local scale variability of seepage and hydraulic conductivity in a shallow gravel bed river seepage rate and direction measured with a seepage metre modified for use in flowing water were greatly variable along a 300 m reach of a shallow gravel bed river and depended primarily on the local scale bed topography the median value of seepage measured at 24 locations was 24 cm day but seepage measured at specific sites ranged from 340 to 237 cm day seepage also varied substantially over periods of hours to days and occasionally reversed direction in response to evolution of the sediment bed vertical hydraulic conductivity was related to seepage direction and was larger during upward seepage than during downward seepage with differences ranging from 4 to 40 in areas of active sediment transport to more than an order of magnitude in areas where current was too slow to mobilize bed sediment seepage was poorly related to hydraulic gradient measured over vertical distances of 0 3 m and appeared to be opposite the hydraulic gradient at 18 of the locations where both parameters were measured results demonstrate the scale dependence of these measurements in coarse grained hyporheic settings and indicate that hydraulic gradients should be determined over a much shorter vertical increment if used to indicate exchange across the sediment water interface published in 2009 by john wiley sons ltd

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

silk producing stream insects and gravel erosion significant biological effects on critical shear stress there is growing evidence for impacts of small stream organisms on the transport of fine sediments however such impacts on the transport of coarser sediments remained unconsidered therefore we studied whether a silk producing stream insect hydropsyche siltalai consolidated gravel through silk fixed among gravel pieces we exposed gravel for 2 months in a stream along a gradient of hydropsyche abundance and measured the critical shear stress for this gravel in a laboratory flume the critical gravel shear stress was very significantly p 7 7 10 11 related to a variable overall biomass describing gravel consolidation by hydropsyche silk we showed that hydropsyche in a range of abundance typical of many streams increases critical shear stress for gravel by as much as a factor of 2 surface gravel arrangement i e the spatial distribution and orientation of different gravel sizes and forms had a less significant impact on critical shear stress while other sediment variables so far considered in erosion had no significant impact future studies on the transport of solids and on habitat conditions should consider the high potential of stream invertebrates to consolidate gravel

changes in hydrologic regime by dams dams have major impacts on river hydrology primarily through changes in the timing magnitude and frequency of low and high flows ultimately producing a hydrologic regime differing significantly from the pre impoundment natural flow regime this paper presents the analysis of pre and post dam hydrologic changes from dams that cover the spectrum of hydrologic and climatic regimes across the united states our overall goals are to document the type magnitude and direction of hydrologic shifts because of impoundment using the entire database for the national inventory of dams nid for dams possessing longstanding u s geological survey usgs gages downstream we identified 21 gage stations that met length of record criteria encompassing an array of types of dams and spanning four orders of magnitude in contributing watershed area to assess hydrologic changes associated with dams we applied a hydrologic model the indicators of hydrologic alteration iha supplemented with orientation statistics for certain hydrograph parameters dams had significant impacts on the entire range of hydrologic characteristics measured by iha for many characteristics the direction and significance of effects were highly consistent across the 21 sites the most significant changes across these sites occurred in minimum and maximum flows over different durations for low flows the 1 day through 90 day minimum flows increased significantly following impoundment the 1 day through 7 day maximum flows decreased significantly across the sites at monthly scales mean flows in april and may tend to decline while mean flows in august and september increase other significant adjustments included changes in annual hydrograph conditions primarily in the number of hydrograph reversals that has generally increased for almost all sites following impoundment the number of high pulses has increased following impoundment but the average length declines the mean rate of hydrograph rise and fall has declined significantly these results indicate that the major pulse of dam construction during the previous century has modified hydrologic regimes on a nationwide scale for large and small rivers

modelling of fine and coarse sediment interaction over alternate bars abstract flume experiments demonstrated an ability to model bed features such as pavement and the alternate bar structure that are commonly encountered in natural gravel bed streams the effect of pool and riffle topography on the infiltration of fines into and their removal from the channel bed was examined fines were deposited first and removed last from the downstream side of the bar and from the pool area when there is no bed motion the flow is capable of completely removing the fines from the pavement but not from the subpavement when the bed is active both pavement and subpavement can be cleaned

