large scale dam removal on the elwha river washington usa river channel and floodplain geomorphic change abstract a substantial increase in fluvial sediment supply relative to transport capacity causes complex large magnitude changes in river and floodplain morphology downstream although sedimentary and geomorphic responses to sediment pulses are a fundamental part of landscape evolution few opportunities exist to quantify those processes over field scales we investigated the downstream effects of sediment released during the largest dam removal in history on the elwha river washington usa by measuring changes in riverbed elevation and topography bed sediment grain size and channel planform as two dams were removed in stages over two years as 10 5 million t 7 1 million m3 of sediment was released from two former reservoirs downstream dispersion of a sediment wave caused widespread bed aggradation of 1 m greater where pools filled changed the river from pool riffle to braided morphology and decreased the slope of the lowermost river the newly deposited sediment which was finer than most of the pre dam removal bed formed new bars largely pebble granule and sand material prompting aggradational channel avulsion that increased the channel braiding index by almost 50 as a result of mainstem bed aggradation floodplain channels received flow and accumulated new sediment even during low to moderate flow conditions the river system showed a two to tenfold greater geomorphic response to dam removal in terms of bed elevation change magnitude than it had to a 40 year flood event four years before dam removal two years after dam removal began as the river had started to incise through deposits of the initial sediment wave 1 2 million t of new sediment 10 of the amount released from the two reservoirs was stored along 18 river km of the mainstem channel and 25 km of floodplain channels the elwha river thus was able to transport most of the released sediment to the river mouth the geomorphic alterations and changing bed sediment grain size along the elwha river have important ecological implications affecting aquatic habitat structure benthic fauna salmonid fish spawning and rearing potential and riparian vegetation the response of the river to dam removal represents a unique opportunity to observe and quantify fundamental geomorphic processes associated with a massive sediment influx and also provides important lessons for future river restoration endeavors

experimental study of incipient motion in mixed size sediment transport rates of five sediments were measured in a laboratory flume three of these sediments had the same mean size the same size distribution shape and different values of grain size distribution standard deviation the critical shear stress for incipient motion of the individual size fractions within these sediments was estimated as that shear stress that produced a small dimensionless transport rate the sorting of the sediment mixture had little effect on the critical shear stress of individual fractions once the median size d50 of the mixture and a fraction s relative size di d50 are accounted for our data combined with previously published data show a remarkably consistent relation between the critical shear stress of individual fractions and the fraction s relative grain size despite a broad variation in the available data of mixture sorting grain size distribution shape mean grain size and grain shape all fractions in a size mixture begin moving at close to the same value of bed shear stress during steady state transport conditions this result is apparently true for transport systems where the transport rates of individual fractions are determined solely by the flow and bed sediment recirculating systems as well as for systems where the fractional transport rates are imposed on the system feed systems this equivalence in initial motion results is important because natural transporting systems often show attributes of both types of behavior in an unknown combination

effects of large floods on sediment transport and reach morphology in the cobble bed sainte marguerite river abstract sediment transport rates were estimated for two flood events on the cobble bed sainte marguerite river in the saguenay region canada morphologic methods were used to derive one set of estimates and a combination of the meyer peter and muller equation with a dimensionless sediment transport ratio after dietrich et al nature 340 1989 215 was used to derive another set of estimates both sets of estimates give consistent results for the first event which had a decade scale return period and for the second event which was the largest flood on record and had a century scale return period the transport occurring during the second event was an order of magnitude greater than that occurring during the first event despite this disparity in the transport intensity of the two events the channel morphology remained qualitatively similar the observed degree of channel stability is attributed to a change of channel pattern and the initiation of bed degradation following channel rectification in the 1960s

grain size and topographical differences between static and mobile armour layers a series of laboratory flume experiments under conditions of sediment starvation zero sediment feeding and recirculation were conducted in order to identify the temporal evolution and surface properties of static and mobile armour layers the experiments were carried out in an 8 m long flume using a bimodal grain size mixture d50 6 2 mm and a range of shear stresses ranging from 4 0 to 8 6 n m 2 the results confirm that a static armour layer is coarser than a mobile one and that the grain size of a mobile armour layer is rather insensitive to changes in the imposed flow strength an analysis of laser scan bed surveys revealed the highly structured and imbricated nature of the static armour layer under these conditions the vertical roughness length scale of the bed diminished and it became topographically less complex at higher forming discharges the topography of mobile armour layers created by rising discharges differed they exhibited a greater roughness length scale and were less organized despite the fact that the grain size of the surface material maintained an approximately constant value during recirculation also the mobile armour tended to create larger cluster structures than static armour layers when formed by higher discharges these differences were mainly due to the transport of the coarser fraction of bed sediments which diminished to zero over the static armour because of being hidden within the bed whereas in the mobile armour the coarser particles protruded into the flow and were actively transported increasing the vertical roughness length scale overall the results show that an examination of the grain size characteristics of armour layers cannot be used to infer sediment mobility and bed roughness detailed elevation models of exposed surfaces of gravel bed rivers are required to provide critical insight on the sediment availability and sedimentation processes copyright 2011 john wiley sons ltd

effective discharge in rocky mountain headwater streams summary whereas effective discharge q eff in mountain streams is commonly associated with a moderate flow such as bankfull discharge q bf this study found that the maximum discharge q max and not bankfull discharge is the channel forming or effective flow for gravel transport in plane bed streams where partial bed mobility causes steep gravel transport rating curves q eff may approach bankfull flow in some step pool channels where gravel moves over a static cobble boulder bed our conclusions are based on magnitude frequency analyses conducted at 41 gauged rocky mountain headwater streams because these gauged streams lacked gravel transport data as is typical comparable streams with measured transport rates were used to develop scaling relations for rating curve exponents with stream and watershed characteristics those scaling relations were then used to estimate the steepness of gravel rating curves at the 41 gauged but unsampled sites the measured flow frequency distributions were characterized by two fitted power functions the steepness of the flow frequency distributions and the estimated steepness of gravel transport relations were combined in magnitude frequency analyses to compute q eff

