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

supply and removal of sediment in a landslide dominated mountain belt central range taiwan abstract a strong coupling between hillslope and valley systems is often inferred for mountain landscapes dominated by bedrock landsliding we reveal the nature of this link using data sets on landsliding and sediment transport from two montane catchments draining the eastern central range of taiwan here the magnitude frequency distribution of landslides can be modeled by a robust power law but this scale invariance is not mirrored in the sediment discharge at the mountain front instead downstream sediment loads reflect a complex response to both sediment supply and ambient hydraulic conditions the rivers do not transport significant amounts of sediment unless it is provided by hillslope mass wasting in the catchment removal of landslide debris is a function of the transport capacity of the stream at the site of entry thus there is a dual supply and transport control on sediment loads in bedrock floored streams over a monitoring period of 25 yr the bulk of the sediment leaving the mountain bel

probabilistic exner sediment continuity equation for mixtures with no active layer the exner equation of sediment continuity is the foundation of river morphodynamics generalization of this equation to mixtures of grain sizes has required the introduction of an active layer i e a buffer layer between the sediment moving in the water column and the immobile substrate below the active layer is defined to be a well mixed layer with no vertical structure that encompasses those grains available to exchange directly with the moving sediment the sediment in the substrate below exchanges with the active layer only as the bed aggrades or degrades the active layer concept is a useful one that has served the research community well for 3 decades however the division of the erodible bed into a discrete active layer and substrate must represent only an approximation of a more general formulation that contains no active layer and in which parameters pertaining to the entrainment from and deposition to the bed vary continuously with depth below the sediment water interface here the probability density function of bed elevation is used to derive a general exner equation of sediment continuity with no discrete layers the formulation is applicable to both sediment mixtures and tracers in uniform sediment although the treatment requires more information than that of the active layer approach it offers the prospect of a better understanding of how streams create a stratigraphic record of their activities through deposition

scour chain employment in gravel bed rivers abstract the dynamics of river beds can be assessed fromscour and fill data obtained from scour chains chains have not been used extensively in gravel bed rivers due to a variety of employment difficulties chain insertion anchoring and relocation can be determined with minimal manpower by utilising a set of tested employment methodologies these are decribed in sufficient detail to allow fluvial geomorphologists ecologists and river engineers to more commonly utilise chains in studies of river stability and river response

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

estimates of the rate of regolith production using and from an alpine hillslope abstract the production of regolith is a fundamental geomorphic process because most surface processes transport only unconsolidated material we use concentrations of the cosmogenic radionuclides crns 10 be and 26 al in regolith and bedrock to deduce the rate of production of regolith on an alpine hillslope in the wind river range wy these calculations are based on a theoretical model which we develop here this model shows that it is important to consider dissolution of regolith in regolith production and in basin averaged erosion rate studies rates of production of regolith are uniform along the hillslope and the mean rates for the entire hillslope deduced from 10 be and 26 al are 14 3 4 0 and 13 0 4 0 m ma 1 respectively rates of production of regolith deduced from 10 be concentrations in regolith mantled bedrock support the rates deduced from regolith concentrations in the alpine environment examined here the rate of production of regolith beneath 90 cm of regolith is nearly twice as fast as the average rate of production of regolith on bare rock surfaces which small et al small e e anderson r s repka j l finkel r 1997 erosion rates of alpine bedrock summit surfaces deduced from in situ 10 be and 26 al earth and planetary science letters 150 413 425 previously documented rock mantled with regolith probably weathers more rapidly than bare rock because the water required for frost weathering is limited on bare rock surfaces because the hillslope examined here is convex with constant curvature and regolith production and thickness are uniform down the slope the regolith volume flux must be proportional to the local slope of the hillside therefore our results are consistent with gilbert s gilbert g k 1909 the convexity of hilltops journal of geology 17 344 350 steady state hillslope hypothesis if tor height and the difference between rates of weathering on bare and regolith mantled rock provide a fair estimate of the age of summit flats steady state hillslope conditions have been attained in less than several million years

