knickzone propagation in the black hills and northern high plains a different perspective on the late cenozoic exhumation of the laramide rocky mountains geomorphic research in the black hills and northern high plains poses an intriguing hypothesis for the cenozoic evolution of this salient of the laramide rockies most recently geologists have appealed to late cenozoic epeirogenic uplift or climate change to explain the post laramide unroofing of the rockies on the basis of field mapping and the interpretation of long valley profiles we conclude that the propagation of knickzones is the primary mechanism for exhumation in the black hills long profiles of major drainages show discrete breaks in the slope of the channel gradient that are not coincident with changes in rock type we use the term knickzones to describe these features because their profiles are broadly convex over tens of kilometers at and below the knickzone the channel is incising into bedrock abandoning a flood plain and forming a terrace above the knickzone the channel is much less incised resulting in a broad valley bottom numerous examples of stream piracy are documented and in each case the capture is recorded in the same terrace level these observations are consistent with migrating knickzones that have swept through black hills streams rearranging drainages in their wake we demonstrate there are two knickzone fronts associated with mapped terraces preliminary field evidence of soil development shows that these terraces are time transgressive in nature our data strongly suggest that knickzone propagation must be considered a viable mechanism driving late cenozoic fluvial incision and exhumation of the northern high plains and adjacent northern rocky mountains

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

controls and limits on bedrock channel geometry 1 data on channel geometry in naturally incising bedrock rivers reveal variable relationships between channel width slope and erosion rate to explain the range of relationships between these parameters we propose an optimized channel geometry model the model reproduces previous theoretical and numerical modeling scaling relationships however to explain the field data the model must incorporate the effects of immobile sediment cover that reduces vertical incision efficiency adding sediment to a system for a given rate of base level fall both widens and steepens the channel we also find that a constant width to depth ratio is only valid for streams where the shear stress required for sediment transport is insignificant compared to the shear required to erode at the imposed base level fall our model offers an explanation of seemingly contradictory field observations of controls on channel width and shows that channel width fundamentally controls the erosive potential of a river especially when it carries a significant bed load

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

an early pliocene thermal disturbance of the main central thrust central nepal implications for himalayan tectonics since the beginning of the collision between india and asia at about 50 ma the convergence in the himalaya has largely been taken up along major thrust zones in this study samples of the lesser himalaya formations up to 10 km below the main central thrust mct and the greater himalaya sequence up to 12 km above the mct have been analyzed by the 40ar 39ar and u pb methods to investigate the thermal history of the mct the ages can be summarized as follows 1 the youngest ages from muscovites 3 1 ma biotites 3 4 ma and hornblendes 4 1 ma all come from within 1 km of the mct 2 there is a marked asymmetry of ages between the footwall and the hanging wall of the mct the maximum mica age in the hanging wall greater himalaya sequence is 13 ma whereas a muscovite 5 km below the mct in the footwall has an age spectrum with a gradient from 400 to 1400 ma 3 five k feldspars from 6 2 to 11 9 km above the mct all give minimum ages in the range 3 0 6 4 ma and 4 a 206pb 238u age on a brannerite from the greater himalayan sequence is 4 8 ma structural and petrologic observations preclude the possibility that this age distance distribution reflects faulting within the greater himalaya sequence and production of the necessary thermal energy by shear heating requires unrealistically high shear stresses infiltration of hot fluids through the mct zone appears to be the best hypothesis to explain these data simple numerical simulations which account for heat transfer by advection within the fluid flow zone and by conduction outside it indicate that the observed age distribution could have been produced by infiltration of hot fluids through the mct zone at circa 5 to 4 ma within the following range of conditions the fluids heated rocks to temperatures in the range 470 to 510 c for less than 1 million years in a region narrower than the entire mct zone the temperature of the thermal disturbance inferred from the 40ar 39ar data is consistent with petrologic data from albitic alteration assemblages in the greater himalaya sequence and the u pb age of the brannerite is similar to the youngest 40ar 39ar hornblende age this scenario hydrothermal heating culminating at about 4 ma is similar to the model of le fort 1981 for the generation of the high himalayan leucogranites in the late oligocene early miocene by dehydration of the footwall rocks of an active thrust in the younger instance we interpret that fluids derived from the footwall rocks of the main boundary thrust migrated upward through the lesser himalaya formations and were subsequently channeled along the mct producing the thermal disturbance

slip rates on the chelungpu and chushiang thrust faults inferred from a deformed strath terrace along the dungpuna river west central taiwan the chelungpu fault produced the september 1999 m w 7 6 chi chi earthquake central taiwan the shortening rate accommodated by this structure integrated over several seismic cycles and its contribution to crustal shortening across the taiwanese range have remained unresolved to address the issues we focus our study on the chelungpu and chushiang thrust faults within the southernmost portion of the chi chi rupture area structural measurements and available seismic profiles are used to infer the subsurface geometry of structures the chushiang and chelungpu faults appear as two splay faults branching onto a common ramp that further north connects only to the chelungpu surface trace we survey a deformed strath terrace along the dungpuna river buried under a 11 540 309 years old fill deposit given this age the dip angles of the faults and the vertical throw determined from the offset of the strath terrace across the surface fault traces we estimate slip rates of 12 9 4 8 and 2 9 1 6 mm yr on the chelungpu and chushiang faults respectively these yield a total shortening rate of 15 8 5 1 mm yr to be absorbed on their common decollement at depth this total value is an upper bound for the slip rate on the chelungpu fault further north where the chushiang fault disappears and transfers shortening to adjacent faults combining these results with the recently constrained shortening rate on the changhua blind thrust reveals that all these frontal faults presently absorb most of the long term horizontal shortening across the taiwanese range they thus stand as the major sources of seismic hazards in this heavily populated area the return period of earthquakes similar to the chi chi event over a 80 km long stretch of the western foothills is estimated to be 64 years this value is an underestimate because it assumes that all the faults locked during the interseismic period slip only during such large events comparison with historical seismicity suggests that episodic aseismic deformation might also play a major role in accommodating shortening

