physically based modeling of bedrock incision by abrasion plucking and macroabrasion 1 many important insights into the dynamic coupling among climate erosion and tectonics in mountain areas have derived from several numerical models of the past few decades which include descriptions of bedrock incision however many questions regarding incision processes and morphology of bedrock streams still remain unanswered a more mechanistically based incision model is needed as a component to study landscape evolution major bedrock incision processes include among other mechanisms abrasion by bed load plucking and macroabrasion a process of fracturing of the bedrock into pluckable sizes mediated by particle impacts the purpose of this paper is to develop a physically based model of bedrock incision that includes all three processes mentioned above to build the model we start by developing a theory of abrasion plucking and macroabrasion mechanisms we then incorporate hydrology the evaluation of boundary shear stress capacity transport an entrainment relation for pluckable particles a routing model linking in stream sediment and hillslopes a formulation for alluvial channel coverage a channel width relation hack s law and exner equation into the model so that we can simulate the evolution of bedrock channels the model successfully simulates various features of bed elevation profiles of natural bedrock rivers under a variety of input or boundary conditions the results also illustrate that knickpoints found in bedrock rivers may be autogenic in addition to being driven by base level fall and lithologic changes this supports the concept that bedrock incision by knickpoint migration may be an integral part of normal incision processes the model is expected to improve the current understanding of the linkage among physically meaningful input parameters the physics of incision process and morphological changes in bedrock streams

importance of a stochastic distribution of floods and erosion thresholds in the bedrock river incision problem 1 fluvial erosion of bedrock occurs during occasional flood events when boundary shear stress exceeds a critical threshold to initiate incision therefore efforts to model the evolution of topography over long timescales should include an erosion threshold and should be driven by a stochastic distribution of erosive events however most bedrock incision models ignore the threshold as a second order detail in addition climate is poorly represented in most landscape evolution models so the quantitative relationship between erosion rate and measurable climatic variables has been elusive here we show that the presence of an erosion threshold when combined with a well constrained probabilistic model of storm and flood occurrence has first order implications for the dynamics of river incision in tectonically active areas first we make a direct calculation of the critical shear stress required to pluck bedrock blocks for a field site in new york second we apply a recently proposed stochastic threshold bedrock incision model to a series of streams in california with known tectonic and climatic forcing previous work in the area has identified a weak relationship between channel gradient or relief and rock uplift rate that is not easily explained by simpler detachment limited models the results with the stochastic threshold model show that even low erosion thresholds which are exceeded in steep channels during high frequency flood events fundamentally affect the predicted relationship between gradient and uplift rate in steady state rivers in a manner consistent with the observed topography this correspondence between theory and data is however nonunique models in which a thin alluvial cover may act to inhibit channel incision in the low uplift rate zone also provide plausible explanations for the observed topography third we explore the broader implications of the stochastic threshold model to the development of fluvial topography in active tectonic settings we suggest that continued field applications of geomorphic models including physically meaningful thresholds and stochastic climate distributions are required to advance our knowledge of interactions among surficial climatic and crustal processes

influence of chemical denudation on hillslope morphology 1 models of hillslope evolution involving diffusion like sediment transport are conventionally presented as an equation in which the changes in land surface elevation or soil thickness are balanced by the divergence of soil transport and tectonic uplift soil production or both these models typically do not include the loss or gain of mass in hillslope soils due to processes of chemical weathering and deposition we formulate a more general depth integrated equation for the conservation of soil mass on a hillslope that includes a term representing chemical deposition or denudation this general depth integrated equation is then simplified to determine the one dimensional form of a steady state hillslope which experiences both mechanical and chemical denudation the differences in morphology between hillslopes only experiencing diffusion like mechanical sediment transport and hillslopes experiencing both diffusion like mechanical sediment transport and chemical denudation are explored under the conditions of a downslope increase in local soil lowering rate due to chemical weathering the hillslope profile will depart from the parabolic shape predicted by models that incorporate only linear diffusion like mechanical sediment transport in addition hillslopes that experience both chemical and mechanical denudation may have a convex concave profile at steady state a necessary condition for such steady state profiles is that the chemical denudation rate must exceed the mechanical denudation rate we further suggest that combinations of other physical parameter values such as total denudation rate average soil depth sediment diffusivity and the increase in soil depth away from the divide that lead to steady state convex concave hillslope profiles may exist in a wide variety of natural settings

drainage basin sensitivity to tectonic and climatic forcing implications of a stochastic model for the role of entrainment and erosion thresholds long term average rates of channel erosion and sediment transport depend on the frequency magnitude characteristics of ood ows that exceed an erosion threshold using a poisson model for rainfall and runoff analytical solutions are developed for average rates of stream incision and sediment transport in the presence of such a threshold solutions are derived and numerically tested for three erosion transport formulas the howard kerby shear stress incision model the bridge dominic sediment transport model and a generic shear stress sediment transport model results imply that non linearity resulting from threshold effects can have a rst order impact on topography and patterns of dynamic response to tectonic and climate forcing this non linearity becomes signi cant when fewer than about half of ood events are capable of detaching rock or sediment predicted morphology and uplift gradient scaling is more closely consistent with observations and laboratory experiments than conventional slope linear or shear linear erosion laws these results imply that particle detachment thresholds are not details that can be conveniently ignored in long term landscape evolution models copyright 2004 john wiley sons ltd

controls on the strength of coupling among climate erosion and deformation in two sided frictional orogenic wedges at steady state 1 many important insights regarding the coupling among climate erosion and tectonics have come from numerical simulations using coupled tectonic and surface process models however analyses to date have left the strength of the coupling between climate and tectonics uncertain and many questions unanswered we present an approximate analytical solution for two sided orogenic wedges obeying a frictional rheology and in a condition of flux steady state that makes explicit the nature and sensitivity of the coupling between climate and deformation we make the simplifying assumption that the wedge grows in a self similar fashion consistent with airy isostasy such that topographic taper is invariant with orogen width tectonic influx rate and climate we illustrate first how and why the form of the erosion rule matters to orogen evolution and then derive a physically based orogen scale erosion rule we show that steady state orogen width crest elevation and crustal thickness are controlled by the ratio of accretionary flux to erosional efficiency to a power dictated by the erosion process remarkably we show that for most combinations of parameters in the erosion law rock uplift rate is more strongly controlled by erosional efficiency than it is by the accretionary flux further assuming frontal accretion with no underplating the spatial distribution of erosional efficiency dictates the relative rock uplift rates on the pro wedge and retro wedge and the time averaged trajectories of rocks through the orogen the restriction to invariant frictional properties is conservative in these respects systems subject to positive feedback between erosion and rheology will exhibit even stronger coupling among climate erosion and deformation than shown here

