holocene shoreline depositional processes at poverty bay a tectonically active area northeastern north island new zealand abstract holocene depositional processes at poverty bay have been affected by 1 tectonism 2 sea level response to climate change 3 fluvial processes responding to 1 and 2 severe storms and volcanic eruptions the height distribution of radiocarbon dated beach and estuarine shells and wood show that during the holocene relative uplift of the land has occurred at the northeastern margin of the poverty bay flats while subsidence has occurred at the southwestern margin three holocene tectonic subregions are distinguished at the poverty bay flats coast a northern gisborne wainui uplift area a central makaraka awapuni static pivotal area and a southern patutahi muriwai downdrop area periods of significant fluvial upstream degradation into gravel terrace deposits and downstream gravel aggradation are dated at ca late last glaciation 12 000 9000 and 7000 5000 bp the 12 000 9000 bp gravel deposits show tilting to the southwest major erosion and infilling events also occurred at ca 1700 and 300 bp holocene marine transgression and progradation shoreline locations are mapped and the coastal processes are reviewed in terms of the tectonic fluvial and eustatic sea level influences

downstream fining in a rapidly aggrading gravel bed river downstream changes in particle size that occur in the waipaoa river a 104 km long gravel bed river in which rapid aggradation in the historic post 1800 period was triggered by the conversion of native forest to pasture are summarized in this paper the textural data presented are unique for a field situation not only because of the spatial resolution and extent of the sampling program but also because they provide information about the pattern of fining at different points in time they are supported by equally comprehensive topographic survey data from which local rates of aggradation can be derived despite variability induced by lateral sediment inputs there is an essentially continuous pattern of fining along the entire length of the river fining occurs in both the fine and coarse size fractions of the bed material the highest rates of fining occur in the larger percentiles of the subsurface bed material and in the surface bed material downstream fining in the waipaoa river appears to be a response to changes in flow hydraulics that are regulated by the concave configuration of the long profile the fining gradient developed rapidly in 45 years it does not appear to be influenced by the rate of aggradation nor the overall rate of sediment supply to the channel system because in the short term aggradation has a negligible impact on the inherited form of the long profile

field evidence for rapid downstream fining of river gravels through selective transport doubts have been expressed about the ability of either abrasion or sorting to explain strong downstream fining of river gravels we describe pronounced fining over a short distance in a scottish river that has no human disturbance or lateral input of water and sediment measured abrasion rates are far too small to explain observed downstream fining but bed load trap measurements and the dispersion of magnetic tracer pebbles in six subreaches both show sorting during transport the observed downstream fining is also simulated well by a numerical sediment routing model using hydraulic and transport laws consistent with our field measurements the geomorphological cause of the strong fining is slope reduction above a local base level control in this common situation the development of downstream fining is part of the river9s tendency to minimize downstream variation in bed load transport rates and can proceed far more rapidly than the major aggradation otherwise required for equilibration

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

numerical simulation of downstream fining by selective transport in gravel bed rivers model development and illustration a numerical model has been developed for the routing of gravel sized sediment along a river channel which is free to adjust both its long profile and surface texture hydraulic calculations use a step backwater approach and sediment transport is predicted with the method of parker 1990a which uses a low degree of size selectivity exchange of sediment between the surface and subsurface is described using the modified exner equation of parker and sutherland 1990 the model is applied to an idealized channel based on the highly concave allt dubhaig scotland in which fining by particle wear is minor the rapid downstream fining observed in this river is closely matched by model predictions after a time equivalent to 102 years under the present flow regime of the river the evolution of the fining pattern during the model run and associated changes in sediment transport and bed aggradation are described it is concluded that strong profile concavity can force rapid downstream fining even though bed load transport is only slightly size selective this run of the model serves as a basis for testing of the sensitivity of downstream fining to alternative choices of parameter values and boundary conditions which are summarized here and will be described in a subsequent paper