strong seasonality in sand loading and resulting feedbacks on sediment transport bed texture and channel planform at mount pinatubo philippines declining sand inputs to a channel with bimodal bed sediment can lead to degradation armoring and reduced bedload transport rates where sand loading is episodic channels may alternate between high sand and low sand conditions with ensuing responses in bed texture and bedload transport rates the effects of episodic sand loading are explored through flow grain size and bedload transport measurements on the pasig potrero river a sediment rich channel draining mount pinatubo philippines sand loading on the pasig potrero river is highly seasonal and channel adjustments between seasons are dramatic in the rainy season inputs from sand rich 1991 eruption deposits lead to active sand bedded braided channels in the dry season many precipitation driven sand sources are cut off leading to incision armoring and significantly lower bedload transport rates this seasonal transition offers an excellent opportunity to examine models of degradation incision and armoring as well as the effectiveness of sediment transport models that explicitly encapsulate the importance of sand on transport rates during the fall 2009 seasonal transition 7 6 km of channel incised and armored carving a 2 3 m deep channel on the upper alluvial fan bedload transport rates measured in the august 2009 rainy season were over four orders of magnitude greater than gravel bedded dry season channels surveyed in january 2010 despite having similar shear stress and unit discharge conditions within dry season incised channels bed armoring is rapid leading to an abrupt gravel sand transition bedload transport rates adjust more slowly creating a lag between armoring and commensurate reductions in transport seasonal channel incision occurred in steps aided by lateral migration into sand rich banks these lateral sand inputs may increase armor layer mobility renewing incision and forming terraces within the incised seasonal channel the seasonal incised channel is currently being reset by precipitation driven sand loading during the next rainy season and the cycle begins again copyright 2012 john wiley sons ltd

sex that moves mountains the influence of spawning fish on river profiles over geologic timescales abstract a key component of resilience is to understand feedbacks among components of biophysical systems such as physical drivers ecological responses and the subsequent feedbacks onto physical process while physically based explanations of biological speciation are common e g mountains separating a species can lead to speciation less common is the inverse process examined can a speciation event have significant influence on physical processes and patterns in a landscape when such processes are considered such as with ecosystem engineers many studies have focused on the short term physical and biological effects rather than the long term impacts here we formalized the physical influence of salmon spawning on stream beds into a model of channel profile evolution by altering the critical shear stress required to move stream bed particles we then asked if spawning and an adaptive radiation event similar to the one that occurred in pacific salmon species could have an effect on channel erosion processes and stream profiles over geological timescales we found that spawning can profoundly influence the longitudinal profiles of stream beds and thereby the evolution of entire watersheds the radiation of five pacific salmon from a common ancestor additionally could also cause significant geomorphic change by altering a wider section of the profile for a given distribution of grain sizes this modeling study suggests that biological evolution can impact landscape evolution by increasing the sediment transport and erosion efficiency of mountain streams moreover the physical effects of a species on its environment might be a complementary explanation for rapid radiation events in species through the creation of new habitat types this example provides an illustrative case for thinking about the long and short term coupling of biotic and abiotic systems

a two dimensional discrete particle model of gravel bed river systems 1 the formation of bed forms in gravel bed rivers acts as a control on stream ecology and the response of rivers to floods available models do not reproduce the range of observed bed forms and do not consider interactions between the bed and flow hydraulics the model presented here considers a gravel bed river as a complex system in which sediment clasts are represented as discrete elements simple and local rules describe the sediment and flow dynamics using a trimodal sediment distribution irregular forms that scale with particle diameter develop without explicit feedback mechanisms because of the tendency of large particles to roll along the bed surface and collect into chains feedback mechanisms such as imbrication increase the effective entrainment threshold of groups of large particles and increase the stability of these imbricate forms a second type of bed form is associated with saltating grains and emerges where particles are transported at a preferred distance the development and maintenance of larger scale bed forms require feedback between the bed and flow properties by allowing mean velocity to adjust to bed morphology and considering the effect of acceleration on turbulence generation and mean velocity profiles we demonstrate the emergence of forms similar in morphology to gravel sheets dunes and riffle pools the model is best used to complement field based studies and is suitable for testing hypotheses of streambed behavior

a surface roughness model for predicting alluvial cover and bed load transport rate in bedrock channels partial alluvial cover in bedrock channels influences downcutting rates and mountain river morphodynamics flume experiments have demonstrated that a wide range of stable cover fractions are possible for a given ratio of sediment supply to transport capacity existing cover models impose different unique relationships between cover fraction and reach scale transport variables e g slope discharge sediment supply and grain size individually these models cannot predict the range of cover behaviors observed experimentally suggesting that additional variables may be important i propose a 1 d model in which bedrock surface roughness is a control on partial alluvial cover in the model surface roughness affects both shear stresses and thresholds of grain motion and therefore sediment transport capacity the roughness of the combined bedrock alluvial bed surface varies with the cover fraction model trends are reasonably consistent with previous experiments over ranges of roughness slope and shear stress when bedrock roughness is much greater than grain size cover increases approximately linearly with the ratio of sediment supply to transport capacity when bedrock roughness is much less than the sediment diameter the model tends to predict abrupt shifts between complete bedrock exposure and alluvial cover this runaway alluviation occurs for model solutions that are unstable to perturbations in supply which is the case when transport capacity decreases as alluvial cover increases in addition a model variant in which sediment supply is controlled by the amount of sediment on the bed suggests that a linear cover relation may be sufficient for modeling sediment flux dependent erosion at landscape scales

