesite pebbles decrease in weight more rapidly than chert 5 abrasion depends strongly on the presence of different sizes because smaller fragments are crushed by larger gravel particles 6 diminution coefficients of andesite and chert obtained from the experiment are in the range of 10 3 10 1 km 1 diminution coefficients from this experiment are one to two orders of magnitude larger than those from previous experiments on abrasion this is mainly because of the larger impact velocities in this experiment diminution coefficients from the experiment are also consistent with those obtained from many japanese rivers on alluvial fans 10 2 10 1 km 1 this result suggests that downstream fining of river bed gravel on alluvial fans can be explained by abrasion

weathering controlled abrasion in a coarse grained meandering reach of the rio grande implications for the rock record abrasion mill analysis of fluvial clast 4 0 mm abrasion in a meandering reach of the rio grande south central colorado indicates that 1 abrasion rates of natural clasts from various locations in the channel and flood plain differ very little 2 abrasion rates in the first few hours are high then drop to much lower levels for these natural clasts and 3 a previously experimentally abraded clast sample otherwise identical to natural samples had a much lower initial abrasion rate than any natural samples the initial high abrasion rate of all natural clasts can be attributed to the presence of an easily abraded weathered layer we interpret the relatively abrupt drop in abrasion rate after a few hours of abrasion to the complete removal of the weathered layer and the exposure of more resistant rock the low initial abrasion rate of the previously experimentally abraded sample supports this interpretation and also indicates that none of the clasts in the river have been abraded as extensively as the experimentally abraded samples from this we infer that clasts have not traveled very far in the channel these results have led to the development of a weathering controlled abrasion model to explain the effect of abrasion on downstream fining size reduction over distance downstream weathering controlled abrasion is essentially a step function in time composed of the alternation of long periods of weathering with no abrasion flood plain storage with very brief but intense periods of mechanical abrasion channel transport this alternation of weathering and abrasion can produce a net abrasion rate sufficient to generate the amount of downstream fining present along the rio grande and also can explain why many abrasion mill experiments which simulate abrasion but not weathering fail to reproduce natural abrasion rates changes in the amount of fluvial downstream fining in an aggrading basin margin and thus in the rock record can result from variation in selective deposition temporary or permanent flood plain storage or from changes in the abrasion rate experiments demonstrate however that the complex interaction between these two processes may in some cases obscure the tectonic signal that is commonly interpreted from basin margin depositional geometries

paradox of downstream fining and weathering rind formation in the lower hoh river olympic peninsula washington although downstream fining of clasts is typical in modern and ancient river systems the hoh river a cobble and boulder bed river in washington state contains surprisingly little downstream fining of the coarsest tail of the grain size distribution along its lower 63 km mean rate of fining of the coarsest size fraction is only 0 24 mm km barely significant given uncertainties in measurement in addition these same clasts have weathering rind thicknesses that change very little along the length of the river they decrease by 0 01 mm km because weathering rinds are well developed in this setting and both tumbler studies and field observations show that abrasion of weathering rinds greatly accelerates clast diminution there seems to be a paradox between predicted and observed downstream fining rates field study shows that detritus is constantly being resupplied to the river by erosion of late glacial materials along the river9s cutbanks and tributaries the clasts supplied range in size but include abundant coarse grains with very thick weathering rinds the continuous resupply of grains strongly attenuates the rate of downstream fining despite the fact that these weathered grains abrade relatively rapidly thus the dominance of reworked glacial debris overwhelms any processes by which grains are reduced in size in the modern river these results suggest that relatively infrequent glaciation can have a long lived effect on river sedimentation

