long term slowdown of river tracer pebbles generic models and implications for interpreting short term tracer studies 1 we propose generic approaches to modeling the reduction in mobility that occurs as tracer pebbles seeded on the surface are mixed to less active locations e g beneath the bed or high on bars or advected downstream to reaches with different shear stress or bed grain size advection slowdown which may be negligible is quantified using a regression equation relating virtual velocity to tracer and reach properties mixing slowdown which is always present is simulated using a markov model of exchanges between three stores and assumptions about relative mean velocities of tracers in each store the model can be calibrated from a full search on one date and at least a surface search on another date calibration to six reaches of allt dubhaig scotland in which tracers were mapped after 2 and 8 years suggests mixing took 3 5 years to approach equilibrium in five reaches and was still proceeding in one simulated slowdown through mixing is far greater than that through advection and goes much of the way to explaining the 50 observed reduction in virtual velocity over 8 years compared to 2 years implications for the design and comparison of tracer pebble experiments are considered

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

morphological effects of different channel forming discharges in a gravel bed river the study analyses the morphological response of a gravel bed river to discharges of different magnitude from moderate events that occur several times a year to a 12 year flood and so defines the range of formative discharges for single morphological units channels bars islands and a range of magnitude of morphological activity from the threshold discharges for gravel transport and minor bar modification up to flows causing major morphological changes the study was conducted on the tagliamento river a large gravel bed river in north eastern italy using two different methods analysis of aerial photographs and field observation of painted gravel particles the available photographs five flights from august 1997 to november 2002 and the two commissioned flights june 2006 and april 2007 do not define periods with a single flood event but the intervals are short enough 11 to 22 months to have a limited number of flood events in each case the fieldwork which involved cross section survey grain size analysis and observation of painted sediments complemented the aerial surveys by allowing analysis of channel response to single flood events substantial morphological changes e g bank erosion of several tens of metres up to more than 100 m associated with flood events with a recurrence interval between 1 1 year and 12 years have been documented multiple forming discharges were defined based on the activity of different morphological units discharges equal to 20 50 of the bankfull discharge are formative for the channels whereas the bankfull discharge 1 1 year flood in this case of the tagliamento river is formative for low bars larger floods but still relatively frequent with a recurrence interval less than five years are required for full gravel transport on high bars and significant morphological changes of islands copyright 2009 john wiley sons ltd

seismicity alert probabilities at parkfield california revisited for a decade the u s geological survey has used the parkfield earthquake prediction experiment scenario document to estimate the probability that earthquakes observed on the san andreas fault near parkfield will turn out to be foreshocks followed by the expected magnitude 6 mainshocks during this time we have learned much about the seismogenic process at parkfield about the long term probability of the parkfield mainshock and about the estimation of these types of probabilities the probabilities for potential foreshocks at parkfield are reexamined and revised in light of these advances as part of this process we have confirmed both the rate of foreshocks before strike slip earthquakes in the san andreas physiographic province and the uniform distribution of foreshocks with magnitude proposed by earlier studies compared to the earlier assessment these new estimates of the long term probability of the parkfield mainshock are lower our estimate of the rate of background seismicity is higher and we find that the assumption that foreshocks at parkfield occur in a unique way is not statistically significant at the 95 confidence level while the exact numbers vary depending on the assumptions that are made the new alert probabilities are lower than previously estimated considering the various assumptions and the statistical uncertainties in the input parameters we also compute a plausible range for the probabilities the range is large partly due to the extra knowledge that exists for the parkfield segment making us question the usefulness of these numbers

bedload entrainment travel lengths step lengths rest periods studied with passive iron magnetic and active radio tracer techniques this paper reports results from bedload transport investigations with active radio and passive iron magnetic tracers in the lainbach a step pool mountain river in bavaria southern germany the spatial distributions of the iron tracers after flood events can be best described by exponential or gamma distributions there is some indication of a tendency of size selective transport of the iron tracers but there is also a considerable amount of scatter in the correlations between weight size and travel length owing to the masking influence of other variables such as the shape of the particles and different positions in the river bed the experiments with artificial magnetic tracers showed that elongated pebbles rods had the longest mean transport distance platy ones discs remained relatively close to the starting points the particles from the pool showed the greatest transport lengths and a 100 per cent chance of being eroded the pebble transmitter system petsy consists of transmitters implanted into individual pebbles a computerized receiver a stationary antenna system with an antenna switchboard and a data logging system the particles do not move continuously but in a series of transport steps and non movement intervals a single value for a given size class is not adequate to describe the critical conditions of entrainment under natural circumstances a probability approach is much more suitable the critical unit discharges total discharge divided by active channel width along the measuring reach are dependent on river bed morphology in the steps bedload needs higher unit discharges to be entrained once entrained the transport of bedload is stochastic in nature and the single particle transport is controlled by the step lengths and the duration of rest periods the distributions of both parameters can be approximated by exponential functions applying the stochastic concept proposed by einstein the mean values of the random variables step length and duration of rest period measured with the petsy technique were used for the simulation of spatial distributions of bedload particles from point sources more field and laboratory data are needed to include varying flow and roughness conditions with tracers representing different particle characteristics

