increased late pleistocene erosion rates during fluvial aggradation in the garhwal himalaya northern india the response of surface processes to climatic forcing is fundamental for understanding the impacts of climate change on landscape evolution in the himalaya most large rivers feature prominent fill terraces that record an imbalance between sediment supply and transport capacity presumably due to past fluctuations in monsoon precipitation and or effects of glaciation at high elevation here we present volume estimates chronological constraints and 10 be derived paleo erosion rates from a prominent valley fill in the yamuna catchment garhwal himalaya to elucidate the coupled response of rivers and hillslopes to pleistocene climate change although precise age control is complicated due to methodological problems the new data support formation of the valley fill during the late pleistocene and its incision during the holocene we interpret this timing to indicate that changes in discharge and river transport capacity were major controls compared to the present day late pleistocene hillslope erosion rates were higher by a factor of 2 4 but appear to have decreased during valley aggradation the higher late pleistocene erosion rates are largely unrelated to glacial erosion and could be explained by enhanced sediment production on steep hillslopes due to increased periglacial activity that declined as temperatures increased alternatively erosion rates that decrease during valley aggradation are also consistent with reduced landsliding from threshold hillslopes as a result of rising base levels in that case the similarity of paleo erosion rates near the end of the aggradation period with modern erosion rates might imply that channels and hillslopes are not yet fully coupled everywhere and that present day hillslope erosion rates may underrepresent long term incision rates

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

the influence of climate on the tectonic evolution of mountain belts simple physical arguments analogue experiments and numerical experiments all suggest that the internal dynamics of actively deforming collisional mountain ranges are influenced by climate however obtaining definitive field evidence of a significant impact of climate on mountain building has proved challenging spatial correlations between intense precipitation or glaciation and zones of rapid rock uplift have indeed been documented in numerous mountain ranges and are consistent with model predictions more compelling evidence such as tectonic changes in response to rather than just coincident with climate change has however rarely been documented triggered by a climate driven increase in erosion rate friction dominated mountain ranges are expected to show a number of simultaneous responses a decrease in the width of the range a temporary increase in sediment yield a persistent increase in the rate of rock uplift and a reduction in the subsidence rate of surrounding basins the most convincing field evidence for such a coordinated response of a mountain range to climate change comes from the european alps and the st elias range of alaska field studies and experimental research during the past two decades have provided considerable evidence for a significant influence of climate on tectonics recent advances suggest that model predictions can guide future fieldwork aimed at substantiating this view

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

late quaternary slip on the santa cruz island fault california the style timing and pattern of slip on the santa cruz island fault were investigated by trenching the fault and by analysis of offset late quaternary landforms a trench excavated across the fault at christi beach on the western coast of the island exposed deformation of latest pleistocene to holocene sediments and pre quaternary rocks recording repeated large magnitude rupture events the most recent earthquake at this site occurred ca 5 ka coastal terraces preserved on western santa cruz island have been dated using the uranium series technique and by extrapolation using terrace elevations and the eustatic record offset of terraces and other landforms indicates that the santa cruz island fault is predominantly left lateral having a horizontal slip rate of not more than 1 1 mm yr and probably about 0 8 mm yr the fault also has a smaller reverse component slipping at a rate of between 0 1 and 0 2 mm yr combined with measurements of slip per event this information suggests a long term average recurrence interval of at least 2 7 k y and probably 4 5 k y and average earthquake magnitudes of mw 7 2 7 5 sense of slip recurrence interval and earthquake magnitudes calculated here for the santa cruz island fault are very similar to recent results for other faults along the southern margin of the western transverse range including the malibu coast fault the santa monica fault the hollywood fault and the raymond fault supporting the contention that these faults constitute a continuous and linked fault system which is characterized by large but relatively infrequent earthquakes

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

on the prediction of channel heads in a complex alpine terrain using gridded elevation data 1 threshold conditions for channel initiation are evaluated by using gridded elevation data derived from a lidar survey a reliable algorithm for the determination of surface flow paths and field observations of channel heads for a study area located in the eastern italian alps these threshold conditions are determined by considering the channel heads observed across a portion of the study area and computing the related values of 1 drainage area a 2 area slope function as2 with s being the local slope and 3 strahler order of surface flow paths extracted from gridded elevation data attention is focused on the dependence of the obtained threshold values on the size of grid cells involved and on the ability of the identified threshold conditions to provide reliable predictions of channel heads across the entire study area the results indicate that the threshold values of a as2 and are all significantly dependent on grid cell size and the uncertainty in the determination of threshold values of is significantly smaller than that affecting the determination of threshold values of a and as2 the comparison between predicted and observed channel heads indicates that the considered methods display variable reliability and sensitivity over different drainage basins and grid cell sizes with a general tendency to predict more channel heads than can be observed in the field acceptable predictions are normally obtained where channel heads are formed essentially by surface runoff more comprehensive methods seem however to be needed to predict channel heads affected by groundwater seeping upward

gravitational sliding of the mt etna massif along a sloping basement geological field evidence and laboratory modelling indicate that volcanoes constructed on slopes slide downhill if this happens on an active volcano then the movement will distort deformation data and thus potentially compromise interpretation our recent gps measurements demonstrate that the entire edifice of mt etna is sliding to the ese the overall direction of slope of its complex rough sedimentary basement we report methods of discriminating the sliding vector from other deformation processes and of measuring its velocity which averaged 14 mm year 1 during four intervals between 2001 and 2012 though sliding of one sector of a volcano due to flank instability is widespread and well known this is the first time basement sliding of an entire active volcano has been directly observed this is important because the geological record shows that such sliding volcanoes are prone to devastating sector collapse on the downslope side and whole volcano migration should be taken into account when assessing future collapse hazard it is also important in eruption forecasting as the sliding vector needs to be allowed for when interpreting deformation events that take place above the sliding basement within the superstructure of the active volcano as might occur with dyke intrusion or inflation deflation episodes

neotectonics of the central nepalese himalaya constraints from geomorphology detrital 40ar 39ar thermochronology and thermal modeling 1 the southern flanks of the central nepalese himalaya correspond to a sharp transition in landscape morphology and bedrock mineral cooling ages that suggests a change in rock uplift rate this transition can be explained by either 1 accretion of footwall material to the hanging wall across a ramp in the decollement separating india from eurasia thereby enhancing rock uplift rates above the zone of accretion or 2 out of sequence surface thrust faulting at the physiographic transition here we use geomorphic data 649 new detrital 40ar 39ar cooling ages and a simple thermokinematic model to evaluate which of these tectonic configurations is most appropriate for the central nepalese himalaya we first define and delineate the physiographic transition in central nepal using maps of knickpoints river steepness indices local relief and the distribution of thick alluvial fill deposits we then report new detrital 40ar 39ar data from two trans himalayan transects each of which suggests a rapid northward increase in the total amount of exhumation across the physiographic transition thermokinematic modeling suggests that either of the two developmental scenarios for the transition is plausible but that an accretion model is viable only under an extremely narrow range of conditions we contend that the physiographic and thermochronologic data in our study area are most simply explained by recent out of sequence surface thrusting within the lesser himalayan metasedimentary sequence approximately 15 30 km south of the mapped surface trace of the main central thrust system an important finding of this work is that there are substantial along strike variations in physiography and thermal history that reflect along strike changes in the degree and location of out of sequence surface thrusting