pebble abrasion during fluvial transport experimental results and implications for the evolution of the sediment load along rivers 1 in actively eroding landscapes fluvial abrasion modifies the characteristics of the sediment carried by rivers and consequently has a direct impact on the ability of mountain rivers to erode their bedrock and on the characteristics and volume of the sediment exported from upland catchments in this experimental study we use a novel flume replicating hydrodynamic conditions prevailing in mountain rivers to investigate the role played by different controlling variables on pebble abrasion during fluvial transport lithology controls abrasion rates and processes with differences in abrasion rates exceeding two orders of magnitude attrition as well as breaking and splitting are efficient processes in reducing particle size mass loss by attrition increases with particle velocity but is weakly dependent on particle size fragment production is enhanced by the use of large particles high impact velocities and the presence of joints based on our experimental results we extrapolate a preliminary generic relationship between pebble attrition rate and transport stage c where fluvial shields stress and c critical shields stress for incipient pebble motion this relationship predicts that attrition rates are independent of transport stage for c 3 and increase linearly with transport stage beyond this value we evaluate the extent to which abrasion rates control downstream fining in several different natural settings a simplified model predicts that the most resistant lithologies control bed load flux and fining ratio and that the concavity of transport limited river profiles should rarely exceed 0 25 in the absence of deposition and sorting

controls on the channel width of rivers implications for modeling fluvial incision of bedrock on the basis of the manning equation and basic mass conser vation principles we derive an expression for scaling the steady state width w of river channels as a function of discharge q channel slope s roughness n and width to depth ratio a w 5 a a1 2 2 3 3 8 q 3 8 s 23 16 n 3 8 we propose that channel width to depth ratio in addition to roughness is a function of the material in which the channel is developed and that where a river is con fined to a given material width to depth ratio and roughness can be assumed constant given these simplifications the expression emulates traditional width discharge relationships for rivers incis ing bedrock with uniformly concave fluvial long profiles more sig nificantly this relationship describes river width trends in terrain with spatially nonuniform rock uplift rates where conventional discharge based width scaling laws are inadequate we suggest that much of observed channel width variability in river channels confined by bedrock is a simple consequence of the tendency for water to flow faster in steeper reaches and therefore occupy smaller channel cross sections we demonstrate that using conventional scaling relationships for channel width can result in underestima tion of stream power variability in channels incising bedrock and that our model improves estimates of spatial patterns of bedrock incision rates

downstream changes in the lithology and grain size of fluvial gravels the watarase river japan evidence of the role of abrasion in downstream fining two processes have been considered to explain downstream grain size reduction in gravel bed rivers hydraulic sorting by which finer grains are transported faster and farther than coarse grains and abrasion by which individual particles are diminished in size most studies on the cause of downstream fining in rivers emphasize hydraulic sorting as the dominant fining mechanism however broken boulders are often observed in the bed material of japanese rivers also grain size changes may be correlated with the difference in resistance to breakdown among lithologies the purpose of this study is to use the lithologic composition of bed material on the watarase river to demonstrate that some downstream fining cannot be explained by hydraulic sorting alone the study reach on the watarase river flows on a dissected alluvial fan which means that the river is no longer aggrading the study reach is influenced only slightly by input of gravel from tributaries bed gravel in the study reach consists of several lithologies andesite quartz porphyry sandstone hornfels and chert the grain size distribution and lithology of the river bed material were examined at nine sites there are two principal results 1 size distribution of the gravel is strongly related to lithology andesite boulders or large cobbles make up the framework sizes in the upstream part of the study reach while chert pebbles make up the framework sizes in the downstream part there are few andesite pebbles or chert boulders in the river bed 2 longitudinal changes in the composition of each size class show selective transportation by lithology in every gravel size because the mobility of gravel depends mainly on its size hydraulic sorting by lithology does not occur within the same size class these results indicate clearly that particle abrasion occurs in the watarase river and is responsible at least in part for the downstream decrease in particle size of bed material

analysis of controls on downstream fining along three gravel bed rivers in the band e golestan drainage basin ne iran abstract in many gravel bed rivers grain size changes exponentially downstream and is mainly related to selective deposition of clasts on the streambed according to size shape and density and abrasion crushing grinding splitting chipping cracking and sandblasting during transport most studies on the cause of downstream fining in rivers emphasize hydraulic sorting or selective transport as the dominant fining mechanism the band e golestan drainage basin with a surface area of about 320 km 2 is located southwest of mashhad in northeast iran this catchment s area consists of three subbasins mayan dehbar and jaghargh that flow toward northeast connecting to the kashafrud river grain size variation and causes for downstream fining were studied at 57 sites compositionally gravels in the study area consist of phyllite slate and a small amount of quartzite consequently these sediments are poorly sorted positively skewed and mainly platykurtic upstream most of the boulders are phyllites while in the downstream part slate makes up the framework of the gravel grain size distributions are weakly bimodality in the upstream and become strongly polymodal downstream based on our study the main factors in downstream fining of grain size in the study area are hydraulic sorting and abrasion