trends of grain sizes on gravel bars in the rio chagres panama abstract we examined the trends of grain sizes along the upper 414 km 2 of the mountainous rio chagres drainage basin in panama gravel bars were sampled along 40 km of the rio chagres and five major tributary streams using a transect pebble count of median diameter lithology and clast rounding although previous investigators have found that downstream fining can be obscured by inputs of colluvial sediment and other local controls in mountain drainages we decided to examine the trends of grain sizes along a tropical mountain river where rapid weathering and high capability of transport might be capable of overriding the input effects of colluvium specifically we tested the hypotheses that downstream fining would be present as a result of selective sorting and that weak felsic particles would decrease in size preferentially to strong mafic particles because of abrasion statistical analyses reveal a weak downstream decrease of sediment size on gravel bars along the study reach of the rio chagres with a sternberg diminution coefficient for felsic and mafic grains of 0 013 and 0 017 respectively felsic clasts have thicker weathering rinds and become rounded downstream faster than mafic particles but tumbling mill tests of abrasion show no significant differences in rate of mass loss in relation to lithology and downstream decreases in grain size are similar between lithologies dividing the study reach into six sub reaches bounded by major tributary junctions we further tested the hypothesis that downstream trends in fining might be obscured at the basin scale by sediment input from tributaries but that trends in grain sizes might be more visible at the reach scale between tributaries we did not find any consistent trends in grain size between tributaries stream width appears to assert a local control on grain size coarse particles are associated with narrow channel reaches whereas smaller particles are associated with wide channel reaches

the morphology of graded gravel rivers a network perspective analytical hydraulic geometry equations are combined with a watershed model to quantify downstrem trends in equilibrium fluvial morphology in gravel streams the hydraulic geometry equations relaet fluvial morphology to discharge bedload transport rate and grain size these independent variables are in turn determined by routing gravel supplied at zero order basins through a channel network as gravel is routed downstream it decreases in size according to an empirical power law the resulting model specifies the discharge bedload transport rate grain size width depth slope and river bed elevation at every point in the drainage basin when the downstream fining law is calibrated using data from central pennsylvania dowstream hydraulic geometry exponents for the width and depth are accurately reproduced longitudinal profiles calculated from the model fit observed profiles in largerwatersheds but are inaccurate in smaller watersheds the model also reproduces hack s 1957 empirical result relating slope to the 0 6 power of a ds where a is the drainage basin area and ds is the mean grain size unfortunately the model requires an equilibrium time scale which is unreasonably long suggesting that the channels of the study area are best described as systems with multiple time scales and multiple rates of response the model probably is successful because the width and depth have relatively short relaxation times and also because downstream trends in discharge and sediment transport rate are controlled by network topology thus downstream hydraulic geometry equations do not necessarily provide strong evidence for the equilibrium of fluvial landscapes rather they reflect the constancy of network topology and the rapid response times of width and depth

slip slidin away a post glacial environmental history of the waipaoa river basin abstract the dramatic changes that occurred to the post glacial landscape in the headwaters of the waipaoa river basin are a consequence of perturbations about the equilibrium that exists between the rate of tectonic uplift and fluvial incision at times when the amount of coarse sediment delivered to channels exceeds the capacity of streams to remove it the channel bed rises at the rate of tectonic uplift once bedload overcapacity is replaced by undercapacity and the alluvial cover is depleted streams reestablish contact with bedrock and recuperate the time lost to fluvial incision the first major perturbation occurred during the final phase of the last glaciation ca 33 17 5 cal ka when aggradation was driven by a climate forced variation in the relative supplies of sediment and water we suggest that the subsequent transformation of channels in the headwaters of the waipaoa river basin from alluvial to bedrock occurred as the atmospheric and oceanic circulation converged on their contemporary patterns ca 12 cal ka a second major perturbation that continues to the present began ca 1910 1912 ce when a massive increase in sediment load was accompanied by a modest increase in water discharge after the native vegetation cover in the headwaters was replaced by pasture the processes of terrace creation and incision are inherently unsteady and in five interim cases incision was arrested by a transient increase in the thickness of the alluvial cover that was a response to climatic forcing events that disrupted the native vegetation cover in the headwaters also modulated patterns of sediment dispersal and accumulation in other parts of the fluvial system and caused rapid storm driven infilling of the poverty bay flats tectonic subsidence dictates the course of the waipaoa river across poverty bay flats which because the modern rate of floodplain construction by vertical accretion is rapid relative to the amount of destruction by lateral channel migration has remained virtually unchanged for the past 100 years during this time the channel assumed a narrower deeper form that is in equilibrium with the contemporary supply of sediment and hydraulic regime

controls of strength and rate of downstream fining above a river base level we report sensitivity tests on a numerical model of the development of downstream fining by size selective transport and deposition above a base level in a gravelbed river with no lateral inputs of water or sediment strong fining requires deposition mainly to the surface otherwise its amount and streamwise distribution are not highly sensitive to any single factor but vary somewhat with hiding factor excess shear stress and bed sorting low initial concavity inhibits fining but different higher concavities generate fairly similar fining profiles lower sediment supply leads to a coarser bed with less fining but there is no simple relationship between fining and aggradation rate the rate of development of fining is sensitive to all these factors but is always rapid initially followed by a far slower approach towards final equilibrium surface grain size adjustment is potentially an important degree of freedom in river response to any environmental change or human impact which causes overloading

