amplified erosion above waterfalls and oversteepened bedrock reaches erosion rate at the upstream end of the flow acceleration zone above a waterfall fr is the froude number at this setting and n ranges between 0 5 1 7 this amplification expression suggests that erosion at the lip could be as much as 2 5 times higher relative to erosion at a normal setting with identical hydraulic geometry utilizing this erosion amplification expression in numerical simulations we demonstrate its impact on reach scale morphology above waterfalls amplified erosion at the lip of a waterfall can trigger the formation of an oversteepened reach whose length is longer than the flow acceleration zone provided incision wave velocity vi at the upstream edge of the flow acceleration zone is higher than the retreat velocity of the waterfall face such an oversteepened reach is expected to be more pronounced when vi increases with increasing slope the simulations also suggest that oversteepening can eventually lead to steady state gradients adjacent to a waterfall lip provided vi decreases with increasing slope flow acceleration above waterfalls can thus account at least partially for prevalent oversteepened bedrock reaches above waterfalls using the cosmogenic isotope cl 36 we demonstrate that incision wave velocity upstream of a waterfall at the dead sea western escarpment is probably high enough for freefall induced oversteepening to be feasible

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

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

interpretation and downstream correlation of bedrock river terrace treads created from propagating knickpoints 1 derivation of an analytical model for the slope of a strath terrace created following an upstream propagating wave of incision reveals that for detachment limited river incision the exponent on channel slope n governs terrace tread slope terrace elevations can increase upstream n 1 downstream n 1 or can be horizontal n 1 numerical modeling confirms these results for temporally evolving knickpoint geometries and for incision due to sudden base level fall and an increase in rock uplift rate except in the case of bedrock river incision with a slope threshold only exceeded during knickpoint propagation a terrace created from transient headward incision contains no information about the paleo channel gradient gradients in rock uplift rate along channels potentially complicate the interpretation of terraces by altering the primary tilt on strath terraces in particular monotonic gradients in rock uplift can produce terrace treads that are apparently folded because gradients in rock uplift rate are common where terraces are not longitudinally traceable care is warranted in terrace correlation in simple tectonic settings where terraces are longitudinally traceable the slope of a strath terrace created from headward incision may provide a means of estimating the dependency of river incision rate on channel slope terraces in four locations argued to have formed from headward incision are parallel or close to parallel with the active channel implying a slope exponent on river incision rate that is much greater than one or a threshold slope for incision that is only exceeded during knickpoint propagation

geomorphic constraints on surface uplift exhumation and plateau growth in the red river region yunnan province china field observations digital elevation model dem data and longitudinal profi le analysis reveal a perched low relief upland landscape in the red river region yunnan province china which correlates to an uplifted regional low relief landscape preserved over the eastern margin of the tibetan plateau as with other major rivers of the plateau margin the red river has deeply incised the lowrelief upland landscape which we interpret to be the remnants of a pre uplift or relict landscape we examine longitudinal river profi les for 97 tributaries of the red river most profi les consist of three segments separated by sharp knickpoints an upper lowgradient channel segment a steeper middle channel segment and a very steep lower channel segment upper channel segments correspond to the relict landscape and have not yet experienced river incision steeper middle and lower segments indicate onset of rapid two phase river incision on the basis of which changes in external forcings such as climate or uplift can be inferred in terms of two end member scenarios two phase incision could be the result of pulsed plateau growth in which relatively slow uplift during the fi rst phase is followed by rapid uplift during the second phase or it could refl ect adjustments of the main channel to changing climate conditions against the backdrop of steady plateau growth reconstruction of the paleo red river indicates 1400 m river incision 1400 1500 m surface uplift and a maximum of 750 m vertical displacement across the northern red river fault elevating the northern ailao shan range above the surrounding relict landscape on the basis of stratigraphic constraints incision along the red river likely began in pliocene time

a demonstration of the importance of bedload transport for fluvial bedrock erosion and knickpoint propagation we provide field evidence for the role of bedload in driving fluvial incision and knickpoint propagation using aerial photographs field surveys and hydrological data we constrain the incision history of a bedrock gorge 1200 m long and up to 20 m deep cut by da an river in western taiwan this reach of the river experienced 10 m of uplift during the 1999 chi chi earthquake for five years following the earthquake bedload was prevented from entering the uplift zone the knickpoint was static and little incision took place bedload transport across the uplift zone resumed in 2004 initiating extremely rapid incision with 620 m of knickpoint propagation and up to 20 m of downcutting by 2008 this change highlights the relative inefficiency of suspended sediment and the dominant role of bedload as a tool for fluvial erosion and knickpoint propagation once bedload tools became available knickpoint propagation was influenced by geological structure lithology and drainage organization in particular a change in dip of the sandstone beds at the site caused a decrease of knickpoint propagation velocity copyright 2012 john wiley sons ltd

