effects of hydraulic roughness on surface textures of gravel bed rivers field studies of forest gravel bed rivers in northwestern washington and southeastern alaska demonstrate that bed surface grain size is responsive to hydraulic roughness caused by bank irregularities bars and wood debris we evaluate textural response by comparing reach average median grain size d50 to that predicted from the total bank full boundary shear stress t0 bf representing a hypothetical reference condition of low hydraulic roughness for a given t0 bf channels with progressively greater hydraulic roughness have systematically finer bed surfaces presumably due to reduced bed shear stress resulting in lower channel competence and diminished bed load transport capacity both of which promote textural fining in channels with significant hydraulic roughness observed values of d50 can be up to 90 smaller than those predicted from t 0 bf we find that wood debris plays an important role at our study sites not only providing hydraulic roughness but also influencing pool spacing frequency of textural patches and the amplitude and wavelength of bank and bar topography and their consequent roughness our observations also have biological implications we find that textural fining due to hydraulic roughness can create usable salmonid spawning gravels in channels that otherwise would be too coarse

temporally and spatially uniform rates of erosion in the southern appalachian great smoky mountains we measured 1 0 be in fluvial sediment samples n 27 from eight great smoky mountain drainages 1 330 km 2 results suggest spatially homogeneous sediment generation on the 10 4 10 5 yr time scale and 100 km 2 spatial scale at 73 11 t km 2 yr 1 equivalent to 27 4 m m y of bedrock erosion this rate is consistent with rates derived from fission track long term sediment budget and sediment yield data all of which indicate that the great smoky mountains and the southern appalachians eroded during the mesozoic and cenozoic at 30 m m y in contrast unroofing rates during the paleozoic orogenic events that formed the appalachian mountains were higher 10 2 m m y erosion rates decreased after termination of tectonically driven uplift enabling the survival of this ancient mountain belt with its deep crustal root as an isostatically maintained feature in the contemporary landscape

the variability of critical shear stress friction angle and grain protrusion in water worked sediments the erodibility of a grain on a rough bed is controlled by among other factors its relative projection above the mean bed its exposure relative to upstream grains and its friction angle here we report direct measurements of friction angles grain projection and exposure and small scale topographic structure on a variety of water worked mixed grain sediment surfaces using a simple analytical model of the force balance on individual grains we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography the friction angle projection and exposure of single grain sizes vary widely from point to point within a given bed surface the variability within a single surface often exceeds the difference between the mean values of disparate surfaces as a result the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution rather than a single value on a given bed the crtitical shear stress distributions of different grain sizes have similar lower bounds but above their lower tails they diverge rapidly with smaller grains having substantially higher median critical shear stresses large numbers of fines trapp ed within pockets on the bed or shielded by upstream grains are effectively lost to the flow our calculations suggest that critical shear stress as conventionally measured is defined by the most erodible grains entrained during transient shear stress excursions associated with the turbulent flow this implies a physical basis for the indeterminacy of initial motion these observations suggest that transport rate shear stress relationships may be controlled in part by the increasing numbers of grains that become available for entrainment as mean shear stress increases they also suggest that bed textures and grain size distributions may be controlled within the constraints of an imposed shear stress and sediment supply regime by the influence of each size fraction on the erodibility of other grain sizes present on the bed

simple method for calculating reach averaged bed load transport abstracta simple and robust method is proposed for calculating reach averaged bed load transport in sand and gravel bed rivers with no calibration the data required are the bed surface d50 and d84 measured with the nontruncated pebble count technique the slope the width and the flow depth or discharge the method is illustrated by comparison with bed load samples measured in 15 river reaches

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

channel widening downstream of valley gorges influenced by flood frequency and floodplain roughness mountain fronts are the locus of significant variations in river width characterized by narrow bedrock gorges opening to wider unconfined alluvial rivers that are often braided the contribution of the abrupt change in river valley confinement in modulating the long term transport capacity and the resulting equilibrium channel width has never been considered here we use a numerical model integrating the full frequency magnitude distribution of streamflow to explore the long term bedload transport capacity of idealized confined and unconfined channels both single thread and braided the model predicts a significant transport capacity loss for a single channel of constant width at the transition between confined gorges and unconfined floodplains that is slightly reduced when floodplain vegetation is present because the total transport capacity of a single thread channel systematically decreases with channel width in a stochastic framework only narrower unconfined channels could compensate for the loss of confinement this prediction contradicts observations of widening ratios ranging from three to eight downstream of gorges in various world rivers we resolve this inconsistency by demonstrating that a braided river made of narrow channels inset in a wide floodplain can maintain the total bedload transport capacity downstream of gorges by increasing the range of competent discharges we also show that riparian vegetation may only enhance bedload transport capacity for highly variable discharge regimes and discuss the relevance of various definitions of representative discharges these results point to the previously unrecognized role of valley confinement in modulating the sediment transport capacity of rivers