time scales of tectonic landscapes and their sediment routing systems abstract in regions undergoing active tectonics the coupling between the tectonic displacement field the overlying landscape and the redistribution of mass at the earth9s surface in the form of sediment routing systems is particularly marked and variable coupling between deformation and surface processes takes place at a range of scales from the whole orogen to individual extensional fault blocks or contractional anticlines at the large scale the attainment of a steady state between the overlying topography and the prevailing tectonic conditions in active contractional orogens requires an efficient erosional system with a time scale dependent on the vigour of the erosional system generally in the range 10 6 10 7 years the catchment fan systems associated with extensional fault blocks and basins of the western usa are valuable natural examples to study the coupling between tectonic deformation landscape and sediment routing systems even relatively simple coupled systems such as an extensional fault block and its associated basin margin fans have a range of time scales in response to a tectonic perturbation these response times originate from the development of uniform steady state relief during the accumulation of displacement on a normal fault c 10 6 years the upstream propagation of a bedrock knickpoint in transverse catchments following a change in tectonic uplift rate c 10 6 years or the relaxation times of the integrated catchment fan system in response to changes in climatic and tectonic boundary conditions 10 5 10 6 years the presence of extensive bedrock or alluvial piedmonts increases response times significantly the sediment efflux of a mountain catchment is a boundary condition for far field fluvial transport but the fluvial system is much more than a simple transmitter of the sediment supply signal to a neighbouring depocentre fluvial systems appear to act as buffers to incoming sediment supply signals with a diffusive time scale c 10 5 10 6 years dependent on the length of the system and the extent of its floodplains stream channels and proximal gravel fans the vocabulary for explaining landscapes would benefit from a greater recognition of the importance of the repeat time and magnitude of perturbations in relation to the response and relaxation times of the landscape and its sediment routing systems landscapes are best differentiated as buffered or reactive depending on the ratio of the response time to the repeat time of the perturbation furthermore landscapes may be regarded as steady or transient depending on the ratio of the response time to the time elapsed since the most recent change in boundary conditions the response of tectonically and climatically perturbed landscapes has profound implications for the interpretation of stratigraphic architecture

size and distance of travel of unconstrained clasts on a streambed we have examined the relation between the mean distance of movement during individual flood events and the mean particle size of unconstrained clasts in gravel bed rivers distance of movement was scaled using the mean distance of movement of the median size group of the surface material size was scaled using the median b axis diameter of the subsurface material the scaled data are fitted with a nonlinear function clasts larger than the median of surface material exhibit a steep decline of mobility with an upper limit for mobility near di d50sub 10 smaller materials exhibit a less sensitive relation we suppose that trapping of small clasts affects their travel distance these results indicate that the movement of free surface stones is size preferred a subordinate analysis of free constrained and buried particles reveals that bed surface structure acts to reduce differential movement

fluvial erosion transport equation of landscape evolution models revisited we present a mesoscale erosion deposition model which differs from previous landscape evolution models equations by taking explicitly into account a mass balance equation for the streamflow the geological and hydrological complexity is lumped into two basic fluxes erosion and deposition and two averaged parameters unit width discharge q and stream slope s the model couples the dynamics of streamflow and topography through a sediment transport length function x q which is the average travel distance of a particle in the flow before being trapped on topography this property reflects a time lag between erosion and deposition which allows the streamflow not to be instantaneously at capacity the so called x q model may reduce either to transport limited or to detachment limited erosion modes depending on x but it also may not we show in particular how it does or does not for steady state topographies long term evolution and high frequency base level perturbations apart from the unit width discharge and the settling velocity the x q function depends on a dimensionless number encompassing the way sediment is transported within the streamflow using models of concentration profile through the water column we show the dependency of this dimensionless coefficient on the rouse number we discuss how consistent the x q model framework is with bed load scaling expressions and einstein s conception of sediment motion

