rates and processes of bedrock incision by the upper ukak river since the 1912 novarupta ash flow in the valley of ten thousand smokes alaska the rates and patterns of bedrock channel incision significantly influence landscape evolution and long term interactions among climate tectonics and erosion unfortunately only sparse field data are available to quantify the controls on river incision rates we exploit the diversion of the upper ukak river by an ash flow in 1912 to measure rates of incision along a newly formed bedrock channel minimum estimates of the rate of incision into intact rock vary from 0 01 to 0 10 m yr 1 this variation reflects differences in channel slope channel width lithologic facies and intensity of jointing as well as the effects of upstream knickpoint migration a streampower type incision model adequately explains the incision rate data provided 1 variations in channel width are prescribed on the basis of field measurements 2 the slope exponent is significantly less than unity n 0 4 0 2 and 3 observed downstream changes in lithologic facies and the intensity of jointing account for the apparent twofold downstream decrease in the coefficient of erosion despite the very rapid rate of incision calibrated stream power erosion coefficients for the ukak river k 2 4 10 4 m 0 2 yr 1 to 9 0 10 4 m 0 2 yr 1 are within the range of previously published estimates two plausible explanations for the low values of the slope exponent n are that incision rate is limited by either 1 a combination of physical weathering and hydrodynamic joint block extraction or 2 block fracture due to bedload impacts modulated on steeper channel segments by suspension of a significant fraction of the sediment load

hydraulic geometry river sediment and the definition of bedrock channels a growing literature deals with bedrock channels erosion processes within them and their role in landscape evolution formal definitions currently in use classify channels as bedrock when alluvial cover is discontinuous or thin this is equated with a physical property of the flow using up to date erosion laws and descriptions of channel dynamics we show that existing definitions are difficult to apply and may lead to conflicting classifications we propose the definition a bedrock channel cannot substantially widen lower or shift its bed without eroding bedrock applied to channels in taiwan this leads to a different classification than previous definitions by analysing at a station hydraulic geometry of these channels we show that the classification exposes different controls of sediment effects on dynamics of channels classified as alluvial and bedrock

helium and co2 soil gas emission from santorini greece soil gas investigation is a useful tool to detect active faults the sudden appearance of soil gas anomalies in zones of deep reaching faults represents a promising potential precursor of earthquakes and volcanic eruptions in volcanic areas the development of soil gas monitoring techniques is particularly important as they can represent together with remote sensing techniques the only geochemical methods that can be safely applied during volcanic unrest when it becomes impossible or too dangerous to sample crater fumaroles a soil gas survey was carried out in june 1993 at the main island of thera in the santorini volcanic complex co2 flux and co2 and helium concentrations were measured at 50 cm depth for 76 points covering the entire island with a spacing of 500 m or less several anomalous soil degassing sites have been detected the main anomalies correspond to the kolumbos line and to the kameni line two volcano tectonic fault systems that controlled all the historic volcanic activity of santorini a third anomaly is related to a gas leaking fault cutting the geothermal field of southern thera soil gas data together with geovolcanological and seismological evidence indicate that the kolumbos and kameni lines are the most probable sites for future volcanic or seismic reactivation and provide the basis for the establishment of a new geochemical monitoring technique at thera

late quaternary mass wasting records in the actively uplifting pa chang catchment southwestern taiwan abstract although dominated by erosion over long term the tectonically active mountains of taiwan commonly contain thick landslide and debris flow gravels capping hillslopes or forming alluvial terraces these deposits and their associated landforms serve to study ancient mass wasting histories and their controls on fluvial processes this study focuses on the pa chang river draining the 50 mm yr 1 are detected further downstream after the recent gravel mining the fact that the obtained bedrock incision rates and patterns are decoupled with the tectonic uplift advises caution in using short term erosion data to interpret the tectonics of the taiwan s mountains

abnormal monsoon years and their control on erosion and sediment flux in the high arid northwest himalaya abstract the interplay between topography and indian summer monsoon circulation profoundly controls precipitation distribution sediment transport and river discharge along the southern himalayan mountain front shf the higher himalayas form a major orographic barrier that separates humid sectors to the south and arid regions to the north during the indian summer monsoon vortices transport moisture from the bay of bengal swirl along the shf to the northwest and cause heavy rainfall when colliding with the mountain front in the eastern and central parts of the himalaya precipitation measurements derived from passive microwave analysis ssm i show a strong gradient with high values at medium elevations and extensive penetration of moisture along major river valleys into the orogen the end of the monsoonal conveyer belt is near the sutlej valley in the nw himalaya where precipitation is lower and rainfall maxima move to lower elevations this region thus comprises a climatic transition zone that is very sensitive to changes in indian summer monsoon strength to constrain magnitude temporal and spatial distribution of precipitation we analyzed high resolution passive microwave data from the last decade and identified an abnormal monsoon year amy in 2002 during the 2002 amy violent rainstorms conquered orographic barriers and penetrated far into otherwise arid regions in the northwest himalaya at elevations in excess of 3 km asl while precipitation in these regions was significantly increased and triggered extensive erosional processes i e debris flows on sparsely vegetated steep hillslopes mean rainfall along the low to medium elevations was not significantly greater in magnitude this shift may thus play an important role in the overall sediment flux toward the himalayan foreland using extended precipitation and sediment flux records for the last century we show that these events have a decadal recurrence interval during the present day monsoon circulation hence episodically occurring amys control geomorphic processes primarily in the high elevation arid sectors of the orogen while annual recurring monsoonal rainfall distribution dominates erosion in the low to medium elevation parts along the shf

struggles with stream power connecting theory across scales abstract two empirically consistent explanations exist for the morphology and dynamics of bedrock rivers the stream power law appears to explain rates of rock incision at large scale large drainage basins or landscape scale but appears to be of limited value in predicting reach scale morphology and dynamics the underlying reason why stream power is such an effective tool for understanding landscape evolution is because it is an explicit representation of the inverse power relation between slope and area in rivers incising bedrock this morphometric relation is nearly universal subject to exogenous contingencies independent testing and validation of the stream power incision model is challenging because of the contingencies that exist in almost all landscapes mechanistic models of erosion have been tested and validated at small scales generally below the reach scale and opportunities exist for independent testing and validation by using them to predict reach scale morphologies predicting larger scale bedrock river morphology requires the assumption that flow sediment fluxes and bedrock erosion processes that occur at smaller scale are adequately averaged at larger scales which is not well supported reconciling these two scales of explanation is one of the grand challenges of fluvial geomorphology unless we accept that theory and explanations will be scale delimited

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

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

tectonic significance of holocene marine terraces in the coastal range eastern taiwan abstract well developed marine terraces on the coast of eastern taiwan between chengkung and hualien can be divided into three morphotectonic sectors 1 i chengkung kueian 2 ii shuilien and its vicinity 3 iii north of shuilien sector i displays at least five terrace levels and has undergone northward tilting and uplift the available 234 u 230 th and 14 c ages suggest that one of its holocene terraces rises to about 40 m and locally to 70 m above msl the average uplift rate for the holocene in this sector decreases northward from 9 10 mm yr to about 2 3 mm yr few traces of former shorelines or marine terraces higher than 5 m are found in sector ii north of shuilien in sector iii terrace elevation increases gradually and indicates an uplift rate of about 2 3 mm yr the uplift pattern agrees with geodetic data obtained between 1985 and 1988 the high uplift rates in sector i may represent compression and shortening of the northwestern corner of the philippine sea plate the low rates in sector ii may correlate with the region where the ryukyu trench meets the eastern coast of taiwan

numerical simulations of bedrock valley evolution by meandering rivers with variable bank material bedrock river valleys are fundamental components of many landscapes and their morphologies from slot canyons with incised meanders to wide valleys with strath terraces may record environmental history several formation mechanisms for particular valley types have been proposed that involve changes in climatic and tectonic forcing but the uniqueness of valley evolution pathways and the long term stability of valley morphology under constant forcing are unknown and are not predicted in existing numerical models for vertically incising rivers because rivers often migrate more rapidly through alluvium than through bedrock we explore the hypothesis that the distribution of bank materials strongly influences river meandering kinematics and can explain the diversity of bedrock river valley morphology simulations using a numerical model of river meandering with vector based bank material tracking indicate that channel lateral erosion rate in sediment and bedrock vertical erosion rate and initial alluvial belt width explain first order differences in bedrock valley type that bedrock bound channels can evolve under steady forcing from alluvial states and that weak bedrock and low vertical incision rates favor wide shallow valleys while resistant bedrock and high vertical incision rates favor narrow deep valleys during vertical incision sustained planation of the valley floor is favored when bedrock boundaries restrict channel migration to a zone of thin sediment fill the inherent unsteadiness of river meandering in space and time is enhanced by evolving spatial contrasts in bank strength between sediment and bedrock and can account for several valley features including strath terraces and underfit valleys commonly ascribed to external drivers