landscape form and millennial erosion rates in the san gabriel mountains ca abstract it has been long hypothesized that topography as well as climate and rock strength exert first order controls on erosion rates here we use detrital cosmogenic 10 be from 50 basins ranging in size from 1 to 150 km 2 to measure millennial erosion rates across the san gabriel mountains in southern california where a strong e w gradient in relief compared to weak variation in precipitation and lithology allow us to isolate the relationship between topographic form and erosion rate our erosion rates range from 35 to 1100 m ma and generally agree with both decadal sediment fluxes and long term exhumation rates inferred from low temperature thermochronometry catchment mean hillslope angle increases with erosion rate until 300 m ma at which point slopes become invariant with erosion rate although this sort of relation has been offered as support for non linear models of soil transport we use 1 d analytical hillslope profiles derived from existing soil transport laws to show that a model with soil flux linear in slope but including a slope stability threshold is indistinguishable from a non linear law within the scatter of our data catchment mean normalized channel steepness index increases monotonically though non linearly with erosion rate throughout the san gabriel mountains even where catchment mean hillslope angles have reached a threshold this non linearity can be mostly accounted for by a stochastic threshold incision model though additional factors likely contribute to the observed relationship between channel steepness and erosion rate these findings substantiate the claim that the normalized channel steepness index is an important topographic metric in active ranges

tectonic and lithologic controls on bedrock channel profiles and processes in coastal california 1 recent theoretical models suggest that topographic characteristics of bedrock channels are products of interactions among tectonics substrate resistance and the climatically modulated erosive ability of the river the degree to which these factors influence the form of channel profiles is poorly quantified at present here we investigate bedrock channels developed across the southern flank of the santa ynez mountains california uniform climate and systematic variations in lithology and rock uplift rate along the range allow comparison of channel morphology between 1 channels eroding rocks of uniform and nonuniform strength and 2 channels experiencing differences in tectonic forcing we combine field observations surveys and analysis of digital data to determine topographic and hydraulic characteristics of bedrock channels at a constant rock uplift rate streams flowing from resistant to less resistant bedrock exhibit highly concave profiles and increased gradients along lower reaches relative to channels developed in uniform bedrock these effects are interpreted as responses to 1 an increase in substrate resistance to channel incision in the upper reaches and 2 transport limited gradients along lower reaches comparisons of channels developed across uniform lithology but experiencing an approximately sevenfold difference in rock uplift rate reveal an approximately twofold increase in gradient and an approximately threefold decrease in width in this landscape the combined channel adjustments of gradient and width are consistent with a fluvial incision model in which channel incision rate is linearly proportional to mean bed shear stress

landslide erosion coupled to tectonics and river incision the steep topography of mountain landscapes arises from interactions between tectonic rock uplift valley incision and landslide erosion on hillslopes an analysis of more than 15 000 landslides in the eastern himalaya mapped from satellite images shows that steep uplands primarily respond to uplift and river incision by increases in landslide erosion rates rather than by steepened hillslope angles

transient fluvial incision in the headwaters of the yellow river northeastern tibet china 1 we utilize topographic analysis of channel profiles combined with field measurements of erosion rates to explore the distribution of channel incision in the anyemaqen shan a broad mountainous region in the northeastern tibetan plateau tributary channels to the yellow river display systematic downstream increases in channel gradient associated with convex upward longitudinal profiles steep lower reaches of channels are associated with rapid 1 m ka incision rates along the yellow river while upstream reaches are associated with relatively slow 0 05 0 1 m ka erosion of soil mantled uplands covariance between erosion rates and channel steepness indices suggest that channels are adjusted to match long wavelength differential rock uplift across the range geologic constraints indicate that rapid incision downstream of the range is associated with excavation of basin fill driven by changes in relative base level farther downstream the upstream limit of this wave of transient incision is marked by a series of knickpoints that are found at nearly the same elevation throughout the watershed consistent with knickpoint migration as a kinematic rather than diffusional wave tributary channel gradients downstream of knickpoints however display a progressive adjustment to increased incision rates that may reflect the influence of increased sediment flux comparison of observed channel profiles to a stream power model of fluvial behavior reveals that the rate of knickpoint propagation can only be explained if the erosional efficiency coefficient k increases during incision our results thus highlight the utility of channel profile analysis to reconstruct the fluvial response to both active tectonism and external changes in base level

fluvial landscape response time how plausible is steady state denudation whether or not steady state topography and denudation are probable states depends on the timescale of system response to tectonic and climatic perturba tions relative to the frequency of those perturbations this paper presents analytical derivations of algebraic relations for the response time of detachment limited fluvial bedrock channel systems both to tectonic and climatic perturbations detachment limited fluvial erosion is described by the stream power incision model and the derivations are limited to the applicability of that model all factors likely to influence system response time that are not adequately captured by the stream power incision model will tend to increase the response time the calculations presented thus provide minimum estimates of landscape response time and therefore over predict the prob ability of attaining and sustaining steady state topography and denudation the central range of taiwan is used as a case study to estimate response times in a landscape often argued to be in steady state model parameters are fit to modern stream profiles by assuming that the topography represents a quasi steady state form estimated response times generally range from 0 25 to 2 5 ma depending on the non linearity of the incision rule and the magnitude and type of perturbation thus it may be reasonably argued that steady state topography and denudation are likely to prevail during periods of climatic stability response time is sufficiently short compared with plate tectonic timescales however rapid climatic fluctuation in the quaternary appears to preclude the attainment of steady state conditions in modern orogens motivation

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

stream capture as driver of transient landscape evolution in a tectonically quiescent setting we use unique fluvial gravel deposits preserved atop a regional drainage divide to confirm the role of stream capture in driving 250 m of incision in the transient roanoke river basin of the appalachian mountains united states gravel provenance constrains the pre capture position of the divide indicating that 225 km 2 of basin area were abruptly connected to the base level of the capturing stream the resulting wave of incision is currently manifest as major knickzones separating adjusting reaches from relict headwaters resembling streams of the new river basin from which the roanoke river was captured the unusual preservation of the unconsolidated gravels on small relict surfaces adjacent to bedrock gorges indicates extreme spatial variability in erosion rates within the roanoke basin which is the first documented example of a transient passive margin basin connected to a capture event by stranded fluvial debris our results show the potential for stream capture across an asymmetric drainage divide to drive major transient incision independent of external forcings such as climate change or tectonic uplift a continuation of this process will lead to eventual capture of 7000 km 2 of the new river basin in the relatively near geologic future