has focused denudation sustained active thrusting at the himalayan topographic front the geomorphic character of major river drainages in the himalayan foothills of central nepal suggests the existence of a discrete west northwest trending break in rock uplift rates that does not correspond to previously mapped faults the 40 ar 39 ar thermochronologic data from detrital muscovites with provenance from both sides of the discontinuity indicate that this geomorphic break also corresponds to a major discontinuity in cooling ages samples to the south are proterozoic to paleozoic whereas those to the north are miocene and younger combined these observations virtually require recent pliocene holocene motion on a thrust sense shear zone in the central nepal himalaya 20 30 km south of the main central thrust field observations are consistent with motion on a broad shear zone subparallel to the fabric of the lesser himalayan lithotectonic sequence the results suggest that motion on thrusts in the toe of the himalayan wedge may be synchronous with deeper exhumation on more hinterland structures in central nepal we speculate that this continued exhumation in the hinterland may be related to intense sustained erosion driven by focused orographic precipitation at the foot of the high himalaya

stable width and depth of straight gravel rivers with heterogeneous bed materials mathematical model for defining stable width and depth of straight gravel rivers is presented in which it is revealed that the major agency for maintaining stable channel is lateral diffusion of longitudinal fluid momentum due to turbulence the primary quantities which determine the stable channel geometry are found to be flow discharge longitudinal free surface slope the median size of bed material and gradation of the bed material increasing gradation is found to increase the depth and decrease the width introduction of one loose and the other rigid bank in laboratory experiments provides good examples to test the model in which four kinds of sand were used to test the gradation effect these laboratory data support the mathematical model presented herein the theory also performs well when it is applied to natural gravel rivers

downstream variation in grain size along an alpine river analysis of controls and processes downstream variation in bed material size was studied along a 115 km reach of the piave river eastern alps italy this river drains a region producing high sediment yields and has a braided pattern along half of its length during the last century the river system has been affected by several human impacts hydroelectric dams diversions gravel mining which have strongly altered flow regime and sediment dynamics the aim of the study is to analyse the changes of bed material size in the downstream direction and to explain these changes in the light of both natural and anthropogenic factors surface material was sampled at 35 sites using the grid by number method the collected data give a detailed picture of the downstream variation in sediment size which is characterised by a complex pattern rather than by a simple decreasing trend an explanation of this variation was found through identification of discontinuities significant steps in the downstream trend and factors which can determine changes in sediment texture both natural lateral sediment sources and anthropogenic factors e g barrages are significant in this river system and they provide an explanation for most of the observed discontinuities seven out of eight discontinuities few tributaries disrupt downstream fining while as expected the barrages produce important changes in sediment texture fining processes were investigated in the lower part of the study reach where the lateral sediment sources and the barrages have minor effects on the bed material exponential models provide acceptable explanations of the downstream variations and give fining coefficients equal to 0 027 and 0 028 km 1 respectively for d50 and d84 evidence suggests that both abrasion and sorting are important in determining the downstream fining observed but it is likely that sorting is the dominant process

geomorphic limits to climate induced increases in topographic relief recognition of the potential for strong dynamic coupling between atmospheric and tectonic processes has sparked intense cross disciplinary investigation and debate on the question of whether tectonics have driven long term climate change or vice versa it has been proposed that climate change might have driven the uplift of mountain summits through an isostatic response to valley incision because isostasy acts to compensate mean elevations the debate hinges on the question of whether climate change can significantly increase topographic relief or more precisely increase the volume of missing mass between summits and ridges here we show that in tectonically active mountain ranges geomorphic constraints allow only a relatively small increase in topographic relief in response to climate change thus although climate change may cause significant increases in denudation rates potentially establishing an important feedback between surficial and crustal processes neither fluvial nor glacial erosion is likely to induce significant isostatic peak uplift