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

dynamic evolution of riverbed potholes in the granitic bedrock of chota nagpur plateau middle part of subarnarekha river basin india abstract potholes are an extraordinary abiotic micro geomorphic feature on the granite and quartzite topography of the subarnarekha river basin these erosional features are carved by the running water actions of river subarnarekha and its tributaries the study has been conducted on 17 sites n 1707 which are exemplary for the development of this micro fluvial feature located on the rejuvenated middle part of subarnarekha river basin hydrological and geological factors are imperative for their formation joint length joint density joint orientation rock hardness variations of fluvial hydraulic bed roughness river bed slope and hydraulic gradient are conducive factors for the development of this geomorphic feature monsoon is the ideal season for carving these features by river turbulence when abrasive work of river is predominant on the river bed monsoon 3000 6500 k g m 2 s 3 non monsoon 0 50 3 0 k g m 2 s 3 the evolutionary sequence of potholes is from round to oval and then elongated with vertical scouring the relation between pothole radius and depth has been established to explain the process of its expansion joint length shows positive relation r2 0 71 with pothole area while joint density has a negative relation r2 0 29 as these are structurally weaker zones characterised by destruction of these features suspended load is also a hindrance in their formation sites with high suspended load are delineated as pothole destructive zones the mean dynamic evolution of circular oval and elongated shaped potholes occurs at spatio temporal level o e 0 005 0 60 m 3 c e 0 01 0 25 m 3 e e 0 005 0 30 m 3 and the initiation of this development depends on inner depth and radius of individual pothole

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

post glacial entrenchment and knickpoint migration of the yarlung tsangpo gorge southeastern tibetan plateau abstract the formation of the yarlung tsangpo river system is commonly thought to be controlled by tectonic processes initiated several million years ago however climate change may also play an important role in guiding drainage evolution here we reconstruct the drainage history of the yarlung tsangpo gorge using field observations age constraints from optically stimulated luminescence and radiocarbon dating paleosalinity reconstruction and grain size analysis of lacustrine deposits and river valley characteristics field observations clay boron derived paleosalinity reconstruction and grain size analysis of lacustrine deposits demonstrate that a closed inland saline lake existed upstream of the yarlung tsangpo gorge implying that cutting of the gorge postdated this paleolake optically stimulated luminescence and radiocarbon dating suggest that the paleolake formed at least 51 ka and terminated after 13 ka with a shift in global climate following the last glacial maximum the paleolake drained by river capture and the gorge entrenchment commenced at the village of jiala northwest of the namche barwa peak approximately 13 10 ka ago the evolution of the yarlung tsangpo occurred on a remarkably short timescale of just tens of thousands of years in response to drainage system reorganization the yarlung tsangpo incised at an average rate of at least 26 mm yr the rapid fluvial incision initiated at jiala 13 ka ago propagated rapidly upstream at 2 12 m yr the yarlung tsangpo knickpoint has not stabilized but will likely continue to migrate upstream

mid holocene erosive episodes in tufa deposits from trabaque canyon central spain as a result of abrupt arid climate transitions during the holocene a series of tufa sediments were deposited from a karstic spring along the trabaque canyon central spain a long term lowering of the water table caused the location of the spring to be displaced from the upper sector of the canyon during the early holocene to the lower sector in the late holocene this progressive shift was in response to decreased precipitation in the region the depositional environments and their geomorphologic relationships were characterized thus five morphosedimentary units msus were identified within the holocene tufa deposits erosive episodes were recognized between msus and as most of the msus partially overlap the older ones are perched in relation to the younger ones the chronology of the msus is based on 14c dates and stable isotopes and total organic carbon content provided environmental information during the period of tufa sedimentation the successive dissection episodes in the trabaque canyon tufa deposits took place during transitioning climate conditions from the relatively wet early holocene to the more arid late holocene during this transitional period in the mid holocene wet phases alternated with more arid conditions causing the largest hydrological regime gradients in the holocene thus the four erosive episodes causing tufa dissection in trabaque during the holocene are interpreted as the result of abrupt transitional periods towards arid conditions

historical bathymetric charts and the evolution of santorini submarine volcano greece historical bathymetric charts are a potential resource for better understanding the dynamics of the seafloor and the role of active processes such as submarine volcanism the british admiralty for example have been involved in lead line measurements of seafloor depth since the early 1790s here we report on an analysis of historical charts in the region of santorini volcano greece repeat lead line surveys in 1848 late 1866 and 1925 1928 as well as multibeam swath bathymetry surveys in 2001 and 2006 have been used to document changes in seafloor depth these data reveal that the flanks of the kameni islands a dacitic dome complex in the caldera center have shallowed by up to 175 m and deepened by up to 80 m since 1848 the largest shallowing occurred between the late 1866 and 1925 1928 surveys and the largest deepening occurred during the 1925 1928 and 2001 and 2006 surveys the shallowing is attributed to the emplacement of lavas during effusive eruptions in both 1866 1870 and 1925 1928 at rates of up to 0 18 and 0 05 km3 a 1 respectively the deepening is attributed to a load induced viscoelastic stress relaxation following the 1866 1870 and 1925 1928 lava eruptions the elastic thickness and viscosity that best fits the observed deepening are 1 0 km and 1016 pa s respectively this parameter pair which is consistent with the predictions of a shallow magma chamber thermal model explains both the amplitude and wavelength of the historical bathymetric data and the present day rate of subsidence inferred from insar analysis

coarse sediment transport in a bedrock channel with complex bed topography 1 independent lithologic and structural controls in fluvial bedrock systems interact with coarse sediment transport processes to play a key role in bedrock incision processes such as abrasion during a 3 year study on the ocoee river in the blue ridge province of the southern appalachians usa we used painted tracer clasts to measure coarse sediment transport dynamics and address whether different scales of morphologic variability in bedrock channels lead to coarse sediment transport processes that differ from alluvial channels at the reach scale folded metasedimentary units are exposed in the channel bed and appear as linear bedrock ribs that vary in amplitude and orientation to flow under similar flow conditions for which size dependent transport has been observed in alluvial channels dimensionless shields stress within 1 5 2 0 times the dimensionless critical shear stress transport distance was a significant function of grain size where bedrock ribs were longitudinal to flow reach 1 and reach 2 however transport distance was not size dependent where bedrock ribs were oblique to flow reach 3 at different intrareach scales the variables characterizing the local bedrock topography and sediment architecture were the best predictors of transport distance in all three reaches therefore bed load transport processes may be influenced by bed forms in bedrock streams bedrock ribs but at potentially smaller scales than bed forms in alluvial channels the strong influence of bedrock ribs on coarse sediment transport suggested that coarse sediment transport processes are controlled by different factors in bedrock channels only when bedrock ribs cross the channel at a high angle to the flow

thresholds of erosion and sediment movement in bedrock channels abstract this study examines the erosive resistance of the discontinuous loose joint blocks of various sizes that comprise the bed material of the etobicoke creek an incised bedrock channel found in an urbanized setting within the city of toronto ontario canada in order to accomplish this both field and laboratory experiments were conducted on the bed material found in this channel the field work involved repeated detailed surveying of the channel bed and estimations of distances of movement of various clasts during many sediment transport events we found that rates of erosion and or deposition can be significant during the snowmelt spring events and that there was a greater loss of bed material over the study reach than was gained in several spots with bed lowering 0 10 m in some locations the field work also showed that the distances moved by the rock clasts scaled directly with shear stresses observed during rain events and that average distances of transport tend to be short the frequency distributions of distance of transport scaled with particle size is log normally distributed flume experiments demonstrated that the clasts offered different levels of resistance to movement depending on their orientation the platy nature of the rock fragments clast orientation and their imbrication significantly affect the threshold of erosion the results have significant implications for the understanding of bedrock rivers from a management perspective especially in the case of those rivers experiencing the effects of urban development in their drainage basins