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

high magnitude and rapid incision from river capture rhine river switzerland 1 landscape evolution is controlled by the development and organization of drainage basins as a landscape evolves drainage reorganization events can occur via river capture or piracy whereby one river basin grows at the expense of another the river downstream of a capture location will generate a transient topographic response as the added water discharge increases sediment transport and erosion efficiency this erosional response will propagate upstream through both the captured and original river basins here we focus on quantifying the impact of drainage reorganization along the rhine aare river system 45 000 km2 during the late pliocene early pleistocene where gravel remnants indicate total incision of 650 m during the last 4 2 myr in the region of the recent aare rhine confluence we develop a numerical model of drainage capture to quantify the range of possible magnitudes of erosion and the transient river response resulting from the reorganization of the rhine river the model accounts for both fluvial incision and sediment transport our model estimates 400 800 m of river elevation change lowering profiles during the last 4 myr due to river capture events providing an important component to the recent exhumation budget of the swiss alpine foreland the model indicates a rapid response to capture events re equilibration timescale of 1 myr the predicted incision magnitudes are consistent with incision measured from the elevation of pliocene and early pleistocene river gravels suggesting that across northern switzerland a significant amount of incision can be explained by drainage reorganization

fluvial incision into bedrock insights from morphometric analysis and numerical modeling of gorges incising glacial hanging valleys western alps france bedrock gorges incising glacial hanging valleys potentially allow measurements of fluvial bedrock incision in mountainous relief using digital elevation models topographic maps and field reconnaissance we identified and characterized 30 tributary hanging valleys incised by gorges near their confluence with trunk streams in the romanche watershed french western alps longitudinal profiles of these tributaries are all convex and have abrupt knickpoints at the upper limit of oversteepened gorge reaches we reconstructed initial glacial profiles from glacially polished bedrock knobs surrounding the gorges in order to quantify the amount of fluvial incision and knickpoint retreat from morphometric analyses we find that mean channel gradients and widths as well as knickpoint retreat rates display a drainage area dependence modulated by bedrock lithology however there appears to be no relation between horizontal retreat and vertical downwearing of knickpoints assuming a postglacial origin of these gorges our results imply high postglacial fluvial incision 0 5 15 mm yr 1 and knickpoint retreat 1 200 mm yr 1 rates that are however consistent with previous estimates numerical modeling was used to test the capacity of different fluvial incision models to predict the inferred evolution of the gorges results from simple end member models suggest transport limited behavior of the bedrock gorges a more sophisticated model including dynamic width adjustment and sediment dependent incision rates predicts present day channel geometry only if a significant supply of sediment from the gorge sidewalls 10 mm yr 1 is triggered by gorge deepening combined with pronounced inhibition of bedrock incision by sediment transport and deposition

hillslope response to tectonic forcing in threshold landscapes hillslopes are thought to poorly record tectonic signals in threshold landscapes numerous previous studies of steep landscapes suggest that large changes in long term erosion rate lead to little change in mean hillslope angle measured at coarse resolution new lidar derived topography data enables a finer examination of threshold hillslopes here we quantify hillslope response to tectonic forcing in a threshold landscape to do so we use an extensive cosmogenic beryllium 10 10 be based dataset of catchment averaged erosion rates combined with a 500 km 2 lidar derived 1 m digital elevation model to exploit a gradient of tectonic forcing and topographic relief in the san gabriel mountains california we also calibrate a new method of quantifying rock exposure from lidar derived slope measurements using high resolution panoramic photographs two distinct trends in hillslope behavior emerge below catchment mean slopes of 30 modal slopes increase with mean slopes slope distribution skewness decreases with increasing mean slope and bedrock exposure is limited above mean slopes of 30 our rock exposure index increases strongly with mean slope and the prevalence of angle of repose debris wedges keeps modal slopes near 37 resulting in a positive relationship between slope distribution skewness and mean slope we find that both mean slopes and rock exposure increase with erosion rate up to 1 mm a in contrast to previous work based on coarser topographic data we also find that as erosion rates increase the extent of the fluvial network decreases while colluvial channels extend downstream keeping the total drainage density similar across the range our results reveal important textural details lost in 10 or 30 m resolution digital elevation models of steep landscapes and highlight the need for process based studies of threshold hillslopes and colluvial channels