quantifying differential rock uplift rates via stream profile analysis despite intensive research into the coupling between tectonics and surface processes our ability to obtain quantitative information on the rates of tectonic processes from topography remains limited due primarily to a dearth of data with which to test and calibrate process rate laws here we develop a simple theory for the impact of spatially variable rock uplift rate on the concavity of bedrock river profiles application of the analysis to the siwalik hills of central nepal demonstrates that systematic differences in the concavity of channels in this region match the predictions of a stream power incision model and depend on the position and direction of the channel relative to gradients in the vertical component of deformation rate across an active fault bend fold furthermore calibration of model parameters from channel profiles argued to be in steady state with the current climatic and tectonic regime indicates that 1 the ratio of exponents on channel drainage area and slope m n is 0 46 consistent with theoretical predictions 2 the slope exponent is consistent with incision either linearly proportional to shear stress or unit stream power n 0 66 or n 1 respectively and 3 the coefficient of erosion is within the range of previously published estimates mean k 4 3 10 4 m 0 2 yr application of these model parameters to other channels in the siwalik hills yields estimates of spatially variable erosion rates that mimic expected variations in rock uplift rate across a fault bend fold thus the sensitivity of channel gradient to rock uplift rate in this landscape allows us to derive quantitative estimates of spatial variations in erosion rate directly from topographic data

testing fluvial erosion models using the transient response of bedrock rivers to tectonic forcing in the apennines italy the transient response of bedrock rivers to a drop in base level can be used to discriminate between competing fluvial erosion models however some recent studies of bedrock erosion conclude that transient river long profiles can be approximately characterized by a transport limited erosion model while other authors suggest that a detachment limited model best explains their field data the difference is thought to be due to the relative volume of sediment being fluxed through the fluvial system using a pragmatic approach we address this debate by testing the ability of end member fluvial erosion models to reproduce the well documented evolution of three catchments in the central apennines italy which have been perturbed to various extents by an independently constrained increase in relative uplift rate the transport limited model is unable to account for the catchments response to the increase in uplift rate consistent with the observed low rates of sediment supply to the channels instead a detachment limited model with a threshold corresponding to the field derived median grain size of the sediment plus a slope dependent channel width satisfactorily reproduces the overall convex long profiles along the studied rivers importantly we find that the prefactor in the hydraulic scaling relationship is uplift dependent leading to landscapes responding faster the higher the uplift rate consistent with field observations we conclude that a slope dependent channel width and an entrainment erosion threshold are necessary ingredients when modeling landscape evolution or mapping the distribution of fluvial erosion rates in areas where the rate of sediment supply to channels is low

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

particle dynamics the continuum of bedrock to alluvial river segments abstract particle dynamics refers to production erosion transport and storage of particulate material including mineral sediment and organic matter particle dynamics differ significantly between the end members of bedrock and alluvial river segments and between alluvial river segments with different grain size distributions bedrock segments are supply limited and resistant to change with relatively slow linear adjustments and predominantly erosion and transport particle dynamics in alluvial segments in contrast are transport limited and dominated by storage of mineral sediment and production of organic matter alluvial segments are resilient to change with relatively rapid multidirectional adjustments and stronger internal influences because of feedbacks between particles and biota bedrock segments are the governors of erosion within a river network whereas alluvial segments are the biogeochemical reactors fundamental research questions for both types of river segments center on particle dynamics which limit network scale incision in response to base level fall bedrock segments and habitat biogeochemical reactions and biomass production alluvial segments these characterizations illuminate how the spatial arrangement of bedrock and alluvial segments within a river network influence network scale resistance and resilience to external changes in relative base level climate and human activities

effects of orographic precipitation variations on the concavity of steady state river profiles the concavity or curvature of river profiles has long been taken to be a fundamental indicator of the underlying processes governing fluvial erosion and thereby of landscape evolution however erosion laws have generally been derived without accounting for the strong orographically driven gradients in precipitation typically found in mountainous regions in addition field measurements have found discrepancies between the form of measured stream profiles and theoretically derived values introducing a simple physically based feedback we find that orographically induced variations in precipitation strongly affect the curvature of steady state river profiles this feedback complicates efforts to infer the form of erosion laws from observed profile concavities but could help explain discrepancies between observations and theory our results demonstrate a strong feedback through which climate influences the form of river profiles and show how such climatic effects act to limit the relief of unglaciated mountain ranges

the effects of precipitation gradients on river profile evolution on the big island of hawai i to better understand how climate affects bedrock river incision and long term landscape evolution in the absence of tectonic forcing we quantify differences in the longitudinal profiles of eroding bedrock channels across the kohala peninsula on the northern tip of the big island of hawai i an orographic rainfall gradient causes mean annual precipitation rates to vary by over an order of magnitude from greater than 4000 mm yr on the wet side to less than 250 mm yr on the dry side of the peninsula channels on the wet side are relatively deeply incised into the surrounding landscape and have developed profiles in which slope increases downstream in upstream reaches convex form and decreases downstream in downstream reaches concave form wet side river channels have relatively large 10 30 m vertical steps or knickpoints in the convex zone in contrast channels on the dry side of the peninsula are more shallowly incised have developed nearly straight longitudinal profiles in which channel slope does not significantly change from upstream to downstream and have smaller knickpoints channel profile form changes from straight to convex concave at a watershed averaged mean annual precipitation rate of between 1300 and 1750 mm yr suggesting a climatic threshold in this landscape above which bedrock incision rates are enhanced valley depth used as a proxy for the magnitude of channel incision increases with increasing mean annual precipitation and is consistent with a similar fluvial incision threshold while the lithology is entirely basalt incision patterns also appear to be affected by spatial differences in bedrock weathering due to both local climate and basalt flow age the older and more weathered polol basalts 260 450 ka are capped by the younger haw basalt series 120 260 ka and we interpret that heterogeneous weathering of these units influences channel form relief sediment production and knickpoint development

a generalized power law approximation for fluvial incision of bedrock channels 1 sediment flux is known to influence bedrock incision rates in mountain rivers although the widely used stream power incision model lacks any explicit representation of sediment flux the model appears to work in a variety of real settings we address this apparent contradiction using numerical experiments to explore the morphology of fluvial landscapes evolved with four different incision models three of which include the influence of sediment flux on incision rate the numerical landscapes have different spatial patterns of uplift and are at steady state we analyze these landscape using the common stream power approach which views incision rates to be primarily a function of the local channel gradient s and the upstream drainage area a we find that incision rates i for these landscapes are well described by an empirical power law equation i k am sn this equation is functionally equivalent to the widely used stream power model with the important distinction that the parameters k m and n are entirely empirical these parameters take on constant values within a single landscape but can otherwise be quite different between landscapes mainly due to differences in the pattern of rock uplift within the drainage in particular the parameters m and n decrease as the rate of rock uplift becomes more focused in the upland part of a mountain belt the parameter m is particularly important in that it describes the sensitivity of a tectonically active mountain belt to changes in precipitation or tectonic accretion it also defines how incision rates will change as the discharge becomes flashier

aridity is expressed in river topography globally it has long been suggested that climate shapes land surface topography through interactions between rainfall runoff and erosion in drainage basins1 4 the longitudinal profile of a river elevation versus distance downstream is a key morphological attribute that reflects the history of drainage basin evolution so its form should be diagnostic of the regional expression of climate and its interaction with the land surface5 9 however both detecting climatic signatures in longitudinal profiles and deciphering the climatic mechanisms of their development have been challenging owing to the lack of relevant global data and to the variable effects of tectonics lithology land surface properties and human activities10 11 here we present a global dataset of 333 502 river longitudinal profiles and use it to explore differences in overall profile shape concavity across climate zones we show that river profiles are systematically straighter with increasing aridity through simple numerical modelling we demonstrate that these global patterns in longitudinal profile shape can be explained by hydrological controls that reflect rainfall runoff regimes in different climate zones the most important of these is the downstream rate of change in streamflow independent of the area of the drainage basin our results illustrate that river topography expresses a signature of aridity suggesting that climate is a first order control on the evolution of the drainage basin a global dataset of river longitudinal profiles shows that river profiles become straighter with increasing aridity and numerical modelling suggests that this can be explained by rainfall runoff regimes in different climate zones