convergent evolution of abrading flow obstacles insights from analogue modelling of fluvial bedrock abrasion by coarse bedload upstream facing convex surfaces ufcs are formed by bedload abrasion in bedrock rivers and indicate the recent significant action of bedload abrasion in causing channel incision beyond this little is known of the dynamics of ufcs and the effect of substrate and bedload properties on rates and distribution of bedload abrasion for these bed roughness elements grain size populations from 1 to 8 cm b axis in 1 or 2 cm bin widths were used to bombard preshaped marble and limestone targets bolted to the base of an annular flume the control of initial shape and lithology of the target and the erodent grain size and lithology were investigated by monitoring the evolution of the target form using laser scanning at predefined time intervals eleven experiment suites were carried out containing three initial target shapes constructed from two lithologies four bedload erodent grain sizes of either granodiorites or limestone or clear water flow all 10 targets abraded by bedload evolved from their initial form into a steady state time invariant form producing ufcss steady state forms were closely similar for all targets despite different initial conditions bedload grain size has a strong control on this equilibrium form related to the transit path of the grains when moving over the target whilst initial target form has only a weak control steady state morphology is achieved more rapidly with harder erodent bedload particles and or softer targets upstream facing convex surface stoss sides were characterised by a brighter sugary granular appearance on the rock forming grain scale increasing erodent grain size for a fixed bedload mass increased the bulk abrasion rate at fixed flow speed and discharge no detectable erosion occurred for a limestone block in clear water flows under the same flow conditions indicating solution and cavitation were insignificant mechanisms of erosion in this study during the experiment suites suspended load abrasion was also found to be an insignificant mechanism in eroding lee or lateral sides in natural settings the initial formation of ufcss can occur for homogenous and or jointed substrates in close association with plucking or alternatively for heterogeneous substrates by variation in substrate erodibility

evidence for holocene megafloods down the tsangpo river gorge southeastern tibet lacustrineandalluvialterracesandsedimentsrecordtheextentofatleasttwoholoceneglaciallydammedlakesimmediatelyupstreamofthe tsangpo rivergorge at the eastern syntaxis of the himalaya the larger lake covered 2835 km 2 with a maximum depth of 680 m and contained an estimated 832 km 3 of water the smaller lake contained an estimated 80 km 3 of water radiocarbon dating of wood and charcoal yielded conventional radiocarbon ages of 8860 f 40 and 9870 f 50 14 c yr b p for the higher set of lake terraces and 1220 f 40 and 1660 f 40 14 cy r b p forsedimentsfromthelowerterraces catastrophicfailureoftheglacialdamsthatimpoundedthelakeswouldhavereleasedoutburstfloods down the gorge of the tsangpo river with estimated peak discharges of up to 1 to 5 10 6 m 3 s 1 the erosive potential represented by the unit stream power calculated for the head of the gorge during such a catastrophic lake breakout indicates that post glacial megafloods down the tsangpo river were likely among the most erosive events in recent earth history d 2004 university of washington all rights reserved

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

upstream facing convex surfaces bedrock bedforms produced by fluvial bedload abrasion bedrock bedforms are a common sight in many bedrock river channels but few process form linkages have been established for the myriad of forms recognised in the literature direct observations from bedrock channels on rates and distributions of erosion involved in bedrock bedform development are rare because of difficulties in making observations in these high energy environments upstream facing convex surfaces ufcs are common in bedrock rivers and are partnered with downstream facing surfaces we have made observations of bedrock morphology aspect and surface texture of ufcss in bedrock river channels exposed at low river stage in taiwan a distinct contrast in morphology and surface texture exists between upstream and downstream facing surfaces on a millimetre scale of observation ufcss have a three dimensional brittle fracture dominated granular surface texture but on a decimetre to metre scale have an overall convex form downstream facing surfaces in contrast have millimetre to micrometre scale rugosity characterised by smooth polished surfaces that on a centimetre to metre scale are concave or undulating with intricately sculpted features a curvilinear topographic feature or crest line marks the change in slope aspect and surface textures between upstream and downstream facing surfaces and is orientated approximately perpendicular to the reach azimuth and therefore mean flow direction we used a system of removable datums and terrestrial laser scanning to monitor the three dimensional erosion of nine bedforms at sites in the liwu river taiwan over a period of one year 2007 2008 all sites had an asymmetric distribution of erosion with higher rates of erosion on upstream facing surfaces and minor to undetectable erosion on downstream facing surfaces with local complexity the crest line mapped the boundary between relatively high and low erosion rates between the upstream facing and downstream facing surfaces respectively observations from unscaled flume experiments show that bedload particles transit a flow obstacle by travelling up the upstream facing surface where erosion is focused and hop over the top of the obstacle our field and flume observations indicate that ufcss observed in natural rivers are produced by bedload abrasion which by being decoupled from the flow can only impact the stoss surface to any significant extent micropitting found on polished downstream facing surfaces indicate that they are dominantly formed by suspended load abrasion the aspect of a surface its position in a channel cross section the topography of the channel upstream and the local topography of the bedrock bedform are significant controls on the location and magnitude of abrasion by bedload and resultant surface textures because of their effect on steering a bedload particle stream the high erosion rates observed indicate that abrasion by bedload is the controlling process in the formation of this variety of bedrock bedform and possibly the channel at large in this setting

erosion of the submarine flanks of the canary islands surveying with multibeam echo sounders around old 1 ma volcanic ocean islands reveals that their submarine flanks contain a strong downslope oriented ridge and valley corrugation which modifies the original volcanic morphology of lava terraces and cones by analogy with canyons in other settings this corrugation was probably caused by channel incision by erosive sedimentary mass flows such as turbidity currents and debris flows we adapt a method first used in subaerial geomorphology to isolate the erosion depth exhumation and apply it to the eroded flanks of the 6 8 ma anaga massif of tenerife the channels formed around this massif divert around local topographic highs these highs which are probably original volcanic cones are therefore preferentially preserved during erosion so that their elevations can be used to construct an artificial reference surface terrain depth was calculated by subtracting this reference surface from measured bathymetry comparison of the terrain depth of the old eroded submarine flank of anaga with that of the young mostly unaltered submarine flank of el hierro allows us to infer the mean depth of anaga s submarine erosion which is 100 m volcanic terrains can be dated by radiometric methods so they also provide a way of quantifying long term denudation rates we infer that submarine denudation of anaga has occurred at comparable rates to that of subaerial lowlands and much slower than denudation of highlands illustrated locally by the more extensive erosion of the subaerial anaga edifice

constant denudation rates in a high alpine catchment for the last 6 kyrs terrestrial cosmogenic nuclides tcn have widely been used as proxies in determining denudation rates in catchments most studies were limited to samples from modern active streams thus little is known about the magnitude and causes of tcn variability on millennial time scales in this work we present a 6 kyrs long high resolution record of 10be concentrations n 18 which were measured in sediment cores from an alluvial fan delta at the outlet of the fedoz valley in the swiss alps this record is paired with a three year time series n 4 of 10be measured in sediment from the active stream currently feeding this fan delta the temporal trend in the 10be concentrations after correction for postdepositional production of 10be was found to be overall constant and in good agreement with the modern river 10be concentration the calculated mean catchment wide denudation rate amounts to 0 73 0 18 mm yr this fairly constant level of 10be concentrations can be caused by a constant denudation rate over time within the catchment or alternatively by a buffered signal in this contribution we suggest that the large alluvial floodplain in the fedoz valley may act as an efficient buffer on holocene time scales in which sediments with different 10be signatures are mixed therefore presumable variations in the 10be signals derived from changes in denudation under a fluctuating holocene climate are only poorly transferred to the catchment outlet and not recorded in the 10be record however despite the absence of high frequency signals we propose that the buffered and averaged 10be signal could be meaningfully and faithfully interpreted in terms of long term catchment averaged denudation rate our study suggests that alluvial buffers play an important role in regulating the 10be signal exported by some alpine settings that needs to be taken into account and further investigated this article is protected by copyright all rights reserved