is the critical shields stress for incipient sediment motion dependent on channel bed slope data from laboratory flumes and natural streams show that the critical shields stress for initial sediment motion increases with channel slope which indicates that particles of the same size are more stable on steeper slopes this observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force processes that might explain this discrepancy are explored using a simple force balance model including increased drag from channel walls and bed morphology variable friction angles grain emergence flow aeration and changes to the local flow velocity and turbulent fluctuations surprisingly increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical shields stress are similar for flume experiments and natural streams and significant variations in bed morphology in flumes is unlikely instead grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed roughness scale to flow depth i e relative roughness a model for the local velocity within the grain roughness layer is proposed based on a 1 d eddy viscosity with wake mixing in addition the magnitude of near bed turbulent fluctuations is shown to depend on the depth averaged flow velocity and the relative roughness extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes

the role of waterfalls and knickzones in controlling the style and pace of landscape adjustment in the western san gabriel mountains california bedrock rivers set the pace of landscape adjustment to tectonic and climatic forcing by transmitting signals of base level change upstream through the channel network and ultimately to hillslopes river incision is typically modeled as a monotonic function of bed shear stress or stream power modulated by sediment tools and cover effects but these models do not apply in channels with steep or vertical bedrock reaches due to changes in flow dynamics hydraulic geometry and bed cover here we investigate how such knickzones oversteepened channel reaches often containing waterfalls influence the propagation of slope break knickpoints that separate relict from adjusting topography and thus the response times of landscapes to external forcing we use a conceptual long profile model to explore the consequences of waterfalls and knickzones on channel response and compare predictions to light detection and ranging lidar topography field observations and cosmogenic radionuclide data from big tujunga creek a 300 km2 watershed in the san gabriel mountains california three prominent knickzones along big tujunga creek characterized by numerous waterfalls show contrasting behavior for the upper knickzone waterfalls align with bands of harder rock exposed on adjacent hillslopes and between waterfalls the channel is mantled by large 2 m boulders indicating knickzone retreat is slow compared to predictions of slope break knickpoint retreat from stream power models enhancing the preservation of an upstream relict landscape the middle knickzone shows evidence for both fast and slow knickzone retreat as well as significant deviations from predictions of uniform tributary knickpoint elevations derived from stream power models the lower knickzone is characterized by a waterfall and knickzone within an incised inner gorge that provide evidence of rapid retreat relative to background channel incision overall we find a pattern of decreasing knickzone and waterfall retreat rate with distance upstream of the range front beyond decreases predicted by simple area dependent celerity models our results highlight that waterfalls and knickzones can both enhance and inhibit landscape adjustment leading to divergent controls on the pace of landscape evolution

paleoseismologic evidence for a 8 ka age of the most recent surface rupture on the eastern sierra madre fault northern los angeles metropolitan region california paleoseismologic data from the sierra madre fault a major north dipping reverse fault that extends for 75 km across the northern edge of the los angeles metropolitan region indicate that the most recent surface rupture on the eastern part of the fault occurred more than 8000 years ago coupled with evidence for a minimum reverse slip rate of 0 6 0 9 mm yr on the strand that we trenched the long elapsed interval since the most recent event suggests that the sierra madre fault breaks during very infrequent large magnitude m w 7 earthquakes events of such large magnitude are much larger than the largest earthquakes that have occurred on any of the los angeles area urban faults during the 200 year long historic period e g the 1971 m w 6 7 san fernando and 1994 m w 6 7 northridge earthquakes and must be considered in future seismic hazard analyses for southern california although more paleoseismologic data are needed to determine whether or not the sierra madre fault ruptures together with adjacent faults available data already show that the raymond fault a west southwest trending left lateral strike slip fault that intersects the central sierra madre fault has ruptured to the surface at least once and possibly several times since the most recent surface rupture on the eastern sierra madre fault moreover if the san andreas fault saf and the sierra madre fault ever rupture together then such events must be exceedingly rare with at least 50 100 saf m w 8 so called big ones occurring between every possible combined saf sierra madre fault event