bedrock incision rock uplift and threshold hillslopes in the northwestern himalayas the topography of tectonically active mountain ranges reflects a poorly understood competition between bedrock uplift and erosion dating of abandoned river cut surfaces in the northwestern himalayas reveals that the indus river incises through the bedrock at extremely high rates 2 12 mm yr 1 in the surrounding mountains the average angles of hillslopes are steep and essentially independent of erosion rate suggesting control by a common threshold process in this rapidly deforming region an equilibrium is maintained between bedrock uplift and river incision with landsliding allowing hillslopes to adjust efficiently to rapid river down cutting

erosion and exhumation in the himalaya from cosmogenic isotope inventories of river sediments the outward erosional flux is a key factor in the tectonic evolution of mountain belts and there is much debate about the feedbacks between tectonics erosion and climate here we use cosmogenic nuclides 10be and 26al analysed in quartz from river sediments from the upper ganges catchment to make the first direct measurements of large scale erosion rates in a rapidly uplifting mountain belt the erosion rates are highest in the high himalaya at 2 7 0 3 mm yr 1 errors fall to 1 2 0 1 mm yr on the southern edge of the tibetan plateau and are 0 8 0 3 to 0 6 mm yr in the foothills to the south of the high mountains these relative estimates are corroborated by the nd isotopic mass balance of the river sediment analysis of sediment from an abandoned terrace suggests that similar erosion rates have been maintained for at least the last few thousand years the data presented here along with data recently published for european river catchments demonstrate that a log linear relationship between relief and erosion rate holds over three orders of magnitude variation in erosion rate and between very different climatic and tectonic regimes the erosion rate estimates from cosmogenic nuclides correlate well with exhumation rates calculated from previously published apatite fission track ages in the indian himalaya this confirms that much of the exhumation in the himalayan mountain chain is now balanced by erosion however exhumation rates calculated from high blocking temperature systems such as 40ar 39ar in muscovite imply lower exhumation rates rocks presently at the surface must have undergone a three to six fold increase in exhumation rate within the last few million years we show how this could be explained either by climatic forcing of erosion rate changes or by tectonics published evidence for equally rapid changes of exhumation rate in the past and the probable diachroneity in the time at which the present exhumation rates accelerated imply that tectonics has moderated at least some of the change in exhumation rates

gravel saltation 2 modeling a lagrangian model for the motion of a sediment particle in a water flow is proposed the particle motion equation is averaged over flow turbulence specialized to the case of coarse sediment particles saltating in water and coupled with a stochastic model for particle collision with the bed model predictions of statistics of gravel saltation generally agree well with experimental observations as an application of the saltation model bed load transport rates are estimated from modeled mean saltation streamwise velocity and a dynamic friction coefficient the results obtained compare favorably with typical bed load equations but they overestimate experimental measurements of gravel transport which would suggest that a bagnoldean formulation for bed load transport of coarse material by saltating particles may not be valid

new constraints on sediment flux dependent river incision implications for extracting tectonic signals from river profiles we present new field data from rivers draining across active normal faults that incise across the same lithology at the fault have been subjected to similar climatic regimes and tectonic settings and were perturbed by a well documented increase in fault slip rate ca 1 ma in spite of these similarities the rivers exhibit markedly different long profiles and patterns of catchment incision we use channel slope and hydraulic geometry data for each river to calculate bed shear stresses b and show that there is no simple relationship between peak b and the relative uplift rates across the faults u which differ by a factor of four the long term average sediment supply to each channel q s estimated from time averaged catchment erosion rates can explain the b versus u data if bedload modulates bedrock incision rate e in a strongly nonlinear way together these field data allow us for the first time to evaluate theoretical predictions of the role of sediment on river profile evolution and to quantify the magnitude of the effect in natural systems