the partitioning of the total sediment load of a river into suspended load and bedload a review of empirical data the partitioning of the total sediment load of a river into suspended load and bedload is an important problem in fluvial geomorphology sedimentation engineering and sedimentology bedload transport rates are notoriously hard to measure and at many sites only suspended load data are available often the bedload fraction is estimated with rule of thumb methods such as maddock s table which are inadequately field tested here the partitioning of sediment load for the pitzbach is discussed an austrian mountain stream for which high temporal resolution data on both bedload and suspended load are available the available data show large scatter on all scales the fraction of the total load transported in suspension may vary between zero and one at the pitzbach while its average decreases with rising discharge i e bedload transport is more important during floods existing data on short and long term partitioning is reviewed and an empirical equation to estimate bedload transport rates from measured suspended load transport rates is suggested the partitioning averaged over a flood can vary strongly from event to event similar variations may occur in the year to year averages using published simultaneous short term field measurements of bedload and suspended load transport rates maddock s table is reviewed and updated long term average partitioning could be a function of the catchment geology the fraction of the catchment covered by glaciers and the extent of forest but the available data are insufficient to draw final conclusions at a given drainage area scatter is large but the data show a minimal fraction of sediment transported in suspended load which increases with increasing drainage area and with decreasing rock strength for gravel bed rivers whereby in large catchments the bedload fraction is insignificant at 1 for sand bed rivers the bedload fraction may be substantial 30 50 even for large catchments however available data are scarce and of varying quality long term partitioning varies widely among catchments and the available data are currently not sufficient to effectively discriminate control parameters

multiband x c l radar amplitude analysis for a mixed sand and gravel bed river in the eastern central andes abstract synthetic aperture radar sar amplitude measurements from spaceborne sensors are sensitive to surface roughness conditions near their radar wavelength these backscatter signals are often exploited to assess the roughness of plowed agricultural fields and water surfaces and less so to complex heterogeneous geological surfaces the bedload of mixed sand and gravel bed rivers can be considered a mixture of smooth compacted sand and rough gravel surfaces here we assess backscatter gradients over a large high mountain alluvial river in the eastern central andes with aerially exposed sand and gravel bedload using x band terrasar x tandem x c band sentinel 1 and l band alos 2 palsar 2 radar scenes in a first step we present theory and hypotheses regarding radar response to an alluvial channel bed we test our hypotheses by comparing backscatter responses over vegetation free endmember surfaces from inside and outside of the active channel bed area we then develop methods to extract smoothed backscatter gradients downstream along the channel using kernel density estimates in a final step the local variability of sand dominated patches is analyzed using fourier frequency analysis by fitting stretched exponential and power law regression models to the 2 d power spectrum of backscatter amplitude we find a large range in backscatter depending on the heterogeneity of contiguous smooth and rough patches of bedload material the sar amplitude signal responds primarily to the fraction of smooth sand bedload but is further modified by gravel elements the sensitivity to gravel is more apparent in longer wavelength l band radar whereas c and x band is sensitive only to sand variability because the spatial extent of smooth sand patches in our study area is typically

quantifying human impacts on rates of erosion and sediment transport at a landscape scale establishing background geologic rates of erosion is prerequisite to quantifying the impact of human activities on earth s surface here we present 10 be estimates of background erosion rates for ten large 10 000 100 000 km 2 river basins in the southeastern united states an area that was cleared of native forest and used intensively for agriculture these 10 be based rates are indicative of the pace at which the north american passive margin landscape eroded before european settlement 8 m m y comparing these background rates to both rates of post settlement hillslope erosion and to river sediment yields for the same basins we find that following peak disturbance late 1800s and early 1900s rates of hillslope erosion 950 m m y exceeded 10 be determined background rates more than one hundred fold although large basin sediment yields during peak disturbance increased 5 10 above pre settlement norms rivers at the time were transporting only 6 of the eroded material work by others suggests that the bulk of historically eroded material remained and still remains as legacy sediment stored at the base of hillslopes and along valley bottoms because background erosion rates such as we present here reflect the rate at which soil is generated over millennial time scales they can inform and enhance landscape management strategies