quantifying frictional variations and erosion in the mexican fold thrust belt abstract the impact of frictional crustal heterogeneities and surface processes on the evolution of the ancient thin skinned mexican fold and thrust belt mftb is investigated and quantified using numerical finite element simulations these simulations explore the evolution total shortening and structural styles of deforming orogenic wedges the models include spatial lateral variations in the internal friction and the topographic erodibility k using the power law incision rule the mftb exhibits paleogeographical elements manifested as lateral facies changes across two platforms units and two sedimentary basins that show different degrees of deformation the observed kinematics of the mftb requires based on the parameters considered in this study a rheologically heterogeneous crust with at least laterally varying internal friction yet only limited syntectonic and post tectonic erosion are necessary to explain the geometries and internal total shortening constrained by field observations and interpretations application of the analytical and homogeneous critical coulomb wedge ccw theory to the mftb closely predicts the external wedge geometry but fails to predict the observed shortening of the individual litho tectonic units and structural styles

dynamic reorganization of river basins river networks evolve as migrating drainage divides reshape river basins and change network topology by capture of river channels we demonstrate that a characteristic metric of river network geometry gauges the horizontal motion of drainage divides assessing this metric throughout a landscape maps the dynamic states of entire river networks revealing diverse conditions drainage divides in the loess plateau of china appear stationary the young topography of taiwan has migrating divides driving adjustment of major basins and rivers draining the ancient landscape of the southeastern united states are reorganizing in response to escarpment retreat and coastal advance the ability to measure the dynamic reorganization of river basins presents opportunities to examine landscape scale interactions among tectonics erosion and ecology

a model for fire induced sediment yield by dry ravel in steep landscapes sediment flux from hillslopes to channels commonly increases following wildfires with implications for the carbon cycle river habitats and debris flow hazards although much of this material is transported via dry ravel existing ravel models are not applicable to hillslopes with gradients greater than the angle of repose which can constitute the majority of mountainous terrain to fill this knowledge gap we develop a continuity model for sediment storage by vegetation dams on steep hillslopes to predict sediment yields following wildfire the maximum volume of sediment stored prior to wildfire is set to be a function of vegetation density the capacity of plants to impound sediment and the contributing hillslope area time is required after fire to establish vegetation and replenish hillslope sediment storage which introduces vegetation regrowth rate soil production rate and fire recurrence interval as important variables that affect ravel yield model results for the san gabriel mountains california predict that sediment yield can increase by several orders of magnitude following fire these results are consistent with field data of ravel yield 30 mm per contributing area of hillslope in 5 months we collected following the 2009 station fire as well as postfire sediment flux recorded by 93 debris basins in contrast to previous work our model shows that heightened postfire sediment yields can be explained by a change in hillslope sediment storage independent of major changes in the soil production rate and landscape form over geomorphic timescales

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

evolution of vertical knickpoints waterfalls with resistant caprock insights from numerical modeling 1 vertical knickpoints waterfalls mark a prominent process transition zone whose governing mechanics is not represented by conventional stream power incision models we examine the evolution of vertical knickpoints with resistant caprock utilizing numerical simulations that explicitly represent 1 face failure mechanisms 2 flow acceleration and amplified erosion above a knickpoint lip 3 deposition and removal of coarse debris below the knickpoint and 4 base level lowering or tectonic uplift rates our model demonstrates that knickpoint retreat rate where the subcaprock is weak or vertically jointed and base level fall rates are steady is likely to become tied to downstream conditions and equal to the downstream incision rate divided by channel gradient mechanically this coupling occurs where the subcaprock reaches a threshold height for failure in shear or buckling or where the weathering rate of the subcaprock is higher than the downstream incision wave velocity vi ds the height of the subcaprock face can influence its gravitational stability and the knickpoint lateral erosion rate and lead to a feedback between downstream incision and retreat rate retreat rate can be lower than vi ds during transients which could be long 106 years and set by the weathering rate of the subcaprock or influenced by lag debris evacuation key variables other than discharge can be important in setting retreat rates these include base level lowering rate the rock strength of stratigraphic units downstream of the knickpoint and the size and flux of sediment contributed from above the knickpoint or from the canyon walls two types of oversteepened reaches can form in association with a vertical knickpoint 1 an upstream free fall induced oversteepened reach whose length is longer than the flow acceleration zone and 2 a downstream coarse debris induced oversteepened reach although the model was constructed with caprock type knickpoints in mind some of its elements and insights are also relevant to homogenous substrates