numerical model predictions of autogenic fluvial terraces and comparison to climate change expectations terraces eroded into sediment alluvial and bedrock strath preserve an important history of river activity river terraces are thought to form when a river switches from a period of slow vertical incision and valley widening to fast vertical incision and terrace abandonment consequently terraces are often interpreted to reflect changing external drivers including tectonics sea level and climate in contrast the intrinsic unsteadiness of lateral migration in rivers may generate terraces even under constant rates of vertical incision without external forcing to explore this mechanism we simulate landscape evolution by a vertically incising meandering river and isolate the age and geometry of autogenic river terraces modeled autogenic terraces form for a wide range of lateral and vertical incision rates and are often paired and longitudinally extensive for intermediate ratios of vertical to lateral erosion rate autogenic terraces have a characteristic reoccurrence time that scales with the time for relief generation there is a preservation bias against older terraces due to reworking of previously visited parts of the valley evolving spatial differences in bank strength between bedrock and sediment reduce terrace formation frequency and length favor pairing and can explain sublinear terrace margins at valley boundaries age differences and geometries for modeled autogenic terraces are consistent in cases with natural terraces and overlap with metrics commonly attributed to terrace formation due to climate change we suggest a new phase space of terrace properties that may allow differentiation of autogenic terraces from terraces formed by external drivers

long term fluvial incision rates and postglacial river relaxation time in the french western alps from 10be dating of alluvial terraces with assessment of inheritance soil development and wind ablation effects studying river long profile development as a response to tectonic and climatic controls requires reliable age dating of paleo profile remnants preserved as river terraces cosmic ray exposure cre dating often represents the only method available to date river terraces but the interpretation of cosmogenic nuclide concentrations is complicated by pre depositional inheritance and post depositional disturbance of the terrace deposits through pedogenesis and surface inflation or deflation here we use cosmogenic 10be measurements to date alluvial terraces in the french western alps in order to estimate river incision rates and to infer river response to climatic fluctuations and tectonic forcing we assess inheritance by constructing 10be concentration vs pebble depth profiles and use a monte carlo technique to estimate terrace ages we find that inheritance is negligible on all terraces enabling us to date terraces as young as 5 ky terraces that predate the last glaciation experienced intense pedogenesis and wind ablation which led to significant scatter in the 10be concentrations of surface samples we assess these effects using a model of 10be ingrowth and show that the oldest cre ages of surface clasts are close to the probable terrace age we study two catchments which have undergone varying degrees of glacial disruption the buech river experienced variations in runoff and sediment flux during the last glaciation as well as occasional ice dammings of its outlet its upper terraces record incision rates averaged over 190 ky of 0 8 mm yr consistent with denudation rates estimated in the surrounding areas and suggesting long term stability of river incision rates climatic forcing is well documented for the drac river which was repeatedly dammed by glaciers during cold periods its postglacial incision history was triggered by an 800 m drop in base level following ice dam disappearance long profile development by knickpoint propagation explains the time lag of 2 5 ky between this base level drop and terrace abandonment upstream as well as subsequent peak incision rates of 6 cm yr followed by a gradual decrease in incision rates the present knickpoint location 55 km upstream from the glacial damming site enables us to calculate a fluvial response time of 15 20 ky controlled by knickpoint propagation rates of several meters per year within the drac river

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

reduction of long term bedrock incision efficiency by short term alluvial cover intermittency fluctuations of the sediment volume stored in mountain channels are driven by stochastic variations of discharge and sediment supply and can inhibit bedrock incision if sediment thickness is too large here i study how this short term stochasticity propagates into the long term reduction of bedrock incision efficiency the cover effect at geological time scales i introduce a new numerical model that resolves sediment transport and bedrock incision at daily time scales and is run for thousands of years it incorporates 1 a transport threshold and daily stochastic variations in water discharge and sediment supply 2 a freely evolving channel width and slope and 3 an explicit treatment of alluvial thickness variations and corresponding bed incision reduction for typical mountain river conditions the model predicts that alluvial cover oscillates between complete and negligible incision reduction in this intermittent regime the long term cover effect is mainly set by the fraction of time spent in full cover and the present day extent of alluvial cover is not representative of long term dynamics the long term integrated cover effect law differs strongly from proposed theoretical and experimental models and it is controlled by sediment supply stochasticity rather than the details of cover development at the hydraulic time scale model results also suggest that steady state channel configuration always depends on sediment supply rate while being never limited by transport capacity or strictly detachment limited these results point out that discharge and sediment supply stochasticity should not be considered less important than the intricate details of incision laws to model long term bedrock channel dynamics

chronology and rates of faulting of ventura river terraces california new evidence concerning the chronology of four late pleistocene terraces of the ventura river near oak view california that are vertically offset and tilted by reverse and reverse flexural slip faults provides a means of estimating rates of fault movement and downcutting by the ventura river radiocarbon ages of charcoal contained within terrace deposits date two of the terraces correlation of soils that have developed on terrace deposits and extrapolation of the rate of vertical displacement of the arroyo parida santa ana fault not a flexural slip fault date the others resulting age estimates for the four main late pleistocene terraces are qt5b 30 000 qt6a 38 000 qt6b 54 000 and qt6c 92 000 yr b p vertical slip rates on flexural slip faults range from 0 3 to 1 1 mm yr and apparently are related to the rate form and mechanics of folding of the north limb of the ayers creek canada larga syncline average rates of downcutting of the ventura river for several intervals during the late pleistocene estimated from the chronology and relative elevation of river terraces north of the arroyo panda santa ana fault upstream from the zone of faulting vary from 0 5 to 1 3 mm yr the range in rates probably reflects variations in local uplift as well as adjustments to changing eustatic sea level climatic conditions and or regional deformation of the western transverse ranges the average rate of downcutting of the ventura river north of the zone of flexural slip faulting is 0 8 mm yr compared with 1 2 to 2 2 mm yr in the deformation zone immediately to the south this apparently indicates that during tectonic deformation there is an approximate balance between the rate of uplift due to faulting and folding and the rate of downcutting by the fluvial system