sedinet a configurable deep learning model for mixed qualitative and quantitative optical granulometry i describe a configurable machine learning framework to estimate a suite of continuous and categorical sedimentological properties from photographic imagery of sediment and to exemplify how machine learning can be a powerful and flexible tool for automated quantitative and qualitative measurements from remotely sensed imagery the model is tested on a large dataset consisting of 400 images and associated detailed label data the data are from a much wider sedimentological spectrum than previous optical granulometry studies consisting of both well and poorly sorted sediment terrigenous carbonate and volcaniclastic sands and gravels and their mixtures and grain sizes spanning over two orders of magnitude i demonstrate the model framework by configuring it in several ways to estimate two categories describing grain shape and population respectively and nine numeric grain size percentiles in pixels from a single input image grain size is then recovered using the physical size of a pixel finally i demonstrate that the model can be configured and trained to estimate equivalent sieve diameters directly from image features without the need for area to mass conversion formulas and without even knowing the scale of one pixel thus it is the only optical granulometry method proposed to date that does not necessarily require image scaling the flexibility of the model framework should facilitate numerous application in the spatio temporal monitoring of the grain size distribution shape mineralogy and other quantities of interest of sedimentary deposits as they evolve as well as other texture based proxies extracted from remotely sensed imagery

on the patterns and processes of wood in northern california streams abstract forest management and stream habitat can be improved by clarifying the primary riparian and geomorphic controls on streams to this end we evaluated the recruitment storage transport and the function of wood in 95 km of streams most drainage areas 2 in northern california crossing four coastal to inland regions with different histories of forest management managed less managed unmanaged the dominant source of variability in stream wood storage and recruitment is driven by local variation in rates of bank erosion forest mortality and mass wasting these processes are controlled by changes in watershed structure including the location of canyons floodplains and tributary confluences types of geology and topography and forest types and management history average wood storage volumes in coastal streams are 5 to 20 times greater than inland sites primarily from higher riparian forest biomass and growth rates productivity with some influence by longer residence time of wood in streams and more wood from landsliding and logging sources wood recruitment by mortality windthrow disease senescence was substantial across all sites mean 50 followed by bank erosion 43 and more locally by mass wasting 7 the distances to sources of stream wood are controlled by recruitment process and tree height ninety percent of wood recruitment occurs within 10 to 35 m of channels in managed and less managed forests and upward of 50 m in unmanaged sequoia and coast redwood forests local landsliding extends the source distance the recruitment of large wood pieces that create jams mean diameter 0 7 m is primarily by bank erosion in managed forests and by mortality in unmanaged forests formation of pools by wood is more frequent in streams with low stream power indicating the further relevance of environmental context and watershed structure forest management influences stream wood dynamics where smaller trees in managed forests often generate shorter distances to sources of stream wood lower stream wood storage and smaller diameter stream wood these findings can be used to improve riparian protection and inform spatially explicit riparian management

implications of the saltation abrasion bedrock incision model for steady state river longitudinal profile relief and concavity the saltation abrasion model predicts rates of river incision into bedrock as an explicit function of sediment supply grain size boundary shear stress and rock strength here we use this experimentally calibrated model to explore the controls on river longitudinal profile concavity and relief for the simple but illustrative case of steady state topography over a wide range of rock uplift rates we find a characteristic downstream trend in which upstream reaches are close to the threshold of sediment motion with large extents of bedrock exposure in the channel bed while downstream reaches have higher excess shear stresses and lesser extents of bedrock exposure profile concavity is most sensitive to spatial gradients in runoff and the rate of downstream sediment fining concavity is also sensitive to the supply rate of coarse sediment which varies with rock uplift rate and with the fraction of the total sediment load in the bedload size class variations in rock strength have little influence on profile concavity profile relief is most sensitive to grain size and amount of runoff rock uplift rate and rock strength influence relief most strongly for high rates of rock uplift analysis of potential covariation of grain size with rock uplift rate and rock strength suggests that the influence of these variables on profile form could occur in large part through their influence on grain size similarly covariation between grain size and the fraction of sediment load in the bedload size class provides another indirect avenue for rock uplift and strength to influence profile form copyright 2008 john wiley sons ltd