river incision into bedrock mechanics and relative efficacy of plucking abrasion and cavitation improved formulation of bedrock erosion laws requires knowledge of the actual processes operative at the bed we present qualitative field evidence from a wide range of settings that the relative efficacy of the various processes of fluvial erosion e g plucking abrasion cavitation solution is a strong function of substrate lithology and that joint spacing fractures and bedding planes exert the most direct control the relative importance of the various processes and the nature of the interplay between them are inferred from detailed observations of the morphology of erosional forms on channel bed and banks and their spatial distributions we find that plucking dominates wherever rocks are well jointed on a submeter scale hydraulic wedging of small clasts into cracks bashing and abrasion by bedload and chemical and physical weathering all contribute to the loosening and removal of joint blocks in more massive rocks abrasion by suspended sand appears to be rate limiting in the systems studied here concentration of erosion on downstream sides of obstacles and tight coupling between fluid flow patterns and fine scale morphology of erosion forms testify to the importance of abrasion by suspended load rather than bedload particles mechanical analyses indicate that erosion by suspended load abrasion is considerably more nonlinear in shear stress than erosion by plucking in addition a new analysis indicates that cavitation is more likely to occur in natural systems than previously argued cavitation must be considered a viable process in many actively incising bedrock channels and may contribute to the fluting and potholing of massive unjointed rocks that is otherwise attributed to suspended load abrasion direct field evidence of cavitation erosion is however lacking in terms of the well known shear stress or stream power erosion law erosion by plucking is consistent with a slope exponent n of 2 3 to 1 whereas erosion by suspended load abrasion is more consistent with a slope exponent of 5 3 given that substrate lithology appears to dictate the dominant erosion process this finding has important implications for long term landscape evolution and the models used to study it

channel changes in badlands stripping of the vegetation and soil from a 13 hectare site in virginia underlain by coastal plain sediments created a rapidly evolving badland topography two types of channels developed 1 sand bed alluvial channels were graded to transport the bed material load supplied from slope erosion with available runoff but they also generally eroded their beds slowly and 2 steeper bedrock floored channels incised rapidly in bedrock channels the erosion rate was proportional to the 4 9ths power of drainage area and the 2 3rds power of gradient these exponents are consistent with a model in which the erosion rate is proportional to the bed shear during high flows due to rapid mass wasting and reduced runoff the alluvial channels became as much as 50 steeper during the winter than the summer with an attendant yearly cycle of winter aggradation and summer entrenchment the gradients their seasonal variability and their downstream hydraulic geometry were consistent with the predictions of total load transport formulas for sand beds and high loads the hydraulic geometry of alluvial channels in the virginia badlands were similar to that on the morrison formation in the western united states

timescales of landscape response to divide migration and drainage capture implications for the role of divide mobility in landscape evolution efforts to extract information about climate and tectonics from topography commonly assume that river networks are static drainage divides can migrate through time however and recent work has shown that divide mobility can potentially induce changes in river profiles comparable to changes caused by variation in rock uplift climate or rock properties we use 1d river profile and 2d landscape evolution simulations to evaluate how mobile divides influence the interpretation of river profiles in tectonically active settings we define a non dimensional divide migration number ndm as the ratio of the timescale of channel profile response to a change in drainage area tda to the timescale of divide migration tdm in simulations of headward divide migration ndm is much less than unity with no measurable perturbation of channel profiles only in simulations configured to induce rapid lateral divide migration are there occasional large stream capture events and zones where localized drainage area loss is fast enough to support ndm values near unity the rapid response of channel profiles to changes in drainage area ensures that under most conditions profiles maintain quasi equilibrium forms and thus generally reflect spatio temporal variation in rock uplift climate or rock properties even during active divide migration this implies that channel profile form may not reliably record divide mobility so we evaluate alternate metrics of divide mobility in our simulations and an example in taiwan we find that simple measures of cross divide contrasts in topography are more robust metrics of divide mobility than measures of drainage network topology

implications of the shear stress river incision model for the timescale of postorogenic decay of topography 1 the reason for the survival of mountainous topography in ancient orogenic belts is a long standing problem in geomorphology and geodynamics we explore the geomorphologic controls on the timescale for the postorogenic decay of topography to address the question of whether there is a viable geomorphologic explanation for the persistence of topography in ancient orogenic belts or whether alternative geodynamic explanations must be sought using both approximate analytical solutions and numerical simulations we show that the standard detachment limited stream power river incision model predicts for reasonable initial topographies relatively short postorogenic decay times of 1 10 myr additional layers of complexity are introduced to this simplest model including isostasy and flexural strength a transition to transport limited conditions during decline and the incorporation of the combined effects of a critical threshold for erosion and the stochastic variability of flood magnitudes each of these additional factors acts to lengthen the decay timescale pure airy isostatic rebound of a thick crustal root results in decay times that are at most a factor of 6 longer than the detachment limited result the transition to transport limited conditions involves partial protection of the bed by a thin layer of alluvium which inhibits erosion and therefore increases decay times by a factor of 2 3 in our analysis finally including critical shear stress results increases decay time by approximately a factor of 20 for the parameter values in our example calculation more importantly however a significant portion of topography remains after predicted lowering rates have dropped to values less than measured rates of denudation in weathering limited low relief environments thus a model combining isostasy a transition to transport limited conditions and a critical shear stress for erosion could account for the presence of residual topography for hundreds of million years all three of these factors can be expected to play a role in natural settings

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

coastal tectonics on the eastern margin of the pacific rim late quaternary sea level history and uplift rates channel islands national park california usa abstract the pacific rim is a region where tectonic processes play a significant role in coastal landscape evolution coastal california on the eastern margin of the pacific rim is very active tectonically and geomorphic expressions of this include uplifted marine terraces there have been however conflicting estimates of the rate of late quaternary uplift of marine terraces in coastal california particularly for the northern channel islands in the present study the terraces on san miguel island and santa rosa island were mapped and new age estimates were generated using uranium series dating of fossil corals and amino acid geochronology of fossil mollusks results indicate that the 2nd terrace on both islands is 120 ka and the 1st terrace on santa rosa island is 80 ka these ages correspond to two global high sea stands of the last interglacial complex marine isotope stages mis 5 5 and 5 1 respectively the age estimates indicate that san miguel island and santa rosa island have been tectonically uplifted at rates of 0 12 0 20 m ka in the late quaternary similar to uplift rates inferred from previous studies on neighboring santa cruz island the newly estimated uplift rates for the northern channel islands are however an order of magnitude lower than a recent study that generated uplift rates from an offshore terrace dating to the last glacial period the differences between the estimated uplift rates in the present study and the offshore study are explained by the magnitude of glacial isostatic adjustment gia effects that were not known at the time of the earlier study set in the larger context of northeastern pacific rim tectonics channel islands uplift rates are higher than those coastal localities on the margin of the east pacific rise spreading center but slightly lower than those of most localities adjacent to the cascadia subduction zone the uplift rates reported here for the northern channel islands are similar to those reported for most other localities where strike slip tectonics are dominant but lower than localities where restraining bends such as the big bend of the san andreas fault result in crustal shortening

does decreasing paraglacial sediment supply slow knickpoint retreat in four rivers in western scotland for which there is a well constrained record of relative base level fall the rate of postglacial bedrock erosion is quantifi ed by measuring the concentration of in situ cosmogenic 10be on strath terraces downstream of headward retreating knickpoints along channel gradients in 10be exposure age show two distinct trends upstream younging and constant age which we interpret as diagnostic of knickpoint retreat and diffusive transport limited incision respectively we show that bedrock channel incision and regional formation of strath terraces began shortly after deglaciation ca 11 5 ka and that knickpoint retreat rates peaked in the early to mid holocene erosion rates have since decreased by two orders of magnitude converging in the late holocene to low rates independent of stream power per unit channel area we infer this regional slowing in postglacial knickpoint retreat to be the result of the depletion of paraglacial sediment supply over the holocene leading to a defi ciency in tools for bedrock erosion our results imply that episodes of major fl uvial erosion may be in tune with glacial cycles and that sediment depletion following glacial interglacial transitions may be an important cause of bedrock erosion rate variations in rivers draining glaciated landscapes