evidence for regional stream aggradation in the central oregon coast range during the pleistocene holocene transition abstract low nearly continuous terraces of similar age are present along streams in drainage basins that range in size from drift creek 190 km 2 to the umpqua river 11 800 km 2 in the oregon coast range radiocarbon ages from near the bose of fluvial sediments underlying these terraces are clustered at about 9000 11 000 14 c yr b p beveled bedrock surfaces straths that underlie the fluvial sediments are 1 8 m above summer stream levels and are present along most of the nontidal reaches of the rivers that we studied where exposed the bedrock straths are overlain by 2 11 m of fluvial sediment that consists of a bottom stratum channel facies of sandy pebble cobble gravel and a top stratum overbank facies of sandy silt or silt eight radiocarbon ages from the fluvial sediments allow correlation of the lowest continuous terrace over a wide area and thus indicate that a regional aggradation episode occurred in coast range drainage basins during the pleistocene holocene transition the cause of such widespread aggradation is unknown but may be related to climate induced changes in the frequency of evacuation of colluvium from hollows which are common in all drainage basins in the region

steepened channels upstream of knickpoints controls on relict landscape response 1 the morphology of a relict landscape provides important insight into erosion rates and processes prior to base level fall fluvial knickpoints are commonly thought to form a leak proof moving boundary between a rejuvenated landscape below and a relict landscape above we argue that fluvial rejuvenation may leak farther upstream depending on the rate and style of knickpoint migration the outer margin of a relict landscape should therefore be used with caution in tectonic geomorphology studies as channel steepening upstream of knickpoints could reduce the relict area we explore the response of the roan plateau to knickpoint retreat triggered by late cenozoic upper colorado river incision multiple knickpoints 100 m waterfalls separate a low relief upper landscape from incised canyons below two digital elevation model data sets 10 m u s geological survey and 1 m airborne laser swath mapping indicate steeper channels above waterfalls relative to concave channels farther upstream the steepened reaches are several kilometers long correspond to doubling of slope and exhibit channel narrowing and an increase in hillslope angle we compare two mechanisms for generating steepened reaches the first uses a recent model for erosion amplification due to flow acceleration at the waterfall lip the second acknowledges that waterfall lips may be limited to the outcrop of a resistant formation subtle structural warping of the stratigraphy can lead to lowering of the waterfall lip as it retreats thus lowering base level for upstream channels results of numerical modeling experiments suggest the latter mechanism is more consistent with our observations of long mildly steepened reaches

new constraints on the late cenozoic incision history of the new river virginia abstract the new river crosses three physiogeologic provinces of the ancient tectonically quiescent appalachian orogen and is ideally situated to record variability in fluvial erosion rates over the late cenozoic active erosion features on resistant bedrock that floors the river at prominent knickpoints demonstrate that the river is currently incising toward base level however thick sequences of alluvial fill and fluvial terraces cut into this fill record an incision history for the river that includes several periods of stalled downcutting and aggradation we used cosmogenic 10be exposure dating aided by mapping and sedimentological examination of terrace deposits to constrain the timing of events in this history 10be concentration depth profiles were used to help account for variables such as cosmogenic inheritance and terrace bioturbation fill cut and strath terraces at elevations 10 20 and 50 m above the modern river yield model cosmogenic exposure ages of 130 600 and 600 950 ka respectively but uncertainties on these ages are not well constrained these results provide the first direct constraint on the history of alluvial aggradation and incision events recorded by new river terrace deposits the exposure ages yield a long term average incision rate of 43 m my which is comparable to rates measured elsewhere in the appalachians during specific intervals over the last 1 ma however the new river s incision rate reached 100 m my modern erosion rates on bedrock at a prominent knickpoint are between 28 and 87 m my in good agreement with rates calculated between terrace abandonment events and significantly faster than 2 m my rates of surface erosion from ancient terrace remnants fluctuations between aggradation and rapid incision operate on timescales of 104 105 year similar to those of late cenozoic climate variations though uncertainties in model ages preclude direct correlation of these fluctuations to specific climate change events these second order fluctuations appear within a longer term signal of dominant aggradation until 2 ma followed by dominant incision a similar signal is observed on other appalachian rivers and may be the result of sediment supply fluctuations driven by the increased frequency of climate changes in the late cenozoic

rapid incision of the colorado river in glen canyon insights from channel profiles local incision rates and modeling of lithologic controls the colorado river system in southern utah and northern arizona is continuing to adjust to the baselevel fall responsible for the carving of the grand canyon estimates of bedrock incision rates in this area vary widely hinting at the transient state of the colorado and its tributaries in conjunction with these data we use longitudinal profiles of the colorado and tributaries between marble canyon and cataract canyon to investigate the incision history of the colorado in this region we find that almost all of the tributaries in this region steepen as they enter the colorado river the consistent presence of oversteepened reaches with similar elevation drops in the lower section of these channels and their coincidence within a corridor of high local relief along the colorado suggest that the tributaries are steepening in response to an episode of increased incision rate on the mainstem this analysis makes testable predictions about spatial variations in incision rates these predictions are consistent with existing rate estimates and can be used to guide further studies we also present cosmogenic nuclide data from the henry mountains of southern utah we measured in situ10be concentrations on four gravel covered strath surfaces elevated from 1 m to 110 m above trachyte creek the surfaces yield exposure ages that range from approximately 2 5 ka to 267 ka and suggest incision rates that vary between 350 and 600 m my these incision rates are similar to other rates determined within the high relief corridor available data thus support the interpretation that tributaries of the colorado river upstream of the grand canyon are responding to a recent pulse of rapid incision on the colorado numerical modeling of detachment limited bedrock incision suggests that this incision pulse is likely related to the upstream dipping lithologic boundary at the northern edge of the kaibab upwarp copyright 2009 john wiley sons ltd