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

fluvial transportation and sedimentation of the fu shan small experimental catchments the fu shan long term ecological site is one of taiwan s mid altitude forest drainage basins this study provides an estimation of the denudation rate for two small catchments within that site data upon which the estimation is based include topographic maps site surveys and measurements on bed dissolved and suspended loads over the period from 1998 to 2000 the result indicates that typhoons and storm rainfalls as well as geomorphic and ancestral conditions have major impacts on hydrological and sediment regimes of these drainage basins the total sediment output measured at weir no 1 indicates a denudation rate of 2 7 mm per year which is 53 of the figure calculated from the data measured at weir no 2 since the geological settings in these basins are similar the result is a demonstration of the nonlinear relationship between basin areas and sediment production in basin no 1 90 of the total sediment output is dissolved load however this figure plunges to 42 in basin no 2 in contrast with the fact that the runoff per unit area in basin no 2 is 35 higher than that in basin no 1 the denudation rates of these two drainage basins are relatively high in comparison with those from other areas around the world

differentiating pleistocene tectonically driven and climate related fluvial incision the sanggan river datong basin north china the sanggan river is an alluvial river flowing through a graben basin system of the northern shanxi rift zone north china during pleistocene times the river reach in the datong basin was affected successively by various external variables such as invasion by basaltic flow along valley faulting and climatic change therefore it provides excellent constraints for differentiating tectonically driven and climate related fluvial incision in the context of tectonic subsidence based on equilibrium profile analysis k ar dating of basalts 0 74 0 41 ma studies of the river terrace and of stream action history we present a conceptual model for differentiating fault driven and climate related fluvial incision by the river the results show that fluvial incision induced by tectonic lowering of the base level due to along valley movement on the sanggan river fault is equal to fault displacement the amount of post basalt fluvial incision of the reach upstream from the lava dam is 23 to 25 m of which the fault driven and climate related incisions are 15 m and 8 to 10 m respectively the former predominating over the latter the total amount of incision in the lava dam reach is 40 to 47 m of which the fault driven and climate related incisions are 10 m and 30 37 m respectively here the latter is predominant over the former since 0 41 0 10 ma the rate of fluvial incision of the lava dam reach of the river has reached 98 115 m ma which is 1 5 2 times as great as those of the reaches upstream and downstream from the lava dam the higher rate of fluvial incision can be attributed to high water levels supplied by the onset and maintenance of backwater conditions in the reach upstream from the lava dam due to the long period of warm and humid climate in this region plucking abrasion and knickpoint migration appear to be the primary erosional processes in the lava dam reach

geomorphic altitudinal zonation of the high mountains of taiwan abstract the central range of taiwan has the characteristics of a high mountain area pleistocene glacial forms crests exceeding the timberline and debris slopes produced by frost weathering estimated denudation rates for taiwan have an average of 5 mm a and are presumed to be in equilibrium with the uplift rate these values are derived either from short term debris measurements in rivers or from fission track dating for a long term evaluation a 5 mm a denudation rate over long timescales seems reasonable but does not indicate whether this rate operated continuously throughout today it is apparent that mass movements and fluvial erosion triggered by typhoons are more intense at altitudes below 3000 m a s l and therefore the denudation rate in these altitudes is very likely higher than 5 mm a above 3000 m precipitation is lower mass movements are rare and above 3400 3500 m a s l frost weathering during winter produces debris slopes low fluvial activity in the seasonal to episodically occupied stream valleys does not significantly contribute to fluvial erosion only in very few places has backward erosion already reached the zone above 3000 m a s l and gravity related processes are active since glacial features are still preserved the denudation rate is considered to have been lower than 5 mm a since the last glacial cycle and denudation in the mountains is not in equilibrium from the top to the lower valleys

late holocene episodic river aggradation along the lao nong river southwestern taiwan an application to the tseng wen reservoir transbasin diversion project abstract the tseng wen reservoir transbasin diversion project the tseng wen project currently the largest water resources scheme in taiwan aims at constructing a tunnel system across the mountains to divert the water of the lao nong river to the tseng wen reservoir the largest reservoir in the island this project was suspended however because the constructed tunnel entrance on the side of the lao nong river has been buried by the 20 30 m aggradation of the river during 2009 typhoon morakot which triggered extensive landslides debris flows and killed 400 people in southwestern taiwan to understand the river history and help assess the risk to the project the fluvial debris flow sequences exposed along the 5 km long river reach upstream of the buried tunnel entrance were examined and a total of 38 radiocarbon dates were obtained for chronological constraints the study is limited by the intrinsic uncertainties of the dates and by the likelihood that much of the stratigraphic evidence has been eroded away or remains unexposed even so this study identified 9 pre historical aggradation events in the past 2 ka each of which had a magnitude comparable to or greater than the 2009 event the most recent event occurred

geophysical prospecting on the santorini islands a program of geophysical research was carried out as a preliminary stage of study of the santorini volcanic group this area is of remarkable geothermal and volcanological interest and the definition of a volcanological structural model is the starting point for an understanding of the local geodynamic processes

late pleistocene differential uplift inferred from the analysis of fluvial terraces southern apennines italy abstract the stratigraphic architecture and morphological assemblage of the pleistocene fluvial terraces contained in two contiguous fluvial valleys are used to understand the spatial distribution and the timing of the differential uplift that affected two different geological and geomorphological settings of an active orogen the study areas both placed in the eastern sector of the southern apennines of italy are the sant arcangelo sedimentary basin and the valsinni ridge anticline pleistocene uplift rate of 0 7 0 9 mm y 1 and historical earthquakes affecting those areas suggest active tectonics based on the synthem units used to classify the fluvial deposits in the field several strath fill and fill cut terraces have been mapped in the middle valleys of the agri and sinni rivers four middle pleistocene high terraces qes are found in the sant arcangelo basin and cut its infill and three late pleistocene low terraces qt are found at both the agri and sinni valley flanks the agri and sinni rivers cross cut the nw se oriented fold and thrust belt of the southern apennines from w to e producing a transverse drainage as a result ten to hundred metre deep gorges and wide floodplains were created in the middle reach of the river valleys computation of the bedrock incision rates from the qes1 qes4 and sqt1 terraces corresponding to 1 2 0 2 mm y 1 at 400 240 ka and 0 8 0 2 mm y 1 in the last 240 ka together with the terrace profile arrangements in the agri and sinni valleys allow for the documentation of i the differential uplift of the study area and ii the age of terrace abandonment corresponding to the beginning age of the vertical incision in the valley floor sediments to form the qt terraces the differential uplift is subsequently discussed in a space and time sequence evolution of the late pleistocene to assess the complex morphotectonic development that occurred in the eastern threshold of the basin the differential uplift of both the sant arcangelo basin and valsinni ridge would appear to indicate that buried fold and thrust structures that affect the mesozoic cenozoic sedimentary nappes are still active and they also controlled the slab retreat processes in the mediterranean region during the late pleistocene

transport dominated sediment loading in taiwanese rivers a case study from the ma an stream abstract we report newly obtained data of total suspended sediment concentrations cs and water discharges from the ma an stream in the eastern taiwan where intermittent supply limited conditions with a minimal constant load were invoked for sediment discharge hovius et al 2000 the main evidence for the supply limited condition is the many zero values of cs corresponding to low flow conditions recorded in the hydrological archive of the water resources agency wra of taiwan such conditions form a distinct mode in the rating relationship between loading and discharge which is discontinuous from the normal mode however our data raise serious doubt regarding the sensitivity of the measurements that produced data in support of the minimal constant load a field sampling from april 4 to may 5 2003 was conducted in the ma an stream during a very dry period our observations showed no zero values even though the sampling period was one of the driest periods in comparison with historical records in

stratigraphic relations of santorini s intracaldera fill and implications for the rate of post caldera volcanism we propose a new volcanological interpretation of santorini s intracaldera fill deposits as revealed by seismic reflection profiles this interpretation along with supplementary gravity and geophysical studies reveals three distinct volcaniclastic units 1 3 from top to bottom these units are attributed to 1 modern infilling sediment 2 shallow marine phreatomagmatic volcanism associated with the relatively recent formation of the kameni islands and 3 downfaulted minoan pyroclastic deposits which formed during caldera collapse towards the end of the bronze age eruption estimates of the volumes of the seismic units and kameni islands yield a dense rock equivalent magma volume of 4 85 0 7 km 3 the average rate of volcanism over the past c 3641 years is estimated at 1 3 10 3 km 3 a 1 and is similar to the rate since the ad 1707 eruption of 1 2 10 3 km 3 a 1 based on historical lava volume estimates