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

temporal profiling of uplift rate along an active fault using river long profile in the kuchchh region western india abstract river long profiles are widely used as tectonic event markers incision uplift rate estimations based on the dating of river terrace or exposed scarp sediment give long term averaged uplift rate values with the limitation of not giving the intermediate phases of highs and lows on the other hand gps based estimates provide only decadal scale uplift rate estimates in our study we adopted the long profile inversion methodology to obtain a temporal profile of uplift rate history over the late pleistocene holocene time period along an active fault the kuchchh mainland fault in kuchchh region western india which has experienced several large magnitude earthquakes in recent past we also used long profiles analysis to quantify the transient response of kuchchh landscape to uplift activity along the kuchchh mainland fault steepness index and chi profiles were estimated to display the spatial variability in landscape disequilibrium integral method based chi profiles were used to obtain the optimum value of basin area exponent of the stream power incision model spim our estimates provide a mean uplift rate value of 1 1 1 4 mm yr along the kuchchh mainland fault during the late pleistocene holocene period which matches with the average uplift rate derived from other independent observations in this region further a chronology of major tectonic phases from the estimated uplift rate time series also corroborates with field based estimates of major incision events in kuchchh region we also tested the robustness and sensitivity of our uplift rate estimates to model parameters such data can be complementary to the existing sparsely both spatially and temporally distributed uplift rate database which is imperative to seismic hazard assessment

relict landscape resistance to dissection by upstream migrating knickpoints expanses of subdued topographies are common at high elevation in mountain ranges they are often interpreted as relict landscapes and are expected to be replaced by steeper topography as erosion proceeds preservation of such relict fragments can merely reflect the fact that it takes time to remove any preexisting topography however relict fragments could also possess intrinsic characteristics that make them resilient to dissection we document here the propagation of a wave of dissection across an uplifted relict landscape in puerto rico using 10be 26al burial dating on cave sediments we show that uplift started 4 ma and that river knickpoints have since migrated very slowly across the landscape modern detrital 10be erosion rates are consistent with these long term rates of knickpoint retreat analysis of knickpoint distribution combined with visual observations along the streambeds indicates that incision by abrasion and plucking is so slow that bedrock weathering becomes a competing process of knickpoint retreat the studied rivers flow over a massive stock of quartz diorite surrounded by an aureole of metavolcanic rocks earlier studies have shown that vegetation over the relict topography efficiently limits erosion allowing for the formation of a thick saprolite underneath such slow erosion reduces streambed load fluxes delivered to the knickpoints as well as bed load grain size both processes limit abrasion compounding the effect of slow abrasion wide joint spacing in the bedrock makes plucking infrequent thus the characteristics of the relict upstream landscape have a direct effect on stream incision farther downstream reducing the celerity at which the relict subdued landscape is dissected we conclude that similar top down controls on river incision rate may help many relict landscapes to persist amidst highly dissected topographies

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

wave train model for knickpoint migration rivers respond to a drop in their base level by incising the topography the upstream propagation of an incision as usually depicted by a knickpoint migration is thought to depend on several parameters such as the drainage area lithology and the amplitude of the base level drop we first investigate the case of the messinian salinity crisis that was characterized by the extreme base level fall 1500 m of the mediterranean sea at the end of the miocene the response of drainage areas of three orders of magnitude 103 to 106 km2 highlights the dominant role of the drainage area with a square root relationship in controlling the knickpoint migration after a base level fall a compilation of mean rates of knickpoint propagation for time durations ranging from 102 to 107 years displays a similar relationship indicating that successive wave trains of knickpoint can migrate in a river first wave trains linked to the release of the alluvial cover and then wave trains related to the bedrock incision which correspond to the real time response of rivers wave trains with very low retreat rates long lived knickpoints 1 my rather correspond to the response time of regional landscape

relationships between block quarrying bed shear stress and stream power a physical model of block quarrying of a jointed bedrock channel we examine interactions among substrate characteristics hydraulic forces and erosive processes acting on a jointed resistant substrate in a flume experiencing supercritical flow we conducted six experimental runs with no sediment in transport constant slope varying discharge varying channel width and varying geometry of concrete blocks forming the channel bed the initial downstream boundary was a vertical knickpoint lip perpendicular to flow observed changes in bed morphology occurred primarily by episodic horizontal knickpoint retreat the knickpoint lip can develop a curve that reflects interlocking of the irregular boundaries between otherwise physically disjointed blocks during rotation of the blocks sometimes a block acts like a keystone in an arch and mobilization of the key block facilitates mobilization of the linked blocks on either side resulting in a large magnitude event in contrast to previous studies where blocks were vertically plucked blocks were removed by sliding from the knickpoint building on previous work we develop a theoretical force threshold for sliding block mobilization which suggests that increased block side length parallel to flow increases block stability whereas increased side length perpendicular to flow decreases block stability stream power was also linked to observed erosion rates as a function of block length consistent with the idea that block loosening rates play a key role in controlling average erosion rates inferred amplification of bed shear stress and pressure at the knickpoint lip stream power and joint spacing correlate with the rate of block erosion the results although limited in scope and scale suggest a linear relationship between bed shear stress and stream power with the rate of block erosion block sliding especially at near vertical knickpoints can play a key or even dominant role in the morphologic dynamics of the river channel

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

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

responses of soil mantled hillslopes to transient channel incision rates 1 channel incision drives hillslope morphology in humid soil mantled landscapes when channel incision rates change numerous hillslope soil properties e g erosion rates soil thickness and soil production adjust in response to this change here we investigate the timescales of adjustment of hillslope soil properties when channel incision rates change in time an idealized soil mantled hillslope linear sediment flux law and soil density equal to bedrock density is investigated and the transient evolution of this hillslope is determined analytically this analysis reveals two dominant adjustment timescales the longer of these two timescales determines the rate at which the entire hillslope adjusts to a change in channel incision rates and is proportional to 4 2 2d where is the length of the hillslope and d is the sediment diffusivity numerical simulations are then used to examine responses of hillslopes that involve nonlinear behavior e g hillslopes that experience nonlinear sediment transport or have soil thicknesses that respond to the soil production function using these numerical models we show that the ratio between the soil density s and the density of the soil parent material can alter the long term response of the hillslope such that the characteristic timescale becomes 4 2 2d s in addition we show that the adjustments of the soil erosion rate the soil production rate and the soil thickness have different characteristic response timescales hillslopes that experience sediment flux that is proportional to the depth slope product respond on longer timescales than hillslopes that experience a linear sediment flux law when channel incision rates increase we illustrate how the spatial pattern of hillslope response to changes in channel incision rates can be used to constrain either channel incision histories or hillslope response timescales if the hillslope response timescale is known the pattern of hillslope disturbance can be used to constrain the celerity of a incision wave as it moves upstream through a channel if the channel incision history is known the hillslope response timescale may be evaluated on the basis of the spatial pattern of hillslope disturbance