solutions of the stream power equation and application to the evolution of river longitudinal profiles 1 erosion by bedrock river channels is commonly modeled with the stream power equation we present a two part approach to solving this nonlinear equation analytically and explore the implications for evolving river profiles first a method for non dimensionalizing the stream power equation transforms river profiles in steady state with respect to uniform uplift into a straight line in dimensionless distance elevation space second a method that tracks the upstream migration of slope patches which are mathematical entities that carry information about downstream river states provides a basis for constructing analytical solutions slope patch analysis explains why the transient morphology of dimensionless river profiles differs fundamentally if the exponent on channel slope n is less than or greater than one and why only concave up migrating knickpoints persist when n 1 at migrating knickpoints slope patches and the information they carry are lost a phenomenon that fundamentally limits the potential for reconstructing tectonic histories from bedrock river profiles stationary knickpoints which can arise from spatially varying uplift rates differ from migrating knickpoints in that slope patches and the information they carry are not lost counterparts to migrating knickpoints called stretch zones are created when closely spaced slope patches spread to form smooth curves in distance elevation space these theoretical results are illustrated with examples from the california king range and the central apennines

1500 years of lake sedimentation due to fire earthquakes floods and land clearance in the oregon coast range geomorphic sensitivity to floods during timber harvest period

denudation processes and rates in the transverse ranges southern california erosional response of a transitional landscape to external and anthropogenic forcing 1 quantification of denudation in the transverse ranges of california permits reconstruction of spatial and temporal variations in erosion that represent both the response and evolution of interacting hillslopes and channels on the southern front of the san gabriel mountains observational records of the infilling of debris basins and dams define twentieth century landscape erosion rates averaging 1 6 and 0 9 mm yr 1 respectively although all major sediment transport occurs during intense winter storms debris production on hillslopes is greatly enhanced by recurrent fires consequently in this populated region anthropogenic fires have augmented the natural erosion rates we perform a global inversion to estimate the role of precipitation intensity burned areas and local slope on catchment denudation rates after subtracting the effects of anthropogenic fires we estimate landscape denudation rates given natural fire ignition rates increased fire during the past century has augmented sediment production in debris basins by an average of 60 and individual basins show increases up to 400 to identify the dominant hillslope erosion processes the volumetric contribution of landslides was estimated using repeat aerial photographs for the same time interval over which the debris basins have been operative between 1928 and 1973 landsliding produced only 10 of the sediment in debris basins even in the long term when infrequent but volumetrically important landslides occur bedrock landslides appear to contribute a maximum of 50 to the long term landscape denudation previous mapping of soil slippage in the san dimas experimental forest within the san gabriel mountains rice et al 1969 rice and foggin 1971 indicates that shallow landsliding is likely to be the dominant modern hillslope erosion process when compared to incision rates derived from a fluvial shear stress model and to exhumation rates based on low temperature thermochronological data blythe et al 2000 modern natural erosion rates are comparable to denudation rates since the pliocene comparisons of modern erosion rates suggest that debris production on hillslopes and first order channels is directly dependent on vegetation cover and precipitation intensity for higher order channels drainage areas 2 km2 only major storms convey the sediments down valley during the past century some temporal decorrelation occurs between small scale and large scale catchments because hillslope produced sediment is stored in second order and higher channels until major storms mobilize it thus the larger fluvial network damps the episodic fire induced hillslope sediment pulses that occur within small watersheds at a longer temporal scale however uplift and denudation may have been sustained sufficiently long in much of the san gabriel mountains for the topography to reach a macroscale steady state in contrast to rapidly eroding 2 mm yr 1 ranges for which quantification of bedrock landsliding will approximate the sediment flux the san gabriel mountains occupy a niche of intermediate rates 0 1 1 0 mm yr 1 in which a broad suite of hillslope processes including shallow seated and deep seated landslides debris flows and wet and dry ravel contribute to the sediment flux