latitudinal trends in modern fluvial erosional efficiency along the andes abstract latitude is a dominant control on climate influencing both temperature and precipitation leading to potential gradients in erosional efficiency previous work focused on the latitudinal control of glacial erosion on topography however the expected latitudinal trend in erosional efficiency in the absence of glacial processes is unknown we parameterize a suite of numerical models using uniform rock uplift and climate from the andes to explore how latitudinal patterns of rainfall could influence the efficiency of fluvial bedrock erosion we use the child landscape evolution model parameterized with storm duration intensity and frequency statistics from ncep ncar reanalysis data along the andes over 31 years to predict latitudinal trends in topography subject only to fluvial and hillslope erosion modeled elevations as a metric for erosional efficiency vary 2 30 fold depending on the erosional and hydrological model parameterization modeled elevation trends generated using stochastic storm generation vary significantly from those generated using mean annual precipitation map the variation suggests map may not capture the true erosional efficiency of the modern climate in some locations but may in others explaining the discrepancy of conclusions in previous studies erosional efficiency is greatest in the tropics 5 15 s where high intensity and frequent storms occur and mid latitudes 30 35 s where less frequent but intense storms occur erosional efficiency is lowest in the subtropics 20 to 25 s where low intensity and infrequent storms occur and upper mid latitudes 45 50 s where low intensity and somewhat frequent storms occur erosional efficiency varies across the width of the orogen because of across orogen effects the results provide further evidence supporting climate s control on erosion processes and topography highlighting the importance of stochastic storm distributions and suggesting that latitudinal controls on rainfall can explain some observed trends in topography the modern latitudinal trend in topography is frequently used as an argument for a glacial buzzsaw however our results suggest more information on the pre glacial distribution of topography is needed to justify such an argument since latitude dependent rainfall regimes may generate similar topographic trends

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

chemical weathering mass loss and dust inputs across a climate by time matrix in the hawaiian islands we determined the total mass loss and rate of chemical weathering from three minimally eroded hawaiian lava flows that are 10 170 and 350 ka old using a backhoe we sampled the entire weathering zone at 28 sites and measured the depletion or enrichment of each major element in each soil horizon relative to parent material we were able to assess the influence of both climate and substrate age on chemical weathering because each flow crosses a precipitation gradient from 600 to 2500 mm yr 1 mass loss rates were highest for the 0 10 ka interval under the wettest climatic conditions 54 t km 2 yr 1 and decreased to near zero in the wet sites during the 10 170 and 170 350 ka intervals not surprisingly weathering rates were lower in drier sites 24 t km 2 yr 1 from 0 10 ka to b 2tk m 2 yr 1 thereafter however the effects of precipitation were non linear there was a precipitation threshold below which mass loss was relatively small and above which mass loss was substantial but insensitive to increased rainfall chemical weathering rates depend on tectonic uplift erosion climate rock type or some combination thereof by working on stable uneroded surfaces of a single rock type across a well constrained precipitation gradient we were able to identify another potential driver the rate of dust deposition although hawai i is one of the least dusty places in the northern hemisphere dust inputs reached 82 of the total mass loss from the weathering zone at some sites and averaged 30 on the 170 ka flow this highlights the potential importance of dust as a component of observed weathering fluxes from catchments worldwide

enhancement of cyanobacterial growth by riverine particulate material abstract particulate material plays a major role in the transport of sparingly soluble nutrients such as p and fe in natural surface waters microbes might gain access to these nutrients either indirectly through particulate dissolution or directly through microbial attack as such it seems reasonable to expect a link between the particulate material concentration and bacterial growth in natural surface waters to explore this link a series of microcosm growth experiments were performed in the presence of a typical freshwater cyanobacteria synechococcus sp grown in dilute bg 11 culture media in the presence and absence of basaltic and continental riverine particulate material results demonstrate that riverine particulates can increase bacterial biomass by 1 triggering bacterial growth in otherwise unfavourable conditions 2 increasing total maximum biomass concentration and 3 inducing bacteria growth during the post exponential phase these effects are found to be enhanced by increasing particulate concentration results also indicate a positive feedback between the nutrient release from the particulates and growing bacteria where dissolving particulates enhance bacterial growth which further promotes particulate dissolution by altering fluid ph microscopic analysis showed direct physical contact between particulates and cyanobacteria suggesting that bacteria attach directly on mineral surfaces to gain required nutrients furthermore frequent bacteria clusters were observed associated with particulates indicating an increasing aggregation of bacteria in the presence of particulate material which may facilitate a higher burial efficiency of organic carbon

knickpoint initiation and distribution within fluvial networks 236 waterfalls in the waipaoa river north island new zealand if knickpoints transmit signals of base level fall in river networks then improvements in our understanding of their retreat rate and basin wide distribution helps constrain the transient response following perturbation many studies of knickpoint retreat focus on the response of trunk streams to base level fall here we examine the response of an entire fluvial network as recorded by 236 active knickpoints distributed within the waipaoa river on the north island of new zealand base level fall within the waipaoa catchment initiated 18 000 years ago in response to a climatically triggered and tectonically exacerbated pulse of incision using observations from field work aerial photo analysis and a digital elevation model dem we study the knickpoint positions within the network we find that 70 of the knickpoints are located at drainage areas between 1 10 5 m 2 and 1 10 6 m 2 and more than half are b1 km upstream of a large change in drainage area for the knickpoints b1 km upstream of large tributary junctions we find that the retreat distances were well correlated with the tributaries drainage areas in order to determine how a pulse of incision distributes itself throughout a fluvial network we develop two simple end member models and compare their behavior to the observed knickpoint distribution in the waipaoa in the first model we propose that a knickpoint initiated at the basin outlet retreats upstream and distributes the signal throughout the network at a rate that is a power law function of drainage area in the second model we propose that knickpoints form near a threshold drainage area below which channels cannot incise with the same efficiency as possible in downstream reaches though neither model addresses the along stream variability in substrate or knickpoint form the misfit between the modeled and the observed knickpoints along stream positions are surprisingly low b1 km 1 25 of the total stream length for knickpoints with drainage areas b1 10 6 m 2 large misfits up to 3 5 km are observed for knickpoints with present day drainage areas greater than 1 10 6 m 2 the large single step in channel elevation that characterizes knickpoints presently observed in tributaries of the waipaoa river may not characterize the base level fall signal that propagated through the trunk streams evidence for progressive rather than instantaneous incision in the trunk streams the knickpoints vicinities to tributary junctions and the equivalent success of the two end member models lead us to conclude that the present positions of the 236 observed knickpoints are largely a consequence of thresholds in channel incision at low drainage areas

orographic precipitation and the relief of mountain ranges 1 the influence of climate on the relief of mountain ranges has long been supposed but has received little quantitative investigation the simplified system of a longitudinal river profile in a bedrock channel coupled to a physically based model of orographic precipitation is used to explore how orographically influenced patterns of precipitation affect the relief of fluvially sculpted mountain ranges it is shown that the impact on the orography depends on the nature of the precipitation regime significant changes in relief also occur for imposed temperature changes of realistic magnitude although the ability of the relief to respond to any given change depends sensitively on its response time and the timescale of the forcing