geomorphic constraints on fault throw rates and linkage times examples from the northern gulf of evia greece we evaluate how the growth and interaction of active normal faults in the sperchios basin and northern gulf of evia greece are recorded by the landscape we demonstrate that patterns in footwall relief along the faults reflect fault segmentation and we show that in this study area fault throw is 2 to 3 times the maximum footwall relief rivers crossing the faults typically have two knickpoints which are unrelated to lithology however their heights measured from the active fault trace vary systematically the height of the upper set of knickpoints scales linearly with the footwall relief of the faults and is typically 85 of the maximum relief the height of the lower set of knickpoints also scales with footwall relief but the heights are consistently lower the existence of two sets of knickpoints suggests that the rivers have been perturbed by two changes in tectonic rates during faulting we interpret the upper knickpoints to represent the initiation and growth of fault generated topography while the lower set of knickpoints reflects a throw rate increase due to fault linkage estimates of throw rate enhancement factor derived from fault interaction theory suggest that the faults increased their rate by a factor of 3 when they linked this constraint combined with the distribution of knickpoint heights allows us to estimate the throw rate and linkage time for the faults the sperchios fault has a maximum throw rate of 1 5 2 0 mm yr while the coastal fault has a maximum throw rate of 0 8 1 2 mm yr

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

knickpoint recession rate and catchment area the case of uplifted rivers in eastern scotland knickpoint behaviour is a key to understanding both the landscape responses to a base level fall and the corresponding sediment fluxes from rejuvenated catchments and must be accommodated in numerical models of large scale landscape evolution knickpoint recession in streams draining to glacio isostatically uplifted shorelines in eastern scotland is used to assess whether knickpoint recession is a function of discharge here represented by its surrogate catchment area knickpoints are identified using ds plots log slope versus log downstream distance a statistically significant power relationship is found between distance of headward recession and catchment area such knickpoint recession data may be used to determine the values of m and n in the stream power law e kamsn the data have too many uncertainties however to judge definitively whether they are consistent with m n 1 bedrock erosion is proportional to stream power and kps should be maintained and propagate headwards or m 0 3 n 0 7 bedrock incision is proportional to shear stress and kps do not propagate but degrade in place by rotation or replacement nonetheless the e scotland m and n values point to the dominance of catchment area discharge in determining knickpoint retreat rates and are therefore more consistent with the stream power law formulation in which bedrock erosion is proportional to stream power copyright 2005 john wiley sons ltd

neogene rejuvenation of central appalachian topography evidence for differential rock uplift from stream profiles and erosion rates abstract the persistence of topography within ancient orogens remains one of the outstanding questions in landscape evolution in the eastern north american appalachians this question is manifest in the outstanding problem of whether topographic relief is in a quasi equilibrium state decaying slowly over many millennia or whether relief has increased during the late cenozoic here we present quantitative geomorphic data from the nonglaciated portion of the susquehanna river drainage basin that provide insight into these end member models analysis of channel profiles draining upland catchments in the northern valley and ridge appalachian plateau blue ridge and piedmont provinces reveals that a large number of streams have well defined knickpoints clustered at 300 600 m elevation but not systematically associated with transitions from weak to resistant substrate cosmogenic 10 be inventories of modern stream sediment indicate that erosion rates are spatially variable ranging from 5 30 m myr above knickpoints to 50 100 m myr below knickpoints overall channel gradients normalized for drainage area scale linearly with catchment averaged erosion rates collectively regionally consistent spatial relationships among erosion rate channel steepness and knickpoints reveal an ongoing wave of transient channel adjustment to a change in relative base level reconstructions of relict channel profiles above knickpoints suggest that higher rates of incision are associated with 100 150 m of relative base level fall that accompanied epierogenic rock uplift rather than a change to a more erosive climate or drainage reorganization channel response timescales imply that the onset of relative base level change predates 3 5 ma and may have begun as early as 15 ma we suggest that adjustment of the channel network was likely driven by changes in mantle dynamics along the eastern seaboard of north america during the neogene

an automated method for producing synoptic regional maps of river gradient variation procedure accuracy tests and comparison with other knickpoint mapping methods abstract the study of abrupt changes in longitudinal river profiles or knickpoints is currently approached through an empirical power law the slope area relationship results based on digital elevation model dem analyses and stream extractions are generally intended to determine crustal uplift rates and identify transient landscape conditions in this article we present an alternative geomorphometric method for locating knickpoints and knickzones based on local slope gradient and curvature attributes intended as a rapid regional scale automated knickpoint detection technique the accuracy of this slope curvature method is tested on two digital elevation grids nasa s srtm ground resolution of 90 m resampled here to 75 m and the aster dem 15 m in the sierra nacimiento new mexico usa a basement cored mountain range recently exhumed by waves of headward drainage integration in response to remote tectonic deformation in the adjacent rio grande rift out of every 10 gradient anomalies detected by the srtm derived numeric routine up to 8 are certifiable knickpoints recognized among a population of georeferenced occurrences surveyed in the field an independent comparison with the slope area method provided a further accuracy test which was particularly useful at sites that could not be validated in the field for practical reasons given the low tectonic activity of the study area the majority of knickpoints was also found to coincide with lithologic boundaries making it difficult without further geomorphological data to single out dynamic knickpoints directly caused by the upstream propagation of channel instabilities relating to base level change

tectonic and climatic controls on knickpoint retreat rates and landscape response times 1 the rate at which transient knickpoints propagate through a landscape fundamentally controls the rate of geomorphic response to tectonic and climatic perturbation here we present knickpoint retreat rates upstream of active faults for 19 bedrock catchments in turkey and 11 bedrock catchments in italy where we have very good constraints on both the magnitude and timing of the tectonic perturbation and where climate histories are well documented we show that the knickpoints have average retreat rates of between 0 2 and 2 mm yr for catchments with drainage areas between 6 and 65 km2 and we test whether differences in rock mass strength and catchment size are sufficient to explain this range in retreat rates our analysis suggests that even accounting for these two variables knickpoint propagation velocities differ markedly and we show that channels crossing faults with higher throw rates have knickpoints that are retreating faster the dependence of knickpoint retreat velocity on throw rate is at least as important as catchment drainage area these results indicate counterintuitively that landscapes forced by large amplitude tectonic perturbations will have shorter response times than those perturbed by smaller amplitude changes the link between the knickpoint propagation velocity and throw rate is largely but not completely explained by channel narrowing in areas of high uplift rate channel steepening upstream of the active faults may explain all of the residual dependency of knickpoint retreat rate on fault throw rate but only if the slope exponent n in the standard stream power model is greater than 1 3 however we cannot rule out a role for sediment supply in driving enhanced knickpoint retreat rate in addition to the well documented channel narrowing effect finally we find that mean knickpoints retreat rates in turkey are only half of those in italy for catchments of equivalent size crossing faults with similar throw rates this difference in fluvial response time is accounted for by long term differences in the ratio of precipitation to infiltration in the two areas over the last 1 my