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

knickpoint retreat and transient bedrock channel morphology triggered by base level fall in small bedrock river catchments the case of the isle of jura scotland a sudden drop in river base level can trigger a knickpoint that propagates throughout the fluvial network causing a transient state in the landscape knickpoint retreat has been confirmed in large

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

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

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

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

quaternary climate change and the formation of river terraces across growing anticlines on the north flank of the tien shan china nested stream terraces warped upward over actively growing anticlines along the north flank of the tien shan in western china appear to record alternating phases of valley widening and incision differences of relative heights between remnants of four separate strath terraces along one river and between two such terraces along another reach 100 to 120 m over the crests of the anticlines we infer that this spacing is due to alternating stages of valley widening and rapid incision associated with climate changes with a periodicity of 100 kyr the crests of the anticlines appear to emerge from the aggrading flanks of the anticlines at an average rate of about 1 mm a the maximum heights of 25 and 35 10 m for the lowest terraces above their projected initial profiles imply ages of roughly 25 kyr and 35 kyr 10 kyr hence they suggest that flood plains which were abandoned to form the terraces developed adjacent to active stream beds during the last glacial period when climates were relatively

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

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

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

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

fluvial terraces of the lower susquehanna river abstract fluvial terraces of the lower susquehanna river offer a unique opportunity to investigate the late stage geologic and geomorphic evolution of the u s atlantic passive margin petrography and elevation distinguish and provide a basis for correlation of two groups of terraces the upland terraces and lower terraces through the piedmont newark basin and great valley downstream correlation to dated upper coastal plain and fall zone fluvial deposits relative weathering and soil profile development characteristics establish terrace age upland terraces tg1 tg2 and tg3 middle to late miocene strath terraces 80 to 140 m above the present channel occur only along the piedmont reach they are underlain by unstratified texturally mature quartz dominated roundstone diamictons lower terraces qtg qt1 qt6 pliocene and pleistocene strath and thin aggradational terraces within 45 m of the present channel are underlain by stratified and unstratified texturally and compositionally immature sand gravel and pebbly silt terrace age and longitudinal profiles suggest complex interactions among relative base level long term flexural isostatic processes climate and river grade our model for terrace genesis requires the susquehanna river to attain and maintain a characteristics graded longitudinal profile over graded time for the u s atlantic margin we propose that straths are continually cut along this graded profile during periods of relative base level stability achieved by slow steady isostatic continental uplift acting in concert with eustatic rise change in an external modulating factor such as eustatic fall or climate change results in fluvial incision and subsequent genesis of strath terraces longitudinal profiles of lower susquehanna river terraces which converge at the river mouth diverge through the piedmont and reconverge north of the piedmont contrast with their hypothesized original concave up profiles progressive and cumulative flexural upwarping of the atlantic margin accounts for terrace profile deformation suggesting flexural isostasy as a first order regional deformation mechanism these results offer new interpretations of terrace age correlation and geologic significance that require modification of previous studies suggesting uplifted or anticlinically warped peneplains on the u s atlantic margin

late quaternary fluvial terraces of the romagna and marche apennines italy climatic lithologic and tectonic controls on terrace genesis in an active orogen abstract we synthesize a new fluvial terrace chronostratigraphy of the bidente and musone rivers cast within a broader european framework which forms the basis of a terrace genesis and river incision model for the northern apennines italy our model supported by terrace long profiles correlation to po foreland sediments 15 new radiocarbon dates and published numeric and relative stratigraphic ages highlights how drainage basin substrate drives concurrent formation of strath terraces in the bidente basin and fill terraces in the musone basin quaternary climate change paces the formative geomorphic processes through unsteady discharges of water and sediment in the weathering limited setting represented by the bidente basin siliciclastic detritus carves broad strath surfaces during glacial climates that are preserved as terraces as the river incises during the transition to an interglacial climate in contrast the transport limited and carbonate detritus dominated musone basin sees valleys deeply buried by aggradation during glacial climates followed by river incision during the transition to an interglacial climate incision of these rivers over the past 1 million years has been both unsteady and non uniform these and all po adriatic draining rivers are proximal to a base level defined by mean sea level and have little room for increasing their longitudinal profile concavities through incision particularly in their lower reaches despite periodic glacio eustatic drawdowns as a result the observed incision is best explained by rock uplift associated with active local fault or fold growth embedded in the actively thickening and uplifting apennine foreland