freshwater megaflood sedimentation what can we learn about generic processes abstract there is growing recognition that sedimentary deposits related to high energy large scale freshwater floods are widespread across the continents and in the main can be related to quaternary outbreak flows from ice dammed lakes such deposits may also occur in the neogene and earlier recognition of these megaflood deposits is hindered by the lack of well conditioned stratigraphic models of depositional successions however descriptions of successions from many locations often exhibit commonalities when compared carefully thus the primary purpose of this paper is to examine and condense the published stratigraphic and sedimentological evidence so as to identify the key signatures of megaflood successions the deposits often are the only record of a flood and so the secondary purpose is to interpret the sedimentary sequences in order to recreate the behaviour of floods through single and multiple events finally some pointers are provided as to those areas of sedimentological research that might be profitably explored in more depth to improve understanding of megaflood dynamics a key characteristic of megaflood deposits is the apparent massive appearance of single units which on closer inspection usually are amalgamated layers of similar grain size distribution individual beds and sets of beds are commonly near horizontal planar and laterally extensive successions evidently are not channelized and extensivity is indicative of simultaneous and coherent deposition over large areas localised large scale clinoforms or smaller scale cross beds are common associated with bar front or dune progradation respectively but otherwise extensivity implies that cross cutting erosional contacts notably steeply inclined examples and small scale scours are not common except immediately around large isolated clasts such as dropstones and ice blocks a common vertical stack of sequences in any one succession consists of 1 basal thick coarse parallel bedded units 2 large scale clinoforms 3 horizontally bedded thin laminated units 4 ripple and dune cross beds 5 silt beds and 6 succession capping debris flow deposits in some cases such a succession usually is indicative of a single cycle of waxing and waning flood flow generally dominated by high concentration suspension deposition onto plane beds the high power allows the transport of fine gravel and coarse sand in suspension leading upon deposition to relatively coarse beds from suspension fall out clay and silt is generally sparse or absent being transported further down the system at the scale of the hydrograph the successions indicate that flows initially accelerate and then decelerate with significant shorter period flow pulses also evident in the sedimentary signatures in parallel bedded sequences primarily during waxing flows bedload transport often was ineffective in sorting sediment due to the high rates of deposition from the highly sediment charged waters resulting in diffuse accretion surfaces at each location similar sequence stacks are common leading to the impression of rhythmic deposition from pulsing flow including from repetitive series of floods although often composed of coarse grains many sequence stacks excluding any debris flows are not dissimilar to turbidity flow sequences the bouma sequence in particular although there is limited experimental evidence for turbidity currents associated with high energy freshwater flows theory does not preclude the deposition of similar sedimentary motifs from high concentration flows such that an analogy between some megaflood sedimentary beds and turbidites is not unreasonable despite considerable variation in megaflood sedimentary successions commonalities occur which are indicative of similar processes of deposition from high energy floods at geographically different locations the changes in style of deposition throughout a succession provide clues as to the nature of the flood flows and this information could be used to improve estimates of flood hydrograph behaviour however hypotheses linking deposit characteristics to flow dynamics are currently ill conditioned and often poorly tested the majority of published studies describe the coarser scale stratigraphy with only passing reference to the fine scale structure of individual beds closer attention to lamination styles particle orientation and syn depositional deformation structures for example should provide information on the flow mechanics at the time of bed deposition thus better consideration of the sedimentology of megaflood deposit beds will lead to better understanding of the controlling hydraulics and should result in better models of flood deposition

downstream patterns of bed material grain size in a large lowland alluvial river subject to low sediment supply 1 a new data set of bimodal subaqueous channel bed sediments was analyzed for longitudinal patterns in grain size it yielded two interesting observations 1 separate fining trends in d50 exist for gravel and fines that overlap for 175 river kilometers and 2 this overlap in fining trends results in a protracted nonabrupt gravel to sand transition these suggest bed patchiness that is interpreted in the context of inherent grain size bimodality localized hydraulic sorting due to spatially heterogeneous and low sediment supply and local deviations in channel gradient the data also show that tributaries have a minor impact on main stem downstream fining due to basin shape and tectonic history in addition subaqueous channel sediments were observed to be finer than nearby sediments collected from exposed bars which may indicate a narrow active zone of transport where bars are disconnected from sediment movement this research has implications for bed material collection and theories of downstream fining