rock slope failure and the river long profile this study examines the geomorphic effects of large 10 6 m 3 rock slope failures on long profiles of rivers in the swiss alps and the new zealand southern alps regression of channel slope versus drainage basin area objectively highlights knickpoints separating incised from aggraded reaches that often correspond to locations of large rock slope failures for a fixed concavity index the highest values of the steepness index and erosion index along a given profile spatially coincide with breach channels cut into formerly river damming rockslide debris as old as 10 k y assuming that the knickpoints do not predate slope failure data show that high profile steepness and inferred specific stream power are not always the cause but often a result of large river blocking rock slope failure in mountain basins omission of rockslide data from slope area plots lowers the steepness index and increases the concavity index on average yet only in few cases more than one standard deviation

frequency dependent landscape response to climatic forcing 1 whereas the existence of pronounced orbitally controlled periodicities is a major feature of earth climate its impact on landscape dynamics remains poorly understood we use a landscape evolution model lem to systematically investigate the response of landscapes to a range of periodic oscillations in precipitation the resulting sediment flux evolution displays a pronounced sensitivity to the period of the input precipitation signal such that for a given erodibility a specific periodicity maximizes the amplitude of the response this optimal period of resonance scales as the inverse of the erodibility but is progressively filtered out of the response when the intensity of hillslope diffusion increases this frequency dependent landscape behavior displayed by our model provides a mechanistic perspective on molnar s 2004 proposition that ubiquitous changes in late cenozoic continental denudation could result directly from modifications in the spectral content of the climatic signal

landslides threshold slopes and the survival of relict terrain in the wake of the mendocino triple junction establishing landscape response to uplift is critical for interpreting sediment fluxes hazard potential and topographic evolution we assess how landslides shape terrain in response to a wave of uplift traversing the northern california coast ranges united states in the wake of the mendocino triple junction we extracted knickpoints landslide erosion rates and topographic metrics across the region modified by mendocino triple junction migration landslide erosion rates mapped from aerial imagery are consistent with modeled uplift and exhumation while hillslope gradient is invariant across the region suggesting that landslides accommodate uplift as predicted by the threshold slope model landslides are concentrated along steepened channel reaches downstream of knickpoints generated by base level fall at channel outlets and limit slope angles and relief we find evidence that landslide derived coarse sediment delivery may suppress catchment wide channel incision and landscape denudation over the time required for the uplift wave to traverse the region we conclude that a landslide cover effect may provide a mechanism for the survival of relict terrain and orogenic relief in the northern californian coast ranges and elsewhere over millennial time scales

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

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

diagenetic variation in the oregon coast range implications for rock strength soil production hillslope form and landscape evolution the mechanisms by which lithology modulates geomorphic processes are poorly known in the oregon coast range ocr rhythmically bedded sandstones of the eocene tyee formation underlie steep soil mantled hillslopes with relatively uniform ridge valley spacing these characteristic landforms are perturbed where diagenetic variations manifest as resistant cliffs here we use petrology rock mechanics and lidar to characterize grain scale variations in rock properties and their influence on rock strength hillslope processes and landscape morphology in two adjacent watersheds petrographic analyses suggest that a suite of diagenetic products in the resistant bedrock account for a 2 5 times increase in tensile strength relative to typical tyee bedrock our reference catchment exhibits negligible resistant outcrops and consistent hillslope gradients and longitudinal valley profiles by contrast the adjacent catchment teems with resistant 1 to 10 m thick noncontiguous sandstone beds that form hanging valleys with gentle upstream hillslopes and anomalously narrow valleys mechanical and topographic analyses suggest that the low fracture density characteristic of these resistant beds may render them relatively impervious to comminution by tree root activity the dominant ocr soil production mechanism based on both hillslope gradient and hilltop curvature erosion models we estimate that hillslopes perched above resistant beds erode at approximately half the pace of hillslopes unencumbered by downstream knickpoints the diagenetic variations likely influence relief at the watershed scale depositional position and diagenetic processes appear to control the occurrence of resistant beds providing a framework to quantify how seemingly subtle variations in rock properties can impose first order controls on landscape form and evolution

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

longitudinal profile development into bedrock an analysis of hawaiian channels analysis of topographic maps of rivers incised into dated hawaiian lava flows shows that the long term average bedrock erosion rate along certain reaches is linearly related to stream power field observations suggest that two processes may control hawaiian channel downcutting 1 stream power dependent erosion including abrasion of the channel bed by transported particles and 2 step wise lowering caused by knickpoint propagation modeling results indicate that a simple stream power dependent erosion law predicts the straight to weakly convex longitudinal profiles characteristic of kauai channels but is insufficient to predict two other characteristic features the upslope propagation of knickpoints and the straight 5 8 channel slopes below the knickpoints thus more than this single transport law is apparently required to model bedrock channel incision field surveys also indicate that significant portions of the channel lengths below the knickpoints are mantled with large boulders we propose that

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

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

new insights into the mechanics of fluvial bedrock erosion through flume experiments and theory river incision into bedrock drives the topographic evolution of mountainous terrain and may link climate tectonics and topography over geologic time scales despite its importance the mechanics of bedrock erosion are not well understood because channel form river hydraulics sediment transport and erosion mechanics coevolve over relatively long time scales that prevent direct observations and because erosive events occur intermittently and are difficult and dangerous to measure herein we synthesize how flume experiments using erodible bedrock simulants are filling these knowledge gaps by effectively accelerating the pace of landscape evolution under reduced scale in the laboratory we also build on this work by providing new theory for rock resistance to abrasion thresholds for plucking by vertical entrainment sliding and toppling and by assessing bedrock analog materials bedrock erosion experiments in the last 15 years reveal that the efficiency of rock abrasion scales inversely with the square of rock tensile strength sediment supply has a dominant control over bed roughness and abrasion rates suspended sediment is an efficient agent of erosion and feedbacks with channel form and roughness strongly influence erosion rates erodibility comparisons across rock concrete ice and foam indicate that for a given tensile strength abrasion rates are insensitive to elasticity the few experiments that have been conducted on erosion by plucking highlight the importance of block protrusion height above the river bed and the dominance of block sliding and toppling at knickpoints these observations are consistent with new theory for the threshold shields stress to initiate plucking which also suggests that erosion rates in sliding and toppling dominated rivers are likely transport limited major knowledge gaps remain in the processes of erosion via plucking of bedrock blocks where joints are not river bed parallel waterfall erosion by toppling and plunge pool erosion feedbacks between weathering and physical erosion erosional bedforms and morphodynamic feedbacks between channel form and erosion rates despite scaling challenges flume experiments continue to provide much needed tests of existing bedrock erosion theory force development of new theory and yield insight into the mechanics of landscapes

