evidence for a mazama like tephra deposited ca 10 000 bp at copper lake banff national park alberta mazama tephra is a widespread mid holocene stratigraphic marker dating to ca 6845 bp and is considered useful for the correlation of holocene events in western north america we present evidence from copper lake alberta for a tephra layer that underlies mazama tephra and appears to be a discrete tephra layer but that is indistinguishable from the conventional mazama tephra by microprobe analysis of glass shards three alternative hypotheses are considered to explain this tephra underlying mazama in situ position of an earlier mazama like tephra recycling of tephras and settling of tephra slabs through the gyttja the size distributions of birch pollen grains indicate that the mazama like tephra is in primary stratigraphic position in the early holocene gyttja and that sediment recycling has not destroyed the integrity of copper lake sediment stratigraphy it is concluded that most probably a mazama like tephra was deposited between 9700 and 10 500 bp if this in situ hypothesis is correct the id

mobility of bed material in harris creek 1 the mobility of bed material is investigated in a small gravel bed river in which the bed is weakly bimodal in texture and relatively stable the bed surface remained intact throughout the freshet and scour was limited to small areas of the bed observations of bedload caught in pit traps and displacement of magnetically tagged particles during two spring freshets permit analysis of the mobility of each size fraction fractional transport rates plot up to three orders of magnitude below the reference transport rate suggested in the literature at low flows no size fraction was fully mobile at intermediate flows shear stress 27 pa sand was fully mobile while larger material remained partially mobile at high flows 43 pa the division between the full and partial mobility regimes occurred around 16 mm which remains finer than the median size of the subsurface material the largest material scarcely moves at mean annual flood magnetic tracers confirmed results obtained from the pit traps three distinct zones of mobility can be defined in fractional transport plots partial mobility full mobility and overpassing suspended critical shear stress for incipient motion varied over an order of magnitude a qualitatively similar result for bimodal sediment was reported by wilcock and mcardell 1993 incipient motion analysis based on the largest grain observed to move yielded an upper envelope implying transport similarity for sizes found on the bed surface and a lower envelope that corresponds with our nonsimilar entrainment criterion

channel adjustments to a succession of water pulses in gravel bed rivers gravel bed rivers commonly exhibit a coarse surface armor resulting from a complex history of interactions between flow and sediment supply the evolution of the surface texture under single storm events or under steady flow conditions has been studied by a number of researchers however the role of successive floods on the surface texture evolution is still poorly understood an experimental campaign in an 18 m long 1 m wide flume has been designed to study these issues eight consecutive runs each one consisting of a low flow period of variable duration followed by a sudden flood water pulse lasting 1 5 h have been conducted the total duration of the experiment was 46 h the initial bed surface was created during a 280 h long experiment focused on the influence of episodic sediment supply on channel adjustments our experiments represent a realistic armored and structured beds found in mountain gravel bed rivers the armor surface texture persists over the duration of the experiment the experiment exhibits downstream fining of the bed surface texture it was found that sorting processes were affected by the duration of low flow between flood pulses since bed load transport is influenced by sediment sorting the evolution of bed load transport is impacted by the frequency of the water pulses short interpulse durations reduce the time over which fine material transported as bed load can be winnowed this in turn contributes to declining reduction of the bed load transport over time while the sediment storage increases

intra event and inter seasonal behaviour of suspended sediment in flash floods of the semi arid northern negev israel abstract a better understanding of the relation between suspended sediment concentration and water discharge is gained by recognizing amongst other things the different impacts of two types of rainfall event wandering discrete convective or convectively enhanced high intensity storms and ubiquitous multi celled low intensity frontal storms in the levant the first type is characteristic of autumn and spring and the flushing of sediment by the instant onset of high intensity rainfall produces a clockwise hysteresis in the suspended sediment water discharge relation the second type is characteristic of winter and can be marked by either anticlockwise or no hysteresis in the suspended sediment water discharge relation sedimentary flushing is rarely apparent suspended sediment concentrations in flash floods resulting from convective storms are high typically 10 4 10 5 mg l 1 but correlation with water discharge is low r 0 57 in contrast frontal storm runoff carries lower suspended sediment concentrations typically 10 3 10 4 mg l 1 and correlation with water discharge is better r 0 78 this division assists in interpreting the high degree of scatter evident in the overall suspended sediment concentration water discharge relation but it reveals that much is yet to be learned about the interplay of the disposition of cellular storm tracks and sediment source areas