influence of rock strength properties on escarpment retreat across passive continental margins we examine the influence of bedrock strength properties imparted by jointing and fracturing on the retreat of erosional escarpments across passive continental margins two types of escarpment are identified and focus is placed on the gorge head knickpoint form of escarpment where upland streams drain across the escarpment along the macleay river system of eastern australia major knickpoints associated with escarpment retreat are currently located about 200 km upstream from the river mouth despite differences in bedrock lithology and order of magnitude differences in drainage areas upstream of the gorge heads implying an average escarpment retreat rate of 2 km m y since the margin formed 85 m y ago field observations from apsley river gorge a macleay river tributary indicate that bedrock jointing profoundly affects the evolution of the gorge headwall and sidewalls where dominant jointing dips into hillslopes failure occurs by block and column toppling conversely where jointing dips out of hillslopes mass wasting occurs by joint parallel rock sliding allowing slopes to remain steep as material is shed from the gorge sidewalls the toppling slopes develop lower overall gradients the gorge becomes asymmetric in cross section and the channel remains nearer to the steeper sidewall affected by joint parallel sliding coupling between fluvial and hillslope processes is essential to maintain headward propagation of the knickpoint gorge head and to facilitate mass wasting from the gorge sidewalls peak stream discharge across the gorge head must exceed a threshold sufficient to transport debris away from the gorge head to keep the knickpoint rock face free of talus to incise the stream channel and to erode the toes of the sidewalls if these conditions on fluvial processes are met then hillslope processes including rockslope failure mechanisms are the rate controlling processes for knickpoint propagation and therefore determine the rate of escarpment retreat across the passive margin

a quantitative evaluation of playfair s law and its use in testing long term stream erosion models playfair s law j playfair illustrations of the huttonian theory of the earth 1802 requires any two tributaries in a river network to lower at the same rate near their junction although this law holds exactly at the junction it is unclear how well it holds in the vicinity of the junction this issue has practical importance because playfair s law has been used to estimate parameters for detachment limited models of erosion if the incision rate of a stream is modelled as amsn where is an erodibility parameter a is the area drained by the stream and s is the local gradient of the channel then the ratio of the parameters m n can be estimated from junctions by assuming that playfair s law holds over the distance used to determine s for each tributary in this paper playfair s law and associated m n estimates are evaluated for simulated basins with constant and temporally varying uplift rates or baselevel lowering rates the results demonstrate that estimates of m n may be biased for basins with upward concave stream profiles because the local slope must be approximated with an average upstream slope in addition when uplift rate varies temporally knickpoints are shown to travel through the basins with constant vertical velocity because incision rates vary within the basin playfair s law only holds exactly at the junctions these effects are more important when slopes are measured over longer distances finally measurement techniques are presented which address these potential biases copyright 2001 john wiley sons ltd

geomorphic response to uplift along the dragon s back pressure ridge carrizo plain california we used high resolution topography geomorphic mapping of active surface processes and geologic mapping to study the topographic and erosional response of small drainage basins to rock uplift along the dragon s back pressure ridge along the san andreas fault in the carrizo plain california we infer the history of deformation experienced by 40 small drainage basins formed in poorly consolidated sedimentary rocks a space for time substitution directly images the erosional and topographic responses to deformation progressive deformation and rock uplift are accompanied by increases in channel steepness and basin relief as uplift ceases channel concavity rapidly increases causing channels to undercut hillslopes this undercutting promotes the consumption of hillslopes by landsliding this undercutting also causes basin relief to be greatest after uplift has stopped this analysis indicates that channels of the dragon s back pressure ridge respond to changes in rock uplift rates over thousands of years whereas hillslope processes may take more than an order of magnitude longer to adjust to changes in rock uplift rates our study directly measures changes in erosional processes due to the initiation and cessation of rock uplift which can typically only be inferred using numerical models by direct field observations