marginal bed load transport in a gravel bed stream sagehen creek california marginal bed load transport describes the condition when relatively few bed particles are moving at any time bed particles resting in the shallowest bed pockets will move when the dimensionless shear stress exceeds a value of about 0 020 as increases the number of bed particles moving increases significant motion of bed particles i e when a substantial fraction of the bed particles are moving occurs when exceeds a value of about 0 060 thus marginal bed load transport occurs over the domain 0 020 0 060 marginal bed load transport rates and associated hydraulic characteristics of sagehen creek a small mountain gravel bed stream were measured on 55 days at discharges ranging from slightly less than one half of the bank full discharge to more than 4 times the bank full discharge dimensionless shear stress varied from 0 032 to 0 042 and bed particles as large as the 80th percentile of the bed surface were transported the relation between reference dimensionless shear stress and relative particle protrusion for sagehen creek was determined by varying ri to obtain the best fit of the parker bed load function to the measured transport rates during the period of record water years 1954 1991 the mean annual quantity of bed load transported past the sagehen creek gage was 24 7 tons forty seven percent of all bed load transported during the 38 years of record occurred in just 6 years during 10 of the 38 years of record essentially no bed load was transported the median diameter of bed load was 26 mm compared to 58 mm in the surface bed material

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

downstream changes in the channel geometry of a large gravel bed river 1 field data obtained on a nearly contiguous segment of the colorado river in western colorado and eastern utah are used to examine the mechanisms driving downstream changes in channel geometry measurements characterizing the bank full hydraulic geometry bed material grain size and average channel gradient were made at closely spaced intervals in 10 alluvial and quasi alluvial reaches covering 260 km of the river these data indicate that changes in surface and subsurface grain sizes are small in relation to the change in channel slope over the full length of the study area the median grain size of the surface sediment decreases by a factor of a little more than 2 whereas the average channel slope decreases by a factor of about 5 the decreases in slope and median grain size are offset by a large increase in bank full depth relative to width such that the bank full shields stress b is constant downstream for the reach as a whole b averages 0 049 which is roughly 50 higher than the threshold for bed load transport

geomorphology and american dams the scientific social and economic context american geomorphologic research related to dams is embedded in a complicated context of science policy economics and culture research into the downstream effects of large dams has progressed to the point of theory building but generalization and theory building are from this research because 1 it is highly focused on a few locations 2 it concerns mostly very large dams rather than a representative sample of sizes 3 the available record of effects is too short to inform us on long term changes 4 the reversibility of changes imposed by dam installation and operation is unknown and 5 coordinated funding for the needed research is scarce in the scientific context present research is embedded in a history of geomorphology in government service with indistinct boundaries between bbasic and appliedq research the federal policy that most strongly influences present geomorphological investigations connected with dams is related to habitat for endangered species because the biological aspects of ecosystems are directly dependent on the substrate formed by the sediments and landforms that are influenced by dams the economic context for research includes large amounts of public funds for river restoration along with substantial private investments in dams and geomorphology is central to these expensive issues the cultural context for research is highly contentious and dominated by advocacy procedures that include intense scrutiny of any geomorphologic research related to dams advocates are likely to use the products of geomorphological research to make cases for their own positions d 2005 elsevier b v all rights reserved