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

variability of rock erodibility in bedrock floored stream channels based on abrasion mill experiments we quantify variations in rock erodibility kr within channel cross sections using laboratory abrasion mill experiments on bedrock surfaces extracted from streams with sandstone bedrock in utah and basaltic bedrock in the hawaiian islands samples were taken from the thalweg and channel margins the latter at a height that is inundated annually for each sample a sequence of abrasion mill experiments was completed to quantify variations in erosion rate with erosion depth erosion rate data from these experiments shows two things first the erosion rate from channel margin samples is greater than for thalweg samples with the greatest difference observed for the rock surface that was exposed in the stream channel second erosion rate decreases with depth beneath the original rock surface by an order of magnitude in most cases the erosion rate becomes steady at depths of 1 3 mm for channel margin samples and 0 1 0 4 mm for thalweg samples because only rock properties and microtopography vary throughout the sequence of mill experiments these results suggest that kr of the bedrock surface exposed in stream channels is higher at the margins than near the channel center and that kr decreases over depths of 1 mm the simplest explanation for these patterns is that kr is enhanced at the bedrock surface and along the channel margins due to the effects of weathering on rock strength and surface roughness we hypothesize that a balance exists between weathering enhanced erodibility and episodic incision to allow channel margins to lower at rates similar to the thalweg

paleohydrology of arid southeastern maui hawaiian islands and its implications for prehistoric human settlement abstract arid slopes on the southeastern side of maui are densely covered with archaeological remains of hawaiian settlement from the late prehistoric to early postcontact period ca a d 1500 1860 permanent habitation sites agricultural features and religious structures indicate perennial occupation and farming in a subregion called kahikinui yet there is presently no year round water source we explore the possibility that postcontact deforestation led to the loss of either 1 perennial channel flow or 2 perennial springs or seeps to investigate the first possibility we estimated ancient peak flows on 11 ephemeral channels in kahikinui using field measurements and paleohydrology peak flow estimates 3 230 m3 s for a given drainage area are smaller than those for current perennial maui streams but are equivalent to gauged peak flows from ephemeral and intermittent streams in the driest regions of hawai i and maui islands this is consistent with the long term absence of perennial channel flow in kahikinui on the other hand others have shown that canopy fog drip in hawai i can be greater than rainfall and thus a large part of groundwater recharge using isolated live remnants and snags we estimate the former extent of the forest upstream from archaeological sites we use rough estimates of the loss of fog drip recharge caused by deforestation and apply a simple steady state hydrologic model to calculate potential groundwater table fall these order of magnitude estimates indicate that groundwater could have fallen by a minimum of several meters abandoning perennial seeps this is consistent with archaeological evidence for former perennial seeps such as stonewalls enclosing potential seeps to protect them although longer term reductions in rainfall cannot be ruled out as a factor deforestation and loss of fog drip recharge are obvious and more immediate reasons for a recent loss of perennial water in kahikinui maui

landscape response to tectonic forcing digital elevation model analysis of stream profiles in the mendocino triple junction region northern california the topographic evolution of orogens is fundamentally dictated by rates and patterns of bedrock channel incision quantitative field assessments of process based laws are needed to accurately describe landscape uplift and denudation in response to tectonics and climate we evaluate and calibrate the shear stress or similar unit stream power bedrock incision model by studying stream profiles in a tectonically active mountain range previous work on emergent marine terraces in the mendocino triple junction region of northern california provides spatial and temporal control on rock uplift rates digital elevation models and field data are used to quantify differences in landscape morphology associated with along strike northwest to southeast changes in tectonic and climatic conditions analysis of longitudinal profiles supports the hypothesis that the study area channels are in equilibrium with current uplift and climatic conditions consistent with theoretical calculations of system response time based on the shear stress model within uncertainty the profile concavity of the trunk streams is constant throughout the study area 0 43 as predicted by the model channel steepness correlates with uplift rate these data help constrain the two key unknown model parameters the coefficient of erosion k and the exponent associated with channel gradient n this analysis shows that k cannot be treated as a constant throughout the study area despite generally homogeneous substrate properties for a reasonable range of slope exponent values n best fit values of k are positively correlated with uplift rate this correlation has important implications for landscape evolution models and likely reflects dynamic adjustment of k to tectonic changes due to variations in orographic precipitation and perhaps channel width sediment load and frequency of debris flows the apparent variation in k makes a unique value of n impossible to constrain with present data

bedrock channel adjustment to tectonic forcing implications for predicting river incision rates we present detailed data of channel morphology for a river undergoing a transient response to active normal faulting where excellent constraints exist on spatial and temporal variations in fault slip rates we show that traditional hydraulic scaling laws break down in this situation and that channel widths become decoupled from drainage area upstream of the fault unit stream powers are 4 times higher than those predicted by current scaling paradigms and imply that incision rates for rivers responding to active tectonics may be significantly higher than those heretofore modeled the loss of hydraulic scaling cannot be explained by increasing channel roughness and is an intrinsic response to tectonic forcing we show that channel aspect ratio is a strongly nonlinear function of local slope and demonstrate that fault induced adjustment of channel geometries has reset hillslope gradients the results give new insight into how rivers maintain their course in the face of tectonic uplift and illustrate the first order control the fluvial system exerts on the locus and magnitude of sediment supply to basins

crustal subsidence rate off hawaii determined from 234u 238u ages of drowned coral reefs a series of submerged coral reefs off northwestern hawaii was formed during largely glacial intervals when the rate of local sea level rise was less than the maximum upward growth rate of the reefs mass spectrometric sup 234 u sup 238 u ages for samples from six such reefs range from 17 to 475 ka and indicate that this part of the hawaiian ridge has been subsiding at a roughly uniform rate of 2 6 mm yr for the past 475 ka the sup 234 u sup 238 u ages are in general agreement with model ages of reef drowning based on estimates of paleo sea level stands derived from oxygen isotope ratios of deep sea sediments but there are disagreements in detail the high attainable precision plus minus 10 ka or better on samples younger than approximately 800 ka large applicable age range relative robustness against open system behavior and ease of analysis for this technique hold great promise for future applications of dating of 50 1 000 ka coral

hanging valleys in fluvial systems controls on occurrence and implications for landscape evolution 1 we document and characterize hanging valleys in a fluvially eroded landscape in eastern taiwan our conceptual model for the initiation of hanging valleys builds on a recently proposed model of bedrock incision in which erosion actually becomes less efficient on very steep channel gradients if a pulse of incision in the main stem outpaces the tributary response the gradients at tributary mouths may therefore pass a threshold value beyond which erosional efficiency declines giving rise to a mismatch between trunk and tributary erosion rates this mismatch is expected at junctions with small tributaries where a step function decrease in drainage area also leads to sharp contrasts in water and sediment flux between trunk and tributary channels the occurrence of hanging valleys in actively uplifting fluvial landscapes such as the central range of taiwan suggests that the most common parameterizations of bedrock erosion which typically assume a monotonic positive correlation between channel gradient and incision rate may be violated in very steep channels in addition hanging valleys could greatly increase the response time of landscapes to tectonic perturbations since catchments above these tributary mouths will be insulated from these perturbations until a new suite of processes e g weathering and rock mass failure wear through the hanging valley lip the results of this study underscore the need for a more complete understanding of bedrock erosion processes and the incorporation of process transitions and threshold conditions into landscape evolution models