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

field measurements of the energy delivered to the channel bed by moving bed load and links to bedrock erosion 1 impact driven fluvial erosion is directly related to the energy delivered to the channel bed by moving bed load using a novel protocol we have measured this energy in four mountain streams in austria and switzerland similar to bed load transport rates the energy delivered to the bed displays large scatter over several orders of magnitude even for a constant discharge we found that only a small fraction 1 of the total energy available to the stream is delivered to the bed and can be used for erosive work empirical predictive equations can be defined for specific sites but there is large site to site variability prediction of energy delivered to the bed using the saltation abrasion model of bedrock erosion only provides the observed trends when measured bed load transport rates are used as input using an empirical transport law calibrated to the conditions at one of the study streams leads to overprediction of delivered energies by more than 2 orders of magnitude this overprediction decreases with increasing discharge and thus at high discharges better predictive results are obtained we find a correlation between the channels bed slope or characteristic grain sizes of the channel bed and the fraction of energy delivered to the bed of the total energy available to the stream this observation provides a tentative link between fundamental fluvial incision processes to the stream power model family that has widely been used to model fluvial bedrock incision in landscape evolution simulations

substrate sediment and slope controls on bedrock channel geometry in postglacial streams the geometry of channels controls the erosion rate of rivers and the evolution of topography following environmental change we examine how sediment slope and substrate interact to constrain the development of channels following deglaciation and test whether theoretical relationships derived from streams reacting to tectonic uplift apply in these settings using an extensive data set of channel geometry measurements from postglacial streams in the scottish highlands we find that a power law width drainage area scaling model accounts for 81 of the spatial variation in channel width substrate influences channel form at the reach scale with bedrock channels found to be narrower and deeper than alluvial channels bedrock channel width does not covary with slope which may be due to downstream variations in sediment flux bedrock channel width to depth ratios increase with discharge or area and sediment flux consistent with increasing bed cover promoting lateral widening we find steep wide and shallow bedrock channels immediately below lakes which we interpret as the result of limited erosion due to a lack of sediment tools where sediment supply is sufficient to exceed transport capacity alluvial channels develop wider shallower geometries constrained primarily by flow hydraulics our results indicate that simple scaling models of channel width with drainage area are applicable at regional scale but locally channel width varies with substrate and in the case of bedrock channels with sediment flux

interpretation of gravity and magnetic data in the phlegraean fields geothermal area naples italy abstract available gravity and magnetic data of the phlegraean fields geothermal area naples italy have been interpreted and the obtained structural models discussed in the light of the other available geological volcanological and geophysical data on the basis of the results of a previous seismic reflection survey in the gulf of naples and in the pozzuoli bay which delineated a basement characterized by a seismic velocity of 4 6 km s it has been possible to evaluate the gravity anomaly connected with the morphology of this horizon 2 7 g cm3 the residual anomaly map obtained after subtraction of the regional long wavelength components relative to mantle and deep crustal structures and the computed components relative to the above mentioned seismic basement shows up as a circular low with an amplitude of 10 mgal centred in the pozzuoli bay this gravity low has been interpreted as due to the occurrence in the centre of pozzuoli bay of light 0 2 g cm3 material with a maximum thickness of about 2 km however a contribution to the anomaly due to a narrow magmatic body intruded in the basement as suggested by volcanological and ground deformation data cannot be excluded the aeromagnetic map of the phlegraean fields is characterized by three main anomalies which have been fitted by superficial tridimensional parallelepipedic bodies schematically representing lava flows and domes their anomalies have been subsequently subtracted from the observed field obtaining as a residual a large anomaly centred in the southwestern area of the pozzuoli bay it has been interpreted as being due to a lowmagnetized body which taking into account the thermal state of the area should represent that part of the pyroclastic sequence which has lost part of its magnetization by thermo chemical alteration

climate driven bedrock incision in an active mountain belt measurements of fluvial bedrock incision were made with submillimeter precision in the east central range of taiwan where long term exhumation rates and precipitation driven river discharge are independently known they indicate that valley lowering is driven by relatively frequent flows of moderate intensity abrasion by suspended sediment is an important fluvial wear process and channel bed geometry and the presence of widely spaced planes of weakness in the rock mass influence erosion rate and style

modeling the evolution of channel shape balancing computational efficiency with hydraulic fidelity 1 the cross sectional shape of a natural river channel controls the capacity of the system to carry water off a landscape to convey sediment derived from hillslopes and to erode its bed and banks numerical models that describe the response of a landscape to changes in climate or tectonics therefore require formulations that can accommodate evolution of channel cross sectional geometry however fully two dimensional 2 d flow models are too computationally expensive to implement in large scale landscape evolution models while available simple empirical relationships between width and discharge do not adequately capture the dynamics of channel adjustment we have developed a simplified 2 d numerical model of channel evolution in a cohesive detachment limited substrate subject to steady unidirectional flow erosion is assumed to be proportional to boundary shear stress which is calculated using an approximation of the flow field in which log velocity profiles are assumed to apply along vectors that are perpendicular to the local channel bed model predictions of the velocity structure peak boundary shear stress and equilibrium channel shape compare well with predictions of a more sophisticated but more computationally demanding ray isovel model for example the mean velocities computed by the two models are consistent to within 3 and the predicted peak shear stress is consistent to within 7 furthermore the shear stress distributions predicted by our model compare favorably with available laboratory measurements for prescribed channel shapes a modification to our simplified code in which the flow includes a high velocity core allows the model to be extended to estimate shear stress distributions in channels with large width to depth ratios our model is efficient enough to incorporate into large scale landscape evolution codes and can be used to examine how channels adjust both cross sectional shape and slope in response to tectonic and climatic forcing

caldera formation on santorini and the physiography of the islands in the late bronze age the caldera of santorini is a composite structure with a subsidence history extending over 100 ka or more geomorphological mapping shows that the present day caldera wall is a complex assemblage of cliff surfaces of different ages and that collapse at santorini has repeatedly exhumed earlier caldera cliffs and unconformities cliffs bounding the southern southeastern and northwestern rims of the caldera are morphologically fresh and probably formed during or soon after the minoan eruption in the late bronze age the well scalloped shape of these cliffs is attributed to large scale rotational landslip around the margins of the minoan caldera the deposit from one landslip is preserved subaerially minoan landslips in southeast santorini detached along the basement unconformity exposing a cliff of the prevolcanic island the caldera wall in the north northeast and east preserves evidence for three generations of cliff those of minoan age and two earlier generations of caldera wall the two early calderas can be dated relative to a well established statigraphy of lavas and tuffs the presence of in situ minoan tephra plastered onto the present day caldera wall provides evidence that these ancient caldera cliffs had already been exhumed prior to the minoan eruption field relationships permit reconstruction of the physiography of bronze age santorini immediately before the minoan eruption the reconstruction differs from some previously published versions and is believed to be the most accurate to date bronze age sa ntorini had a large flooded caldera formed 21 ka ago this caldera must have acted as an excellent harbour for the bronze age inhabitants of the island the 3 6 ka minoan eruption deepened and widened the extant caldera the volume of minoan collapse 25 km3 is in good agreement with published estimates for the volume of discharged magma if between 5 and 8 km3 of minoan ignimbrite ponded as intracaldera tuff