effects of sediment transport and seepage direction on hydraulic properties at the sediment water interface of hyporheic settings summary relations between seepage flux and hydraulic properties are difficult to quantify in fluvial settings because of the difficulty in measuring these variables in situ tests conducted in a 1 5 m diameter by 1 5 m tall sediment filled tank indicate that hydraulic gradient increased and hydraulic conductivity k decreased following the onset of downward seepage but both parameters were little changed following the onset of upward seepage reductions in k during downward seepage were more pronounced when surface water current was sufficient to mobilize sediment on the bed averaged ratios of k determined during upward seepage to k determined during downward seepage k up k down through a sand and gravel bed increased from 1 4 to 1 7 with increasing surface water velocity and decreased to slightly greater than 1 when the sediment bed became fully mobile k up k down for tests conducted with a silt veneer on the bed surface was greater than 2 for all but the fastest surface water velocities substantial reductions in k also were associated with a silt floc that formed on the bed surface during and following test runs although the silt floc was typically less than 0 5 mm in thickness most of the hydraulic gradient was distributed across this thin layer k of the thin silt floc was reduced by two to three orders of magnitude relative to the underlying sediment directional bias in k and relation between k and surface water velocity require the presence or absence of a layer of lower k sediment at or near the bed surface without which no reduction in k and corresponding increase in hydraulic gradient can occur at the bed surface the lack of prior observation of the consistent bias in k associated with seepage direction is somewhat surprising given the numerous studies where k has been measured in fluvial settings but may be explained by the small value of the bias relative to the typical uncertainty associated with field determinations of k if shown to exist in field settings this bias and its relation to fluvial processes will be relevant to many studies conducted in hyporheic settings that require determination of fluxes across the sediment water interface

temporal and spatial scales of geomorphic adjustments to reduced competency following flow regulation in bedload dominated systems abstract because of the combined effects of reduced sediment transport capacity and competency following flow regulation morphological changes are expected to occur in channels downstream from dams and specifically at tributary junctions where local inputs of water and sediment occur using a combination of historical aerial photographs mainstem and tributary channel pebble counts and hec ras flow modeling for two watersheds in south central vt one unregulated and the other regulated since 1961 we document the time series of post regulation channel narrowing and associated bar growth due to the influx of tributary sediment channel adjustments at regulated tributary junctions have been significant in ca 50 years following impoundment with channels downstream of the confluences narrowing over 15 after an initial ca 20 year lag before the onset of accelerated narrowing moreover flow modeling suggests that downstream of regulated confluences the modern median grain size d50 along the channel bed is immobile no significant channel narrowing has occurred either above or below unregulated tributary junctions or on the mainstem upstream of regulated confluences however greater channel sediment fining is observed upstream of regulated confluences than above unregulated confluences thus the primary mode of mainstem channel adjustment differs up and downstream of regulated tributaries these confluence effects have occurred where the tributary drainage area is only 0 2 times that of the mainstem well below the threshold ratio of 0 6 required for significant geomorphic effects at unregulated confluences highlighting the geomorphic scale shift of dams lastly we evaluate the downstream length required for a river to recover from the impacts of impoundment and demonstrate that even distal locations are impacted by flow regulation unlike the impacts of flow regulation in the western us where channel incision and bar erosion predominate following impoundment we find that in situations where bed incision is minimal and where sediment loads are low but bed caliber high bar growth and channel narrowing are significant adjustments at tributary junctions following impoundment therefore at our sites the effects of dams on reduced competency may be more profound than on reduced sediment transport capacity highlighting the importance of geologic and geomorphic settings in understanding fluvial responses to impoundment

critical discharge for entrainment of poorly sorted gravel entrainment criteria for gravel sized particles on river beds are usually defined in terms of shear stress but some workers have proposed that critical discharge relationships are more reliable and more convenient the critical shear stress in poorly sorted gravels is known from recent work to depend much more on relative than absolute grain size it is shown that this effect can be included in a semi theoretical equation for the critical unit discharge to move individual grain sizes in a poorly sorted bed comparisons with published data on critical discharges in a variety of rivers show that both the sensitivity of critical discharge to grain size and the absolute levels of critical discharge are predicted quite well however any prediction of critical discharge is sensitive to the assumptions made first about flow resistance and secondly about the critical stress to move average sized particles