tectonic uplift threshold hillslopes and denudation rates in a developing mountain range studies across a broad range of drainage basins have established a positive correlation between mean slope gradient and denudation rates it has been suggested however that this relationship breaks down for catchments where slopes are at their threshold angle of stability because in such cases denudation is controlled by the rate of tectonic uplift through the rate of channel incision and frequency of slope failure this mechanism is evaluated for the san bernardino mountains california a nascent range that incorporates both threshold hillslopes and remnants of pre uplift topography concentrations of in situ produced cosmogenic 10 be in alluvial sediments are used to quantify catchment wide denudation rates and show a broadly linear relationship with mean slope gradient up to 30 above this value denudation rates vary substantially for similar mean slope gradients we propose that this decoupling in the slope gradient denudation rate relationship marks the emergence of threshold topography and coincides with the transition from transport limited to detachment limited denudation the survival in the san bernardino mountains of surfaces formed prior to uplift provides information on the topographic evolution of the range in particular the transition from slope gradient dependent rates of denudation to a regime where denudation rates are controlled by rates of tectonic uplift this type of transition may represent a general model for the denudational response to orogenic uplift and topographic evolution during the early stages of mountain building

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

modeling of knickpoint retreat on the roan plateau western colorado 1 the roan plateau in western colorado constitutes a natural experiment for studying landscape response to a drop in base level late cenozoic incision of the upper colorado river led to elevational isolation of the plateau and initiation of a wave of incision into its southern edge knickpoints oversteepened reaches that contain waterfalls 60 110 m in height mark the upstream extent of this headward propagating wave that this incision has occurred in a laterally extensive well stratified and essentially flat lying bedrock and in an area with relatively uniform climate implies that it should serve as a good test of existing knickpoint propagation models we predict the locations of knickpoints by using a stream power based celerity model in which knickpoint recession rate is a power function of drainage area and is proportional to rock susceptibility to erosion models of the parachute and roan drainages 17 and 16 knickpoints respectively show expected rapid initial knickpoint propagation rates which decline as drainage area decreases stepwise at tributary junctions the modeled positions of knickpoints match well with observed features using a single combination of parameters to model retreat in both drainage basins we compare our celerity model results with past studies and explore how longitudinal profile analysis may be used to derive independently the exponent on drainage area in the celerity model

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

uplift analysis on a pericratonic region an example in the sierras de cordoba 29 34 s argentina abstract the sierras de cordoba argentina is a mountain belt located in the most distal pericratonic part of the south central andean foreland in the easternmost end of the sierras pampeanas pampean ranges broken foreland although this region is considered an excellent example for settings where basement thrusting generates relief and intermontane basin systems hundreds of kilometers away from the cordilleran front the driving mechanisms and main controls on topography are still poorly understood given that the landscape and particularly the rivers react to changes in climate and tectonics on timescales between 103 and 106 yrs we used dem based morphometry elevation patterns local relief and analysis of 242 longitudinal stream profiles to evaluate the main controls on modern topography we focus on the morphologies of the main streams and their tributaries using metrics of the channel concavity and normalized channel steepness index ksn the distribution of local relief concavity and ksn values was compared with the present day topography tectonic structures rock type and modern rainfall patterns the stream longitudinal profiles show homogeneous concavities and ksn background values with some local anomalies local structures east and west vergent thrusts in the sierras de cordoba seem to be the most fundamental controls on the formation of these anomalies slope break knickpoints as well as deep incision high local relief this suggests localized and accelerated rock uplift rates which produce localized increases in the gradient along the longitudinal stream profile it is important to notice that the highest ksn values were reported on the eastern range flanks coincident with the highest values of mean annual rainfall however high ksn values appear to be maintained even with greater rainfall our study provides new insights on the neogene sierra de cordoba uplift evolution which will assist future sampling and modelling cosmogenic nuclides analysis thermochronology and gps analysis

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

experimental study of knickpoint and longitudinal profile evolution in cohesive homogeneous material a series of experiments were conducted in homogeneous bedrock to study knickpoint and longitudinal profile evolution knickpoints are created by successive drops in base level which simulate intermittent uplift channel morphology and flow characteristics vary systematically along a knickpoint reach as the knickpoint lip is approached width decreases but depth velocity and bottom shear stress increase as a result knickpoints are rapidly destroyed through the process of 1 knickpoint replacement and 2 subsequent knickpoint inclination knickpoint replacement consists of two morphologically distinct erosional reaches the knickpoint face below the knickpoint lip which decreases in slope and an incising reach above the knickpoint lip which increases in slope knickpoint replacement is a significant process in the evolution of knickpoints in homogeneous bedrock within experimental limits this study supports the notion that knickpoints do not undergo parallel head ward retreat in homogeneous bedrock given intermittent uplift except in those possible cases of extensive and pervasive bedrock jointing only the erosional sediment transport discontinuity above the knickpoint is developed in cohesive sediments below the knickpoint a steep bedrock channel slope low sediment load and rapid decrease in grain size of sediment load preclude development of the aggradational sediment transport discontinuity knickpoint evolution can be characterized by three general models parallel retreat replacement and inclination these models are based on the following criteria within the knickpoint reach 1 relationship between bottom shear stress 0 and critical shear stress needed to initiate motion c 2 nature of the bed load transport discontinuities and 3 spatial variability of bedrock resistance to fluvial erosion

climatic control of denudation in the deglaciated landscape of the washington cascades erosion in the washington cascades mountains was dominated by glacier activity until the end of the last glacial maximum when fluvial incision and mass wasting became the dominant drivers an analysis of millennial scale rates of denudation in the range suggest erosion is sensitive both to spatial variations in precipitation and inherited topography

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

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

channel planform geometry and slopes from freely available high spatial resolution imagery and dem fusion implications for channel width scalings erosion proxies and fluvial signatures in tectonically active landscapes article i nfo the rapid expansion of high spatial resolution optical sensors and imagery over the last decade presents ex ceptional opportunities for quantifying visible attributes of geomorphic systems in this study we detail a simple robust methodology changeom to extract continuous channel width and centerline datasets for single thread channels using freely available high spatial resolution imagery currently available in google earth and bing maps comparisons with a global dataset of field and lidar derived channel widths indicate minimal errors associated with the imagery and changeom methodology b1 overall while examples from the goriganga river indian himalaya and the yakima river wa usa emphasize the benefit of em pirical width values over established channel width scalings in deciphering fluvial responses to complex landscape forcings in tectonically active regions additionally accurate centerline delineation from the changeom methodology provides improved sinuosity measurements and when fused with coarse resolution digital elevation models dems removes along profile shortening and coincident increases in reach scale channel slope lastly comparisons of aster gdem v2 srtm v4 1 and lidar channel profiles extracted in moderate to high relief regions demonstrate the inferiority of the aster gdem for channel slope calcula tions despite the apparent spatial resolution advantages 9 the methodology presented here will facili tate new discoveries in the fluvial environment that have historically been difficult due to access and imagery resolution issues and provide greater perspective on channel signatures and responses to a host of landscape forcings especially in tectonically active bedrock and lower order drainage systems