size and distance of travel of unconstrained clasts on a streambed we have examined the relation between the mean distance of movement during individual flood events and the mean particle size of unconstrained clasts in gravel bed rivers distance of movement was scaled using the mean distance of movement of the median size group of the surface material size was scaled using the median b axis diameter of the subsurface material the scaled data are fitted with a nonlinear function clasts larger than the median of surface material exhibit a steep decline of mobility with an upper limit for mobility near di d50sub 10 smaller materials exhibit a less sensitive relation we suppose that trapping of small clasts affects their travel distance these results indicate that the movement of free surface stones is size preferred a subordinate analysis of free constrained and buried particles reveals that bed surface structure acts to reduce differential movement

unequal mobility of gravel and sand in weakly bimodal river sediments bed load was trapped during flood events over a 20 month period at the lower end of the allt dubhaig a small river in scotland with rapid downstream fining of gravel bed material on a slowly aggrading concave long profile the channel bed near the trap is predominantly gravel with a secondary sand mode total transport in each event depended mainly on peak shear stress rather than duration over a threshold bed load was mainly sand in smaller events bimodal in intermediate events and mainly gravel in the biggest floods mean and maximum grain diameter both increased with peak shear stress but in different ways analysis of fractional transport rates and maximum grain size in relation to peak shear stress suggests that gravel transport is slightly size selective but sand transport is close to equal mobility the slight selectivity in gravel transport is consistent with previous field studies of near equilibrium unimodal beds and supports assumptions made in the numerical model of hoey and ferguson 1994 which successfully simulates the observed amount of downstream fining over the 2 5 km upstream of the bed load trap

riparian vegetation and fluvial geomorphic processes abstract riparian vegetation and fluvial geomorphic processes and landforms are intimately connected parts of the bottomland landscape relations among vegetation processes and landforms are described here for representative streams of four areas of the united states high gradient streams of the humid east coastal plain streams great plains streams and stream channels of the southwestern united states vegetation patterns suggest that species distributions in the humid east are largely controlled by frequency duration and intensity of floods along channelized streams vegetation distribution is largely controlled by variation in fluvial geomorphic processes cycles of degradation and aggradation in response to increases in channel gradient associated with channelization similarly riparian vegetation of great plains streams may be controlled by fluxes in sediment deposition and erosion along braided streams patterns of riparian vegetation in semi arid regions may be most closely related to patterns of water availability unlike most other streams in more humid environments channel equilibrium conditions control stability of the coincident fluvial landform and attendant vegetation pattern throughout the continent in most situations riparian vegetation patterns are indicative of specific landforms and thus of ambient hydrogeomorphic conditions

observations on sediment mobility in a large gravel bed river article this study investigates sediment mobility in a large gravel bed river tagliamento river northeastern italy field data were used to identify the morphological effectiveness of a range of flows floods with recurrence interval b1 to 3 5 year and for a detailed analysis of the partial transport condition the analyses were carried out on three cross sections where a number of areas representative of different morphological units main and secondary channels low and high bars islands were painted grain size of the painted area was measured using a photographic method monitoring of bed mobility during the study period supplied new photographs of the painted areas measurements of size and travel distance of the mobilized particles and estimate of flow depth after each flood event our analyses have shown that with dimensionless shear stress around 0 073 17 n m 2 the morphological effect over the painted areas was primarily partial transport while with stresses n0 1 25 n m 2 the bed experienced full mobility i e all the painted particles were transported or totally buried full sediment mobility was observed on lower morphological units i e main and secondary channels under floods with very high frequency rib1 y whereas on the low bars full mobility occurred under floods with a recurrence interval slightly n1 year instead fine deposition and partial transport conditions were observed on the higher bars even after the largest of the monitored floods ri 3 5 y the analysis of the partial transport condition relies on more than 3500 measurements of particle travel distance travel distances range from a few centimetres up to 22 m the transported particles were generally finer than the painted sediments and the analysis of the sediment transport ratio revealed that partial transport occurred under equal mobility conditions for particles finer than 32 mm and under size selective mobility for coarser fractions this appears to be corroborated for db32 mm by the insensitiveness of particle transport distance to particle sizes even if the exponent 1 of the relationship between the dimensionless shear stress and the maximum transported diameter would suggest that all fractions are transported under equal mobility conditions