model experiments on mobile paved gravel bed streams laboratory experiments which model streams with poorly sorted gravel beds at reduced scale reveal that pavement is a mobile bed phenomenon it acts to decrease the inherent difference in mobility between large and small grains by overrepresenting the percentage of large grains exposed to the flow pavement can coexist with the motion of all available grain sizes because motion is sporadic so that at any given time only a small percentage of surface grains are actually in motion particles in the immediate subpavement are only occasionally moved and deeper particles are hardly ever moved the experiments were conducted at constant discharges they provide accurate quantitative models of two field streams in that they obey the same bedload law as field data both sediment feed and recirculating configurations were utilized the only systematic difference between the two is in the behavior of the finer grains

in situ δ18o and mg ca analyses of diagenetic and planktic foraminiferal calcite preserved in a deep sea record of the paleocene eocene thermal maximum we report 18o and minor element mg ca sr ca data acquired by high resolution in situ secondary ion mass spectrometry sims from planktic foraminiferal shells and 100 500 m sized diagenetic crystallites recovered from a deep sea record odp site 865 of the paleocene eocene thermal maximum petm the 18o of crystallites 1 2 pee dee belemnite pdb is 4 8 higher than that of planktic foraminiferal calcite 3 6 pdb while crystallite mg ca and sr ca ratios are slightly higher and substantially lower than in planktic foraminiferal calcite respectively the focused stratigraphic distribution of the crystallites signals an association with petm conditions hence we attribute their formation to early diagenesis initially sourced by seafloor dissolution burndown ensued by reprecipitation at higher carbonate saturation the mg ca ratios of the crystallites are an order of magnitude lower than those predicted by inorganic precipitation experiments which may reflect a degree of inheritance from donor phases of biogenic calcite that underwent solution in the sediment column in addition sims 18o and electron microprobe mg ca analyses that were taken within a planktic foraminiferal shell yield parallel increases along traverses that coincide with muricae blades on the chamber wall the parallel 18o and mg ca increases indicate a diagenetic origin for the blades but their 18o value 0 5 pdb is lower than that of crystallites suggesting that these two phases of diagenetic carbonate formed at different times finally we posit that elevated levels of early diagenesis acted in concert with sediment mixing and carbonate dissolution to attenuate the 18o decrease signaling petm warming in whole shell records published for site 865

channel bed mobility response to extreme sediment loading at mount pinatubo since the 1991 eruption of mount pinatubo the specific sediment yields from watersheds draining its slopes have been the highest ever recorded in spite of this overwhelming sediment load rivers inundated by pyroclastic flow deposits delivered almost half of the initial deposits to downslope alluvial and or debris fans by 1996 although most of this transport occurred by hyperconcentrated flows and debris flows lahars very high sediment transport rates also characterize low flow measured flow velocities and depths channel properties and the size of both mobile and stable clasts in the sacobia and pasig potrero rivers indicate median bed surface grain sizes of 2 to 8 mm grain size independent dimensionless critical shear stresses c of 0 016 to 0 041 and manning s n values of 0 017 to 0 024 values well below those previously reported for steep mountain channels the dramatic bed surface fining bed mobility and selective transport in these extremely sediment rich channels indicate that changes in grain size c values and bed roughness can increase transport capacity in response to high sediment supply our observations also suggest viewing the apparently contradictory concepts of an equal threshold of bed mobility and grain size dependent selective transport as end member concepts that apply to channels with low or intermittent and high or continuous sediment supply respectively

alternate bar response to sediment supply termination 1 sediment supply is widely held to be one of the primary controls on bar topography in alluvial channels yet quantitative linkages between sediment supply and bar topography are not well developed we explore the conditions under which alternate bars form and how they respond to the elimination of sediment supply in two linked laboratory experiments the first set of experiments was conducted in a 28 m long 0 86 m wide flume channel using a unimodal sand gravel mix the second set of experiments was conducted at field scale in a 55 m long 2 74 m wide channel using a unimodal gravel mixture in both experiments alternate bars and patchy surface grain size distributions developed under steady flow and sediment supply conditions the cessation of the sediment supply induced a reduction in the surface grain size heterogeneity and the bars were eliminated in both flumes mean boundary shear stress had declined but were capable of moving sediments after the bars disappeared albeit at relatively small rates compared to when the bars were present in the smaller flume the previously stationary bars migrated out of the flume and were not replaced with new bars a nearly featureless bed formed with limited surface grain size heterogeneity a slightly coarsened surface and a slightly reduced slope in the larger flume the formation of alternate bars was induced by an imposed upstream flow constriction and as such the bars did not migrate termination of sediment supply led to progressive erosion of bed topography and loss of the bars coarsening of the bed surface loss of bed texture patchiness and significant slope reduction the original alternate bar topography redeveloped when the sediment supply was restored once sufficient deposition had occurred to reconstruct the original channel slope this shows that the bar loss was reversible by establishing the previous conditions and highlights the importance of sediment supply for bar formation the role of sediment supply in bar formation and stability is not often recognized in stream restoration our results suggest that the loss of sediment supply can significantly affect alternate bar topography and that considerable volumes of sediment may be needed restore channel bars

the trouble with shear stress abstract predicted bedload fluxes often have large errors partly because calculations usually do not include spatial and temporal flow variability sediment motion is driven by a temporal distribution of applied fluid forces which are often simplified into time averaged flow velocities or shear stresses in sediment transport equations spatial variations in these flow parameters are also often neglected given the nonlinearities in the relation between flow and sediment flux the reach averaged sediment flux calculated with an average velocity or shear stress will not necessarily equal the measured spatially averaged bedload flux such spatial and temporal flow variations are ignored with the commonly used reach averaged shear stress ds that is a function of the flow depth and channel slope to demonstrate the uncertainties with using reach averaged shear stresses and even local shear stresses to predict bedload transport rates we have three goals first we outline the derivations and assumptions involved with commonly used estimates of shear stress second we summarize the literature on the uncertainties involved with using shear stress and other space and time averaged flow parameters to predict sediment flux third we use a new data set from experiments to add further support to a growing body of literature that suggests ds may not be linked to the mechanics of bedload transport we show considerable uncertainties exist in estimating a reach averaged shear stress by comparing ds and near bed shear stresses calculated from velocity profiles reynolds stresses and local turbulence for example ds was systematically greater than these near bed stresses even for experiments without large bed roughness we also find that correlations between local time averaged sediment fluxes and local reynolds stresses or turbulence intensities are relatively poor the near bed time averaged local flow velocity has the strongest relation with the time averaged local bedload flux because it is closely related to the forces that actually drive sediment transport however sediment transport rates in individual experiments could still vary by orders of magnitude for a given value of this flow velocity suggesting that additional work is needed to identity a parameter or distributions of parameters that will best predict sediment fluxes