evidence from cosmic ray exposure cre dating for the existence of a pre minoan caldera on santorini greece cosmic ray exposure cre dating was performed on the caldera cliffs of santorini with the aim of detecting cliff segments predating the minoan eruption 17th century bce the methodology involved the determination of in situ produced cosmogenic 36cl concentration in basaltic to rhyodacitic whole rocks cropping out in the cliffs after the samples were processed following the chemical protocol of 36cl preparation for silicate rocks 36cl concentrations were measured by accelerator mass spectrometry ams important challenges during the implementation procedure were related to large amounts of radiogenic 36cl complex modeling of inherited 36cl and dominance of the thermal and epithermal low energy neutron capture production pathway nevertheless quantitative assessments on the basis of the contribution of the low energy neutron capture pathway percent to the total production rate validated the calculated cre dates current cre ages demonstrate that an ancient caldera existed on pre minoan santorini occupying at least the northern half of the modern day caldera

a thermomechanical wedge model of taiwan constrained by fission track thermochronometry abstract we present 31 new apatite fission track aft ages for the island of taiwan that when combined with existing aft and zircon fission track zft data provide regional spatial coverage of the island with respect to low temperature thermochronometry the overall pattern of zft and aft ages in taiwan exhibits unreset ages in the southern and western portions of the island and reset ages predominantly in the central range and eastern taiwan this pattern supports interpretations of the orogen kinematics as reflecting a crustal scale wedge with a southward propagating collision zone in this model new material is accreted to the wedge from the west and is transferred to the east with the greatest exhumation occurring along the eastern margin as recorded in the reset ages in the east and unreset ages in the west the southward propagating collision is consistent with reset ages in the north where erosional exhumation has been ongoing for longer and unreset ages in the south where the younger collision implies less time for erosional exhumation despite the variation in the age of the collision along the strike of the island the widths of the aft and zft reset age zones remain nearly constant between 23 00 n to 24 00 n and 23 20 n to 24 00 n respectively suggesting that the orogen is in an exhumational steady state over these regions with respect to the aft and zft thermochronometers we use the fission track data in conjunction with observations of crustal structure crustal fabric and heat flow measurements to constrain a time dependent two dimensional thermomechanical model of orogen evolution by accounting for the heat transfer tectonic and erosion processes needed to predict aft and zft ages we are able to investigate the relationship between the measured ages and the tectonic characteristics of the orogen with our model we conclude that 1 roughly half of the material accretion in taiwan occurs through underplating over an approximately 40 km wide region 2 current average erosion rates are 3 3 mm yr in the eastern central range and 2 3 mm yr over the whole island 3 the collision has been propagating southward at a rate between 20 and 51 km ma over the past 2 3 ma and 4 central taiwan is in a topographic thermal and exhumational steady state

late quaternary landscape evolution and genesis of the 2009 catastrophic landslide in the hsiao lin area southwestern taiwan abstract with cumulative rainfall of 1700 mm typhoon morakot triggered a catastrophic landslide at hsiao lin 9 august 2009 which dammed the river buried the village and killed 400 people we conducted a geomorphic study to understand the origin of this landslide in the context of the long term evolution of the hillslope the landslide originated from a broad poorly drained slope 21 23 and ran down along two first order channels the erosion up to 80 m thick has created a concave shape slope and exposed mostly loose debris which implies that the landslide was mainly the reworking of ancient colluviums the genesis of the concave shape slope where the source sediments of the landslide resided reflects the weakness of the mudstone shale bedrock underlying this slope in contrast to the sandstone based transport area the two channels in the transport area also follow old faults or joints given that the source area has been subject to erosion during large landslides the subsequent deposition here was likely achieved by minor colluvial processes this deposition continued until the catastrophic failure in 2009 which could be controlled by 1 the amount of sediments deposited which determined the slope angle and 2 the headward expansion of the channels from the transport area which enhanced groundwater convergence and later provided routes for the landslide materials to run downslope extensive mass wasting motions had occurred in the area as shown by the prevalence of 20 m thick landslide debris flow sequences on the lower part of the hillslope which are dated 21 14 9 13 7 and 12 0 ka by the radiocarbon method these mass wasting events had hindered the long term incision of the trunk river in response to the rapid tectonic uplift at least the 12 0 ka event which caused extensive mud flow deposition had dammed the river overall this study shows how bedrock lithology and structures controlled the shape of a hillslope and caused extremely unsteady mass wasting transport in the active mountains of taiwan

enhanced supply of fossil organic carbon to the okinawa trough since the last deglaciation 1 significantly older 14c ages by 2500 7900 years are found for sedimentary total organic carbon toc when compared to ages of codeposited surface dwelling foraminifera in the southern okinawa trough this age discrepancy increases with rising sea level since the last glacial maximum a progressive shift in toc 13c toward more negative values with rising sea level reflects an increasing fractional contribution of terrestrial organics soil organics plant debris and or fossil organics to the buried organic pool organic matter previously stored on the east china sea shelf during sea level lowstand and riverine material from taiwan may be the sources that cause the 13ctoc to shift to more terrestrial values during the holocene when sea level is above 40 m 13ctoc values stabilize within a narrow range 22 3 to 22 8 while age discrepancies continue to increase and less chemically weathered sediments are deposited the increase in age discrepancy between toc and foraminifera in the holocene may be due to a wetter climate that drove higher rates of physical weathering on taiwan and greater transport rates of fossil organic c bearing lithogenic sediment to the ocean the climate impact on the relative delivery of fossil and nonfossil toc in depositional settings influenced by fluvial sources should be considered in interpretations of sedimentary c isotope records

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

threshold of erosion of submarine bedrock landscapes by tidal currents since sea level stabilized 7000 yr bp shelf seas experiencing semi diurnal tides will have been affected by streaming four times per day if tidal erosion of bedrock were even only marginally efficient the 10 million streamings since then should have left geomorphological imprints we examine high resolution multibeam sonar data from three areas with extreme tidal currents the minas passage bay of fundy experiencing 8 knot surface tidal currents was surveyed in 2007 with a multibeam sonar in an area near to transverse dunes which are evidence for bedload transport the data show local overhanging surfaces near to the sediment rock contact potentially created by abrasion by saltating particles however they are uncommon in the straits of messina where surface currents reach 10 knots surveying revealed ridges lying oblique to the flow that are not obviously broken into separate outcrops by erosion in the bristol channel uk sonar data collected where currents reach 3 4 knots at 1 5 m above the bed reveal outcrops of limestone with superimposed sand dunes but only minor rounding of blocks holocene tidal currents have apparently been generally ineffective at eroding bedrock we examine this issue further by compiling extreme tidal streams around the uk and from them estimate shear stresses representing a macro tidal environment where peak surface currents reach 9 7 knots those data are compared with shear stresses in mountainous rivers where long term rates of erosion are comparable with tectonic uplift rates and are thus geomorphologically significant whereas river stresses reach 102 103 pa the largest tidal stresses are generally 101 and only rarely approach 102 pa too small for quarrying to operate generally however the vertical faces in the minas passage may represent the onset of abrasion given this limited evidence for abrasion we explore conditions in the geological past for tides that may have locally eroded bedrock

magnitude frequency distributions of boundary shear stress along a rapidly eroding bedrock river 1 the magnitude frequency distribution of boundary shear stress frames erosion rates in bedrock rivers but empirical constraints are rare particularly for extreme floods here we present measurements of mean stress b and its frequency distribution along a fast eroding river in taiwan we construct rating functions of discharge and hydraulic geometry using high resolution satellite images of flood patterns river stage time series topographic profiles and post flood field surveys the method allows us to assess the spatiotemporal variation in b along the channel the boundary shear stress pdf p b has a steep power law tail and includes semiannual floods generating b 100 200 pa and 50 year 3000 m3 s 1 events driving b 300 pa all such floods contribute to modification of the coarse alluvial cover and erosion of the bedrock bed given the steep tail decay in p b the rapid channel incision probably owes more to the exceptional frequency of moderate shear stresses than to the magnitude of the extremes