size selective entrainment of bed load in gravel bed streams entrainment of mixed size gravel bed material was studied in nine reaches of three high power streams in scotland and norway paired measurements of at a point shear stress estimated from velocity profiles and bed load transport by hand held sampler were made they extended to very high values 400 n m 2 3 5 kg m x s x analyses of maximum bed load diameter mean bed load diameter transport rates of individual size fractions and tracer pebble movements all show some dependence of threshold shear stress for entrainment on absolute particle size despite strong relative size effects precise equal mobility of all sizes was approached in the data set with the highest shear stresses and transport rates size selective transport in the streams studied is also indicated by clear downstream and downbar reductions in surface sediment size over distances too short for abrasion to be significant

virtual rate and mean distance of travel of individual clasts in gravel bed channels travel distances in gravel bed rivers of tagged particles of various sizes are related to excess stream power estimated for peak discharge mean distance of movement irrespective of grain size is weakly correlated with stream power especially near the threshold of movement there may be several reasons for the weak correlation including variable effects of bed structure varying magnitudes of sediment mobilizing events and sampling problems grain size itself is of marginal significance the virtual rate of travel calculated using total time for which the flow is larger than that needed to initiate clast movement also bears a weak relation to the excess stream power over the period better results are obtained by relating the virtual rate of travel to the first peak of the flow event only this implies that the initial seeding of the tagged particles dominates the observations nonetheless an underlying general relation is indicated by the data which are derived from a wide range of flow regime types

the relationship between shear stress and the shaping of the bed of a pebble loaded river la rulles ardenne summary geomorphological modifications in the bed of a small pebble loaded river were related to different parameters such as flow velocity and shear stress experiments carried out with marked pebbles of known diameter demonstrated that shear stress calculated from the energy grade line slope and the hydraulic radius is a reliable criterion in explaining erosion and transport of the pebble load increase in shear stress with discharge entrainment varies according to site at times of low flow shear stress is weak in the pools allowing transitory accumulation of fine deposits and pebbles these originate mainly from erosion downstream of riffles where shear stress is already considerable at times of discharge near bankfull stage shear stress becomes considerable in the pools so that pebbles deposited at lower discharge rates are then displaced however the coarser elements are halted on the counter slope of the pools and upstream of riffles since shear stress there is then weaker than in the pools riffles thus appears as stopping places with only small elements 1 5 cm diameter able to cross them at times of high discharge the size of these elements thus provides an estimate of the true competence of the river

entrainment displacement and transport of tracer gravels the mass and size distribution of grain entrainment per unit bed area may be measured by replacing a volume of the bed with tracer gravels and observing the mass difference before and after a transport event this measure of spatial entrainment is relevant to any process involving size selective exchange of sediment between transport and bed and may be directly used in calculations of sediment transport rate using an elementary relation for fractional transport components presented here this relation provides a basis for evaluating tracer data collected by different methods and may be used to provide physical insight regarding the expected behaviour of tracer grains the variation with grain size of total displacement length l ti depends on the degree of mobilization of the individual fractions on the bed surface l ti is independent of d i for smaller fully mobile sizes and decreases rapidly with d i for larger fractions in a state of partial transport in which a portion of the surface grains remain immobile through the flow event the boundary between fully and partially mobile grain sizes increases with flow strength these inferences are supported by values of l ti calculated from flume experiments and provide a physical explanation for a summary relation between l ti and d i based on field data 1997 john wiley sons ltd