quantifying sand provenance and erosion marsyandi river nepal himalaya we use petrographic and mineralogical data on modern sediments to investigate erosion patterns in the marsyandi basin of the central himalaya a privileged natural laboratory in which a series of multidisciplinary geomorphological sedimentological geochemical and geochronological studies have been recently carried out to unravel the interrelationships between tectonic climatic and sedimentary processes in high relief orogenic belts although relative erosion patterns are effectively constrained by analyses of replicate samples along six successive tracts of the marsyandi river uncertainties are caused by potential compositional variation between the monsoon and post monsoon season estimates of erosion rates are significantly affected by poor knowledge of total sediment flux through the basin ourresults support focusederosion of the southern tectonically lower part of the greater himalayain the hangingwall of the mct zone where thesummer monsoon reachesitspeak intensity up to 5 m a andsediment yieldsanderosion ratesreach14 100 3400t km 2 and 5 1 1 2 mm a erosion rates sharply decrease southward in low relief lesser himalayan units 1 6 0 6 mm a and also progressivelydecreasenorthwardsinthehigh altitude tectonically upperpartofthegreaterhimalaya whererainfalldecreasesrapidly to b2 m a even areas of extreme topography such as the manaslu granite are characterized by relatively low erosion rates 2 4 0 9 mm a because precipitations become too scarce to feed significant ice flux and glacial activity monsoonal rainfall decreases further tob0 5 m a in the tethys himalayan zone farther north where erosion rates are 1 mm a coupling between erosion and peak monsoonal rainfall along the southern front of the greater himalaya is consistent with both channel flow models of tectonic extrusion

downstream coarsening in headwater channels 1 field data from four mountain drainage basins in western washington document systematic downstream coarsening of median bed surface grain size d50 and a subsequent shift to downstream fining at a drainage area of about 10 km2 analyses of network wide patterns of unit stream power derived from both channel surveys and digital elevation models reveal maximum unit stream power that in all four study areas roughly corresponds with both the grain size maxima and an inflection in the drainage area slope relation thought to represent the transition from debris flow dominated channels to fluvially dominated channels our results support the hypothesis that basin wide trends in d50 are hydraulically controlled by systematic variations in unit stream power in addition to lag deposits forced by mass wasting processes the similar relations found in our four study areas suggest that the tendency for downstream coarsening may be ubiquitous in headwater reaches of mountain drainage basins where debris flow processes set the channel gradient

particle comminution defines megaflood and superflood energetics abstract the concept of megafloods and superfloods was introduced at the end of the last century to define exceptionally large discharge floods primarily those associated with the failure of quaternary ice dams these floods exceeded by one or two magnitudes historically recorded floods megafloods having peak discharges equalling or exceeding 1 m m3 s 1 herein the sediment associations of exceptional floods are found to be distinctive being dominated by comminuted smashed grain size distributions which contrast to the sediment deposits of more moderate floods a consideration of the energetics of moderate floods superfloods and megafloods with respect to the entrainment dis entrainment suspension and comminution of coarse gravel allows thresholds of energy expenditure to be proposed that reflect general transformations in flood behaviour that relate to distinctive flood deposits a threshold of energy expenditure 20 kj s 1 m 2 is proposed to separate moderate floods from superfloods the latter by this definition can be 0 1 m m3 s 1 this threshold separates floods competent to entrain cobbles and boulders with limited comminution of the load from superfloods that comminute the suspended cobbles and boulders to produce fine granules in suspension the granules are subsequently deposited to form distinct depositional units that are characteristic of superfloods and megafloods the energetic threshold is consistent with the lower range of power expenditures associated with the original definition of a megaflood in terms of peak fluid flux significance exceptional floods occurred in prehistoric times the largest are referred to as megafloods or superfloods to distinguish such exceptional floods from other large floods megafloods were defined as having a peak discharge exceeding 1 m m3 s 1 however many reports of megafloods are flows with a peak discharge smaller than 1 m m3 s 1 recent research has shown that megafloods and superfloods deposit distinctive layers of smashed gravel which is not the case for smaller floods the gravel is smashed by exceptionally high energy expenditures consideration of the energetics of floods to transport gravel allows a robust quantitative definition of superflood based on the energy needed to smash a range of gravel sizes