mass accumulation rates in asia during the cenozoic summary this work establishes estimates of mass accumulation rates in 18 mostly offshore sedimentary basins in asia since the beginning of the cenozoic 66 ma the estimates were derived from isopach maps cross sections and drill holes or stratigraphic columns assuming regional similarity of the strata average solid phase volumes and accumulation rates were calculated for nine epochs approximately corresponding to geological periods palaeocene 66 58 ma eocene 58 37 ma oligocene 37 30 and 30 24 ma miocene 24 17 17 11 and 11 5 ma pliocene 5 2 ma and quaternary 2 0 ma these rates shed new light on the geological history of asia since the onset of the collision of india with asia 50 ma the overall average accumulation rates curve for asian sedimentary basins since the beginning of the tertiary shows an exponential form with slow accumulation rates less than 0 5 106 km3 myr 1 until the beginning of the oligocene more than 15 myr after the onset of the collision from the oligocene onwards rates increase quickly in an exponential manner reaching their maximum values in the quaternary more than 1 5 106 km3 myr 1 from these observations we suggest that extrusion and crustal shortening are complementary processes that have been successively dominant throughout the india eurasia collision history at smaller scales one may distinguish between independent histories at the subcontinental and basin scales this permits a comparison of the relative importance of tectonic and climatic erosion processes affecting the different mountain belts of asia during the cenozoic

physically based model of downstream fining in bedrock streams with lateral input 1 bed load particles in bedrock streams receiving lateral input from hillslopes may or may not show a clear monotonic pattern of size reduction in the downstream direction both abrasion and selective sorting may play important roles in generating downstream fining the objective of this study is to develop a physically based model of downstream fining in bedrock streams with lateral input based on both processes a surface based gravel transport relation for the size mixture is employed to account for the effect of selective sorting differential transport while the model produces silt and sand by abrasion it is also assumed to loosely capture particle fracturing via a lumped abrasion coefficient embodied in sternberg s law the model is here tested against field data from vieux habitants river in guadeloupe island which is located in the caribbean sea the river shows clear downstream fining and this pattern is captured reasonably well by the model the model results indicate that abrasion including fracturing is solely responsible for the downstream fining pattern for most of the study reach of the vieux habitants river sensitivity analysis of the model has also been performed the model results suggest that in general selective sorting by differential transport can play a role in downstream fining only in cases of streams with relatively fine gravel sizes and lower slopes the results also indicate that abrasion including fracturing and selective sorting can be equally important for downstream fining in bedrock rivers the results also suggest that future work should explicitly consider fracturing comminution separately from abrasion wear rather than lumping them in a sternberg type coefficient

numerical and analytical models of cosmogenic radionuclide dynamics in landslide dominated drainage basins 1 in many tectonically active regions on earth landslides dominate sediment delivery to channels while cosmogenic radionuclides crn represent a valuable tool to document basin averaged erosion rates a better understanding of crn dynamics in these systems is required before erosion rates can be reliably inferred from crn concentrations the stochastic nature of landslides results in variability of crn concentrations in both alluvial and hillslope samples that is both difficult to document and poorly understood to guide sampling strategies we developed a 2 d plan view model to simulate the influence of landslides on crn dynamics we employ a sediment mixing model in which concentrations of crns from material eroded by both landslides and steady processes are combined to evaluate how best to infer erosion rates from crn concentrations this model is further used to test a new analytical solution for the distribution of crn concentrations on hillslopes in a landslide prone system results suggest that for landscapes dominated by deep 5 m landslides only large drainage basins 100 km2 provide consistently reliable estimates of erosion rates from crn concentrations for landscapes dominated by shallow landslides 0 7 2 m smaller basins can produce reliable estimates results further suggest that the distribution of concentrations from hillslope samples could be used to deduce the relative contribution of material derived from landslide and steady processes these findings underscore the need to consider carefully the sources of natural variability in sampling and analyzing crns if properly done the variability can reveal useful information about geomorphic processes in landslide prone areas