sediment transport processes in pool riffle sequences this paper reviews the published data for sediment transport in pool riffle sequences which suggests that a velocity or shear stress reversal hypothesis does not explain all of the published evidence of sediment transport this conclusion is explored in more detail using observations of sediment transport from the regulated river north tyne sediment transport rates at discharges of 30 per cent bankfull are initially highest on riffles as discharge increases sediment transport rates in pools rise sharply and values locally exceed those on riffles tracing experiments using a combination of magnetic and painted tracers are used to show the downstream dispersal of fine and coarse sediments through the pool riffle sequence coarser particles experience longer transport paths and faster virtual rates of travel in pools during bankfull floods analysis of entrainment data reveals evidence for additional restraint operating on riffles and causing higher dimensionless entrainment thresholds an appraisal of the possible mechanisms controlling sediment entrainment transport and deposition in pool riffle sequences is made which reveals evidence for the operation of a combination of hydraulic sedimentological and interactive processes a descriptive model of sediment transport processes in pool riffle sequences is presented

partial transport of a sand gravel sediment grains of a single size within a mixed size bed are entrained over a range of flows within this range some grains exposed on the bed surface are active entrained at least once over the duration of a transport event while the remaining surface grains are immobile a condition we define as partial transport we demonstrate the existence and domain of partial transport using observations of grain entrainment on time series of bed photographs of flume experiments with a widely sorted sand gravel mixture the active proportion of the bed surface increases with bed shear stress 0 at a given 0 90 of the active grains are entrained when the cumulative mass transported exceeds approximately 4 times the active mass on the bed mobilization of grains in a size fraction increases from 10 to 90 over a range of 0 of a factor of 2 the bounds of this range increase with grain size di so that at a given 0 sizes over a range of a factor of 4 are in a state of partial transport fractional transport rates are independent of di for fully mobilized fractions and decrease rapidly with di for partially mobile fractions partial transport is associated with substantial transport rates of finer fully mobile sizes limits both the rate and size distribution of grain exchange with the bed subsurface and may be the dominant transport condition in many gravel bed rivers

mobility of river tracer pebbles over different timescales 1 tracer pebbles are widely used to learn about gravel transport along rivers movement over short times and distances is dominated by factors controlling entrainment relative particle size and shear stress movement at longer scales also involves depositional factors burial and reexposure and exchange between channels bars and other depositional environments we mapped mixed size tracers in six reaches of a small scottish river after 2 and 8 years to investigate differences in relative and absolute mobility and infer the importance of burial and exchange patterns of relative mobility according to size and shear stress both within and between reaches did not change significantly some local bunching of tracers was apparent in both surveys with redistribution from pools into riffles and bars the main change was that virtual velocities were 50 lower and estimated gravel fluxes were also lower in the longer term this slowdown is attributed to vertical mixing giving decreased mobility as surface seeded tracers become buried long term storage in bars and other less active parts of the system and in this channel advection of tracers downstream onto a finer bed giving higher relative size

bedload entrainment in low gradient paraglacial coastal rivers of maine u s a implications for habitat restoration abstract the rivers of coastal maine flow through mainstem lakes and long low gradient reaches that break the continuum of bedload transport expected in nonparaglacial landscapes stream erosion of glacial deposits supplies coarse sediment to these systems the land use history includes intensive timber harvest and associated dam construction which may have altered the frequency of substrate mobilizing events these watersheds are vital habitat for the last remaining wild anadromous atlantic salmon in the united states future adjustments in channel morphology and habitat quality via natural stream processes or restoration projects depend on erosion transport and deposition of coarse sediment these factors motivate our study of competence at four sites in the sheepscot and narraguagus watersheds three of the four sites behaved roughly similarly with particle entrainment during intervals that include winter ice and spring flood conditions and relatively minor bed mobilization during moderate floods in the summer and fall with a recurrence interval of 2 3 years the fourth site on the sheepscot river mainstem exhibits more vigorous entrainment of marked particles and more complex three dimensional channel morphology this contrast is partially due to local geomorphic conditions that favor high shear stresses particularly relatively steep gradient but also likely to nourishment of the bedload saltation system by recruitment from an eroding glacial deposit upstream our results suggest that the frequency and magnitude of bedload transport are reach specific depending on factors including local channel geometry upstream sediment supply and transport and formation of anchor ice this presents a challenge for stream practitioners in this region different reaches may require contrasting management strategies our results underscore the importance of understanding channel processes at a given site and assessing conditions upstream and downstream as a prerequisite for conducting habitat restoration projects