late quaternary slip rate gradient defined using high resolution topography and 10be dating of offset landforms on the southern san jacinto fault zone california 1 recent studies suggest the san jacinto fault zone may be the dominant structure accommodating pa na relative plate motion however because the late quaternary slip history of the southern san andreas fault system is insufficiently understood it is difficult to evaluate the partitioning of deformation across the plate boundary and its evolution landforms displaced by the clark fault of the southern san jacinto fault zone were mapped using high resolution airborne laser swath topography and selected offset landforms were dated using cosmogenic 10be beheaded channels at rockhouse canyon displaced by 500 70 m and 220 70 m have been dated to 47 8 ka and 28 9 ka respectively farther south near the southern santa rosa mountains an alluvial deposit displaced by 51 9 m has been dated to 35 7 ka from these sites the slip rate of the clark fault is determined to diminish southward from 8 9 2 0 to 1 5 0 4 mm yr this implies a slip rate decrease along the clark fault from anza southeastward to its surface termination near the salton trough where slip is transferred to the coyote creek fault and additional deformation is compensated by folding and thrusting in the basin these data suggest that since 30 to 50 ka the slip rate along the southern san jacinto fault zone has been lower than or equivalent to the rate along the southernmost san andreas fault accordingly either the slip rate of the san jacinto fault has substantially decreased since fault initiation or fault slip began earlier than previously suggested

explicit treatment of inheritance in dating depositional surfaces using in situ 10be and 26al we describe a new strategy for dating depositional landscape surfaces using in situ produced cosmogenic radionuclides crns that removes the complication of nuclide inheritance by clasts prior to depositon two amalgamated samples each consisting of 30 clasts one from the surface and one from a fixed depth in the subsurface constrain this crn inheritance and date the surface the inheritance may be used to estimate minimum exhumation rates and maximum transport times within the geomorphic system we test the technique using sup 10 be and sup 26 al to date the third fr3 of five terraces along the fremont river utah and the third wr3 of 15 along the wind river wyoming whereas effective ages based solely upon the surface samples yield 118 138 ka fr3 and 93 ka wr3 the subsurface samples reveal that inheritance accounts for 22 wr3 to 43 fr3 of the total crn concentration taking this into account yeilds terrace ages of nearly 61 81 ka for fr3 and nearly 67 76 ka for wr3 we explore the dependence of age estimates on the accumulation history of the terrace silt caps 22 refs 3 figs 1 tab

hillslope response to knickpoint migration in the southern appalachians implications for the evolution of post orogenic landscapes the southern appalachians represent a landscape characterized by locally high topographic relief steep slopes and frequent mass movement in the absence of significant tectonic forcing for at least the last 200 ma the fundamental processes responsible for landscape evolution in a post orogenic landscape remain enigmatic the non glaciated cullasaja river basin of south western north carolina with uniform lithology frequent debris flows and the availability of high resolution airborne lidar dems is an ideal natural setting to study landscape evolution in a post orogenic landscape through the lens of hillslope channel coupling this investigation is limited to channels with upslope contributing areas 2 7 km2 a conservative estimate of the transition from fluvial to debris flow dominated channel processes values of normalized hypsometry hypsometric integral and mean slope vs elevation are used for 14 tributary basins and the cullasaja basin as a whole to characterize landscape evolution following upstream knickpoint migration results highlight the existence of a transient spatial relationship between knickpoints present along the fluvial network of the cullasaja basin and adjacent hillslopes metrics of topography relief slope gradient and hillslope activity landslide frequency exhibit significant downstream increases below the current position of major knickpoints the transient effect of knickpoint driven channel incision on basin hillslopes is captured by measuring the relief mean slope steepness and mass movement frequency of tributary basins and comparing these results with the distance from major knickpoints along the cullasaja river a conceptual model of area elevation and slope distributions is presented that may be representative of post orogenic landscape evolution in analogous geologic settings importantly the model explains how knickpoint migration and channel hillslope coupling is an important factor in tectonically inactive i e post orogenic orogens for the maintenance of significant relief steep slopes and weathering limited hillslopes copyright 2011 john wiley sons ltd