cosmogenic 21ne analysis of individual detrital grains opportunities and limitations we use a numerical model describing cosmogenic nuclide acquisition in sediment moving through the upper gaub river catchment to evaluate the extent to which aspects of source area geomorphology and geomorphological processes can be inferred from frequency distributions of cosmogenic 21ne 21nec concentrations in individual detrital grains the numerical model predicts the pathways of sediment grains from their source to the outlet of the catchment and calculates the total 21nec concentration that each grain acquires along its pathway the model fully accounts for variations in nuclide production due to changes in latitude altitude and topographic shielding and allows for spatially variable erosion and sediment transport rates model results show that the form of the frequency distribution of 21nec concentrations in exported sediment is sensitive to the range and spatial distribution of processes operating in the sediment s source areas and that this distribution can be used to infer the range and spatial distribution of erosion rates that characterise the catchment the results also show that lithology can affect the form of the 21nec concentration distribution indirectly by exerting control on the spatial pattern of denudation in a catchment model results further indicate that the form of the distribution of 21nec concentrations in the exported sediment can also be affected by the acquisition of 21nec after detachment from bedrock in the diffusive hillslope and or advective fluvial domains however for such post detachment nuclide acquisition to be important this effect needs to at least equal the nuclide acquisition prior to detachment from bedrock copyright 2009 john wiley and sons ltd

the role of sediment in controlling steady state bedrock channel slope implications of the saltation abrasion incision model abstract the saltation abrasion model is a mechanistic model for river incision into bedrock by saltating bedload which we have previously derived and used experimental data to constrain all parameter values here we develop a method for applying the saltation abrasion model at a landscape scale and use the model as a reference for evaluating the behavior of a wide range of alternative incision models in order to consider the implications of the saltation abrasion model as well as other models for predicting topographic steady state channel slope to determine the single valued discharge that best represents the effects of the full discharge distribution in transporting sediment and wearing bedrock we assume all runoff can be partitioned between a low flow and a high flow discharge in which all bedload sediment transport occurs during high flow we then use the gauged discharge record and measurements of channel characteristics at a reference field site and find that the optimum discharge has a moderate magnitude and frequency due to the constraints of the threshold of grain motion and bed alluviation by high relative sediment supply incision models can be classified according to which of the effects of sediment on bedrock incision are accounted for using the predictions of the saltation abrasion model as a reference we find that the threshold of motion is the most important effect that should be represented explicitly followed in order of decreasing importance by the cover effect the tools effect and the threshold of suspension effect models that lack the threshold of motion over predict incision rate for low shear stresses and under predict the steady state channel slope for low to moderate rock uplift rates and rock strengths models that lack the cover effect over predict incision rate for high sediment supply rates and fail to represent the degree of freedom in slope adjustment provided by partial bed coverage models that lack the tools effect over predict incision rate for low sediment supply rates and do not allow for the possibility that incision rate can decline for increases in shear stress above a peak value overall the saltation abrasion model predicts that steady state channel slope is most sensitive to changes in grain size such that the effect of variations in rock uplift rate and rock strength may affect slope indirectly through their possible but as yet poorly understood influence on the size distribution of sediments delivered to channel networks by hillslopes

is it possible to quantify pebble abrasion and velocity in rivers using terrestrial cosmogenic nuclides 1 attrition rates and mean transport velocities of coarse fluvial sediments averaged over long periods of time 100 a are yet to be fully quantified a knowledge of long term transport rates would allow us to predict the response of fluvial systems to changes in external conditions while determining clast attrition rates would improve our understanding of fluvial abrasion processes the concentration of terrestrial cosmogenic nuclides tcns in boulders can be modified during river transport by attrition and temporary exposure which are two competing processes in order to evaluate the potential of tcns to quantify these processes a stochastic model of tcn concentration evolution in clasts of different sizes is developed from exhumation on a hillslope to a sampling point in a river the model includes shallow landslides or movement in a regolith and episodic transport of clasts in a sediment layer on the river bed we study the downstream evolution of tcn concentrations for a population of clasts sourced from a localized tcn rich lithology on a hillslope when attrition is strong the model predicts that the variance and maximum of tcn concentrations decrease downstream such a trend is possible only if the dominant hillslope erosion corresponds to shallow landslides natural variability of hillslope processes can hamper the trend a way to limit the scatter is to sample the biggest clasts on the contrary if clast attrition is small and river transport slow tcn concentrations increase downstream in this case combining the tcn concentrations of clasts gathered at several river stations should provide a method to estimate their mean transport rate our results offer guidelines to interpret the downstream evolution of tcn concentration in pebbles and at the surface of boulders