holocene strath terraces climate change and active tectonics the clearwater river basin olympic peninsula washington state the 400 km 2 clearwater river basin located on the pacific flank of the actively uplifting olympic mountains of western washington state contains a well preserved flight of holocene fluvial terraces we have collected a large data set of numeric ages from these terraces that is used to elucidate the geomorphic fluvial active tectonic and climatic processes that operate at holocene spatial and temporal scales detailed field mapping reveals three prominent holocene straths and their overlying terrace deposits terrace ages fall into three broad ranges ca 9000 11 000 yr b p qt4 4000 8000 yr b p qt5 and 0 3000 yr b p qt6 terrace deposit stratigraphy sedimentology and age distributions allow us to consider two alternative models for their genesis the favored model states that the terrace ages are coincident with lateral incision of the clearwater channel emplacement of the terrace alluvium and the carving of the straths vertical incision of the clearwater channel was primarily relegated to the brief 1000 yr intervals when we have no record of terraces alternatively the straths were carved as the channel incised vertically during the brief time periods between dated terrace deposits and the terrace ages record a subsequent long time of alluviation atop the straths and concomitant termination of vertical incision in both models we envision a clearwater river channel at or near capacity with a temporally variable rate of both lateral and vertical incision small deviations from this at capacity condition are driven by variations in the liberation and delivery of hillslope sediment to the channel we consider several causes for variable hillslope sediment flux in this tectonically active setting including holocene climate change and ground accelerations related to earthquakes holocene rates of vertical incision are reconstructed along nearly the entire clearwater valley from the wide distribution of dated terraces incision rates clearly increase upstream mimicking a pattern documented for pleistocene terraces in the same basin however the rates are 2 3 times those determined for the pleistocene terraces the faster holocene incision rates may be interpreted in terms of an increase in the rates of rock uplift however we favor an alternative explanation in which the holocene rates represent a channel rapidly reacquiring its stable graded concavity following protracted periods of time in the pleistocene when it could not accomplish any vertical incision into tectonically uplifted bedrock because the channel was raised above the bedrock valley bottom by climatically induced alluviation these results illustrate how even in tectonically active settings representative rates of rock uplift inferred from studies of river incision should be integrated over at least one glacial interglacial cycle

knickpoint evolution in a vertically bedded substrate upstream dipping terraces and atlantic slope bedrock channels we conducted flume experiments to investigate the nature of knickpoint and bedrock channel longitudinal profile evolution for a channel reach underlain by a vertically bedded substrate of alternating resistance this research was motivated by the presence behavior and apparent persistence of impressive knickpoints formed in steeply dipping or steeply foliated bedrock in atlantic slope drainages of the appalachian mountains the experiments were carried out in a 10 m long wooden box flume filled with a very fine to medium grained sand alluvial substrate and a single 30 cm wide ridge of vertically oriented lacustrine silt clay varves designed to simulate a bedrock channel reach numerous control experiments established a stable meandering channel form and transport gradient from which knickpoints were produced by a single instantaneous base level fall at the channel mouth knickpoint evolution in the alluvial material downstream of the bedrock reach was dominated by inclination of the knick point face this resulted in a rapid transition from a waterfall at the point of base level fall to a broad convex zone spanning the entire lower alluvial channel reach a waterfall and plunge pool reformed at the contact between the lower alluvial reach and the simulated bedrock ridge where the knickpoint shortened and steepened the knickpoint then migrated upstream through the bedrock reach by a combination of parallel retreat and vertical channel incision the knickpoint evolution process resulted in the formation of upstream dipping strath terraces an uncommon landform possibly present in the holtwood gorge of the susquehanna river as base level fall effects were transmitted to the alluvial channel above the bedrock reach a complex channel response accompanied by pulses of sediment alternately buried and excavated the bedrock reach these results illustrate the complex behavior associated with knickpoint evolution the unsteadiness of the bedrock channel erosion process and the significant lag times that may exist in natural fluvial systems that are in the process of adjusting to base level fall

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

long river profiles tectonism and eustasy a guide to interpreting fluvial terraces along three rivers at the mendocino triple junction northern california strath cut and fill terraces have formed in response to tectonic and eustatic processes detailed surveying and radiometric dating at multiple sites indicate that lower reaches of the rivers are dominated by the effects of oscillating sea level primarily aggradation and formation of fill terraces during sea level high stands alternating with deep incision during low stands a eustasy driven deposi tional wedge extends tens of kilometers upstream on all rivers tapering to zero thickness this distance is greater than expected from studies of the effects of check dams on much smaller streams elsewhere due in part to the large size of these rivers however the change in gradient is nearly identical to other base level rise studies the depositional gradient is about half that of the original channel middle to upper reaches of each fiver are dominated by the effects of long term uplift primarily lateral and vertical erosion and formation of steep unpaired strath terraces exposed only upstream of the depositional wedge vertical incision at a rate similar to that of uplift has occurred even during the present sea level high stand along rivers with highest uplift rates strath terraces have steeper gradients than the modem channel bed and do not merge with marine terraces at the river mouth consequently they cannot be used to determine altitudes of sea level high stands strath formation is a continuous process of response to long term uplift and its occurrence varies spatially along a river depending on stream power and hence position upstream strath terraces are found only along certain parts of a coastal stream upstream of the aggradational effects of oscillating sea level and far enough downstream that stream power is in excess of that needed to transport the prevailing sediment load for a given size fiver the greater the uplift rate the greater the rate of vertical incision and consequently the less the like lihood of strath terrace formation and preservation

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

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

fluvial archives a valuable record of vertical crustal deformation abstract the study of drainage network response to uplift is important not only for understanding river system dynamics and associated channel properties and fluvial landforms but also for identifying the nature of crustal deformation and its history in recent decades geomorphic analysis of rivers has proved powerful in elucidating the tectonic evolution of actively uplifting and eroding orogens here we review the main recent developments that have improved and expanded qualitative and quantitative information about vertical tectonic motions the effects of horizontal deformation are not addressed channel long profiles have received considerable attention in the literature and we briefly introduce basic aspects of the behaviour of bedrock rivers from field and numerical modelling perspectives before describing the various metrics that have been proposed to identify the information on crustal deformation contained within their steady state characteristics then we review the literature dealing with the transient response of rivers to tectonic perturbation through the production of knickpoints propagating through the drainage network inverse modelling of river profiles for uplift in time and space is also shown to be very effective in reconstructing regional tectonic histories finally we present a synthetic morphometric approach for deducing the tectonic record of fluvial landscapes as well as the erosional imprint of tectonic forcing sedimentary deposits such as fluvial terrace staircases are also considered as a classical component of tectonic geomorphology we show that these studies have recently benefited from rapid advances in dating techniques allowing more reliable reconstruction of incision histories and estimation of incision rates the combination of progress in the understanding of transient river profiles and larger more rigorous data sets of terrace ages has led to improved understanding of river erosion and the implications for terrace profile correlation i e extrapolation of local data to entire profiles finally planform changes in fluvial systems are considered at the channel scale in alluvial rivers and regional level in terms of drainage reorganisation examples are given of how numerical modelling can efficiently combine with topographic data to shed new light on the dis equilibrium state of drainage systems across regional drainage divides