a statistical framework to quantify spatial variation in channel gradients using the integral method of channel profile analysis 1 we present a statistical technique for analyzing longitudinal channel profiles our technique is based on the integral approach to channel analysis drainage area is integrated over flow distance to produce a transformed coordinate which has dimensions of length assuming that profile geometry is conditioned by the stream power law defined as e kamsn where e is erosion rate k is erodibility a is drainage area s is channel gradient and m and n are constants the slope of a transformed profile in elevation space should reflect the ratio of erosion rate to channel erodibility raised to a power 1 n this quantity is often referred to as the channel steepness and represents channel slope normalized for drainage area our technique tests all possible contiguous segments in the channel network to identify the most likely locations where channel steepness changes and also identifies the most likely m n ratio the technique identifies locations where either erodibility or erosion rates are most likely to be changing tests on a simulated landscape demonstrate that the technique can accurately retrieve both the m n ratio and the correct number and location of segments eroding at different rates where model assumptions apply tests on natural landscapes illustrate how the method can distinguish between spurious channel convexities due to incorrect selection of the m n ratio from those which are candidates for changing erodibility or erosion rates we also show how given erosion or uplift rate constraints the method can be used to constrain the slope exponent n

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

objective extraction of channel heads from high resolution topographic data fluvial landscapes are dissected by channels and at their upstream termini are channel heads accurate reconstruction of the fluvial domain is fundamental to understanding runoff generation storm hydrology sediment transport biogeochemical cycling and landscape evolution many methods have been proposed for predicting channel head locations using topographic data yet none have been tested against a robust field data set of mapped channel heads across multiple landscapes in this study four methods of channel head prediction were tested against field data from four sites with high resolution dems slope area scaling relationships two techniques based on landscape tangential curvature and a new method presented here which identifies the change from channel to hillslope topography along a profile using a transformed longitudinal coordinate system our method requires only two user defined parameters determined via independent statistical analysis slope area plots are traditionally used to identify the fluvial hillslope transition but we observe no clear relationship between this transition and field mapped channel heads of the four methods assessed one of the tangential curvature methods and our new method most accurately reproduce the measured channel heads in all four field sites feather river ca mid bailey run oh indian creek oh piedmont va with mean errors of 11 7 5 and 24 m and 34 3 12 and 58 m respectively negative values indicate channel heads located upslope of those mapped in the field importantly these two independent methods produce mutually consistent estimates providing two tests of channel head locations based on independent topographic signatures

an automated knickzone selection algorithm kz picker to analyze transient landscapes calibration and validation streams commonly respond to base level fall by localizing erosion within steepened convex knickzone reaches localized incision causes knickzone reaches to migrate upstream such migrating knickzones dictate the pace of landscape response to changes in tectonics or erosional efficiency and can help quantify the timing and source of base level fall identification of knickzones typically requires individual selection of steepened reaches a process that is tedious subjective and has no efficient means to measure knickzone size we construct an algorithm to automate this procedure by selecting the bounds of knickzone reaches in a space drainage area normalized framework an automated feature calibrates algorithm parameters to a subset of knickzones handpicked by the user the algorithm uses these parameters as consistent criteria to identify knickzones objectively and then the algorithm measures the height length and slope of each knickzone reach we test the algorithm on 1 10 and 30 m resolution dems of six catchments trunk stream lengths 2 1 5 4 km on santa cruz island southern california on the 1 m dem algorithm selected knickzones confirm 93 of handpicked knickzone positions n 178 to a spatial accuracy 100 m 88 to an accuracy within 50 m and 46 to an accuracy within 10 m using 10 and 30 m dems accuracy is similar 88 86 to 100 m and 82 to 50 m n 38 36 respectively the algorithm enables efficient regional comparison of the size and location of knickzones with geologic structures mapped landforms and hillslope morphology thereby facilitating approaches to characterize the dynamics of transient landscapes

hillslope derived blocks retard river incision the most common detachment limited river incision models ignore the effects of sediment on fluvial erosion yet steep reaches of mountain rivers often host clusters of large 1 m blocks we argue that this distribution of blocks is a manifestation of an autogenic negative feedback in which fast vertical river incision steepens adjacent hillslopes which deliver blocks to the channel blocks inhibit incision by shielding the bed and enhancing form drag we explore this feedback with a 1 d channel reach model in which block delivery by hillslopes depends on the river incision rate results indicate that incision dependent block delivery can explain the block distribution in boulder creek colorado the proposed negative feedback may significantly slow knickpoint retreat channel adjustment and landscape response compared to rates predicted by current theory the influence of hillslope derived blocks may complicate efforts to extract base level histories from river profiles

controls and feedbacks in the coupling of mountain channels and hillslopes mountain channels can be strongly coupled with adjacent hillslopes exchanging both mass and energy however hypotheses of the underlying cause and effect relations are based on indirect observations that do not resolve the mechanics of channel hillslope coupling at the process scale here we present direct observational data of a coupled channel hillslope system in the catchment area of the erlenbach a mountain stream in switzerland a slow moving landslide flanking this alpine stream failed after a flood had eroded an alluvial step in the channel at its base representing evidence for an upsystem link in channel hillslope coupling progressive accumulation of landslide debris in the channel resulted in a renewed step stabilizing the hillslope and restoring the channel step in a downsystem link thus upsystem and downsystem coupling mechanisms are joined in a negative feedback cycle in this cycle debuttressing and rebuttressing due to channel bed erosion and alluviation are the dominant controls on hillslope stability based on an order of magnitude estimate it is plausible that the observed feedback mechanism is a relevant process in the production of coarse 2 mm sediment in the erlenbach

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

using hilltop curvature to derive the spatial distribution of erosion rates 1 erosion rates dictate the morphology of landscapes and therefore quantifying them is a critical part of many geomorphic studies methods to directly measure erosion rates are expensive and time consuming whereas topographic analysis facilitates prediction of erosion rates rapidly and over large spatial extents if hillslope sediment flux is nonlinearly dependent on slope then the curvature of hilltops will be linearly proportional to erosion rates in this contribution we develop new techniques to extract hilltop networks and sample their adjacent hillslopes in order to test the utility of hilltop curvature for estimating erosion rates using high resolution 1 m digital elevation data published and new cosmogenic radionuclide analyses in the feather river basin california suggest that erosion rates vary by over an order of magnitude 10 to 250 mm kyr 1 hilltop curvature increases with erosion rates allowing calibration of the hillslope sediment transport coefficient which controls the relationship between gradient and sediment flux having constraints on sediment transport efficiency allows estimation of erosion rates throughout the landscape by mapping the spatial distribution of hilltop curvature additionally we show that hilltop curvature continues to increase with rising erosion rates after gradient limited hillslopes have emerged hence hilltop curvature can potentially reflect higher erosion rates than can be predicted by hillslope gradient providing soil production on hilltops can keep pace with erosion finally hilltop curvature can be used to estimate erosion rates in landscapes undergoing a transient adjustment to changing boundary conditions if the response timescale of hillslopes is short relative to channels