constrained 1d joint inversion of seismic surface waves and p refraction traveltimes we present a joint inversion scheme that couples p wave refraction and seismic surface wave data for a layered subsurface an algorithm is implemented with a damped least squares approach the estimated parameters are s and p wave velocities and layer thicknesses while densities are assumed constant during inversion the coupling is both geometric and physical layer thicknesses are the same for sand p wave velocity profiles and p wave velocities enter in both forward algorithms sensitivity analysis performed on synthetic data reveals that surface wave dispersion curves can be sensitive also to p wave velocity of some layers especially for poisson s ratio values smaller than about 0 35 allowing synergic resolution of this parameter applications on both synthetic and field data show that the proposed approach mitigates the hidden layer problem of seismic refraction and leads to more accurate results than individual inversions also for surface waves additional constraints on the objective function on a priori poisson s ratio values allow unrealistic and not admissible vp and vs values to be avoided such constraints were applied in one field case considering the a priori information available about water table depth it is also shown that estimation of porosity can help the selection of the proper constraint on a priori poisson s ratio

transpressional tectonics and stream terraces of the gobi altay mongolia we studied the patterns rates and evolution of fluvial terraces and fault system during the building process of an intracontinental transpressional mountain in the gobi altay mongolia by analyzing incisions and offsets of fluvial terraces and alluvial fans we show that the massif has grown by outward migration of thrust faults through time on the northern flank the present bounding thrust fault began its activity 600 ka ago while a more internal sub parallel fault was still active until 200 100 ka vertical offset of an alluvial fan abandoned 100 ka ago allows an estimate of 0 1 mm yr upper pleistocene holocene uplift rate the morphology of the catchment piedmont system strongly suggests a periodical formation of the alluvial surfaces controlled by the climatic pulses at the beginning of the wet interglacial periods the abandonment of the alluvial terraces lags by several thousand years the abandonment of the alluvial fans showing a diachronous incision propagating upstream the incision rate deduced from the different elevations of straths exceeds of one order of magnitude the rock uplift rate this excess is mostly due to ongoing drainage network growth at the core of the massif and incision due to alluvial apron entrenchment near the outlet this implies that fluvial response is mainly controlled by drainage growth interaction with piedmont and cyclic climatic variations rather than by rock uplift

from source to sink preserving the cosmogenic 10be derived denudation rate signal of the bolivian andes in sediment of the beni and mamore foreland basins abstract the denudation rate signal of the bolivian andes as measured by cosmogenic 10 be in sediment is preserved in the floodplain of adjacent foreland basins even though these basins store the sediment for thousands of years this conclusion is drawn from comparing published andean source area denudation rates with new cosmogenic 10 be data as measured in the floodplains of the large beni and mamore basins for the entire beni basin including the sediment producing andes and the vast flooded plains of the foreland the cosmogenic nuclide derived denudation rate is 0 45 0 06 mm yr while that of the mamore basin is 0 55 0 19 mm yr by comparison the respective andean source areas erode at averaged rates of 0 37 0 06 mm yr upper beni and at 0 56 0 09 mm yr upper mamore we notice a decrease in variability of denudation rate with increasing spatial scale as small scale processes are averaged out sediment mixing within the floodplain damps scatter of nuclide derived denudation rates picked up in the source area on the temporal scale a remarkable agreement between cosmogenic nuclide derived rates averaging over a few kiloyears with those from fission track dating averaging over millions of years seems to suggest that cosmogenic nuclide derived denudation rates capture the long term erosional characteristics of the mountain belt the sum of these observations suggests that any sample collected along a river traversing a floodplain will yield the denudation rate of the source area this finding opens the unique possibility of constraining cosmogenic nuclide derived paleo sediment budgets for these large basins as the long term spatially averaged denudation rate signal of the sediment producing area is preserved in sedimentary archives