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

late cenozoic flexural deformation of the middle u s atlantic passive margin despite the century long recognition of regional epeirogeny along the middle atlantic passive margin relatively few studies have focused on understanding postrift uplift mechanisms here we demonstrate that epeirogenic uplift of the central appalachian piedmont and subsidence of the salisbury embayment represent first order flexural isostatic processes driven by continental denudation and offshore deposition our results show that regional epeirogenic processes present on all atlantic type passive margins are best resolved by specific stratigraphic and geomorphic relationships rather than topography a simple one dimensional geodynamic model constrained by well dated baltimore canyon trough coastal plain and lower susquehanna river piedmont stratigraphy simulates flexural deformation of the u s atlantic margin the model represents the passive margin lithosphere as a uniformly thick elastic plate without horizontal compressive stresses that deforms flexurally under the stress of strike averaged vertically applied line loads model results illustrate a complex interaction among margin stratigraphy and geomorphology the isostatic response to denudational and depositional processes and the modulating influence of exogenic forces such as eustasy the current elevation with respect to modern sea level of fluvial terraces and correlative coastal plain deposits or unconformities is successfully predicted through the synthesis of paleotopography eustatic change and margin flexure results suggest that the middle u s atlantic margin landward of east coast magnetic anomaly is underlain by lithosphere with an average elastic thickness of 40 km flexural rigidity d 4 1023 n m the margin experiences an average long term denudation rate of approximately 10 m m y and the piedmont has been flexurally upwarped between 35 and 130 meters in the last 15 m y long term isostatic continental uplift resulting from denudation and basin subsidence resulting from sediment loading are accommodated primarily by a convex up flexural hinge physiographically represented by the fall zone our results elucidate an inherent danger in using topography alone to constrain late stage passive margin deformation mechanisms only through careful synthesis of field stratigraphic and geomorphic elements such as fluvial terraces coastal plain deposits and offshore stratigraphy can age control be extended from the offshore depositional setting to the erosionally dominated continent this study demonstrates that despite a relatively subdued topography the middle u s atlantic margin experiences progressive flexural isostatic deformation similar to that proposed for high relief margins characterized by great escarpments thus margin topographic diversity remains a function of other factors such as lithospheric composition and or structure supracrustal stratigraphy and structure degree of drainage integration drainage divide migration and climate

effects of land use change on channel morphology in northeastern puerto rico between 1830 and 1950 much of northeastern puerto rico was cleared for agriculture runoff increased by 50 and sediment supply to the river channels increased by more than an order of magnitude much of the land clearance extended to steep valley slopes resulting in widespread gullying and landslides and a large load of coarse sediments delivered to the stream channels a shift from agriculture to industrial and residential land uses over the past 50 yr has maintained the elevated runoff while sediment supply has decreased allowing the rivers to begin removing coarse sediment stored within their channels the size abundance and stratigraphic elevation of in channel gravel bar deposits increases channel depth decreases and the frequency of overbank flooding increases downstream along these channels this is presumed to be a transient state and continued transport will lead to degradation of the bed in downstream sections as the channel adjusts to the modern supply of water and sediment a downstream decrease in channel size is contrary to the expected geometry of self adjusted channels but is consistent with the presence of partially evacuated sediment remaining from the earlier agricultural period reverse downstream decreasing channel morphology is not often cited in the literature although consistent observations are available from areas with similar land use history identification of reverse channel morphology along individual watercourses may be obscured in multiwatershed compilations in which other factors produce a consistent but scattered downstream trend identification of reverse channel morphology along individual streams in areas with similar land use history would be useful for identifying channel disequilibrium and anticipating future channel adjustments

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

numerical modeling of fluvial strath terrace formation in response to oscillating climate many river systems in western north america retain a fluvial strath terrace rec ord of discontinuous downcutting into bedrock through the quaternary their importance lies in their use to interpret climatic events in the headwaters and to determine long term incision rates terrace formation has been ascribed to changes in sediment supply and or water discharge produced by late quaternary climatic fluctuations we use a one dimensional channel evolution model to explore whether temporal variations in sediment and water discharge can generate terrace sequences the model includes sediment transport vertical bedrock erosion limited by alluvial cover and lateral valley wall erosion we set limits on our modeling by using data collected from the terraced wind river basin two types of experiments were performed constant period sinusoidal input histories and variable period inputs scaled by the marine 18 o rec ord our simulations indicate that strath terrace formation requires input variability that produces a changing ratio of vertical to lateral erosion rates straths are cut when the channel floor is protected from erosion by sediment and are abandoned and terraces formed when incision can resume following sediment cover thinning high sediment supply promotes wide valley floors that are abandoned as sediment supply decreases in contrast wide valleys are promoted by low effective water discharge and are abandoned as discharge increases widening of the valley floors that become terraces occurs over many thousands of years the transition from valley widening to downcutting and terrace creation occurs in response to subtle input changes affecting local divergence of sediment transport capacity formation of terraces lags by several thousand years the input changes that cause their formation our results suggest that use of terrace ages to set limits on the timing of a specific event must be done with the knowledge that the system can take thousands of years to respond to a perturbation the incision rate calculated in the field from the lowest terrace in these systems will likely be higher than the rate calculated by using older terraces because the most recent fluvial response in the field is commonly downcutting associated with declining sediment input since the last glacial maximum this apparent increase in incision rates is observed in many river systems and should not necessarily be interpreted as a response to an increase in rock uplift rate