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

variations of lateral bedrock erosion rates control planation of uplifting folds in the foreland of the tian shan nw china fluvial planation surfaces such as straths commonly serve as recorders of climatic and tectonic changes and are formed by the lateral erosion of rivers a process that remains poorly understood here we present a study of kilometer wide fluvially eroded low relief surfaces on rapidly uplifting folds in the foreland of the southwestern tian shan a combination of field work digital elevation model analysis and dating of fluvial deposits reveals that despite an arid climate and rapid average rock uplift rates of 1 3 mm yr rivers cut extensive 1 2 km wide surfaces with typical height variations of 2 6 kyr the extent of this beveling varies in space and time such that different beveling episodes affect individual structures between times of planation beveled surfaces are abandoned incised and deformed across the folds in a challenge to models that link strath cutting and abandonment primarily to changes in river incision rates we demonstrate that lateral erosion rates of antecedent streams crossing the folds have to vary by more than 1 order of magnitude to explain the creation of beveled platforms in the past and their incision at the present day these variations do not appear to covary with climate variability and might be caused by relatively small much less than an order of magnitude changes in sediment or water fluxes it remains uncertain in which settings variations in lateral bedrock erosion rates predominate over changes in vertical erosion rates therefore when studying fluvial planation and strath terraces variability of both lateral and vertical erosion rates should be considered

inner gorge slot canyon system produced by repeated stream incision eastern alps significance for development of bedrock canyons abstract many inner bedrock gorges of the alps show abrupt downstream changes in gorge width as well as channel type and gradient as a result of epigenetic incision of slot canyons many slot canyons also are associated with older gorge reaches filled with quaternary deposits the age of slot canyons and inner bedrock gorges however commonly is difficult to constrain for the inner bedrock gorge system of the steinberger ache catchment eastern alps active slot canyons as well as older abandoned gorge reaches filled with upper wurmian proglacial deposits record three phases of gorge development and slot canyon incision a 234u 230th age of cement of 29 7 1 8 ka in fluvial conglomerates onlapping the flank of an inner gorge fits with late wurmian valley bottom aggradation shortly before pleniglacial conditions in addition the age indicates that at least the corresponding canyon reach must be older during advance of ice streams in the buildup of the last glacial maximum lgm the catchment was blocked and a proglacial lake formed bedrock gorges submerged in that lake were filled with fluviolacustrine deposits during the lgm the entire catchment was overridden by ice during post glacial reincision streams largely found again their preexisting inner bedrock canyons in some areas however the former stream course was missed and a slot canyon formed the distribution of pleistocene deposits the patterns of canyon incision and the mentioned u th cementation age however together record a further discrete phase of base level rise and stream incision well before the lgm the present course of steinberger ache and its tributaries is a patchwork of 1 slot canyons incised during post glacial incision 2 vestiges of slot canyons cut upon an earlier middle to late wurmian cycle of base level rise and fall 3 reactivated reaches up to 200 m in width of inner bedrock gorge that are watershed at present and more than at least 30 ka in age and 4 abandoned sediment filled reaches of bedrock canyons that also must be older than 30 ka and that are exposed alongside the active streams multi cyclic bedrock canyon systems composed of reaches of markedly different ages may be common in mountain ranges subject to glaciations and or mass wasting

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

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

water and sediment discharge from small mountainous rivers taiwan the roles of lithology episodic events and human activities national science council of taiwan nsc96 2628 m 001 028 u s national science foundation and naval oceanographic office

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

spatial coincidence of rapid inferred erosion with young metamorphic massifs in the himalayas a spatially distributed rate of erosion index ei based on models of bedrock river incision documents a strong spatial correspondence between areas of high erosion potential and young metamorphic massifs as well as structural highs throughout the himalayas the ei is derived from slopes and drainage areas calculated from a hydrologically cor rected digital elevation model gtopo30 combined with precipitation data iiasa to generate synthetic annual stream discharges these variables drive three generalized pro cess models to produce ei maps that while differing in detail provide an internally con sistent spatially continuous index of large scale erosion rates the large spatial variation in potential erosion rates in the himalayas suggested by the ei patterns contrasts with the uniform convergence of the indian subcontinent if these ei gradients persist through time they support the emerging view of a positive feedback between localized rapid ero sion and upward advection of lower crust

earthquake triggered increase in sediment delivery from an active mountain belt in tectonically active mountain belts earthquake triggered landslides deliver large amounts of sediment to rivers we quantify the geomorphic impact of the 1999 mw 7 6 chi chi earthquake in taiwan which triggered 20 000 landslides coseismic weakening of substrate material caused increased landsliding during subsequent typhoons most coseismic landslides remained confined to hillslopes downslope transport of sediment into the channel network occurred during later storms the sequential processes have led to a factor of four increase in unit sediment concentration in rivers draining the epicentral area and increased the magnitude and frequency of hyperpycnal sediment delivery to the ocean four years after the earthquake rates of hillslope mass wasting remain elevated in the epicentral area

correlation of fluvial terraces and temporal steady state incision on the onshore makran accretionary wedge in southeastern iran insight from channel profiles and 10be exposure dating of strath terraces we describe and summarize the geomorphology of fluvial terraces along the four major rivers draining the central onshore makran accretionary wedge in iran and describe uplifted marine terraces on the coast of this region thirty five strath terraces at different sites were dated using in situ produced cosmogenic 10 be concentrations from surfaces and depth profiles these new measurements reveal abandonment ages between ca 15 and 380 ka the age distribution allows determining the chronology of terrace levels and establishing regional correlations between two major regional levels the geographically widespread correspondence suggests that these two levels result from a regional climatically driven force systematic dating also provides evidence for time and spatial variations in incision rate which enables distinguishing between a regionally uniform incision rate of 0 3 0 4 mm a and higher local incision and or uplift 0 8 mm a rates these spatial changes are consistent with where localized tectonic activity is recognized in the field we also dated using 14 c in shells four uplifted marine terraces on the coast of the study area comparing incision rates derived from strath terraces 0 3 mm a with published uplift estimates from marine terraces 0 2 mm a reveals that fluvial rivers responded to a regional long term incision and surface uplift pattern this rate reflects the tectonically steady state of the wedge on a regional scale locally high incision rates delineate active faults and folds indicating that perfect steady state is unlikely on short length scales

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

revised estimates for the volume of the late bronze age minoan eruption santorini greece the late bronze age minoan eruption of santorini greece occurred from within an existing caldera low temperature pyroclastic flow emplacement on shallow slopes outside the caldera can only be consistent with the caldera being filled with eruption products that are not preserved field observations and seismic reflection surveys suggest that this missing material has been downfaulted the volume of the caldera infill is estimated as 18 26 km 3 dense rock equivalent dre increasing the total of the minoan eruption deposits to 78 86 km 3 dre and making it the largest known holocene eruption this study highlights a general mechanism whereby caldera eruption volumes could be significantly underestimated

climatic variability of the sub surface sea temperatures in the aegean black sea system and relation to meteorological forcing non smoothed yearly temperature records with minimal statistical uncertainties are constructed for winter and summer of the period 1950 2000 in two areas in the aegean sea for the sub surface layer of 80 120 m and two areas in the black sea for the sub surface layer of sigma theta isopycnals between 14 5 and 15 4 the specific areas are selected mostly because of the dense hydrographic data coverage they have during the period 1950 2000 two trend regimes appear in both seas a period of decreasing sea temperatures from the early mid 1960s to the early mid 1990s and an apparent warming afterwards trends in sea temperatures correlate with trends in the north atlantic oscillation nao and partly the east atlantic west russian eawr indexes but the signs of nao and or eawr cannot sufficiently justify the winter to winter temperature changes in the entire study area in examining the wind flows in the sea level pressure maps for characteristic winters in which local peaks in the sea temperature records occur we identify particular sea level pressure structures that are not accounted for by the typical north atlantic or east atlantic west russia positive or negative dipoles in addition there are winters when the siberian high induces local maxima in sea temperatures in the study area a spectral coherence analysis of the unfiltered winter sea temperature and the corresponding teleconnection nao eawr records shows that common spectral and coherence peaks exist at 5 6 9 10 and 15 17 years