sediment supply controls equilibrium channel geometry in gravel rivers in many gravel bedded rivers floods that fill the channel banks create just enough shear stress to move the median sized gravel particles on the bed surface d50 because this observation is common and is supported by theory the coincidence of bankfull flow and the incipient motion of d50 has become a commonly used assumption however not all natural gravel channels actually conform to this simple relationship some channels maintain bankfull stresses far in excess of the critical stress required to initiate sediment transport we use a database of 300 gravel bedded rivers and 600 10be derived erosion rates from across north america to explore the hypothesis that sediment supply drives the magnitude of bankfull shear stress relative to the critical stress required to mobilize the median bed surface grain size formula see text we find that formula see text is significantly higher in west coast river reaches 2 35 n 96 than in river reaches elsewhere on the continent 1 03 n 245 this pattern parallels patterns in erosion rates and hence sediment supplies supporting our hypothesis we find a significant correlation between upstream erosion rate and local formula see text at sites where this comparison is possible our analysis reveals a decrease in bed surface armoring with increasing formula see text suggesting channels accommodate changes in sediment supply through adjustments in bed surface grain size as also shown through numerical modeling our findings demonstrate that sediment supply is encoded in the bankfull hydraulic geometry of gravel bedded channels through its control on bed surface grain size

sediment supply and channel morphology in mountain river systems 1 relative importance of lithology topography and climate 1 quantifying landscape scale variations in sediment supply to streams and rivers is fundamental to our understanding of both denudational processes and stream channel morpho dynamics previous studies have linked a variety of sediment supply proxies to climatic topographic or geologic factors but few have connected these directly to the characteristics of fluvial systems draining these landscapes here we correlate landscape controls on sediment supply to observed sedimentology and channel patterns through direct measurements of water and sediment fluxes in over 80 drainage basins ranging in area from 1 4 to 35 000 km2 in the northern rocky mountains usa these data show that the relative sediment supply defined by the bankfull sediment concentration is dominated by basin lithology while exhibiting little correlation to factors such as relief mean basin slope and drainage density bankfull sediment concentrations bed load and suspended load increase as much as 100 fold as basin lithology becomes dominated by softer sedimentary and volcanic rocks relative to basins with more resistant lithologies as bed load concentrations increase stream beds become less armored and bed load grain size coarsens at very high sediment concentrations bed surface subsurface and bed load grain sizes converge and a transition from single thread to to braided channel patterns is commonly observed

channel stability in bed load dominated streams with nonerodible banks inferences from experiments in a sinuous flume 1 experimental and computational evidence shows that bed state adjustments can accommodate a twofold to fourfold change in sediment supply without otherwise affecting the morphology of a sinuous gravel bed stream channel with nonerodible banks in contrast previously published experiments of the same kind but with erodible channel banks suggest that changes in sediment supply are taken up almost exclusively by changes in channel sinuosity and hence the reach average water surface gradient it appears that stream channel response to changes in sediment supply is strongly conditioned by the nature of the stream boundary and that determines which of several potential adjustments are possible

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

effects of sediment supply on surface textures of gravel bed rivers using previously published data from flume studies we test a new approach for quantifying the effects of sediment supply i e bed material supply on surface grain size of equilibrium gravel channels textural response to sediment supply is evaluated relative to a theoretical prediction of competent median grain size d9 50 we find that surface median grain size d50 varies inversely with sediment supply rate and systematically approaches the competent value d9 at low equilibrium transport rates furthermore equilibrium transport rate is a power function of the difference between applied and critical shear stresses and is therefore a power function of the difference between competent and observed median grain sizes d9 and d50 consequently we propose that the difference between predicted and observed median grain sizes can be used to determine sediment supply rate in equilibrium channels our analysis framework collapses data from different studies toward a single relationship between sediment supply rate and surface grain size while the approach appears promising we caution that it has been tested only on a limited set of laboratory data and a narrow range of channel conditions