implications of sediment flux dependent river incision models for landscape evolution 1 developing a quantitative understanding of the factors that control the rate of river incision into bedrock is critical to studies of landscape evolution and the linkages between climate erosion and tectonics current models of long term river network incision differ significantly in their treatment of the role of sediment flux we analyze the implications of various sediment flux dependent incision models for large scale topography in an attempt 1 to identify quantifiable and diagnostic differences between models that could be detected from topographic data or from the transient responses of perturbed systems and 2 to explain the apparent ubiquity of mixed bedrock alluvial channels in active orogens although certain forms of the various models can be discarded as inconsistent with morphological data we find that the relative intrinsic concavity indices of detachment and transport limited systems defined herein largely dictate whether the various models can be tied to distinctive steady state morphologies preliminary data suggest that no such diagnostic differences may exist and other methods must be developed to test models accordingly we develop and explore differences in the scaling behavior of topographic relief and the extent of detachment versus transport limited channels as a function of rock uplift rate that may allow discrimination among various models further we explore potentially diagnostic differences in the rates and patterns of transient channel response to changes in rock uplift rate in addition to general differences between detachment and transport limited systems our analysis identifies an interesting hysteresis in landscape evolution hybrid channels at the threshold between detachment and transport limited conditions are expected to act as detachment limited systems in response to an increase in rock uplift rate or base level fall and as transport limited systems in response to a decrease in rock uplift rate especially during postorogenic topographic decline the analyses presented set the stage for field studies designed to test quantitatively the various river incision models that have been proposed

transport slopes sediment cover and bedrock channel incision in the henry mountains utah 1 field data from channels in the henry mountains of utah demonstrate that abundant coarse sediment can inhibit fluvial incision into bedrock by armoring channel beds the cover effect we compare several small channels that share tributary junctions and have incised into the same sedimentary bedrock unit navajo sandstone but contain differing amounts of coarse diorite clasts owing to the spatial distribution of localized sediment sources bedrock channels that contain abundant clasts diorite rich have steeper longitudinal slopes than tributaries of these channels with smaller drainage areas and less sediment diorite poor the diorite poor tributaries have incised more deeply to lower average slopes and have more reach scale slope variability which may reflect bedrock properties longitudinal sediment sorting and incision at lower sediment supply diorite rich channels have less bedrock exposed and smoother longitudinal profiles than diorite poor channels we find that 1 coarse sediment can mantle bedrock channel beds and reduce the efficiency of incision validating the hypothesized cover effect in fluvial incision models 2 the channel slope needed to transport the sediment load can be larger than that needed to erode bedrock suggesting that the slope of incising bedrock channels can become adjusted to the sediment load 3 when abundant sediment is available transport capacity rather than thresholds of motion can be dominant in setting bedrock channel slope and 4 cover effects can be important even when moderate amounts of bedrock are exposed in channel beds

quaternary sediment in the yichang area implications for the formation of the three gorges of the yangtze river sediment data from the yichang area in the jianghan basin of hubei province in china suggest deposition in a lacustrine environment prior to 0 75 ma b p followed by incision of the yangtze river the earliest quaternary yunchi formation accumulated in an alluvial fan to fan delta environment the subsequent shanxiyao formation was deposited in an environment that changed from fan delta to lacustrine the distribution of sedimentary facies suggests the presence of a lake in the yichang area prior to 0 75 ma b p the lack of sediments contemporaneous with the yunchi and shanxiyao formations in other areas of the jianghan basin suggests that this ancient lake was limited to the yichang area this lake predates the present yangtze river in the yichang area and the jianghan basin provenance studies of gravels in the yunchi and shanxiyao formations as well as gravels in terraces and the channel of the yangtze river indicate a variety of sediment sources but suggest that no material from the area west of the three gorges had been carried into the yichang area prior to 0 75 ma b p the yangtze river cut through three gorges area only after 0 75 ma b p

geochemical evidence for the paleoproterozoic arc back arc basin association and its importance in understanding the evolution of the central indian tectonic zone abstract the central indian tectonic zone citz is considered to be one sector in a series of paleoproterozoic orogens that stitched together the supercontinent columbia the paleoproterozoic mahakoshal belt forms the foundation of the citz metabasalts and metaclastic rocks of the mahakoshal belt have been studied for major elements and trace elements including the rare earth elements ree metabasalts of the lower saleemanabad formation are grouped into two types however both types of metabasalt depict a back arc basalt character by showing enrichments in large ion lithophile elements liles like k rb cs and ba and light rare earth elements lrees but show depletions in high field strength elements hfses like nb ta y and ti and hree heavy rare earth elements in comparison to the n morb this is further supported by some key elemental ratios like ba nb nb ta la nb eu eu nb nb and ree patterns the geochemistry of the metaclastic rocks of the lower saleemanabad middle parsoi and upper dudhmaniya formations of the mahakoshal belt show an arc signature rather than the cratonic signature in the bundlekhand ttg and n morb normalised diagrams the arc signature calc alkaline series of metaclastic rocks is further supported by various tectonic discrimination diagrams the overall study indicates that the mahakoshal belt was formed as a back arc rift basin very close to continental arcs on the hinterland of the bundelkhand craton at the initial stage of subduction as a result of trench rollback followed by thermal relaxation and basin inversion the sediment supply started from the continental arcs lying south of the mahakoshal basin furthermore the present study strengthens the mechanism of subduction related outgrowth of columbia along its continental margins and implies that the citz is a major precambrian crustal boundary that began as a mahakoshal back arc rift by northward subduction