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

erosion timescales derived from u decay series measurements in rivers the relative importance of the factors influencing weathering of continental rocks has been a topic of debate for the last few decades the principal reasons are the lack of reliable proxies for chemical weathering and the difficulty in constraining actual physical denudation rates in this study 234u 238u 230th 238u and 226ra 238u were measured by tims and by mc icp ms in the dissolved and suspended loads of rivers from the mackenzie basin northwest territories canada the data show a complementary nature between 234u 238u 230th 238u in the dissolved and suspended loads while 226ra is characterized by a more complex behavior modeling of fractionation of u series nuclides in the particulate matter and the corresponding dissolved phase permits us to constrain the duration of chemical erosion for the suspended load currently sampled in the watershed 9 28 10 ka as well as the rates of release of u series nuclides the results also imply significant recent changes of chemical erosion rates and underline the impact of the last glaciation on current continental fluxes of northern latitude rivers such as the mackenzie river

landslide rupture and the probability distribution of mobilized debris volumes 1 landslide driven erosion is controlled by the scale and frequency of slope failures and by the consequent fluxes of debris off the hillslopes in this paper we tackle the magnitude frequency part of the process and develop a theory of initial slope failure and debris mobilization that reproduces the heavy tailed distributions probability density function or pdfs observed for landslide source areas and volumes landslide rupture propagation is treated as a quasi static noninertial process of simplified elastoplastic deformation with strain weakening debris runout is not considered the model tracks the stochastically evolving imbalance of frictional cohesive and body forces across a failing slope and uses safety factor concepts to convert the evolving imbalance into a series of incremental rupture growth or arrest probabilities a single rupture is simulated with a sequence of weighted coin tosses with weights set by the growth probabilities slope failure treated in this stochastic way is a survival process that generates asymptotically power law tail pdfs of area and volume for rock and debris slides predicted scaling exponents are consistent with analyses of landslide inventories the primary control on the shape of the model pdfs is the relative importance of cohesion over friction in setting slope stability the scaling of smaller shallower failures and the size of the most common landslide volumes are the result of the low cohesion of soil and regolith whereas the negative power law tail scaling for larger failures is tied to the greater cohesion of bedrock

cover effect in bedrock abrasion a new derivation and its implications for the modeling of bedrock channel morphology the sediment load of a bedrock river plays an important role in the fluvial incision process by providing tools for abrasion the tools effect and by covering and thereby protecting the bed the cover effect we derive a new formulation for the cover effect in which the fraction of exposed bed area falls exponentially with increasing sediment flux or decreasing transport capacity and explore its consequences for the model of bedrock abrasion by saltating bed load erosion rates predicted by the model are higher than those predicted by earlier models in a closed system the maximum erosion rate is predicted to occur when sediment supply is equal to transport capacity for a flat bed by optimizing the channel geometry to minimize the potential energy of the stream and using representative values for both discharge and grain size we derive equations for the geometry of a bedrock river and explore how predictions for width slope and bed cover vary as functions of drainage area rock uplift rate and rock strength the equations predict a dependence of channel width on drainage area similar to the relations using a simple shear stress incision law the slope area relationship is predicted to be concave up in a log log regime with a curvature dependent on uplift rate however this curvature does not deviate sufficiently from a straight line to allow discrimination between models using empirical data dependence of channel width and slope on rock uplift rate can be separated into two domains for low uplift rates channel geometry is largely insensitive to uplift rate due to a threshold effect at high uplift rates there is a power law dependence bed cover is predicted to increase progressively downstream and to increase with increasing uplift rate in our model the width to depth ratio is a function of both tectonic and climatic forcing this indicates that the scaling between channel width and bed slope is neither a unique indicator of tectonic forcing at steady state nor a signature of transience or steady state we conclude that sediment effects need to be taken into account when modeling bedrock channel morphology

formation of fluvial hanging valleys theory and simulation 1 although only recently recognized hanging tributary valleys in unglaciated tectonically active landscapes are surprisingly common stream power based river incision models do not provide a viable mechanism for the formation of fluvial hanging valleys thus these disequilibrium landforms present an opportunity to advance our understanding of river incision processes in this work we demonstrate that thresholds apparent in sediment flux dependent bedrock incision rules provide mechanisms for the formation of hanging valleys in response to transient pulses of river incision we simplify recently published river incision models in order to derive analytical solutions for the conditions required for hanging valley formation and use these results to guide numerical landscape evolution simulations analytical and numerical results demonstrate that during the response to base level fall sediment flux dependent incision rules may create either temporary or permanent hanging valleys these hanging valleys form as a consequence of 1 rapid main stem incision oversteepening tributary junctions beyond some threshold slope or 2 low tributary sediment flux response during the pulse of main stem incision thus limiting the tributary s capacity to keep pace with main stem incision the distribution of permanent and temporary hanging valleys results from four competing factors the magnitude of base level fall the upstream attenuation of the incision signal the lag time of the sediment flux response and the nonsystematic variation in tributary drainage areas within the stream network the development of hanging valleys in landscapes governed by sediment flux dependent incision rules limits the transmission of base level fall signals through the channel network ultimately increasing basin response time