assessment of channel changes due to long term bedload supply decrease roubion river france this paper presents the observations made in the watershed of the upper roubion river which drains a submediterranean mountainous environment and in two of its main tributaries the soubrion and bine creeks our aim was to analyze contemporary morphological and sedimentary changes in response to a progressive decrease in bedload supply this decrease is linked to an increase of forest cover in the watershed during the 20th century following rural depopulation and to a correlated decrease in peak flows several variables were systematically measured width slope bed level change sediment size along 23 km of river whereas dendrochronological data floodplain cross sections and estimated recurrence intervals of sediment entrainment based on critical shear stress calculations were also produced for specific sites to complete our analysis evidence of three major phenomena was obtained on both the roubion and its tributaries i bed degradation ii development of stable pavement in incised reaches and iii channel narrowing associated with the development of alluvial forest on recently abandoned portions of the pre existing active channel forest development began in the 1950s these three phenomena show positively covariant relationships that lead us to propose a descriptive model of mountain fluvial system adjustments to progressive decrease of bedload

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

millennial slip rate of the longitudinal valley fault from river terraces implications for convergence across the active suture of eastern taiwan the longitudinal valley fault is a key element in the active tectonics of taiwan it is the principal structure accommodating convergence across one of the two active sutures of the taiwan orogeny to understand more precisely its role in the suturing process we analyzed fluvial terraces along the hsiukuluan river which cuts across the coastal range in eastern taiwan in the fault s hanging wall block this allowed us to determine both its subsurface geometry and its long term slip rate the uplift pattern of the terraces is consistent with a fault bend fold model our analysis yields a listric geometry with dips decreasing downdip from about 50 to about 30 in the shallowest 2 5 km the holocene rate of dip slip of the fault is about 22 7 mm yr this rate is less than the 40 mm yr rate of shortening across the longitudinal valley derived from gps measurements the discrepancy may reflect an actual difference in millennial and decadal rates of convergence an alternative explanation is that the discrepancy is accommodated by a combination of slip on the central range fault and subsidence of the longitudinal valley floor the shallow listric geometry of the longitudinal valley fault at the hsiukuluan river valley differs markedly from the deep listric geometry illuminated by earthquake hypocenters near chihshang 45 km to the south we hypothesize that this fundamental along strike difference in geometry of the fault is a manifestation of the northward maturation of the suturing of the luzon volcanic arc to the central range continental sliver

coupling between hillslopes and channels in upland fluvial systems implications for landscape sensitivity illustrated from the howgill fells northwest england abstract the sensitivity of upland fluvial systems depends on the magnitude and frequency of sediment and flood producing events modified by the internal coupling characteristics of the system this paper assesses the role of hillslope channel coupling for the sensitivity of upland geomorphic systems using evidence from a 30 year monitoring programme of geomorphic change in the carlingill valley howgill fells northwest england in the hillslope zone there is little sediment supply to the stream system locally slope failures occur in response to extreme events six such events in 30 years within the ca 6 km 2 carlingill valley in the footslope coupling zone basally induced gullies are major sediment sources to the stream sediment production events occur ca 30 times per year feeding sediment to basal debris cones stream floods which can entrain these sediments occur once every ca 2 5 years stream channel morphology is adjusted to this regime downstream of gullies channels are wide and braided elsewhere they are narrow and single thread as the gullies develop however this coupling weakens and the eroding slopes eventually stabilise by revegetation over the 30 year monitoring period there has been a progressive trend towards gully stabilisation and an associated reduction in channel instability massive destabilisation may occur in response to rare extreme flood events such an event return period 100 years occurred in neighbouring langdale and bowderdale valleys in june 1982 that event destabilised the system causing slope failures fan deposition and in some places a switch in channel style to wide braided channels since 1982 there has been a progressive recovery by slope stabilisation and single thread sinuous channels have become reestablished a different style of extreme event occurred in carlingill in october 1998 in response to the wettest week in the 30 year period a slope failure fed debris flows 400 m downslope almost coupling with the channel system future climatic change could render the system prone to destabilisation through either extreme flood events or through major slope failures

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

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

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

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

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

fluvial incision and tectonic uplift across the himalayas of central nepal the pattern of fluvial incision across the himalayas of central nepal is estimated from the distribution of holocene and pleistocene terraces and from the geometry of modern channels along major rivers draining across the range the terraces provide good constraints on incision rates across the himalayan frontal folds sub himalaya or siwaliks hills where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces farther north and upstream in the lesser himalaya prominent fill terraces were deposited probably during the late pleistocene and were subsequently incised the amount of bedrock incision beneath the fill deposits is generally small suggesting a slow rate of fluvial incision in the lesser himalaya the terrace record is lost in the high range where the rivers are cutting steep gorges to complement the terrace study fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate this approach allows quantification of the effect of variations in channel slope width and discharge on the incision rate of a river the determination of incision rates requires an additional lithological calibration the two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared in the sub himalaya river incision is rapid with values up to 10 15 mm yr it does not exceed a few millimeters per year in the lesser himalaya and rises abruptly at the front of the high range to reach values of 4 8 mm yr within a 50 km wide zone that coincides with the position of the highest himalayan peaks sediment yield derived from the measurement of suspended load in himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range the observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat ramp flat geometry of the main himalayan thrust fault the morphology of the range reflects a dynamic equilibrium between present day tectonics and surface processes the sharp relief together with the high uplift rates in the higher himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the tibetan plateau driven by late cenozoic climate change or late miocene reactivation of the main central thrust