sediment and rock strength controls on river incision into bedrock recent theoretical investigations suggest that the rate of river incision into bedrock depends nonlinearly on sediment supply challenging the common assumption that incision rate is simply proportional to stream power our measurements from laboratory abrasion mills support the hypothesis that sediment promotes erosion at low supply rates by providing tools for abrasion but inhibits erosion at high supply rates by burying underlying bedrock beneath transient deposits maximum erosion rates occur at a critical level of coarse grained sediment supply where the bedrock is only partially exposed fine grained sediments provide poor abrasive tools for lowering bedrock river beds because they tend to travel in suspension experiments also reveal that rock resistance to fluvial erosion scales with the square of rock tensile strength our results suggest that spatial and temporal variations in the extent of bedrock exposure provide incising rivers with a previously unrecognized degree of freedom in adjusting to changes in rock uplift rate and climate furthermore we conclude that the grain size distribution of sediment supplied by hillslopes to the channel network is a fundamental control on bedrock channel gradients and topographic relief

downstream variations in the width of bedrock channels field surveys of channel width w and drainage area a in bedrock channel reaches reveal relationships where w 5 ca b similar to the classic hydraulic geometry of alluvial channels data from five mountain channel networks support the assumption used in many landscape evolution models that an alluvial hydraulic geometry relationship where b 5 0 3 0 5 holds for bedrock channel systems although there is substantial local variability in channel width in bedrock channel systems there is no systematic difference in width versus drainage area relations for the surveyed bedrock and alluvial reaches in sedimentary lithologies in coastal oregon and washington in contrast bedrock channels were narrower and therefore had deeper flow than alluvial channels with equal drainage areas in the granite and limestone terrain of the yuba river california in addition data from the mokelumne river show that bedrock channel width decreases substantially downstream at the contact between relatively weak limestone and more erosion resistant granite but that channel slope does not change appreciably across contacts between these two lithologies data from coastal oregon drainage basins further show systematic channel widening after flood flows and debris flow impacts we conclude that downstream variations in the width of bedrock channels generally follow traditional hydraulic geometry relations but also reflect the local influence of longitudinal patterns of bedrock erosivity and disturbance history

evaluating a 2d image based computerized approach for measuring riverine pebble roundness abstract the geometrical characteristics of pebbles are important features to study transport pathways sedimentary history depositional environments abrasion processes or to target sediment sources both the shape and roundness of pebbles can be described by a still growing number of metrics in 2d and 3d or by visual charts despite new developments existing tools remain proprietary and no pebble roundness toolbox has been available widely within the scientific community the toolbox developed by roussillon et al 2009 automatically computes the size shape and roundness indexes of pebbles from their 2d maximal projection plans using a digital camera this toolbox operates using 2d pictures taken of pebbles placed on a one square meter red board allowing data collection to be quickly and efficiently acquired at a large scale now that the toolbox is freely available for download 1 an evaluation is needed to help end users sample process and interpret the data we first assess the influence of pebble position and of the resolution and enhancement of pictures and errors associated with the sampling size second we run experimental flume tests of roundness trends that confirm the great potential of the toolbox for river geomorphic diagnoses as well as for targeting sediment sources we also validate the correlation between roundness metric trends and particle abrasion