rare earth elements in fine grained sediments of major rivers from the high standing island of taiwan thirty eight sediment samples from 15 primary rivers on taiwan were retrieved to characterize the rare earth element ree signature of fluvial fine sediment sources compared to the three large rivers on the chinese mainland distinct differences were observed in the ree contents upper continental crust normalized patterns and fractionation factors of the sediment samples the average ree concentrations of the taiwanese river sediments are higher than those of the changjiang and huanghe but lower than the zhujiang light rare earth elements lrees are enriched relative to heavy rare earth elements hrees with ratios from 7 48 to 13 03 we found that the variations in la lu ucc gd lu ucc and la yb ucc gd yb ucc are good proxies for tracing the source sediments of taiwanese and chinese rivers due to their distinguishable values our analyses indicate that the ree compositions of taiwanese river sediments were primarily determined by the properties of the bedrock and the intensity of chemical weathering in the drainage areas the relatively high relief and heavy rainfall also have caused the rees in the fluvial sediments from taiwan to be transported to the estuaries down rivers from the mountains and in turn delivered nearly coincidently to the adjacent seas by currents and waves our studies suggest that the ree patterns of the river sediments from taiwan are distinguishable from those from the other sources of sediments transported into the adjacent seas and therefore are useful proxies for tracing the provenances and dispersal patterns of sediments as well as paleoenvironmental changes in the marginal seas

bedload to suspended load ratio and rapid bedrock incision from himalayan landslide dam lake record about 5400 cal yr bp a large landslide formed a n400 m tall dam in the upper marsyandi river central nepal the resulting lacustrine and deltaic deposits stretched n7 km upstream reaching a thickness of 120 m 14 c dating of 7 wood fragments reveals that the aggradation and subsequent incision occurred remarkably quickly 500 yr reconstructed volumes of lacustrine 0 16 km 3 and deltaic 0 09 km 3 deposits indicate a bedload to suspended load ratio of 1 2 considerably higher than the 1 10 that is commonly assumed at the downstream end of the landslide dam the river incised a new channel through 70 m of greater himalayan gneiss requiring a minimum bedrock incision rate of 13 mm yr over last 5400 yr the majority of incision presumably occurred over a fraction of this time suggesting much higher rates the high bedload ratio from such an energetic mountain river is a particularly significant addition to our knowledge of sediment flux in orogenic environments 2007 university of washington all rights reserved

increase in basin sediment yield from landslides in storms following major seismic disturbance the landslide area along the tachia river catchment of central taiwan was investigated using the remote sensing images of various typhoon and earthquake events taken from 1996 to 2004 and the sediment discharge measured at hydrometric stations our findings indicate that 88 of the coseismic landslides triggered by the 1999 chi chi earthquake were first time occurrences after the chi chi earthquake 59 of the landslide area was reactivated during typhoon toraji and 66 during typhoon mindulle the landslides prone to reactivation were on the formations with closely spaced discontinuities it is suggested that further rock mass defects in the formations were added or opened by the earthquake and that the landslide areas continued to extend until the end of 2004 post 1999 rainstorms delivered large amounts of colluvial sediment into the main channel leading to a 2 fold increase in post seismic sediment discharge while the precipitation was only half that of the pre earthquake rate

late quaternary intensified monsoon phases control landscape evolution in the northwest himalaya the intensity of the asian summer monsoon circulation varies over decadal to millennial time scales and is reflected in changes in surface processes terrestrial environments and marine sedi ment records however the mechanisms of long lived 2 5 k y intensified monsoon phases the related changes in precipitation distribution and their effect on landscape evolution and sedimen tation rates are not yet well understood the arid high elevation sectors of the orogen correspond to a climatically sensitive zone that currently receives rain only during abnormal i e strength ened monsoon seasons analogous to present day rainfall anom alies enhanced precipitation during an intensified monsoon phase is expected to have penetrated far into these geomorphic threshold regions where hillslopes are close to the angle of failure we as sociate landslide triggering during intensified monsoon phases with enhanced precipitation discharge and sediment flux leading to an increase in pore water pressure lateral scouring of rivers and ov ersteepening of hillslopes eventually resulting in failure of slopes and exceptionally large mass movements here we use lacustrine deposits related to spatially and temporally clustered large land slides 0 5 km 3 in the sutlej valley region of the northwest him alaya to calculate sedimentation rates and to infer rainfall patterns during late pleistocene 29 24 ka and holocene 10 4 ka inten sified monsoon phases compared to present day sediment flux measurements a fivefold increase in sediment transport rates re corded by sediments in landslide dammed lakes characterized these episodes of high climatic variability these changes thus em phasize the pronounced imprint of millennial scale climate change on surface processes and landscape evolution

bedrock erosion and changes in bed sediment lithology in response to an extreme flood event the 2013 colorado front range flood abstract in this study we use before and after flood observations of bed sediment lithology to determine how the lithological composition of bed sediment changes in response to an extreme flood event and use the post flood measurements and observations to estimate values of critical shear stresses needed to erode shale bedrock and protective bank vegetation in the study streams we present results from measurements of bed sediment lithology taken after the 2013 colorado front range flood and compare these results with the same measurements taken before the flood these two data sets provide a unique opportunity to determine how bed sediment lithology changes following an extreme flood event and allows insight into sources of channel bed sediment after the flood bed sediment lithology shifted distinctly towards the lithological distribution of the total catchment before the flood bed sediment in most locations was primarily composed of granitic material while after the flood the streams were more likely to have metamorphic volcanic or sedimentary rocks in the bed sediment reflecting more input from more non local hillslopes compared to before the flood our post flood measurements and observations allowed us to estimate threshold critical shear stress for the plucking of shale bedrock 212 pa and the removal of large cottonwood trees 364 pa in downstream sections of the study streams bank vegetation plays a critical role in protecting bedrock banks from lateral erosion such that only the largest flood events can remove vegetation and expose bedrock on channel banks to potential erosion we found that this large flood event plucked shale bedrock from both the channel bed and banks and that the shale bed sediment then quickly breaks down and is transported away by small flows as such this data set allowed us to make conclusions about the effect of large flood events on the long term evolution of lateral and vertical erosion in bedrock rivers

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

accuracy assessment of lidar derived dems of bedrock river channels holtwood gorge susquehanna river 1 we evaluate the accuracy of 1 m lidar derived dems of an exposed bedrock channel and use these high resolution terrain models to calculate erosional fluxes between four levels of strath terraces dated previously with 10be recent investigations into the timing rates and processes by which rivers incise their bedrock channels have greatly enhanced our understanding of landscape evolution however measuring channel geometries in 3 dimensions is difficult thus volumes of bedrock eroded during incision events are rarely considered although our analysis successfully demonstrates the proportionality between incision rates and erosional fluxes for terraces high above the channel floor in this gorge unusually high river flows on the date of acquisition inundated lower bedrock surfaces preventing accurate volume estimates to ensure optimal low flow lidar coverage for fluvial environments we discuss methods to improve the scheduling of data acquisition by using archived discharge records and predictive models based on real time data

partitioning sediment flux by provenance and tracing erosion patterns in taiwan we critically evaluate the potential and limitations of an alternative way to calculate erosion rates based on petrographic and mineralogical fingerprints of fluvial sediments coupled with gauged sediment fluxes our approach allows us to apportion sediment loads to different lithological units and consequently to discriminate erosion rates in different tectonic domains within each catchment our provenance data on modern taiwanese sands indicate focused erosion in the backbone range and tananao complex of the retrowedge lower rates are inferred for the northern part of the island characterized by tectonic extension and for the western foothills in the prowedge the principal factor of uncertainty affecting our estimates is the inevitably inaccurate evaluation of total sediment load because only the suspended flux was measured another is the assumption that suspended load and bed load are derived from the same sources in fixed proportions additional errors are caused by the insufficiently precise definition of lithologically similar compositional end members and by the temporal variability of sediment composition at the outlet of each catchment related to the spatial variability of erosional processes and triggering agents such as earthquakes typhoons and landslides to evaluate the robustness of our findings we applied a morphometric technique based on the stream power model the results obtained are broadly consistent with local discrepancies ascribed to poorly constrained assumptions and choices of scaling parameters our local erosion estimates are consistent with gps uplift rates measured on a decadal timescale and generally higher than basin wide results inferred from cosmogenic nuclide and thermochronology data