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

tectonics from fluvial topography using formal linear inversion theory and applications to the inyo mountains california tectonic activity generates topography and the variability of tectonic forcing is responsible for topographic patterns and variability of relief in fluvial landscapes despite this basic relation the inverse problem by which features of the topography are used for inferring tectonic uplift rates has proven challenging here we develop formal linear inversion schemes to infer a record of the rate of relative uplift as a function of space and time from the long profiles of rivers the relative uplift rate is the difference between the rates of rock uplift and of the base level change the inversion schemes are based on a closed form analytic solution to the transient linear stream power model and to increase model resolution they make use of the multiplicity of information made available by multiple rivers and their tributaries the distribution of the fluvial response time to tectonic perturbations is a key component of the inversion scheme as this determines which tectonic events are preserved in the topography we develop two inversion parameterizations that differ in their assumptions about the tectonic forcing space invariant and time space variability with an assumed spatial distribution the inversion schemes are applied to the inyo mountains an uplifted block along the western boundary of the basin and range province in california inversion results indicate that the range has been experiencing an acceleration of the relative uplift in the past 2 3 ma we use the inversion results to constrain the paleotopography and paleo erosion rate along the range and to recover the throw rate history along the fault that bounds the inyo range

use of a regional relict landscape to measure vertical deformation of the eastern tibetan plateau 1 field work and topography analysis show that remnant local areas of a low relief landscape or erosion surfaces are geographically continuous across the southeastern tibetan plateau margin we correlate these remnant surfaces as a paleolandscape that formed at low elevation remnants of this paleolandscape are preserved because incision of the fluvial system has been largely limited to major rivers and principal tributaries and has not yet progressed throughout the entire fluvial network the incomplete adjustment of the fluvial system signals initiation of rapid bedrock incision into a developing plateau margin and erosional denudation is concentrated in the major river channels this interpretation contradicts earlier notions that low gradient regional topography is the product of regional elevation reduction by intense landscape dissection due to the presence of several large southeast flowing rivers the modern altitude of the reconstructed paleolandscape or relict landscape constrains the vertical displacement of the plateau surface in response to crustal thickening and subsequent erosion during the lateral growth of the tibetan plateau regional preservation of the relict landscape is consistent with minor surface disruption by late cenozoic thrust faults and folds and supports a model of distributed lower crustal thickening because significant erosion is limited to narrow river gorges an increase in the plateau elevation due to isostatic rebound is minor therefore we propose that the modern elevation of the relict landscape reflects isostatically compensated thickening of the lower crust

arc parallel extension and localization of volcanic complexes in guadeloupe lesser antilles 1 subduction of atlantic seafloor under the caribbean plate causes shallow earthquakes within the lesser antilles volcanic arc such earthquakes above the subduction interface show strike slip or normal fault plane solutions the latter with e w striking nodal planes to better assess seismic hazard and the coupling between volcanism and tectonics we investigated faulting related to overriding plate deformation in the guadeloupe archipelago using aerial photographs satellite spot images and topographic maps 1 25000 scale we mapped active and middle to late pleistocene fissures and normal fault systems that cut the uplifted coral platforms grande terre and marie galante and the volcanic rocks of basse terre the available marine geophysical data show that the faults extend offshore to bound submarine rifts the e w striking 1500 m deep v shaped marie galante rift separates the two islands of marie galante and grande terre normal faults in the north of grande terre appear to mark the similarly v shaped western termination of the 5000 m deep n 50e to n130 e striking desirade graben three shallow m 5 5 earthquakes 6 may 1851 29 april 1897 3 august 1992 appear to have ruptured segments of the marie galante rift boundary faults the young la grande decouverte volcanic complex of basse terre including the 1440 a d soufriere dome lies within the western termination of the marie galante rift the ancient volcanic shoulders of the rift buttress the active dome to the north and south which may explain why major prehistoric sector collapses and pyroclastic avalanches have been directed southwestward into the caribbean sea or southeastward into the atlantic ocean the marie galante rift is typical of other troughs transverse to the northeastern edge of the caribbean plate we interpret such troughs which are roughly orthogonal to the arc to result from slip partitioning and extension perpendicular to plate convergence that they disappear southward implies that they result from interaction between the caribbean and north american plates