transport of moderately sorted gravel at low bed shear stresses the role of fine sediment infiltration a reliable estimation of sediment transport in gravel bed streams is important for various practical engineering and biological studies e g channel stability design bed degradation aggradation restoration of spawning habitat in the present work we report original laboratory experiments investigating the transport of gravel particles at low bed shear stresses the laboratory tests were conducted under unsteady flow conditions inducing low bed shear stresses with detailed monitoring of the bed topography using a laser scanner effects of bed surface arrangements were documented by testing loose and packed bed configurations effects of fine sediments were examined by testing beds with sand artificial fine sand or cohesive silt infiltrated in the gravel matrix analysis of the experimental data revealed that the transport of gravel particles depends upon the bed arrangement the bed material properties e g size and shape consolidation index permeability and the concentration of fine sediments within the surface layer of moving grains this concentration is directly related to the distribution of fine particles within the gravel matrix i e bottom up infiltration or bridging and their transport mode i e bedload or suspended load compared to loose beds the mobility of gravel is reduced for packed beds and for beds clogged from the bottom up with cohesive fine sediments in both cases the bed shear stress for gravel entrainment increases by about 12 on the other hand the mobility of gravel increases significantly bed shear stress for particle motion decreasing up to 40 for beds clogged at the surface by non cohesive sand particles

bed load transport of mixed size sediment fractional transport rates bed forms and the development of a coarse bed surface layer fractional transport rates bed surface texture and bed configuration were measured after a mixed size sediment had reached an equilibrium transport state for seven different flow strengths in a recirculating laboratory flume fractional transport rates were also measured at the beginning of each run when the bed was well mixed and planar the start up observations allow us to describe the variation of fractional transport rates with bed shear stress for a constant bed surface texture and bed configuration the start up and equilibrium observations together allow for the first time an unambiguous description of the mutual adjustment among the transport the bed configuration and the bed surface as the transport system moves toward equilibrium we find that a substantial interaction exists among the transport bed surface and bed configuration bed forms and a coarse surface layer coexist over a range of bed shear stress under some flow conditions the size and shape of the bed forms are controlled by the presence of the coarse surface layer at higher flows the coarse surface layer is eliminated by scour in the lee of the bed forms if the bed surface is defined as that over which the bed forms move a coherent relation between the bed surface texture and the transport grain size distribution may be defined at equilibrium the transport rates of all fractions were not equally mobile defined as identical transport and bed grain size distributions although equal mobility was approached for runs in which the bed shear stress was more than twice that for initial motion of the mixture under some flow conditions the transport was observed to adjust away from equal mobility as the bed adjusted from a well mixed start up condition to an equilibrium state development of a partial static armor wherein some individual grains become essentially immobile even though other grains in the same fraction remain in transport is suggested to explain these adjustments between the transport and bed surface grain size distributions constraints on equilibrium mixed size sediment transport are defined the special conditions for which equal mobility must hold and the relevance to natural conditions of flume results and the equal mobility concept are discussed

flow resistance under conditions of intense gravel transport a study of flow resistance was undertaken in a channelized reach of the north fork toutle river downstream of mount st helens washington hydraulic and sediment transport data were collected in flows with velocities up to 3 m s and shear stresses up to 7 times the critical value needed for bed load transport details of the flow structure as revealed in vertical velocity profiles indicate that weak bed load transport over a plane gravel bed has little effect on flow resistance the plane gravel bed persists up to stresses 3 times critical at which point irregular bed forms appear bed forms greatly increase flow resistance and cause velocity profiles to become distorted the latter arises as an effect of flows becoming depth limited as bed form amplitude increases at very high rates of bed load transport an upper stage plane bed appeared velocity profiles measured in these flows match the law of the wall closely with the equivalent roughness being well represented by ks 3d84 of the bed load the effects noted here will be important in very large floods or in rivers that are not free to widen such as those cut into bedrock

geomorphology and endangered fish habitats of the upper colorado river 2 linking sediment transport to habitat maintenance alluvial reaches of the colorado river near grand junction colorado provide important habitat for the endangered colorado squawfish this paper examines recent changes in the geomorphology of the colorado river and addresses the question of what can be done to improve existing fish habitats observations of channel change during periods of above average runoff from 1993 through 1995 indicate minor scour and fill of the main channel but more significant scour and enlargement of side channel and backwater habitats and flushing of fine sediment from the bed modeled relations between discharge and dimensionless shear stress indicate that the threshold of bed load transport 0 03 occurs at about half the bank full discharge these flows are exceeded about 26 days per year on average the bank full discharge produces an average of 0 047 and is exceeded about 8 days per year the width and depth of the channel thus appear to be set by a discharge that produces an average boundary shear stress of 1 5 times greater than the critical shear stress the effective discharge is slightly less than the bank full discharge and approximately 80 of the total sediment load is carried by the highest 10 of all discharges