erosional response to northward propagating crustal thickening in the coastal ranges of the u s pacific northwest we measured basin scale erosion rates using cosmogenic 10 be concen trations in quartz from fluvial sediment in rivers draining the coastal mountain ranges of the u s pacific northwest between 40 and 47 n apparent erosion rates are 0 1 to 0 2 mm yr 1 throughout the oregon coast ranges north of 43 n and increase to the south to 0 6 to 1 1 mm yr 1 in the northern california coast ranges near 40 n we propose that these observations display the erosional response to northward migrating crustal thickening associated with subduction of the mendocino triple junction north south variations in erosion rate range elevation and metrics of landscape relief and steepness are consistent with the hypotheses that i their primary cause is northward migrating crustal thickening ii erosion rates are strongly controlled by topographic relief and weakly if at all controlled by climate and iii the dependence of erosion on relief is nonlinear and obeys a threshold relief relationship

geologic and physiographic controls on bed material yield transport and channel morphology for alluvial and bedrock rivers western oregon the rivers of western oregon have diverse forms and characteristics with channel substrates ranging from continuous alluvial gravel to bare bedrock analysis of several measurable morphologic attributes of 24 valley reaches on 17 rivers provides a basis for comparing nonalluvial and alluvial channels key differences are that alluvial reaches have greater bar area greater migration rates and show systematic correlation among variables relating grain size to bed material transport capacity we relate these differences between channel types to bed material transport rates as derived from a coupled regional analysis of empirical sediment yield measurements and physical experiments of clast attrition during transport this sediment supply analysis shows that overall bed material transport rates for western oregon are chiefly controlled by 1 lithology and basin slope which are the key factors for bed material supply into the stream network and 2 lithologic control of bed material attrition from in transport abrasion and disintegration this bed material comminution strongly affects bed material transport in the study area reducing transport rates by 50 90 along the length of the larger rivers in the study area a comparison of the bed material transport estimates with the morphologic analyses shows that alluvial gravel bed channels have systematic and bounding relations between bed material transport rate and attributes such as bar area and local transport capacity by contrast few such relations are evident for nonalluvial rivers with bedrock or mixed bed substrates which are apparently more influenced by local controls on channel geometry and sediment supply at the scale of western oregon the physiographic and lithologic controls on the balance between bed material supply and transport capacity exert far reaching influence on the distribution of alluvial and nonalluvial channels and their consequently distinctive morphologies and behaviors differences germane for understanding river response to tectonics and environmental perturbations as well as for implementing effective restoration and monitoring strategies

channel responses to varying sediment input a flume experiment modeled after redwood creek california article i nfo storage flume aggradation degradation at the reach scale a channel adjusts to sediment supply and flow through mutual interactions among channel form bed particle size and flow dynamics that govern river bed mobility sediment can impair the beneficial uses of a river but the timescales for studying recovery following high sediment loading in the field setting make flume experiments appealing we use a flume experiment coupled with field measurements in a gravel bed river to explore sediment transport storage and mobility relations under various sediment supply conditions our flume experiment modeled adjustments of channel morphology slope and armoring in a gravel bed channel under moderate sediment increases channel bed elevation increased and sediment output increased but channel planform remained similar to pre feed conditions during the following degradational cycle most of the excess sediment was evacuated from the flume and the bed became armored under high sediment feed channel bed elevation increased the bed became smoother mid channel bars and bedload sheets formed and water surface slope increased concurrently output increased and became more poorly sorted during the last degradational cycle the channel became armored and channel incision ceased before all excess sediment was removed selective transport of finer material was evident throughout the aggradational cycles and became more pronounced during degradational cycles as the bed became armored our flume results of changes in bed elevation sediment storage channel morphology and bed texture parallel those from field surveys of redwood creek northern california which has exhibited channel bed degradation for 30 years following a large aggradation event in the 1970s the flume experiment suggested that channel recovery in terms of reestablishing a specific morphology may not occur but the channel may return to a state of balancing sediment supply and transport capacity published by elsevier b v