applications of grain pivoting and sliding analyses to selective entrapment of gravel and to flow competence evaluations models of sediment threshold by grain pivoting or sliding over underlying particles are examined in order to explore their application to evaluations of selective entrainment of gravel by flowing water of special interest is whether such process based models provide satisfactory evaluations of flow competence and the movement of large clasts by floods a detailed derivation is undertaken focusing first on the fluid flow and forces at the particle level the resulting threshold equation for the particle level velocity is then modified to yield the mean entrainment stress for the flow as a whole this approach is appropriate for considerations of selective entrainment of grains of varying sizes within a deposit the sorting being due to their relative projection distances above the bed and the dependence of their pivoting angles on grain size and shape the resulting threshold equations contain a number of coefficients e g drag and lift whose values are poorly known but can be constrained by requiring agreement with the shields curve for the threshold of grains in uniform deposits if pivoting coefficients based on laboratory measurements with tetrahedral arrangements of particles are used in the models smaller degrees of selective sorting are predicted than found in the field measurements of gravel entrainment however if reasonable modifications of those coefficients are made for expected field conditions then the models yield good agreement with the data sliding models where sorting is due entirely to projection distances of the grains above the bed yield somewhat poorer agreement with the field data however the sliding models may have support from laboratory experiments on gravel entrainment in that the data and theoretical curves have similar concave trends the existing measurements lack documentation of the mechanisms of grain movement so it is not possible to conclusively determine the relative importance of grain pivoting versus sliding in spite of such uncertainties the results are encouraging and it is concluded that pivoting and sliding models for grain entrainment do have potential for field computations of selective entrainment and flow competence

analysis of bedload transport characteristics of idaho streams and rivers field data are essential in evaluating the adequacy of predictive equations for sediment transport each dataset based on the sediment transport rates and other relevant information gives an increased understanding and improved quantification of different factors influencing the sediment transport regime in the specific environment data collected for 33 sites on 31 mountain streams and rivers in central idaho have enabled the analysis of sediment transport characteristics in streams and rivers with different geological topographic morphological hydrological hydraulic and sedimentological characteristics all of these streams and rivers have armored poorly sorted bed material with the median particle size of surface layer coarser than the subsurface layer the fact that the largest particles in the bedload samples did not exceed the median particle size of the bed surface material indicates that the armor layer is stable for the observed flow discharges generally bankfull or less and in some cases two times higher than bankfull discharge the bedload transport is size selective the transport rates are generally low since sediment supply is less than the ability of flow to move the sediment for one range of flow discharges or the hydraulic ability of the stream is insufficient for entrainment of the coarse bed material detailed analyses of bedload transport rates bedload and bed material characteristics were performed for each site the obtained results and conclusions are used to identify different influences on bedload transport rates in analyzed gravel bed rivers copyright 2008 john wiley sons ltd

subsidence control on river morphology and grain size in the ganga plain the ganga plain represents a large proportion of the current foreland basin to the himalaya the himalayan sourced waters irrigate the plain via major river networks that support approximately 10 percent of the global population however some of these rivers are also the source of devastating floods the tendency for some of these rivers to flood is directly linked to their large scale morphology in general the rivers that drain the east ganga plain have channels that are perched at a higher elevation relative to their floodplain leading to more frequent channel avulsion and flooding in contrast those further west have channels that are incised into the floodplain and are historically less prone to flooding understanding the controls on these contrasting river forms is fundamental to determining the sensitivity of these systems to projected climate change and the growing water resource demands across the plain here we present a new basin scale approach to quantifying floodplain and channel topography that identifies areas where channels are super elevated or entrenched relative to their adjacent floodplain we explore the probable controls on these observations through an analysis of basin subsidence rates sediment grain size data and sediment supply from the main river systems that traverse the plain yamuna ganga karnali gandak and kosi rivers subsidence rates are approximated by combining basement profiles derived from seismic data with known convergence velocities results suggest a more slowly subsiding basin in the west than the east grain size fining rates are also used as a proxy for relative subsidence rates along the strike of the basin the results also indicate higher fining rates and hence subsidence rates for given sediment supply in the east by integrating these observations we propose that higher subsidence rates are responsible for a deeper basin in the east with perched low gradient river systems that are relatively insensitive to climatically driven changes in base level in contrast the lower subsidence rates in the west are associated with a higher elevation basin topography and entrenched river systems recording climatically induced lowering of river base levels during the holocene

a process based classification system for headwater streams in this paper we develop a classification system for small headwater streams predicated upon an understanding of the dominant physical processes acting in the channel and its basin and the conditions under which these processes operate the variables used to build the classification are hillslope gradient valley width as compared to channel width channel gradient channel depth and sediment size using physical laws and morphologic relationships we recognize domains in which various processes dominate channels within the same process domain can be expected to behave in a similar manner in transporting sediment and water and responding to and recovering from basin disturbance therefore we use the domains to recognize distinct channel types this classification system provides the foundation for building an approach for identifying basins and streams that are comparably sensitive to landuse