modeling the effects of weathering on bedrock floored channel geometry 1 field and modeling studies suggest that bedrock channels equilibrate to base level change through geometry and slope adjustment to imposed discharge sediment supply and substrate erodibility conditions in this study we model the influence of bedrock weathering on channel geometry and slope as mean peak discharge qm and uplift rate u vary we find that channels in which weathering is allowed to increase erodibility are wider deeper and less steep than nonweathering channels with the same initial conditions while fixed erodibility channels maintain similar width depth ratios regardless of qm or u the width depth ratio of weathering channels is sensitive to uplift rate at low uplift rates weathering outpaces erosion and channels obtain similar width depth ratios but are wider and less steep than fixed erodibility channels with equal initial conditions at high uplift rates erosion outpaces weathering and erodibility remains near the unweathered value with channel shape and slope nearly identical to a fixed erodibility channel with equal initial conditions weathering channels differ most from fixed erodibility channels at intermediate uplift rates with greater width depth ratios and lower slopes than fixed erodibility channels with the same initial conditions our results support the hypothesis that cross channel variations in erodibility created by weathering may be an important control on channel geometry and provide guidance for further testing of this hypothesis in natural systems

drainage basin responses to climate change recent investigations have shown that the extent of the channel network in some drainage basins is controlled by a threshold for overland flow erosion the sensitivity of such basins to climate change is analyzed using a physically based model of drainage basin evolution the golem model simulates basin evolution under the action of weathering processes hillslope transport and fluvial bedrock erosion and sediment transport results from perturbation analyses reveal that the nature and timescale of basin response depends on the direction of change an increase in runoff intensity or a decrease in vegetation cover will lead to a rapid expansion of the channel network with the resulting increase in sediment supply initially generating aggradation along the main network followed by downcutting as the sediment supply tapers off by contrast a decrease in runoff intensity or an increase in the erosion threshold will lead to a retraction of the active channel network and a much more gradual geomorphic response cyclic changes in runoff intensity are shown to produce aggradational degradational cycles that resemble those observed in the field cyclic variations in runoff also lead to highly punctuated denudation rates with denudation concentrated during periods of increasing runoff intensity and or decreasing vegetation cover the sediment yield from threshold dominated basins may therefore exhibit significant variability in response to relatively subtle environmental changes a finding which underscores the need for caution in interpreting modern sediment yield data

observations on the role of lithology in strath terrace formation and bedrock channel width field observations from western washington and eastern tibet indicate a strong lithologic influence on strath terrace formation and highlight the response of bedrock channel width to spatial gradients in bedrock erodibility and or rock uplift rates measurements of local bedrock bed and bank erosion rates together with observations of the role of weathering processes on erosion of siltstones and sandstones illustrate a mechanism that underlies a conceptual model that predicts a strong lithological control on strath terrace formation direct measurements of bedrock erosion rates in lithologies susceptible to accelerated erosion upon sub aerial exposure show that lateral rates of bedrock bank erosion can substantially exceed vertical incision below the perennial flow level due to an asymmetry in erodibility between perennially submerged rock and rock exposed to cyclic wetting and drying on bedrock channel walls under such conditions the rapid weathering mediated lowering of bedrock exposed above baseflow promotes development of a beveled bedrock platform that could become a strath terrace upon abandonment in contrast to previous studies channel widths measured across a substantial gradient in lithology slope and long term erosion rates in eastern tibet do not follow conventional relations where channel width scales as a power law function of drainage area instead as they flow across a zone of rapid uplift these channels systematically narrow relative to the predicted width extrapolated from traditional power law relations field studies from a wide range of geological settings and lithologies support the interpretation that in general strath terraces tend to be more extensive in rivers flowing over weak sedimentary rock and tend to be poorly developed and or preserved where rivers flow over hard erosion resistant rock

age of 360 m reef terrace hawaii and the rate of late pleistocene subsidence of the island observations from a manned submersible vehicle indicate that the 360 m reef terrace northwest of the island of hawaii is a drowned coral reef the preferred uranium series age of coralline algae collected from the reef face is 120 5 ka this age agrees with the notion that the reef was drowned during the sea level rise following the major lowstand of the sea that occurred at 145 ka oxygen isotope stage 6 this drowning pattern is similar to the previously determined radiocarbon age of 13 ka for drowning of the 150 m reef off west hawaii which drowned during the sea level rise following the last major lowstand of the sea at 18 ka oxygen isotope stage 2 estimated average subsidence of the reef site off northwest hawaii is 2 7 mm yr since final drowning of the 360 m reef

hillslope and channel evolution in a marine terraced landscape santa cruz california a flight of marine terraces along the central california coastline provides a unique setting for the study of topographic evolution wavecut platforms mantled by 2 6 m of marine terrace cover deposits are separated by 10 50 m tall decaying sea cliffs paleocliff edges become more rounded with age yet the details of the profiles and frequent bedrock exposure on the upper slopes imply weathering limited transport five bedrock stream channels etched through the marine terrace sequence display one to three distinct convexities in their longitudinal profiles detailed hand level surveys of the hillslopes and of the stream channel longitudinal profiles constrain hillslope evolution and channel incision components of a numerical model of landscape evolution we account for regolith production as a function of regolith depth in accord with the field observation that hillslope processes are presently dominated by the activities of burrowing rodents the transport process is taken to be diffusive stream incision is assumed to be controlled by stream power for which we use the surrogate of local drainage area slope product best fits of the numerical model to field data imply hillslope diffusivity is 10 m2 kyr 1 regolith production rate on bare bedrock is 0 3 m kyr 1 and falls off rapidly with regolith cover and the constant controlling the efficiency of stream incision is 5 to 7 10 7m 1 kyr1

limits of downstream hydraulic geometry adjustments to flow width depth and velocity in response to changes in discharge are commonly characterized by using downstream hydraulic geometry relationships the spatial limits of these relationships within a drainage basin have not been systematically quantified where the erosional resistance of the channel substrate is sufficiently large hydraulic driving forces presumably will be unable to adjust channel form data sets from 10 mountain rivers in the united states panama nepal and new zealand are used in this study to explore the limits of downstream hydraulic geometry relationships where the ratio of stream power to sediment size d 84 exceeds 10 000 kg s3 downstream hydraulic geometry is well developed where the ratio falls below 10 000 kg s3 downstream hydraulic geometry relationships are poorly developed these limitations on downstream hydraulic geometry have important implications for channel engineering and simulations of landscape change