stabilizing self organized structures in gravel bed stream channels field and experimental observations stable reticulate structures which we call stone cells have been observed in cobble gravel channel beds with low bed material transport rates experiments show that such structures develop simultaneously with the armor layer during an extended period when flows do not exceed the shields threshold by more than 2 times so that bed material transport is low they are constructed by particles moving from less stable positions into more stable configurations against each other intermediate developments include clusters and stone lines they reduce sediment transport by orders of magnitude and are evidently a major stability promoting mechanism in gravel channels the timescale for their development suggests that the boundaries of many gravel bed channels are not in equilibrium with recent competent flows but reflect the history of recent dominant flows

a continuous holocene glacial record inferred from proglacial lake sediments in banff national park alberta canada abstract sediment cores from three proglacial lakes in northern banff national park alberta preserve a record of holocene glacial activity upvalley which is more continuous and better dated than available surficial records dating of the cores is based on tephrochronology and 16 ams14c ages of terrestrial macrofossils all cores contain a threefold sequence of lacustrine sediments overlying a late pleistocene diamicton basal lacustrine sediments 10 10014c yr old contain little organic matter sediment composition indicates a large glacigenic contribution a sharp increase in organic content marks the beginning of the altithermal interval at all three lakes this transition occurred abruptly at about 10 10014c yr b p at crowfoot lake and possibly more gradually at the other lakes altithermal sediments contain relatively little glacigenic material and during most of the altithermal glaciers may have been absent above crowfoot and bow lakes glaciers draining into hector lake appear to have persisted through the altithermal a subsequent decrease in organic content in each lake reflecting increased clastic sedimentation marks the end of the altithermal and the onset of neoglacial ice advances the transition took place between about 5800 and 400014c yr b p and may be time transgressive beginning earlier in hector lake than in crowfoot lake changing neoglacial clastic sedimentation rates through the neoglacial interval indicate two main periods of increased glacier extent between ca 3000 and 1800 varve yr ago ca 2900 190014c yr b p and during the last several hundred years during the intervening period glaciers were less extensive but much more extensive than during the recessions of the altithermal interval

experimental study of the coarse surface development effect on the bimodal bed load transport under unsteady flow conditions abstractgravel bed rivers display a surface layer that is coarser than the substrate lining below this armored surface the armored layer that is developed under certain flow conditions may be attributed as water worked sediment bed both the threshold of incipient motion and the bed load transport are affected by the presence of this layer in this study the coarse surface development and its effect on the sediment transport were investigated experimentally in a rectangular flume four antecedent flow rates were combined with an input hydrograph without sediment feeding the reference shear stress of individual fractions and the grain size distribution were determined after the development of the coarse surface due to the antecedent flow then a triangular shaped hydrograph was generated for both the intact surface and the coarse surface cases it was seen that the maximum bed load transport values obtained in unsteady flow experiments were highly dependent on the coarse surface formed by the anteceden

the effect of hydrographs on bed load transport and bed sediment spatial arrangement 1 field measurements of sediment transport in gravel bed rivers often reveal hysteretic effects due to differences in sediment availability between the rising and falling limbs of a flood hydrograph however only a small number of flume studies have analyzed the dynamics of sediment transport during hydrographs three types of stepped hydrographs with contrasting durations and magnitudes are simulated here under sediment recirculation conditions bed load transport rate and grain size have been measured continuously the dynamic behavior of the surface armor layer has been explored by analyzing digital photographs and laser scanner surveys of the bed surface taken during hydrographs the results indicate that sediment transport during the falling limb was lower than during the rising limb in all of the three types of hydrographs this reduction is more evident for the low magnitude hydrographs the grain size of the bed remained virtually constant throughout the hydrographs but the grain size of transported sediments exhibited a counterclockwise hysteresis also a significant increase in the reference shear stress for sediment entrainment was measured during the falling limb of hydrographs additionally a detailed analysis of partial transport dynamics of the bed surface sediments reveals a reduction in sediment mobility during the falling limb of the hydrographs the difference in sediment entrainment and transport before and after the peak of the hydrographs appears to be caused by a change in the organization of the surface sediments an analysis of detailed laser scan bed surveys reveals a phase of bed restructuring lower vertical roughness clast rearrangement during the falling limb of hydrographs consequently changes in the degree of organization and complexity of the bed surface are likely the cause of the reduced mobility of sediments and thus of the reduced sediment transport rate during the falling limb of the hydrographs