experiments on the effect of hydrograph characteristics on vertical grain sorting in gravel bed rivers 1 desert ephemeral gravel bed streams typically have bed surfaces that are relatively unarmored compared to the substrate below while gravel bed streams in humid and snowmelt areas typically have well armored surfaces the degree of armoring can be characterized in terms of an armor ratio defined as the ratio of the surface median size to the substrate median size a set of field data shows desert ephemeral gravel bed streams with armor ratios ranging from 0 5 to 2 4 and with an average value of 1 2 the armor ratio of snowmelt fed gravel bed streams in the same set ranges from 2 to 7 with an average value of 3 4 the reason for this difference is sought in terms of differing hydrological characteristics and sediment supply regimes thirteen experiments were conducted to study the formation of armoring under a range of hydrological conditions the experiments have two limiting cases a relatively flat hydrograph that represents conditions produced by continuous snowmelt and a sharply peaked hydrograph that represents conditions associated with flash floods all constant hydrograph experiments developed a well armored structured surface while short asymmetrical hydrographs did not result in substantial vertical sorting all symmetrical hydrographs show some degree of sorting and the sorting tended to become more pronounced with longer duration sediment supply appears to be a first order control on bed surface armoring while the shape of the hydrograph plays a secondary role

the concept of transport capacity in geomorphology the notion of sediment transport capacity has been engrained in geomorphological and related literature for over 50 years although its earliest roots date back explicitly to gilbert in fluvial geomorphology in the 1870s and implicitly to eighteenth to nineteenth century developments in engineering despite cross fertilization between different process domains there seem to have been independent inventions of the idea in aeolian geomorphology by bagnold in the 1930s and in hillslope studies by ellison in the 1940s here we review the invention and development of the idea of transport capacity in the fluvial aeolian coastal hillslope debris flow and glacial process domains as these various developments have occurred different definitions have been used which makes it both a difficult concept to test and one that may lead to poor communications between those working in different domains of geomorphology we argue that the original relation between the power of a flow and its ability to transport sediment can be challenged for three reasons first as sediment becomes entrained in a flow the nature of the flow changes and so it is unreasonable to link the capacity of the water or wind only to the ability of the fluid to move sediment secondly environmental sediment transport is complicated and the range of processes involved in most movements means that simple relationships are unlikely to hold not least because the movement of sediment often changes the substrate which in turn affects the flow conditions thirdly the inherently stochastic nature of sediment transport means that any capacity relationships do not scale either in time or in space consequently new theories of sediment transport are needed to improve understanding and prediction and to guide measurement and management of all geomorphic systems

effects of bed forms and large protruding grains on near bed flow hydraulics in low relative submergence conditions in mountain rivers bedforms large relatively immobile grains and bed texture and topographic variability can significantly alter local and reach averaged flow characteristics the low relative submergence of large immobile grains causes highly three dimensional flow fields that may not be represented by traditional shear stress flow velocity and turbulence intensity equations to explore the influence of large protruding grains and bedforms on flow properties we conducted a set of experiments in which we varied the relative submergence while holding the sediment transport capacity and upstream sediment supply constant flow and bed measurements were conducted at the beginning and end of each experiment to account for the absence or presence of bedforms respectively detailed information on the flow was obtained by combining our measurements with a 3d numerical model commonly used velocity profile equations only performed well at the reach scale when shallow flow effects and the roughness length of the relatively mobile sediment were considered however at the local scale large deviations from these profiles were observed and simple methods to estimate the spatial distribution of near bed shear stresses are likely to be inaccurate zones of high turbulent kinetic energy occurred near the water surface and were largely controlled by the immobile grains and plunging flow the reach averaged shear stress did not correlate to depth or slope as commonly assumed but instead was controlled by the relative boulder submergence and degree of plunging flow for accurate flow predictions in mountain rivers the effects of bedforms and large boulders must be considered

bedload sheets in heterogeneous sediment field observations in streams with beds of coarse sand and fine gravel have revealed that bedload moves primarily as thin migrating accumulations of sediment and coarse grains cluster at their leading edge these accumulations are one or two coarse grains high and are much longer 0 2 0 6 m long in sand 0 5 2 0 m in fine gravel than their height the authors propose the term bedload sheet for these features and the authors argue that they result from an instability inherent to bedload movement of moderately and poorly sorted sediment in essence coarse particles in the bedload slow or stop each other trap finer particles in their interstices and thus cause the coarse particles to become mobile again bedload sheets develop on the stoss side of dunes causing the dune to advance incrementally with the arrival of each sheet successive deposition of coarse sediment from the leading edge followed by fine sediment may generate the grain size sorting that distinguishes cross bedding available flume experiments and field observations indicate that bedload sheets are a common but generally unrecognized feature of heterogeneous sediment transport

the relative stability of salmon redds and unspawned streambeds where female salmon build nests redds streambed material is mixed fine sediment is winnowed and bed material is moved into a tailspill mound resembling the shape of a dune completed redd surfaces are coarser and better sorted than unspawned beds which is thought to increase redd stability because larger grains are heavier and harder to move and sorting increases friction angles for mobility however spawning also loosens sediment and creates topography that accelerates flow which can increase particle mobility we address these factors controlling the relative stability of redds and unspawned beds in flume experiments where redds were constructed with a technique that mimics the nesting behavior of female salmon although redds exhibited relatively coarse surfaces measured entrainment forces indicate particle loosening by spawning lowered grain resistance to motion by 12 37 on average compared to unspawned beds in addition for the same discharges boundary shear stress was 13 41 higher on a redd due to flow convergence on the tailspill visual measurements of particle entrainment further indicated redd instability as bed average shear stress was 22 lower at incipient motion and 29 lower at the discharge that mobilized all grain sizes on a redd overall results demonstrate redds are unstable compared to unspawned beds which increases the risk of scour for buried eggs but may facilitate fine sediment flushing and improve the quality of spawning gravels for future generations of spawners therefore managing salmon returns to increase streambed disturbance may be an effective tool for reducing sedimentation impacts on salmon reproduction this article is protected by copyright all rights reserved