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

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

influence of precipitation phase on the form of mountain ranges observations of precipitation fall speeds and precipitation patterns suggest that precipitation phase rain versus snow is a significant control on the relationship between precipitation patterns and topography due to the potential for increased downwind advection of snow relative to rain a coupled model of orographic precipitation and surface erosion shows that for a range of climate variables steady state precipitation patterns vary from nearly uniform and maximizing over the highest topography to highly spatially variable closely coupled to topography and reaching a maximum on low slopes precipitation patterns are a first order control on modeled range scale and ridge valley scale relief channel concavity and the position of the drainage divide an association between cool climates spatially uniform precipitation and efficient erosion of high topography is indicated the importance of precipitation phase to the evolution of precipitation patterns and topography further demonstrates the fundamental importance of the coupled climate erosion and tectonic system in the evolution of mountain topography

expression of active tectonics in erosional landscapes understanding the manner and degree to which topography in active mountain ranges reflects deformation of the earth s surface remains a first order goal of tectonic geomorphology a substantial body of research in the past decade demonstrates that incising channel systems play a central role in setting relationships among topographic relief differential rock uplift rate and climatically modulated erosional efficiency this review provides an introduction to the analysis and interpretation of channel profiles in erosional mountain ranges we show that existing data support theoretical expectations of positive monotonic relationships between channel steepness index a measure of channel gradient normalized for downstream increases in drainage area and erosion rate at equilibrium and that the transient response to perturbations away from equilibrium engenders specific spatial patterns in channel profiles that can be used to infer aspects of the forcing these aspects of channel behavior lay the foundation for a series of case studies that we use to illustrate how focused quantitative analysis of channel morphology can provide insight into the spatial and temporal dynamics of active deformation although the complexities of river response to climate lithology and uplift patterns mean that multiple interpretations of topographic data alone will always possible we show that application of stream profile analysis can be a powerful reconnaissance tool with which to interrogate the rates and patterns of deformation in active mountain belts

predictions of steady state and transient landscape morphology using sediment flux dependent river incision models 1 recent experimental and theoretical studies support the notion that bed load in mountain rivers can both enhance incision rates through wear and inhibit incision rates by covering the bed these effects may play an important role in landscape evolution and in particular the response of river channels to tectonic or climatic perturbation we use the channel hillslope integrated landscape development child numerical model with two different bedrock incision models that include the dual role of the sediment flux to explore the transient behavior of fluvial landscapes both models predict that steady state channel slopes increase in landscapes with higher rock uplift rates however the incision models predict different transient responses to an increase in uplift rate and the behavior of each incision model depends on both the magnitude of change in uplift rate and the local drainage area in some cases the transient channel behavior is indistinguishable from that predicted for transport limited alluvial rivers in other cases knickpoints form in some or all of the drainage network as predicted by the detachment limited stream power model in all cases the response in the lower parts of the network is highly dependent on the response in the upper parts of the network as well as the hillslopes as the upper parts of the network send more sediment downstream channel incision rates may rise or fall and slopes in the lower parts of the channel may in fact decrease at times during the transient adjustment to an increase in rock uplift rate in some cases channel incision in the upper parts of the network ceases during the transient while the hillslopes adjust to the new uplift rate drainage density may also change as a function of uplift rate our results suggest that if the sediment flux strongly controls bedrock incision rates then 1 the transient fluvial response will take longer than predicted by the detachment limited stream power model 2 changes in channel slope may be much more complex than predicted by the detachment limited stream power model and 3 changes in the fluvial system will be closely tied to sediment delivery from the hillslopes importantly our results outline quantitative differences in system behavior produced by competing models and provide a framework for identifying locations in natural systems where differences in channel morphology can be used to discern between competing fluvial erosion models

exploring landscape sensitivity to the pacific trade wind inversion on the subsiding island of hawaii the interaction of the subsiding limb of the hadley circulation and the easterly north pacific trade winds establishes a persistent thermal inversion at about 2000 m above sea level in the subtropical pacific the inversion restricts convective rainfall to the lower elevations of the windward flank of the island of hawaii creating an order of magnitude vertical rainfall gradient as well as high interbasin variability in precipitation in the high rainfall zone streams are incised tens to hundreds of meters below the surface of the volcanic shield we use a digital elevation model and 1 d numerical modeling to assess whether deep incision on the flank of mauna kea is tied to the elevation of the trade wind inversion the 83 channels examined can be well fit in aggregate by all models that account for differences in precipitation between basins specifically the maximum depth of incision in each drainage is a power function of precipitation weighted drainage area with an exponent of individual longitudinal stream profiles are generally better fit by models that acknowledge both along channel precipitation variability and the subsidence of the island through that gradient but this relationship is not clearly demonstrable for many channels we argue that island subsidence through the dry to wet precipitation gradient has resulted in less cumulative discharge in the lower reaches of the drainages over their lifetime relative to a nonsubsiding case this reduces differential erosion between the wet and dry reaches by 20 30 over 300 kyr making the longitudinal profiles less sensitive to the very strong climate gradient

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

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

discharge discharge variability and the bedrock channel profile long term bedrock incision is driven by daily discharge events of variable magnitude and frequency with ineffective events below an incision threshold we explore theoretically how this short term stochastic behavior controls long term steady state incision rates and bedrock channel profiles combining a realistic frequency magnitude distribution of discharge with a deterministic detachment limited incision model in which incision rate is a power function of basal shear stress above a critical shear stress our model predicts a power law relationship between steady state slope and drainage area consistent with observations the exponent of this power law is independent of discharge mean and variability while the amplitude factor which controls mountain belt relief is a power law function of mean runoff with an exponent of 0 5 and a complex function of runoff variability in accordance with evidence that incision occurs between 6 and 20 of time in rapidly incising rivers 1 mm yr our model predicts that channel steepness is virtually insensitive to runoff variability runoff variability can only decrease channel steepness for very slow incision rates and or weak lithologies the relationship between channel steepness and incision rate is always a power law whose exponent depends on the channel cross sectional geometry and runoff variability this contradicts models neglecting discharge stochasticity in which the steepness incision scaling is set by the incision law exponent our results suggest that changes in climate variability cannot explain an increase in bedrock incision rates during the late cenozoic within the context of a detachment limited model

incision and channel morphology across active structures along the peikang river central taiwan implications for the importance of channel width river morphology and dynamics are strongly influenced by active tectonics we report channel dynamics for the peikang river which flows through the hsuehshan range in central taiwan using a digital elevation model and field surveys we constrain channel morphology for an 90 km stretch of river to calculate unit stream power and boundary shear stress along the river path incision rates are estimated with optically stimulated luminescence dating of sand deposited on strath terraces we find a strong correlation between unit stream power shear stress and incision rate but only if variation in channel width is considered a calibrated river incision rule implies river incision rates of 9 13 5 mm yr upstream of the meiyuan and tili faults and suggests that one or both of these structures are presently active our results indicate that the shuilikeng fault is also actively deforming as incision rates increase to 6 10 mm yr across it compared to 1 4 mm yr in adjacent reaches prominent narrowing across the shuilikeng fault and the absence of significant gradient variation indicate that channel width is a first order morphological adjustment to differential incision only when the channel width to depth ratio reaches a minimum does the channel slope significantly adjust to local changes in base level as is the case upstream of the meiyuan and tili faults