the influence of relative sediment supply on riverine habitat heterogeneity abstract the diversity of aquatic habitats in streams is linked to physical processes that act at various spatial and temporal scales two components of many that contribute to creating habitat heterogeneity in streams are the interaction between sediment supply and transport capacity and the presence of local in stream structures such as large woody debris and boulders data from previously published flume and field studies and a new field study on tributaries to the south yuba river in nevada county california usa were used to evaluate the relationship between habitat heterogeneity local in stream structural features and relative sediment supply habitat heterogeneity was quantified using spatial heterogeneity measures from the field of landscape ecology relative sediment supply as expressed by the sediment supply transport capacity ratio which controls channel morphology and substrate textures two key physical habitat characteristics was quantified using a dimensionless bedload transport ratio q calculated q values were plotted against an ecologically meaningful heterogeneity index shannon s diversity index measured for each study reach as well as the percent area of in stream structural elements the results indicate two potential mechanisms for how relative sediment supply may drive geomorphic diversity in natural river systems at the reach scale when less mobile structural elements form a small proportion of the reach landscape the supply capacity ratio dictates the range of sediment textures and geomorphic features observed within the reach in these settings channels with a moderate relative sediment supply exhibit the highest textural and geomorphic diversity in contrast when less mobile structural elements are abundant forced local scour and deposition creates high habitat heterogeneity even in the presence of high relative sediment supply

effective sediment transporting discharge from magnitude frequency analysis the long term effectiveness of a geomorphic event of a particular magnitude is the product of the effect of a single event of that magnitude times the frequency with which it recurs magnitude frequency analysis if discharge frequency of a stream is assumed to be log normally distributed and if sediment transport rate is assumed to be a power function of discharge then transport effectiveness attains a maximum value at some discharge termed here the effective discharge that moves the most sediment over the long term the effective discharge and its recurrence interval may be predicted if both assumptions are correct analysis of 55 u s streams demonstrates that the observed and the predicted recurrence intervals of the effective discharge are in poor agreement this lack of agreement results from the failure of the power function of discharge to predict adequately the sediment transport rate above some high discharge threshold or thresholds the observed recurrence interval of the effective discha

the structure of the kohistan arc terrane in northern pakistan as inferred from gravity data abstract modelling of gravity data taken across the kohistan island arc terrane in northern pakistan can be used to constrain the shape and thickness of the arc over 600 new gravity measurements were made across the kohistan island arc terrane in northern pakistan these data were taken along traverses normal to the structures bounding the arc and were reduced to terrain corrected bouguer values the reduced data were then modelled using standard two dimensional modelling techniques the southern margin of the arc the main mantle thrust mmt dips to the north at approximately 45 and gradually flattens out at a depth of 7 9 km the northern margin of the arc the main karkoram thrust mkt also dips towards the north but at a shallower initial angle 15 from the models the arc terrane now appears to be around 7 9 km thick with the thicker sections occurring closer to the southern margin the proposed model in particular the angle of the mmt and the mkt may have been significantly affected by the recent and rapid uplift that is occurring along the nanga parbat haramosh massif

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

channel flow competence and sediment transport in upland streams in southeast australia bedload transport data from planebed and step pool reach types are used to determine grain size transport thresholds for selected upland streams in southeast australia morphological differences between the reach types allow the effects of frictional losses from bedforms microtopography and bed packing to be incorporated into the dimensionless critical shear stress value local sediment transport data are also included in a regime model and applied to mountain streams to investigate whether empirical data improve the delineation of reach types on the basis of dimensionless discharge per unit width q and dimensionless bedload transport q b instrumented planebed and step pool sites are not competent to transport surface median grains d50s at bankfull discharge qbf application of a locally parametrized entrainment equation to the full range of reach types in the study area indicates that the majority of cascades cascade pools step pools and planebeds are also not competent at qbf and require a 10 year recurrence interval flood to mobilize their d50s consequently the hydraulic parameters of the regime diagram which assume equilibrium conditions at bankfull are ill suited to these streams and provide a poor basis of channel delineation modifying the diagram to better reflect the dominant transported bedload size equivalent to the d16 of surface sediment made only slight improvements to reach delineation and had greatest effect on the morphologies with smaller surface grain sizes such as forced pool riffles and planebeds likewise the corey shape factor was incorporated into the regime diagram as an objective method for adjusting a base dimensionless critical shear stress c50b to account for lithologically controlled grain shape on bed packing and entrainment however it too provided only minor adjustments to reach type delineation