experiments on surface structure and partial sediment transport on a gravel bed eight flume experiments were conducted to study the development of bed surface texture and structure in the presence of partial bed material transport the experiments have two phases a no feed degradational phase followed by a feeding phase a surface structure of irregular reticulate stone nets and clusters was developed before sediment feeding commenced bed load transport equaled or slightly exceeded the fed supply except at the highest feed rate the bed structure was maintained but bed surface texture fined with increasing sediment load the two phenomena may coexist because the largest grains on the bed moved only very sporadically the actual sediment transport rates were much less than the expected rate calculated from the ratio of bed surface grain size to transported grain size the difference reflects the increase in bed stability introduced by the bed structure between 17 and 47 of the bed shear stress is estimated to be carried by the structure 4 being absorbed by the load while the bed grains carried the balance of the stress bed material transport is exceedingly sensitive to bed surface structure and grain size which raises concerns about the realizable precision of grain size measurements and characterization of the structure

sediment supply and the development of the coarse surface layer in gravel bedded rivers the bed surface of most gravel rivers is considerably coarser than the sub surface or the gravel load transported over it a phenomenon affecting river dynamics morphology and ecology the coarse surface layer often called an armour or pavement has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1 2 and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3 4 where bedload supply is cut off below dams selective erosion causes coarsening5 6 using a simple quantitative model we propose that surface coarsening develops in gravel bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load we present laboratory results which support this hypothesis and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse less active bed it may therefore be possible to relate the degree of river bed surface coarsening to sediment supply resulting from land use

coarser and rougher effects of fine gravel pulses on experimental step pool channel morphodynamics understanding how steep mountain rivers respond to natural and anthropogenic sediment supply perturbations is important for predicting effects of extreme events e g floods and landslides and for restoring rivers to more natural conditions using flume experiments we show that stabilized step pool like channel beds can respond to pulses of finer gravel by becoming even coarser and rougher than before adding finer gravel initially reduces bed roughness and also increases the mobility of previously stable bed grains small and intermediate diameter clasts are then preferentially winnowed from the bed surface leaving behind higher concentrations of even larger clasts ultimately this results in both a coarser and rougher bed our experiments demonstrate that steep river beds become stable through the coevolution of bed roughness and surface grain size distribution and that these morphological variables can be sensitive to the history of upstream sediment supply

formation and dynamics of vegetated fluvial landforms follow the biogeomorphological succession model in a channelized river feedback between hydrogeomorphological processes and riparian plants drives landscape dynamics and vegetation succession in river corridors we describe the consequences of biogeomorphological feedback on the formation and dynamics of vegetated fluvial landforms based on observations from the channelized isere river in france the channel was laterally confined with embankments and mostly straightened from the beginning of the 1970s to the end of the 1990s alternate bars were progressively but heavily colonized by vegetation this context presented an exceptional opportunity to analyse temporal adjustments between fluvial landforms and vegetation succession from bare gravel bars to mature upland forest as the consequence of biogeomorphological interactions based on a gis analysis of aerial photographs between 1948 and 1996 we show that the spatiotemporal organization of vegetated bars within the river channel observed in 1996 resulted from a bioconstruction and biostabilization effect of vegetation and interactions between bars of varying age size and mobility field measurements in 1996 reflected how a strong positive feedback between sedimentary dynamics and riparian vegetation succession resulted in the construction of the vegetated bars a highly significant statistical association of geomorphological and vegetation variables rv of co inertia analysis 0 41 p 0 001 explained 95 of the variability in just one axis supporting the existence of very strong feedback between geomorphological changes i e the transformation of small bare alternate bars to fluvial landforms covered by mature upland forest and vegetation succession such dynamics reflect the fluvial biogeomorphological successions model as described by the authors earlier 2020 john wiley sons ltd

on why gravel bed streams are paved bedload transport in poorly sorted gravel bed streams downstream of dams is considered bedload and typical bed material subpavement size distributions are observed to be similar it follows that the coarse half of the subpavement moves through a reach at a rate near that of the fine half since coarser grains are intrinsically less mobile than fine grains it follows that some mechanism must act to nearly equalize mobility it is hypothesized that the pavement seen in gravel bed streams at low flow is in fact in place during typical transport events capable of moving all available sizes this pavement can provide the equalizing mechanism by exposing proportionally more coarse grains to the flow field data are used to quantify this concept and to develop a predictive relation for river pavement the model indicates that pavement should be absent in most sand bed streams in agreement with observation

mobilization of coarse surface layers in gravel bedded rivers by finer gravel bed load 1 additions of sand to gravel beds greatly increase the mobility and flux of gravel however it is not known how additions of finer gravel to coarser gravel beds will affect the mobility of bed material here we examine the effect of fine gravel pulses on gravel bed material transport and near bed flow dynamics in a series of flume experiments bed material refers exclusively to sediment in the channel prior to the pulse introduction the observations indicate that fine sediment pulses tend to migrate downstream in low amplitude waves as the waves pass over the gravel bed the interstitial pockets in the bed material surface fill and coarse gravel particles are entrained this increases bed material transport rates and causes a distinct shift from a selective mobility transport regime where particles coarser than the bed material median 8 mm make up 30 of the load to an equal mobility transport regime where bed materials coarser than 8 mm and finer than 8 mm are transported in equal proportions the only possible source for this coarser bed load material is the sediment bed suggesting that portions of the coarse surface layer are being mobilized observations of near bed velocity and turbulence suggest that fine gravel pulses cause fluid acceleration in the near bed region associated with a reduction in the level of turbulence produced at the sediment boundary this accelerated fluid at the bed increases drag exerted on coarse surface layer particles promoting their mobilization our findings suggest that in general finer bed sediment not just sand can mobilize coarser sediment and that expressions for the influence of sand on bed mobility need to be generalized on the basis of grain ratios