bedrock channel geometry along an orographic rainfall gradient in the upper marsyandi river valley in central nepal channel width we focus on small catchments 0 6 12 4 km 2 along a more than tenfold gradient in monsoon rainfall rainfall data are gathered from a dense weather network and calibrated satellite observations the pattern of quaternary exhumation is inferred from apatite fission track cooling ages and rock compressive strength is measured in the field bedrock channel widths surveyed at high scour indicators scale as a power law function of discharge w a qw that is estimated by combining rainfall data with 90 m digital topography the results suggest that power law width scaling models apply 1 to regions with pronounced rainfall gradients 2 to tributary catchments distributed across a climatically diverse region and 3 to large rapidly denuding orogens an analysis of rainfall data indicates that the regional gradient of rainfall during storms that drive erosive discharge events is about half as large as the gradient of seasonal rainfall across the same area finally numerical models in which the maximum rainfall is displaced significantly downstream from the headwaters predict a midcatchment zone of relatively rapid decreases in channel gradient and increases in channel concavity that are driven by locally enhanced discharge because differential rock uplift can produce analogous changes in gradients the influence of rainfall gradients should be assessed before tectonic inferences are drawn

analysis of small scale gravel bed topography during armoring 1 in evaluating the resistance of sediment particles to entrainment by the action of the flow in a river the grain geometry is usually characterized using representative sizes this approach has been dictated initially by lack of physical insight but more recently by the lack of analytical tools able to describe the 3 d nature of surface grain organization on water worked sediment beds laboratory experiments are presented where mixed grain size beds were mobilized under a range of hydraulic and sediment input conditions detailed bed topography was measured at various stages statistical tools have been adopted which describe the degree of surface organization on water worked sediment bed surfaces the degree of particle organization and the bed stability can be evaluated in relative terms using the properties of the probability density distribution of the bed surface elevations and in absolute terms using a properly defined 2 d structure function the methods described can be applied directly to natural water worked surfaces given the availability of appropriate bed surface elevation data sets

modeling the influence of rainfall gradients on discharge bedrock erodibility and river profile evolution with application to the big island hawai i motivated by convex concave bedrock river profiles developed across a climate gradient on the wetside of the kohala peninsula of the big island of hawai i we numerically model how rainfall gradients may influence longitudinal fluvial incision patterns first we model transient profile adjustment with two tectonic boundary conditions subsidence and uplift in this generalized analysis we assume that rainfall gradients only influence incision by modifying the relation between upstream drainage area and local discharge using a detachment limited model downstream increases in rainfall lead to profile convexities during transient adjustment in both tectonic settings and this is the opposite of the predicted increase in profile concavity that would develop in a steady state uplifting profile a transport limited erosion model develops only concave channel profiles without clear signatures of the rainfall pattern second we model the development of convex concave transient profiles and incision patterns on kohala using a detachment limited model if rainfall gradients only influence incision through the local discharge reasonable rainfall gradients can only develop channel convexities that are much smaller than those observed instead we hypothesize that local bedrock erodibility increases with the degree of rainfall dependent chemical weathering when local erodibility is assumed to scale with local rainfall rate the model can produce convex concave profiles similar to those observed in kohala our results suggest that changes in local bedrock erodibility due to local climate dependent weathering may be an important mechanism by which climate influences landscape form and rates of evolution this hypothesis requires further testing in this study area and beyond

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

the impact of climate on the biogeochemical functioning of volcanic soils abstract rainfall and the amount of water available to leach ions from soil are among the most important features determining mineral weathering secondary mineral synthesis and soil chemical properties along an arid to humid climosequence on kohala mountain hawaii we sampled 16 soil profiles and found that weathering and soil properties change in a nonlinear fashion with increased rainfall the lavas are influenced by a strong rain shadow with mean annual precipitation map averaging 160 mm near the coast and rising to 3000 mm near the summit a temperature decline from 24 to 15 c with increasing elevation is matched by lower potential evapotranspiration et a water balance model monthly precipitation minus monthly et defines three broad climate zones along the sampling transect an arid zone with moisture deficit in every month an intermediate zone with moisture deficit during low rainfall summer months and moisture surplus during high rainfall winter months and a humid zone with moisture surplus during every month the annualized water balance can be ratioed with the integrated porosity of the top meter of soil to provide a leaching index the index reaches 1 total filling of the pore space on an annual basis at about 1400 mm map index values 1 imply intense leaching conditions because of pore water replacement in these 170 ka soils leaching losses of soluble base cations and si are nearly complete at index values 1 whereas only 60 of al has been lost at index values 1400 mm soil buffering capacity has been totally exhausted leading to low ph and low ecec the nonlinear decline in ecec is irreversible under natural conditions base cation depleted soils will remain so even if the climate shifts to drier conditions in contrast a climate shift to wetter conditions can drastically modify surface chemical properties existing in the drier soils as weathering depletes primary minerals elements are lost to leaching and surface chemistry is modified there is a time dimension implied in climate gradient studies soils forming in recently rejuvenated landscapes contain more primary minerals and should experience loss of buffering capacity at higher rainfall loss of buffering capacity means that biological acidity will move more deeply into the vadose zone or into the aquatic system the details of this transfer depend on present and past climate and the age and erosional stability of landscapes

subsidence and volcanism of the haleakala ridge hawaii abstract side looking sonar gloria mapping has revealed a series of four arcuate bands of high sonic backscatter on the crest of the haleakala ridge a major rift zone ridge extending 135 km east of the island of maui dredge recovery indicates that the shallowest of these bands is a drowned coral reef and the deeper bands are also inferred to be coral reefs the reefs occur above a prominent submarine bench 1500 2500 m deep on the ridge h terrace that marks the shoreline at the end of vigorous shield building of haleakala volcano when lava flows ceased crossing and reworking the shoreline since their growth these reefs have subsided as much as 2200 m and have tilted systematically about 20 m km southward as a result of post reef volcanic loading on the island of hawaii whose center of mass is about directly south of the haleakala ridge the 234u 238u age of the dredged coral is 750 13 ka in reasonable agreement with an age of 850 ka for the underlying h terrace previously estimated from its relationship to other dated reefs to the southwest basalt glass fragments dredged from the haleakala ridge below the h terrace are tholeiitic and contain high sulfur indicative of eruption in water deeper than 200 m basalt glass fragments associated with the reefs above the h terrace are dominantly tholeiitic and contain intermediate sulfur contents indicative of subaqueous eruption in shallow near shore conditions one alkalic glass fragment was recovered above the h terrace these relations indicate that the morphologic end of shield building as recorded by construction of the h terrace was not accompanyed by a change from tholeiitic to alkalic basalt instead tholeiite eruptions continued for some time before the erupted lava became alkalic

geochemistry of the hawi lavas kohala volcano hawaii hawi lavas form the late stage alkalic cap on kohala volcano and range in composition from hawaiite to trachyte new detailed field mapping of kohala and reinterpretation of previously published age data suggest that there was no significant eruption hiatus between the hawi and underlying pololu shield lavas as was previously suggested mineral and whole rock chemical data are consistent with a crystal fractionation origin for the hawaiite to trachyte compositional variation observed within the hawi lavas plagioclase clinopyroxene ti magnetite olivine and apatite fractionation are needed to explain this variation the clinopyroxene fractionation may have occurred at moderate pressure because it is virtually absent in these lavas and is not a near liquidus phase at pressures of less than 8 kb plagioclase would be buoyant in the hawi hawaiite magmas so a mechanism like dynamic flow crystallization is needed for its fractionation and to account for the virtual absence of phenocrysts in the lavas hawi lavas are distinct in sr and nd isotopic ratios and or incompatible element ratios from the pololu lavas thus they were derived from compositionally distinct sources compared to other suites of hawaiian alkalic cap lavas hawi lavas have anomalously high concentrations of phosphorus and rare earth elements these differences could be due to greater apatite content in the source for the hawi lavas