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

stream terrace genesis implications for soil development genesis of three distinct types of stream terraces can be understood through application of the concepts of tectonically induced downcutting base level of erosion complex response threshold of critical power diachronous and synchronous response times and static and dynamic equilibrium climatic and tectonic stream terraces are major terraces below which flights of minor complex response degradation terraces can form these three types of terraces can be summarized by describing a downcutting aggradation renewed downcutting sequence for streams with gravell bedload by tectonically induced downcutting streams degrade to achieve and maintain a dynamic equilibrium longitudinal profile at the base level of erosion lateral erosion bevels bedrock beneath active channels to create major straths that are the fundamental tectonic stream terrace landform aggradation events record brief reversals of long term tectonically induced downcutting because they raise active channels they may be considered as major the result of climatic perturbations or minor the result of complex response model types of perturbations climatically controlled aggradation followed by degradation leaves an aggradation surface this type of fill terrace tread is the fundamental climatic stream terrace landform aggradation surfaces may be buried by subsequent episodes of deposition unless intervening tectonically induced downcutting is sufficient for younger aggradation surfaces to form below older surfaces raising of the active channel by either tectonic uplift or by climatically induced aggradation provides the vertical space for degradation terraces to form first in alluvial fill and then in underlying bedrock along tectonically active streams these are complex response terraces because they result from interactions of dependent variables within a given fluvial system pauses in degradation to a new base level of erosion and or minor episodes of backfilling lead to formation of complex response fill cut and strath or of fill terraces fill cut terraces are formed in alluvium they are complex response terraces because they are higher than the base level of erosion good exposures and dating are needed to distinguish static equilibrium complex response minor strath terraces from dynamic equilibrium tectonic major straths strath terraces may be regarded as complex response terraces where degradation rates between times terrace tread formation exceed the long term uplift rate for the reach based on ages and positions of tectonic terraces late quaternary global climatic changes control aggradation events and even the times of cutting of major tectonic straths because the base level of erosion can not be attained during times of climatically driven aggradation degradation events most terrace soils form on treads of climatic and complex response terraces aggradation surfaces may provide an ideal flight of terraces on which to study a soils chronosequence each aggradation event is recorded by a single relict soil where tectonically induced downcutting is sufficient to provide clear altitudinal separation of the terrace treads multiple paleosols are typical of tectonically stable regions where younger aggradation events spread alluvium over treads of older climatic terraces pedons on a climatic terrace in a small fluvial system commonly are roughly synchronous variations of soil properties that can be attributed to temporal differences will be minor compared to altitudinally controlled climatic factors climatic terraces of adjacent watersheds also should be roughly synchronous correlatable variations of soil properties that can be attributed to temporal differences will be minor compared to lithologic and climatic factors between different watersheds such generalizations may not apply to basins with sufficient relief that geomorphic responses to climatic changes occur at different and overlapping times and to large rivers whose widely separated reaches are characterized by different response times to climatic perturbations soils on climatic terraces of distant watershedswill not be synchronous if their respective aggradation events occur during full glacial times and interglacial times soils on some complex response terraces may be diachronous within a given fluvial system and typically are diachronous between watersheds

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

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

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

how rivers react to large earthquakes evidence from central taiwan earthquakes and bedrock river incision are fundamental processes in the evolution of tectonically active landscapes yet little work has focused on understanding how a bedrock river responds to a single large earthquake data from the 1999 m w 7 6 chi chi earthquake in central taiwan show dramatic differences in river response that depend on the proximity to the rupture zone near the fault scarp vertical ground deformation intensifi es river incision on a time scale of years to decades while distal to the fault landslides induced by the earthquake mantle bedrock on the river bed with sediment impeding incision for decades to centuries as the material is evacuated this surprising spatial and temporal variability in bedrock incision caused by earthquakes has implications for the paleoseismic interpretation of bedrock terraces and for the evolution of tectonically active landscapes

a fluvial record of long term steady state uplift and erosion across the cascadia forearc high western washington state six late quaternary river terraces preserved along the clearwater river in northwestern washington state provide a 140 ka record of long term incision and uplift across the western side of the cascadia forearc high terrace ages are constrained by weathering rind and radiocarbon dating and by correlation to dated coastal glacio fluvial deposits and the global eustatic curve the terraces overlie flat bedrock surfaces called straths which represent uplifted segments of the river channel bedrock incision is measured by the height of a strath relative to the adjacent modern river channel the straths along the clearwater show an upstream increase in bedrock incision ranging from 0 at the coast to a maximum of 110 m in the headwaters the incision at any point along the profile increases systematically with strath age the calculated incision rates range from 10 ky the clearwater river valley has maintained a steady state profile defined by a long term balance in the rates of incision and rock uplift upstream divergence of terraces is best explained by an increase in the rate of rock uplift from the coast toward the central part of the range these results are consistent with other evidence indicating a long term steady state balance between the accretionary influx and the erosional outflux for this part of the cascadia subduction wedge since 14 ma these results help show how terrace deposits form in tectonically active landscapes the dominantly fluvial clearwater drainage was forming straths while alpine glaciers were advancing in adjacent drainages in turn the straths were buried during the transition to interglacial times because of increased sediment supply due to local deglaciation and because of eustatic highstands that forced aggradation in the lower reach of the drainage and across the continental shelf as well the fluvial system shows strong forcing by the glacial climate cycle even so the river appears to have returned to the same valley profile during each cycle of strath cutting thus bedrock incision is clearly unsteady at time scales shorter than the glacial climate cycle 100 ky but appears to be relatively steady when averaged over longer time scales a simple kinematic model is used to examine how uplift of the cl our analysis indicates that the accretionary flux into the wedge occurs mainly by frontal accretion and not by underplating if accretion occurred entirely at the front of the wedge the present west coast should be moving to the northeast at 3 m ky relative to a fixed puget sound this prediction is in good agreement with offset of a 122 ka sea cliff preserved at the southwest side of the clearwater valley profile in this case the long term margin perpendicular shortening would account for 20 to 35 percent of the geodetically measured northeast southwest shortening across the olympic mountains