identifying dynamic equilibrium of an undeveloped alluvial stream by extremal hypotheses abstract the indeterminate channel problem arises from uncertainty in finding a closure relation for alluvial channels created by self organizing erosional and depositional processes extremal hypotheses have been proposed as one potential approach to closing the system of governing equations for alluvial channels many different extremal hypotheses have been presented but no substantive evidence has been developed to select which hypothesis may be most appropriate for natural alluvial river systems this paper evaluates the ability of ten extremal hypotheses to identify dynamic equilibrium across a geomorphic gradient in the remote and undeveloped mid latitude watershed of rio murta chile this study a introduces extremal hypotheses b describes the field site and geomorphic conditions and c examines which extremal hypotheses are supported by the field data in identifying the evolutionary trend toward dynamic equilibrium the extremal hypotheses that identified dynamic equilibrium within the geomorphic gradient in the field are 1 minimum kinetic energy 2 minimum specific stream power 3 maximum friction factor and 4 maximum total friction factor which collectively support minimizing kinetic energy of the system

predicting natural channel patterns based on landscape and geomorphic controls in the columbia river basin usa 1 based on known relationships of slope discharge valley confinement sediment supply and sediment caliber in controlling channel patterns we developed multivariate models to predict natural channel patterns across the 674 500 km2 columbia river basin usa we used readily available geospatial data sets to calculate reach slopes 2 year flood discharge and valley confinement as well as to develop hypothesized landscape level surrogates for sediment load and caliber relative slope percent of drainage area in alpine terrain and percent of drainage area in erosive fine grained lithologies using a support vector machine svm classifier we found that the four channel patterns were best distinguished by a model including all variables except valley confinement 82 overall accuracy we then used that model to predict channel pattern for the entire basin and found that the spatial distribution of straight meandering anabranching and braided patterns were consistent with regional topography and geology a simple slope discharge model distinguished meandering channels from all other channel patterns but did not clearly distinguish braided from straight channels 68 overall accuracy addition of one or more of the hypothesized sediment supply surrogates improved prediction accuracy by 4 14 over slope and discharge alone braided and straight channels were most clearly distinguished on an axis of relative slope whereas braided and anabranching channels were most clearly distinguished by adding percent alpine area to the model

development and prediction of bed armoring the major purposes of this study are to use available data to predict 1 the development of bed armoring and 2 the final composition of grain size distribution first the different factors influencing the incipient motion for nonuniform sediment size distribution are examined then the procedures for simulating the grain size distribution of the armor layer are developed based on modifications of gessler s procedure and einstein s hiding factor three regression equations are derived to predict the median size d d3 d0 and d d8 d4 of the final armor bed given the initial sediment bed size distribution and flow conditions a modified shields diagram for incipient motion for nonuniform sediment sizes is also suggested

flow and sediment dynamics in the vegetated secondary channels of an anabranching river the loire river france this study investigates the hydrological and sedimentological mechanisms occurring in the vegetated secondary channels of an anabranching river affected by incision the loire river france during and after flood events that occurred between 2000 and 2003 observations and measurements were performed on a vegetated secondary channel located in the study site of brehemont 790 km downstream the source morphological changes and sediment dynamics were analysed using low elevation airborne photographs topographic and bathymetric surveys and scour chains the hydraulic behaviour of the channel was also analysed by measurements performed on flow velocity and direction during different flood stages in order to quantify the influence of woody vegetation on flow resistance the roughness of bands of trees was determined from measurements performed on the field the impact of the disruption of armour layers on bedload pulses the variation of sedimentary processes during a single flood event and the fixation of bedforms by vegetation are all identified as key processes influencing the behaviour of the study channel topographic surveys demonstrate that sediment dynamics is substantial in the upstream part of the channel and that sediment budgets are different according to the temporal scale considered moreover an asymmetrical behaviour of the secondary channel is demonstrated reduced quantities of sediment deposited and preserved in the vegetated zones contrast with material by passing observed in the third order channels flow velocity and direction measurements indicate that these parameters vary according to the water level and to the morphological units of the channel pools riffles vegetated areas during low flows scouring and export of particles from the secondary channel are a consequence of reduced sediment supply from the main channel of the loire river for these water levels sedimentation occurs in pools where velocity and turbulence decrease whereas third order channels are subjected to erosion during high discharges large quantities of sediment available in the main channel feed the temporary stores formed by riffles and bars in the secondary channel the vegetated area located in the downstream part of the secondary channel deflects current trickles at low discharges and decreases flow velocity during high water levels the sedimentary accretion observed in this area exerts a feedback on flow and sedimentary processes d 2005 elsevier b v all rights reserved

breakup and reestablishment of the armour layer in a large gravel bed river below dams the lower ebro changes in armour layer during floods under supply limited conditions are little known this paper describes the breakup and the reestablishment of the bed armour layer in the regulated gravel bed ebro river during a flooding period the study was conducted over a 28 km study reach from 2002 to 2004 the surface subsurface and bed load grain size distribution constitute the bases for the analysis of bed armouring dynamics the results indicate that the magnitude of floods controlled the degree of armouring of the river bed the initial mean armouring ratio was 2 3 with maximum values reaching 4 4 floods in the winter of 2002 2003 q8 caused the breakup of the armour layer in several sections this resulted in the erratic bed load pattern observed during the december 2002 flushing flow and in the increase in bed load transport during successive events most grain size classes were entrained and transported causing river bed incision the mean armouring ratio decreased to 1 9 in contrast during low magnitude floods in 2003 2004 q2 the coarsest fractions 64 mm did not take part in the bed load while finer particles were winnowed thus surface deposits coarsened as a result the armour layer was reestablished i e the mean armouring ratio increased to 2 3 and the supply of subsurface sediment decreased the supply and transport of bed material appear to be in balance in the river reach immediately below the dam in contrast the transport of medium and finer size classes in the downstream reaches was higher than their supply from upstream a phenomenon that progressively reduced their availability in the river bed surface hence the armour layer reworking