displacement and transport of marked pebbles cobbles and boulders during floods in a steep mountain stream in a small experimental catchment of the dolomites rio cordon 5 km2 field observations have been carried out on the movement of various sized bed material particles displacement length of 860 marked pebbles cobbles and boulders 0 032 d 0 512 m has been measured along the river bed during individual snowmelt and flood events in the periods 1993 1994 and 1996 1998 floods were grouped into two categories the first includes ordinary events which are characterized by peak discharges with a return period of 1 5 years and by an hourly bedload rate not exceeding 20 m3 h 1 the second refers to exceptional events with a return period of 50 60 years a flood of this latter type occurred on 14 september 1994 with a peak discharge of 10 4 m3 s 1 and average hourly bedload rate of 324 m3 h 1 the variation according to grain size of total displacement length li depends on the degree of mobilization of the individual fractions of the bed surface li is independent of di for smaller fully mobile grain sizes and decreases rapidly with di for larger fractions in a state of partial transport sustained selective transport without a supply of sediment from upstream leads to the development of a stable coarse armoured surface through progressive winnowing of finer material from the bed surface with supply unlimited conditions for transport both the occurrence of extreme events and the duration of a sequences of ordinary floods play an important role in the degree of mobilization of the individual fractions of the bed copyright 2004 john wiley sons ltd

when does bedload transport begin in steep boulder bed streams the paper presents the results of field measurements of critical conditions for bedload motion conducted in the rio cordon a steep boulder bed stream in the italian alps under conditions of high reynolds numbers and low relative submergence poorly explored before two methods have been used to determine threshold of motion the displacement of marked clasts and the flow competence approach which uses the largest grain size diameter transported by each flood event the high values of confirm the great relevance of non bedload effective shear stresses in step pool streams given by the additional form drag associated with this morphology relative submergence effects on the dimensionless critical shear stress have been quantified by considering the relative submergence ratio rh d84 and the major effect of relative size on the mobility of each particle in steep widely graded bed mixtures has been evaluated finally the dimensionless critical unit discharge has also been adopted in the regression equations as the critical hydraulic parameter because it may represent an easier parameter to use than the critical shear stress for steep rough mountain rivers copyright 2006 john wiley sons ltd

tracer pebble movement along a concave river profile virtual velocity in relation to grain size and shear stress over 1400 tracer pebbles 16 256 mm in diameter were tracked for 2 years in six reaches of allt dubhaig scotland a small gravel bed river along which shear stress and bed surface grain size decrease toward a local base level pebble movement was size selective both within and between reaches within reaches the decrease in mean travel distance with increasing grain size is strongest in the coarse tail of the size distribution particle shape has a minor secondary effect a nondimensional grain velocity averaged over the duration of competent flow is used to compare different size classes and reaches over 90 of its variance is explained by relative grain size and reach shields stress the pattern of size selectivity is consistent with single event tracer results elsewhere bedload trap data from our distal reach and the concept of partial mobility it provides a mechanism for strong downstream fining by selective transport and deposition along rivers in which stress declines toward base level the nondimensional prediction equation for grain velocity may be of use in other rivers but requires testing

entrainment of gravel from naturally sorted riverbed material the critical dimensionless shear stress ci required to entrain a given size particle di was computed from extensive bedload transport measurements in three self formed rivers that have naturally sorted gravel and cobble bed material the value of ci was determined to be significantly affected by the size distribution of the riverbed material for bed particles between 0 3 to 4 2 times the median diameter of the subsurface bed material d50 the average critical dimensionless shear stress ci is equal to ci 0 0834 di d50 0 872 with a coefficient of determination of 0 980 for bed particles larger than 4 2 times d50 ci appears to approach a constant value of approximately 0 020 in a noneroding channel thus ci varies from 0 25 to 0 020 for a given particle depending on the ratio of di to d50 previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0 25 to 0 020 for a given particle size this analysis indicates that virtually all of the variation is due to differences in the subsurface bed material size distribution this analysis shows that ci varies almost inversely proportional to the particle diameter for a nonuniform bed material therefore bed particles between 0 3 to 4 2 times d50 are entrained at nearly the same discharge an investigation of the hydraulic characteristics and bed material mobility of 24 self formed gravel bed rivers in colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less particles as large as the 90th percentile fraction were entrained by the bankfull discharge in nine of the rivers studied consequently in these rivers a significant fraction of the riverbed was entrained rather frequently over a period of years although the transport rate was very small