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

a systematic analysis of eight decades of incipient motion studies with special reference to gravel bedded rivers data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size t c 50 calculated t c 50 values were stratified by initial motion definition median grain size type surface subsurface or laboratory mixture relative roughness and flow regime a traditional shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition t c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel shields curves with the latter generally underlying the former values derived from competence functions define a separate but poorly developed field while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous rather than time averaged critical shear stresses the available data indicate that for high critical boundary reynolds numbers and low relative roughnesses typical of gravel bedded rivers reference based and visually based studies have t c50 ranges of 0 052 0 086 and 0 030 0 073 respectively the apparent lack of a universal t 50 for gravel bedded rivers warrants great care in choosing defendable t c50 values for particular applications

vertical movements of the crust case histories from the northern appalachian basin evidence of former deep burial of ordovician to devonian strata of the northern appalachian basin has been obtained from various techniques of study including fluid inclusion homogenization temperatures 18 o and vitrinite reflectance diagenetic minerals indicate paleotemperatures of 100 200 c maximum depths of burial were calculated from the estimated paleotemperatures a gradient of 26 c km was assumed silurian strata of the basin are interpreted to have reached maximum burial depths of 5 0 km devonian strata in the catskill mountains had former burial depths of 6 5 km lower ordovician carbonate sequences were buried to 7 km middle ordovician strata had paleodepths of 5 km and devonian carbonate strata had paleodepths from 4 5 to 5 km if these strata were buried deeper than previously thought unexpectedly large amounts of uplift and erosion ranging from 4 3 to 7 km must also have taken place to bring these strata to the present land surface the occurrence of such large scale vertical movements of the crust and lithosphere must be recognized in paleogeographic reconstructions such drastic changes represent isostatic unroofing with widespread implications for paleogeography of a kind unrecognized at present

stable channels with mobile gravel beds regime type equations for mobile gravel bed rivers are presented based on data obtained from 62 stable gravel bed river reaches in the united kingdom multiple regression techniques are applied to derive equations relating reach average and riffle values of width mean and maximum depth slope velocity sinuosity and riffle spacing to bankfull discharge bed and bank material characteristics valley slope bank vegetation type and an independent estimate of bankfull bed load transport rates although discharge has a dominant control on channel geometry these equations indicate that bed load discharge also has a significant influence particularly with regard to channel slope bank vegetation has a major control on width and velocity while depth velocity and slope are storngly affected by bed material size reasons for these results are considered in terms of the physical processes controlling channel adjustment the application of these regime equations is discussed and particular consideration is given to the design of sinuous channels with a pool riffle bed topograph

mountain erosion over 10 yr 10 k y and 10 m y time scales we used cosmogenic 10 be to measure erosion rates over 10 k y time scales at 32 idaho mountain catchments ranging from small experimental watersheds 0 2 km 2 t o large river basins 35 000 km 2 these long term sediment yields are on average 17 times higher than stream sediment fluxes measured over 10 84 yr but are consistent with 10 m y erosion rates measured by apatite fission tracks our results imply that conventional sediment yield measurements even those made over decades can greatly underestimate long term average rates of sediment delivery and thus overestimate the life spans of engineered reservoirs our observations also suggest that sediment delivery from mountainous terrain is extremely episodic sporadically subjecting mountain stream ecosystems to extensive disturbance

assessing the effect of vegetation related bank strength on channel morphology and stability in gravel bed streams using numerical models bank strength due to vegetation dominates the geometry of small stream channels but has virtually no effect on the geometry of larger ones the dependence of bank strength on channel scale affects the form of downstream hydraulic geometry relations and the meandering braiding threshold it is also associated with a lateral migration threshold discharge below which channels do not migrate appreciably across their floodplains a rational regime model is used to explore these scale effects it parameterizes vegetation related bank strength using a dimensionless effective cohesion cr the scale effects are explored primarily using an alluvial state space defined by the dimensionless formative discharge q and channel slope s which is analogous to the q s diagrams originally used to explore meandering braiding thresholds the analyses show that the effect of vegetation on both downstream hydraulic geometry and the meandering braiding threshold is strongest for the smallest streams in a watershed but that the effect disappears for q 106 the analysis of the migration threshold suggests that the critical discharge ranges from about 5 m3 s to 50 m3 s depending on the characteristic rooting depth for the vegetation the analysis also suggests that where fires frequently affect riparian forests channels may alternate between laterally stable gravel plane bed channels and laterally active riffle pool channels these channels likely do not exhibit the classic dynamic equilibrium associated with alluvial streams but instead exhibit a cyclical morphologic evolution oscillating between laterally stable and laterally unstable end members with a frequency determined by the forest fire recurrence interval copyright 2008 john wiley sons ltd