simulation of individual particle movement in a gravel streambed a two dimensional simulation model of travel distances of individual particles in a gravel bed river is presented the model is based on a number of rules which include particle size entrainment trajectory distance of movement and entrapment particle interactions are controlled by resistance fields defined about each obstacle and critical elevation defined in the model resistance fields particle dropping and critical elevation rules control particle interactions the interaction rules cause the particles to develop pebble clusters stone cells and transverse structures transverse ribs the simulated travel distances of individual particles are consistent with reported field results individual particle travel distances were simulated using two different models one without interactions between the individual particles and the stationary bed and one with interactions the case without interactions demonstrates the random nature of sediment transport and narrow ranges of travel distances wider ranges of travel distances similar to those for natural situations were obtained for the cases with interactions the more intense the interaction between the mobile stones and the stationary ones the wider the range of distances of travel for a given particle size modelling the mean travel distance yielded a result similar to that published previously which was based on empirical data well developed bed surface structures were obtained for relatively poorly sorted sediment with intense interactions between particles transverse structures developed when relatively large particles were allowed to move copyright 2002 john wiley sons ltd

tracer pebble movement along a concave river profile virtual velocity in relation to grain size and shear stress over 1400 tracer pebbles 16 256 mm in diameter were tracked for 2 years in six reaches of allt dubhaig scotland a small gravel bed river along which shear stress and bed surface grain size decrease toward a local base level pebble movement was size selective both within and between reaches within reaches the decrease in mean travel distance with increasing grain size is strongest in the coarse tail of the size distribution particle shape has a minor secondary effect a nondimensional grain velocity averaged over the duration of competent flow is used to compare different size classes and reaches over 90 of its variance is explained by relative grain size and reach shields stress the pattern of size selectivity is consistent with single event tracer results elsewhere bedload trap data from our distal reach and the concept of partial mobility it provides a mechanism for strong downstream fining by selective transport and deposition along rivers in which stress declines toward base level the nondimensional prediction equation for grain velocity may be of use in other rivers but requires testing

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

late cenozoic deformation and uplift of the western flank of the altiplano evidence from the depositional tectonic and geomorphologic evolution and shallow seismic activity northern chile at 19 30 s 1 we analyze the west vergent thrust system wts along the western flank of the altiplano in northern chile 18 s 21 s in our study area 19 20 s 19 50 s the wts consists of three thrust propagation monocline folds flexures developing growth strata the relative uplift accommodated by the flexures is rapid between 26 and 8 ma 0 1 mm yr diminishing to 0 02 mm yr after 8 ma approximately 2000 m of relative surface uplift was accommodated by the flexures since the late oligocene sedimentological and geomorphological analysis shows that westward tilting of the forearc occurred after 10 ma coeval with the shifting of deformation from the altiplano to the sub andean zone where the underthrusting of the brazilian craton would have resulted in crustal thickening surface uplift in the orogen and westward ductile subcrustal flow forearc tilting is accommodated by east vergent thrusts ets issued from the benioff zone beneath the central depression emerging into the western cordillera contributing 500 1400 m of surface uplift the wts connects the ets in the brittle ductile crustal transition 25 km depth continuing farther east as the altiplano low velocity zone configuring the western altiplano as a crustal scale fault bend fold forearc tilting would be caused by westward ductile flow in the lower crust pushing the rigid forearc in the ets meanwhile between 19 s and 21 s the wts accommodates dextral strike slip and 3 km of n s shortening occurred in the coastal cordillera transcurrence and strain partitioning are probably the result of slight plate convergence obliquity strong coupling within the interplate zone westward continental concavity and high elevation opposing horizontal contraction