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

particle size variations between bed load and bed material in natural gravel bed channels particle sizes of bed load and bed material that represent materials transported and stored over a period of years are used to investigate selective transport in 13 previously sampled natural gravel bed channels the ratio d of median particle size of bed material to the transport and frequency weighted mean of median bed load size decreases to unity with increasing drainage area bank full discharge dimensionless stream power and bed material sorting in channels with high values of d significant volumes of fine bed load are transported during discharges that are less than bank full which is commonly associated with general entrainment of the coarse pavement in many gravel bed channels this indicates transport of fine bed load over a more stable substrate of coarser bed material the apparent breakdown in equal mobility of the bed as a whole may be caused by areal segregation of poorly sorted bed material into superiorly sorted patches of varying mean size likely sources of selectively transported fine bed load include fine patches that have low entrainment thresholds and high virtual particle velocities a simple sediment budget applied to measurements from three channels indicates that velocities of material from fine patches in pools relative to velocities of average bed material are high in low order channels and decrease distally as more bed material that represents the bed as a whole is accessed for bed load by deeper annual scour

effective discharge for suspended sediment transport of the ganga river and its geomorphic implication abstract effective discharge q e for suspended sediment transport in the alluvial reaches of the ganga river in the western ganga plains wgp has been computed using analytical and an alternative magnitude frequency approach thirty years of mean monthly discharge data from various sites of the ganga river have been assessed and the abundance of discharge occurrence has been determined our analysis shows that less than 40 of the flow causes effective sediment transport in the ganga and this can be considered as the effective discharge for suspended sediment transport alternatively 50 of the sediment load for all studied sites is moved by a discharge varying between 14 and 40 of the total discharge effective discharges calculated over the period of record are well below the bankfull discharges q b a few events are close to the bankfull level but with a high return period ri 40 years and therefore not effective to transport most of the available sediments our computation shows that the mean annual discharge ri 2 33 yrs can transport only 0 to 10 of the total available sediments the computation of effective discharge also provided important insights to understand the linkage between hydrology and channel morphology sediment storage and removal processes which are reflected in sediment budget cause changes in cross sectional area channel bathymetry at various sites but the channel margins are not affected a high ratio of bankfull to effective discharge q b q e forces the flow lines to be concentrated to the thalweg position and channels are incised our study also implies that incision and aggradation of the river valley during a relatively long period are caused by changes in effective discharge we argue that the valley incision and filling episodes in the western ganga plains at late quaternary timescales in response to monsoonal fluctuations were primarily affected by changes in the effective discharges of the rivers

hydrological controls on the morphogenesis of low energy meanders cher river france summary river restoration is a key issue for european hydrosystems that were modified over centuries by human activities this is particularly true for numerous low energy rivers flowing in the plateaus of western europe because of this limited energy their potential in terms of autogenic restoration a priori appears strongly restricted this study examines the conditions under which the morphogenesis on one of these systems occurs in relation to various hydrologic conditions two complementary approaches are combined on three reaches of the meandering cher river france firstly we examine at a pluri decadal scale the control of duration frequency and intensity of floods on the planimetric erosion bank retreat mainly secondly we estimate the range of effective discharge for bedload transport the results show that the morphogenesis is controlled by low magnitude hydrological events two major controlling factors are suggested low differential of energy between small and large floods peculiar to low energy rivers of mid latitude and low critical discharges for lateral erosion and bedload mobilization for these reasons the ability of the alluvial cher river to self restore its fluvial dynamics seems to be relatively high