midlatitude ionospheric disturbances during magnetic storms and substorms 1 we present periodic 2 hour magnetospheric and ionospheric oscillations observed with multiple space based and ground based instruments during the severe magnetic storm of 25 september 1998 periodic particle injections were observed at geosynchronous orbit ground magnetometers registered very large as high as 3000 nt magnetic deviations in the nightside auroral zone the millstone hill incoherent scatter radar at magnetic latitude 55 measured periodic enhancements of ionospheric eastward ion velocity from midnight to 1000 mlt the periodic variations in the magnetometers and millstone hill radar were well correlated with the magnetospheric particle injections the observations are consistent with the occurrence of periodic magnetospheric substorms during a magnetic storm during each substorm cycle the ionospheric auroral zone expanded to the latitude of the millstone hill radar so the radar was periodically within the auroral zone and detected the auroral plasma return flows after substorm activity stopped the millstone hill radar measured near noon in phase periodic oscillations of the ionospheric f region electron density and temperature which may be due to particle precipitation from the ring current and or plasmasphere into the midlatitude ionosphere we suggest that magnetospheric substorms during magnetic storms have a cycle time of 2 hours the magnetosphere can sustain similar oscillations for several cycles even though the substorm activity has stopped all the observed periodic variations in the geosynchronous particle injections in the auroral magnetic deviations and in the ionospheric ion velocity are related to the periodic magnetospheric substorms and oscillations

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

smartrock transport in a mountain stream bedload hysteresis and changing thresholds of motion bedload movement is fundamentally probabilistic our quantitative understanding of gravel transport is particularly limited when flow conditions just exceed thresholds of motion in part because of

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

the role of vegetation and large wood on the topographic characteristics of braided river systems riparian vegetation and large wood play a crucial role in shaping rivers on the one hand vegetation tends to increase bank stability reduce channel width and reduce the braiding index in gravel bed rivers conversely large wood tends to increase channel dynamics and promote avulsions the effects of vegetation and large wood have been rarely simulated together in flume experiments in this paper we present a series of experimental runs conducted in a large flume facility using cylindrical wooden dowels and alfalfa seedlings to represent logs and vegetation respectively in order to investigate their role on determining the topographical nature of braided river systems a terrestrial laser scanner was used to measure in detail the topography of the channels and bars and distribution of the elevations and second order structure functions were used to explore the topography that large wood and vegetation imposed on the braided pattern in the flumes results show that vegetation reduced the braiding index and produced wider and deeper channels the standard deviation of the bed elevations was higher than in flumes without vegetation large wood alone was not effective in changing the braiding index or the standard deviation of the elevations when vegetation and large wood acted together in the flume the braiding index reached a minimum value and the skewness of the distribution of elevation increased revealing shallower pools if compared with runs conducted with only vegetation

variation in the reference shields stress for bed load transport in gravel bed streams and rivers 1 the present study examines variations in the reference shear stress for bed load transport r using coupled measurements of flow and bed load transport in 45 gravel bed streams and rivers the study streams encompass a wide range in bank full discharge 1 2600 m3 s average channel gradient 0 0003 0 05 and median surface grain size 0 027 0 21 m a bed load transport relation was formed for each site by plotting individual values of the dimensionless transport rate w versus the reach average dimensionless shear stress the reference dimensionless shear stress r was then estimated by selecting the value of corresponding to a reference transport rate of w 0 002 the results indicate that the discharge corresponding to r averages 67 of the bank full discharge with the variation independent of reach scale morphologic and sediment properties however values of r increase systematically with average channel gradient ranging from 0 025 0 035 at sites with slopes of 0 001 0 006 to values greater than 0 10 at sites with slopes greater than 0 02 a corresponding relation for the bank full dimensionless shear stress bf formulated with data from 159 sites in north america and england mirrors the relation between r and channel gradient suggesting that the bank full channel geometry of gravel and cobble bedded streams is adjusted to a relatively constant excess shear stress bf r across a wide range of slopes