experiments on downstream fining of gravel i narrow channel runs we present the results of three laboratory experiments in which longitudinally sorted deposits were formed by feeding poorly sorted sediment at the upstream end of a narrow 45 m long channel the input sediment had a median size of 6 mm and included significant amounts of material up to 64 mm and down to 0 2 mm water discharge was constant at 49 l s and sediment discharge varied from 0 048 to 0 19 kg s downstream fining was produced in all three runs the variation in sediment feed rate had relatively little effect on the fining profiles in all three runs the formation of a longitudinally sorted deposit was mediated by the formation of a coarse surface layer the surface layer remained at the top of the deposit during aggradation by continually reforming itself at the deposit surface the coarse surface layer fined by approximately a factor of 2 in d90 and d50 consistently in all three experiments the deposit subsurface fined less with d90 fining more strongly than d50 the short channel length a

width adjustment in experimental gravel bed channels in response to overbank flows 1 we conducted a series of flume experiments to investigate the response of self formed gravel bed channels to floods of varying magnitude and duration floods were generated by increasing the discharge into a channel created in sand and gravel sized sediment with a median grain size of 2 mm flooding increased the shields stress along the channel perimeter causing bank erosion and rapid channel widening the sediment introduced to the channel by bank erosion was not necessarily deposited on the channel bed but was rather transported downstream a process likely facilitated by transient fining of the bed surface at the end of each experiment bank sediments were no longer in motion partial bed load transport characterized the flat bed portion of the channel and the shields stress approached a constant value of 0 056 about 1 2 times the critical shields stress for incipient motion furthermore the discharge was entirely accommodated by flow within the channel the creation of a stable channel entirely eliminated overbank flows we speculate that similar processes may occur in nature but only where bank sediments are non cohesive and where channel narrowing processes cannot counteract bank erosion during overbank flows we also demonstrate that a simple model of lateral bed load transport can reproduce observed channel widening rates suggesting that simple methods may be appropriate for predicting width increases in channels with non cohesive unvegetated banks even during overbank flows last we present a model for predicting the equilibrium width and depth of a stable gravel bed channel with a known channel forming shields stress

origin of hysteresis in bed form response to unsteady flows 1 field and laboratory studies indicate that changes in riverbed morphology often lag changes in water discharge this lagged response produces hysteresis in the relationship between water discharge and bed form geometry to understand these phenomena we performed flume experiments to observe the response of a sand bed to step increases and decreases in water discharge for an abrupt rise in discharge we observed that bed forms grew rapidly by collision and merger of bed forms migrating with different celerities growth rate slowed as bed forms approached equilibrium with the higher discharge regime after an abrupt discharge drop bed form decay occurred through formation of smaller secondary bed forms in equilibrium with the lower discharge which cannibalized the original relict features we present a simple model framework to quantitatively predict time scales of bed form adjustment to flow changes based on equilibrium bed form heights lengths and celerities at low and high flows for rising discharge the model assumes that all bed form collisions result in irreversible merger due to a dispersion of initial celerities for falling discharge we derive a diffusion model for the decay of relict high stage features our models predict the form and time scale of experimental bed form adjustments additional experiments applying slow and fast triangular flood waves show that bed form hysteresis occurs only when the time scale of flow change is faster than the modeled and measured bed form adjustment time we show that our predicted adjustment time scales can also be used to predict the occurrence of bed form hysteresis in natural floods

late quaternary history of the lake o hara region british columbia an evaluation of sedimentation rates and bulk amino acid ratios in lacustrine records lake o hara subalpine and opabin lake alpine are situated directly adjacent to a high section of the continental divide in the central canadian rocky mountains core samples recovered from the lakes show a consistent stratigraphy comprising gyttja and underlying inorganic clastic sediments the gyttja contains bridge river 2350 years bp and mazama 6800 years bp tephras and is separated from the lower clastic sediments by a sharp conformable contact radiocarbon dates obtained from conifer needles extracted from directly above the contact indicate that deglaciation had proceeded upvalley from the o hara basin priorto ca 10 100 years bp preliminary palaeobotanical and macrofossil data suggest that a pinus abies forest with lesser picea was established in the vicinity of lake o hara by this time consequently the minimum age of moraine systems situated downvalley from lake o hara is late wisconsinan mean annual sedimentation rates were derived from sediment thickness data from 14 lake o hara an