geologic constraints on bedrock river incision using the stream power law denudation rate in unextended terranes is limited by the rate of bedrock channel incision often modeled as work rate on the channel bed by water and sediment or stream power the latter can be generalized as ka m s n where k represents the channel bed s resistance to lowenng whose variation with lithology is unknown a is drainage area a surrogate for discharge s is local slope and m and n are exponents whose values are debated we address these uncertainties by simulating the lowering of ancient river profiles using the finite difference method we vary m n and k to match the evolved profile as closely as possible to the corresponding modern river profile over a time period constrained by the age of the mapped paleoprofiles we find at least two end member incision laws ka 0 3 0 5 s for australian rivers with stable base levels and k f a 0 1 0 2 s n for rivers in kauai subject to abrupt base level change the long term lowering rate on the latter expression is a function of the frequency and magnitude of knickpoint erosion characterized by k f incision patterns from japan and california could follow either expression if they follow the first expression with m 0 4 k varies from 10 7 10 6 m 0 2 yr for granite and metamorphic rocks to 10 5 10 4 m 0 2 yr for volcaniclastic rocks and 10 4 10 2 m 0 2 yr for mudstones this potentially large variation in k with lithology could drive strong variability in the rate of long term landscape change including denudation rate and sediment yield

the effect of wet dry weathering on the rate of bedrock river channel erosion by saltating gravel abstract previous work has shown that the bedrock erosion rate e because of collisions of saltating bedload can be expressed by e qb 1 pc where qb is the sediment transport rate pc is the extent of alluvial cover and is the abrasion coefficient however the dependence of the abrasion coefficient on the physical characteristics of the bedrock material is poorly known and in particular the effects of wet dry weathering on the saltation abrasion bedrock incision has not been specifically characterized observation suggests that the typical wet dry cycling of exposed bedrock in river beds gives rise to cracks and voids that are likely to alter the incision rate of the material when subjected to impacts of moving sediment in this study flume experiments are performed to develop an understanding of how wet dry cycling affects the rock tensile strength and the bedrock erosion rate to represent the physical effects of weathering boring cores taken from natural bedrock channel are exposed to artificial wet dry cycles the experimental results suggest the following 1 the abrasion coefficient for fresh bedrock is estimated by 1 0 10 4 t 2 d ksb 0 5 where t is the tensile strength d is the diameter of colliding gravel and ksb is the hydraulic roughness height of bedrock 2 the tensile strength of the bedrock decreases exponentially as a result of repeated wet dry cycles t t0 exp ctnwa0 t0 where t0 is the initial tensile strength wa0 is the initial normalized rate of water absorption n is the number of wet dry cycles and ct is a constant 3 the erosion rate of fresh bedrock depends on the inverse of the square of tensile strength but the erosion rate of weathered bedrock depends on the 1 5 power of tensile strength

does climate change create distinctive patterns of landscape incision 1 incised fluvial systems are typically interpreted as recording geologically recent changes in either climate or tectonics however few diagnostic tools exist to evaluate whether particular incised landscapes primarily reflect climatic or tectonic perturbation here we summarize the results of a simple fluvial sediment transport model that allows us to contrast patterns of fluvial incision driven by changes in hydrology and sediment flux climate with those driven by changes in rock uplift patterns relative to sea level tectonics our modeling suggests that there may be diagnostic differences between the spatial and temporal patterns of incision caused by these different processes in particular incision driven by climate change is most commonly accompanied by downstream migrating waves of incision and decreases in channel gradient while under most circumstances incision driven by tectonics will be accompanied by upstream migrating incision and increases in channel gradient we apply our modeling to a case study of the north american high plains where regionally elevated surfaces east of the rockies have been incised up to 500 m by the fluvial systems draining the core of the range although this incision has been interpreted as reflecting a tectonic rejuvenation of the high plains our analysis suggests that climate change is at least as plausible an explanation to explain the incision of this landscape diagnostic differences between the climate and tectonic end member scenarios might be obtained via detailed study of the spatial and temporal patterns of terrace abandonment providing a potentially fruitful target for field investigation

response of bedrock channel width to tectonic forcing insights from a numerical model theoretical considerations and comparison with field data the morphology of bedrock river channels is controlled by climatic and tectonic conditions and substrate properties knowledge of tectonic controls remains scarce this is partly due to slow tectonic rates and long response times of natural channels and partly due to the difficulty in isolating and constraining tectonic forcing conditions in the field to study the effect of tectonic forcing on channel geometry we have developed a numerical model of the cross sectional evolution of a detachment limited channel its predictions are matched by an analytical model based on the assumption of the minimization of potential energy expenditure using these models we illustrate how local tectonics can alter the observed width discharge scaling and discuss published field data in light of our findings except for one case the models fail to correctly describe field observations of well constrained cases this implies that the shear stress stream power family of models is too simple to describe the behavior of natural channels additional complexities such as sediment effects and discharge variability exert a strong control on channel morphology and need to be taken into account in the modeling of channel dynamics and steady state

climatic forcing on channel profiles in the eastern cordillera of the coroico region bolivia abstractorographic precipitation has a large impact on channel morphology and rock uplift via a positive feedback to erosion we show that in the eastern cordillera of bolivia channel concavities reach their highest values where annual precipitation increases in the downstream direction exceeding 3000 mm the steepest channels are upstream of this zone of high concavity where precipitation rates are 1000 mm yr 1 channels exhibit graded forms both upstream and downstream of this transient reach we conclude that the prolonged effect of orographic erosion and related tectonic uplift is the preservation of channels with extreme concavities in the eastern cordillera

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

climatic control of bedrock river incision bedrock river incision drives the development of much of earth s surface topography and thereby shapes the structure of mountain belts and modulates earth s habitability through its effects on soil erosion nutrient fluxes and global climate although it has long been expected that river incision rates should depend strongly on precipitation rates quantifying the effects of precipitation rates on bedrock river incision rates has proved difficult partly because river incision rates are difficult to measure and partly because non climatic factors can obscure climatic effects at sites where river incision rates have been measured here we present measurements of river incision rates across one of earth s steepest rainfall gradients which show that precipitation rates do indeed influence long term bedrock river incision rates we apply a widely used empirical law for bedrock river incision to a series of rivers on the hawaiian island of kaua i where mean annual precipitation ranges from 0 5 metres to 9 5 metres ref 12 over 70 per cent of the global range and river incision rates averaged over millions of years can be inferred from the depth of river canyons and the age of the volcanic bedrock both a time averaged analysis and numerical modelling of transient river incision reveal that the long term efficiency of bedrock river incision across kaua i is positively correlated with upstream averaged mean annual precipitation rates we provide theoretical context for this result by demonstrating that our measurements are consistent with a linear dependence of river incision rates on stream power the rate of energy expenditure by the flow on the riverbed these observations provide rare empirical evidence for the long proposed coupling between climate and river incision suggesting that previously proposed feedbacks among topography climate and tectonics may occur