coarse sediment dynamics in a large glaciated river system holocene history and storage dynamics dictate contemporary climate sensitivity the gravel bedded white river drains a 1279 km2 basin in washington state with lowlands sculpted by continental glaciation and headwaters on an actively glaciated stratovolcano chronic aggradation along an alluvial fan near the river s mouth has progressively reduced flood conveyance in order to better understand how forecasted climate change may influence coarse sediment delivery and aggradation rates in this lowland depositional setting we assessed the contemporary delivery and routing of coarse sediment through the watershed this assessment was based on a rich set of topographic sedimentologic and hydrologic data from the past century with a focus on repeat high resolution topographic surveys from the past decade we found that most of the lower river s contemporary bed load flux originates from persistent erosion of alluvial deposits in the lower watershed this erosion is a response to a drop in local base level caused by a major avulsion across the fan in 1906 and then augmented by subsequent dredging the 1906 avulsion and modern disequilibrium valley profiles reflect landscape conditioning by continental glaciation and a massive mid holocene lahar in the proglacial headwaters infrequent large sediment pulses have accomplished most of the observed coarse sediment export with exported material blanketing downstream valley floors during typical floods transported bed material is largely sourced from erosion of these valley floor deposits throughout the watershed we observe decadal scale coarse sediment dynamics strongly related to the filling or emptying of valley scale sediment storage over 102 104 yr time scales often in response to major disturbances that either emplace large deposits or influence their redistribution paraglacial responses in large watersheds are suggested to be inherently complicated and punctuated as a result of internal landform interactions and stochastic threshold dependent events we argue in combination that holocene disturbance storage dynamics and human flow modification make coarse sediment fluxes in the lower white river relatively insensitive to decadal climate variability results highlight the degree to which river sensitivity to contemporary disturbance climatic or otherwise may be contingent on local and idiosyncratic watershed histories underscoring the need to unpack those histories while demonstrating the utility of watershed scale high resolution topography toward that end

andean growth and monsoon winds drive landscape evolution at sw margin of south america abstract in the atacama desert the driest place on earth located at the subduction margin of the andes the landscape evolves very slowly and changes in tectonic or erosion processes remain for a long time in the memory of topography at latitude 19 30 s a threshold between exoreic and endoreic drainage regimes is clearly associated with the latitudinal gradient imposed by the modern monsoon carrying humidity from the atlantic and disposed obliquely over catchments draining the andes to the pacific we summarize the geomorphic geological and climatic data in the threshold area we then use these data to constrain numerical experiments of drainage evolution data and experimental results are consistent with the development of a flat low energy morphology close to sea level interrupted at 10 ma by tectonic uplift prevailing to the present suggesting trench ward relief growth by incorporation of the coastal atacama region to the andes mountain belt

lithological controls on hillslope sediment supply insights from landslide activity and grain size distributions the volumes rates and grain size distributions of sediment supplied from hillslopes represent the initial input of sediment delivered from upland areas and propagated through sediment routing systems moreover hillslope sediment supply has a significant impact on landscape response time to tectonic and climatic perturbations however there are very few detailed field studies characterising hillslope sediment supply as a function of lithology and delivery process here we present new empirical data from tectonically active areas in southern italy that quantifies how lithology and rock strength control the landslide fluxes and grain size distributions supplied from hillslopes landslides are the major source of hillslope sediment supply in this area and our inventory of 2800 landslides reveals that landslide sediment flux is dominated by small shallow landslides we find that lithology and rock strength modulate the abundance of steep slopes and landslides and the distribution of landslide sizes outcrop scale rock strength also controls the grain sizes supplied by bedrock weathering and influences the degree of coarsening of landslide supply with respect to weathering supply finally we show that hillslope sediment supply largely determines the grain sizes of fluvial export from catchments and that catchments with greater long term landslide rates deliver coarser material therefore our results demonstrate a dual control of lithology on hillslope sediment supply by modulating both the sediment fluxes from landslides and the grain sizes supplied by hillslopes to the fluvial system

sediment residence times in catchments draining to the gulf of carpentaria northern australia inferred by uranium comminution dating abstract uranium u isotopes are useful for constraining the timescales of weathering and erosion processes the 234u 238u activity ratio parentheses denote activity ratio of fine grained detrital minerals is proposed to record the time elapsed since mineral grains were reduced to

selective sorting and abrasion of river gravel ii applications most gravel rivers show a tendency for characteristic grain size to decrease in the downstream direction over scales of tens or hundreds of kilometers it has been surmised that this downstream fin