evolution of stress fields and faulting in seismic zones measurements indicate that stress magnitudes in the crust are normally limited by the frictional equilibrium on pre existing optimally oriented faults fault zones where these limitations are frequently reached are referred to as seismic zones fault zones in the crust concentrate stresses because their material properties are different from those of the host rock most fault zones are spatially relatively stable structures however the associated seismicity in these zones is quite variable in space and time here we propose that this variability is attributable to stress concentration zones that migrate and expand through the fault zone we suggest that following a large earthquake and the associated stress relaxation shear stresses of a magnitude sufficient to produce earthquakes occur only in those small parts of the seismic zone that because of material properties and boundary conditions encourage concentration of shear stress during the earthquake cycle the conditions for seismogenic fault slip migrate from these stress concentration regions throughout the entire seismic zone thus while the stress concentration regions continue to produce small slips and small earthquakes throughout the seismic cycle the conditions for slip and earthquakes are gradually reached in larger parts of and eventually the whole seismogenic layer of the seismic zone prior to the propagation of an earthquake fracture that gives rise to a large earthquake the stress conditions in the zone along the whole potential rupture plane must be essentially similar this follows because if they were not then on entering crustal parts where the state of stress was unfavourable to this type of faulting the fault propagation would be arrested the proposed necessary homogenisation of the stress field in a seismic zone as a precursor to large earthquakes implies that by monitoring the state of stress in a seismic zone its large earthquakes may possibly be forecasted we test the model on data from iceland and demonstrate that it broadly explains the historical as well as the current patterns of seismogenic faulting in the south iceland seismic zone

transport and storage of bed material in a gravel bed channel during episodes of aggradation and degradation a field and flume study the dynamics of sediment transport capacity in gravel bed rivers is critical to understanding the formation and preservation of fluvial landforms and formulating sediment routing models in drainage systems we examine transport storage relations during cycles of aggradation and degradation by augmenting observations of three events of channel aggradation and degradation in cuneo creek a steep 3 gravel bed channel in northern california with measurements from a series of flume runs modeling those events an armored single thread channel was formed before feed rates were increased in each aggradation run output rates increased as the channel became finer and later widened steepened and braided after feed rates were cut output rates remained high or increased in early stages of degradation as the incising channel remained fine grained and later decreased as armoring intensified if equilibrium was not reached before sediment feed rate was cut then a rapid transition from a braided channel to a single thread channel caused output rates for a given storage volume to be higher during degradation than during aggradation variations in channel morphology and surface bed texture during runs that modeled the three cycles of aggradation and degradation were similar to those observed in cuneo creek and provide confidence in interpretations of the history of change cuneo creek aggraded rapidly as it widened shallowed and braided then degraded rapidly before armoring stabilized the channel such morphology driven changes in transport capacity may explain the formation of flood terraces in proximal channels transport storage relations can be expected to vary between aggradation and degradation and be influenced by channel conditions at the onset of changes in sediment supply published in 2011 this article is a us government work and is in the public domain in the usa

latest pleistocene and early holocene fluctuations of glaciers in the canadian and northern american rockies in the canadian and northern american rockies small scale fluctuations of glaciers during and after late wisconsinan deglaciation have been identified using stratigraphy of superimposed glacial deposits geomorphology and records of glacial advance in lake and peat sediments several authors have proposed that valley glacier readvances of relatively modest scale followed the late wisconsinan maximum evidence for some of these is difficult to evaluate from the literature and no deposits recording such readvances have been numerically dated several minimum limiting radiocarbon dates for deglaciation suggest ice had retreated to within a few tens of km of present limits by ca 12 ka bp and to very near present limits by ca 11 ka bp one or more minor glacial advances or stillstands during recession evidently occurred in valley heads prior to 11 ka bp part of the evidence consists of moraines a few km downvalley from subsequently emplaced little ice age and crowfoot moraines the piper lake moraine in the mission mountains of montana is overlain by glacier peak g tephra and thus has a minimum age of 11 200 14c years bp the crowfoot advance a minor advance of about the same magnitude as little ice age advances occurred between ca 11 300 and 10 000 14c years bp and is thought to be related to the younger dryas climatic reversal crowfoot moraines have been found from jasper national park on the north to the mission mountains on the south crowfoot elas were an average of 40 m lower than modern elas around the canada u s border there have been past proposals of significant early holocene glacial advances in the rockies i e after crowfoot time but before the lia but recent evidence indicates that glaciers then were less extensive than at present