the role of non uniform roughness in the formation of sand ribbons abstract sand ribbons typically are shallow water sedimentary features characterized as thin elongate bands of sand overlying a coarser lag sediment which is exposed between them the ribbons are oriented along the direction of the dominant current and are formed by a secondary circulation pattern that includes a small component of the boundary shear stress directed toward the center of the sand ribbons a perturbation expansion coupled with flume experiments indicates that this circulation pattern is linked to differences in the bottom roughness between the sand ribbons and the adjacent coarser strips the non uniform turbulence field arising from this roughness distribution drives a secondary helical flow the expansion indicates that there is a preferred ratio of sand ribbon spacing to water depth 4 1 for unstratified channel flow conditions the secondary flow pattern forces the downstream flow to readjust so that the boundary shear stress over the smooth sand strips is considerably smaller than the mean stress this serves to retard the movement of sand through such a system

tectonic sedimentology sediment systems deciphering global to local tectonics tectonic sedimentology has broad appeal because it can be applied at many time and length scales such as sedimentary consequences of changing rates of plate creation at mid ocean ridges continent continent collisions plateaux uplift active plate margins extensional rift tectonics and the birth growth and death of individual folds and faults tectonic sedimentology deals with the direct influence of neotectonics on sedimentation and basin development and with the inverse problem of reconstructing the order and development of palaeotectonic structures this review attempts to show how the scope and importance of the subject has developed over the last decade and is divided into five themes i at the largest tectonic scale constraints on mid ocean ridge activity and continent continent collisions come from a precise chemical analyses of fluid inclusions in marine halite deposits dating back to ca 550 ma b sr isotopic ratios of marine phanerozoic calcitic fossils and c chemical composition of archaean and proterozoic banded iron formations ii at regional scales constraints on the timing and extent of continental plateau uplift incision and cratonic tilting are given by a palaeoaltimetry estimates from oxygen isotope depletion in carbonate palaeosols b radiometric dating of palaeowater table speleothems and c preferential river migration iii at basin scales catchment processes are emphasized as key controls on sediment flux from tectonic uplands to sedimentary basins the inadequacy of sediment transport algorithms based on tractive stress and flow power to describe erosion incision and sediment transport during the most formative catchment hydrological events is emphasized iv rates of basin scale fault and fold growth displacement death and basement uplift are provided by application of facies analysis and sequence stratigraphy to accurately dated sediments in sedimentary basins v the effects of changing climate and varying basin base level in modifying tectonic influences on erosion and sedimentation are examined in such cases experimental and numerical modelling using multi variate parameters of subsidence rate sediment supply and sea level change can shed light on the problem future developments will undoubtedly improve such models hopefully establishing full linkage and feedback between sediment water supply and changing climate within an overall tectonic framework that may itself be changing

barium in twilight zone suspended matter as a potential proxy for particulate organic carbon remineralization results for the north pacific abstract this study focuses on the fate of exported organic carbon in the twilight zone at two contrasting environments in the north pacific the oligotrophic aloha site 22 45 n 158 w hawaii studied during june july 2004 and the mesotrophic subarctic pacific k2 site 47 n 161 w studied during july august 2005 earlier work has shown that non lithogenic excess particulate ba baxs in the mesopelagic water column is a potential proxy of organic carbon remineralization in general baxs contents were significantly larger at k2 than at aloha at aloha the baxs profiles from repeated sampling five casts showed remarkable consistency over a period of three weeks suggesting that the system was close to being at steady state in contrast more variability was observed at k2 six casts sampled reflecting the more dynamic physical and biological conditions prevailing in this environment while for both sites baxs concentrations increased with depth at k2 a clear maximum was present between the base of the mixed layer at around 50 and 500 m reflecting production and release of baxs larger mesopelagic baxs contents and larger bacterial production in the twilight zone at the k2 site indicate that more material was exported from the upper mixed layer for bacterial degradation deeper compared to the aloha site furthermore application of a published transfer function dehairs f shopova d ober s veth c goeyens l 1997 particulate barium stocks and oxygen consumption in the southern ocean mesopelagic water column during spring and early summer relationship with export production deep sea research ii 44 1 2 497 516 relating oxygen consumption to the observed baxs data indicated that the latter were in good agreement with bacterial respiration calculated from bacterial production these results corroborate earlier findings highlighting the potential of baxs as a proxy for organic carbon remineralization the range of poc remineralization rates calculated from twilight zone excess particulate ba contents also compared well with the depth dependent poc flux decrease as recorded by neutrally buoyant sediment traps except in one case out of four this discrepancy could indicate that differences in sinking velocities cause an uncoupling of the processes occurring in the fine suspended particle pool from those affecting the larger particle pool which sustains the vertical flux thus rendering comparison between both approaches risky

coarse sediment transport in mountain streams in colorado and wyoming usa since the early 1990s us forest service researchers have made thousands of bedload measurements in steep coarse grained channels in colorado and wyoming usa in this paper we use data from 19 of those sites to characterize patterns and rates of coarse sediment transport for a range of channel types and sizes including step pool plane bed pool riffle and near braided channels this effort builds upon previous work where we applied a piecewise regression model to 1 relate flow to rates of bedload transport and 2 define phases of transport in coarse grained channels earlier the model was tested using bedload data from eight sites on the fraser experimental forest near fraser colorado the analysis showed good application to those data and to data from four supplementary channels to which the procedure was applied the earlier results were however derived from data collected at sites that for the most part have quite similar geology and runoff regimes in this paper we evaluate further the application of piecewise regression to data from channels with a wider range of geomorphic conditions the results corroborate with those from the earlier work in that there is a relatively narrow range of discharges at which a substantial change in the nature of bedload transport occurs the transition from primarily low rates of sand transport phase i to higher rates of sand and coarse gravel transport phase ii occurs on average at about 80 per cent of the bankfull 1 5 year return interval discharge a comparison of grain sizes moved during the two phases showed that coarse gravel is rarely trapped in the samplers during phase i transport moreover the movement and capture of the d16 to d25 grain size of the bed surface seems to correspond with the onset of phase ii transport particularly in systems with largely static channel surfaces however while there were many similarities in observed patterns of bedload transport at the 19 studied sites each had its own bedload signal in that the rate and size of materials transported largely reflected the nature of flow and sediment particular to that system published in 2005 by john wiley sons ltd