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

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

influence of the sediment supply texture on morphological adjustments in gravel bed rivers the role played by the texture of the sediment supply on channel bed adjustments in gravel bed rivers is poorly understood to address this issue an experimental campaign has been designed flume experiments lasting 96 h in a 9 m long 0 60 m wide have been performed with different sand gravel mixtures as feed textures the response of the surface texture has been found to be highly dependent on the grain size distribution of the feed when the feed texture included gravel the finest fractions of the sediment supply infiltrate beneath the surface conversely sand remains on the surface when the feed texture lacks gravel this different textural response becomes obscured when water discharge increases further the sediment transport rate approaches the feed rate differently depending on the content of gravel in the feed texture when a small proportion of gravel is part of the feed texture bed load transport rate asymptotically approaches the feed rate however when a significant fraction of gravel is part of the feed grain size distribution bed load transport rate approaches the feed rate by following an oscillatory path these findings have been verified in terms of a one dimensional numerical model this modeling reveals that the higher the differences in mobility among the grain sizes contained in the feed texture the more evident is the nonasymptotic transient trend toward equilibrium

linking bed morphology changes of two sediment mixtures to sediment transport predictions in unsteady flows flume experiments were conducted to measure bed morphology adjustments in sand gravel and sand silt sediment mixtures during repeated hydrographs and to link these changes to sediment transport patterns over multiple time scales sediment composition and hydrograph flow magnitude greatly influenced channel morphology which impacted sediment yield hysteresis and transport predictions bed load yields were larger and more variable for the sand silt mixture as gravel in the sand gravel sediment inhibited grain entrainment limited bed form growth and acted to stabilize the bed hysteresis patterns varied due to bed form and surface structure adjustments as well as the stabilizing effect of antecedent low flows using half the data set a dimensionless fractional transport equation was derived based on excess shear stress dimensionless reference shear stresses were estimated in two ways as bulk values from all transport measurements and by applying a separate limb approach in which values were estimated for each limb of each hydrograph for the other half of the data set transport predictions with the separate limb approach were more accurate than those from six existing transport equations and the fractional relationship applied with bulk reference shear stresses thus hydrograph limb dependent dimensionless reference shear stress links changing bed morphology and sediment transport providing a parameter to improve transport predictions during individual flood events and in unsteady flow regimes this approach represents a framework with which to develop site specific transport relationships for varying flow regimes particularly in cases where detailed bed morphology measurements are not feasible and heterogeneous sediment complicates bed structure over time

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

late quaternary environmental change in the lake o hara region yoho national park british columbia abstract palaeoenvironmental reconstructions for the upper cataract brook valley are derived from radiocarbon and tephra dated cores recovered from subalpine lake o hara and alpine opabin lake located near the continental divide in the central canadian rocky mountains late quaternary vegetational and climatic changes are based on pollen and macrofossil data and are supplemented by preliminary diatom data the postglacial colonizing vegetation prior to ca 10 100 yr b p at lake o hara and before ca 8530 yr b p at opabin lake was a shrub herb community dominated by artemisia gramineae and alnus pioneering forests at both sites were composed of pinus cf albicaulis flexilis and abies with lesser abundances of picea and pinus cf contorta timberline remained above the elevation of opabin lake at least 90 m above modern timberline elevation during the period ca 8500 yr b p to ca 3000 yr b p in response to warmer climatic conditions associated with restricted glacial activity forest compositions resembling the modern subalpine picea abies forest had developed by the end of this period the period ca 3000 yr b p to present was marked by deteriorating climatic conditions associated with renewed glacial activity in the opabin cirque and declining timberlines to below the elevation of opabin lake glacigenic sediments in the opabin record from the post bridge river ca 2350 yr b p interval are associated with reduced diatom production and a change in dominant diatom taxa to those characteristic of turbid water conditions and unstable catchment areas

the incidence and nature of bedload transport during flood flows in coarse grained alluvial channels a continuous record reveals that the incidence of bedload in a coarse grained river channel changes from flood to flood long periods of inactivity encourage the channel bed to consolidate sufficiently so that bedload is largely confined to the recession limb of the next flood wave but when floods follow each other closely the bed material is comparatively loose and offers less resistance to entrainment in this case substantial amounts of bedload are generated on the rising limb this is confirmed by values of bed shear stress or stream power at the threshold of initial motion which can be up to five times the overall mean in the case of isolated floods or those which are the first of the season this produces a complicated relationship between flow parameters and bedload and explains some of the difficulties in establishing bedload rating curves for coarse grained channels besides this the threshold of initial motion is shown to occur at levels of bed shear stress three times those at the thresholds of final motion this adds further confusion to attempts at developing predictive bedload equations and clearly indicates at least one reason why equations currently in use are unsatisfactory bedload is shown to be characterized by a series of pulses with a mean periodicity of 1 7 hours in the absence of migrating bedforms it is speculated that this well documented pattern reflects the passage of kinematic waves of particles in a slow moving traction carpet the general pattern of bedload including pulsations is shown to occur more or less synchronously at different points across the stream channel

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