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

downstream fining in a network of gravel bedded rivers a network based routing model is used to determine how spatial variations in sediment supply influence rates of downstream fining in a small watershed in central pennsylvania gravel of a specified size distribution and lithology is supplied to the network at zero order basins once in transport the total sample weight and moments of the grain size distribution are modified according to empirical functions of transport distance contributions from each zero order basin are summed to determine the total population of sizes and lithologies at any location in the study area resistant quartz sandstone ridges supply sediment an order of magnitude larger than other lithologies because these ridges occur primarily in the basin headwaters spatial variations in supply strongly influence rates of downstream fining model calculations indicate that spatial variations in supply alone account for over 80 of the decrease in mean grain size along the main channel

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

erosion thresholds and suspended sediment yields waipaoa river basin new zealand characteristics of the suspended sediment load in the waipaoa river new zealand and in two of its tributaries mangatu and te arai rivers are examined for evidence of different dominant erosion processes in the basins upstream different shapes of the suspended sediment concentration water discharge relations lead to differences in long term average yield event yield magnitude frequency relations and relative importance of large flows and rare events in the mangatu river frequent runoff events 2 1 are relatively more important to the long term yield 11 540 t km yr half of the long term average load is transported by events with return periods less than 1 year and there is little evidence of an erosion threshold limitation on sediment supply this is consistent with the predominance in the mangatu basin of hillslope erosion processes that involve scour by surface and channelized runoff particularly gully erosion this contrasts with the te arai river where sediment concentration tends to be much lower at low and moderate flows frequent runoff events transport less of the long term yield 4600 t km 2 yr than do rarer large magnitude flood events event sediment yields are an order of magnitude lower during events with subannual return periods and half of the long term average load is transported during events with a return period of 2 years some of these characteristics appear to result in part from two populations of runoff events in the te arai basin nonetheless they are consistent with field evidence that most of the sediment supplied to the te arai stream network is generated by shallow landslides which are activated once a rainfall threshold is exceeded

floodplain construction by recent rapid vertical accretion waipaoa river new zealand the rate of vertical accretion typically 14 18 mm h 1 during eight floods in the waipaoa river basin with recurrence intervals of 5 to 60 years was determined by relating the floodplain stratigraphy at mcphail s bend to the 1948 1995 flood history overbank deposits remaining after a flood that occurred in march 1996 suggest a rate of vertical accretion of 15 mm h 1 by contrast because the flow velocity across the floodplain was too high to permit deposition from suspension during the record flood of march 1988 the rate of vertical accretion was only 6 mm h 1 the sequence of deposition is highly discontinuous and the rapid vertical accretion is a response to a late 19th to early 20th century phase of deforestation in the headwaters that probably initiated a far greater change in suspended sediment yield than in discharge cross section surveys conducted since 1948 indicate that the high suspended sediment load of the waipaoa river also promoted in channel deposition which effected a progressive reduction in bankfull channel width although due to the overbank deposition channel capacity remained constant 1998 john wiley sons ltd

selective grain entrainment by a current from a bed of mixed sizes a reanalysis the selective entrainment of grains by a current from a bed of mixed sizes and densities is important to grain sorting processes that lead to the formation of placers in sands and to armored beds in gravels existing field and laboratory measurements of selective entrainment depart systematically from the standard threshold curves such as that of shields which are based on experiments with nearly uniform grain sizes entrainment measurements from beds of mixed sizes form trends which obliquely cross the threshold curves for uniform grains the crossing point being roughly at the median diameter of the size distribution due to this crossover the coarser size fractions of the distribution require lower flow stresses for their entrainment than if they formed uniform beds while the finer size fractions require higher stresses than uniform beds with deposits having medians in the range of medium sand through gravel the larger the individual particle within the mixed sizes the greater the flow stress required for its entrainment in this case the smaller grains are preferentially entrained possibly leading to bed armoring in contrast if the median is in the fine to medium sand range the smallest grains in the bed of mixed sizes are the most difficult to entrain due to sheltering by the larger grains where grain density is also involved this sorting can lead to the formation of placers a variety of empirical relationships is considered to quantify the evaluation of selective entrainment from deposits of mixed sizes but no single formulation is satisfactory for all data sets

origin of fluvial grain size trends in a foreland basin the pocono formation on the central appalachian basin abstract a widely recognized phenomenon of modern and ancient river systems is downstream decrease in grain size over the past two decades theoretical formulations observations of modern rivers flume studies and numerical models have significantly increased our understanding of sediment transport in mixed grain size river systems these have established that downstream fining in modern and ancient rivers can be attributed to a combination of selective sorting abrasion rate and accumulation rate however no detailed multiple grain size sediment transport model has been used to address how much subsidence rate sediment flux water discharge hydraulic geometry and the mechanics of sediment transport influence grain size distribution and facies belt development in ancient fluvial systems we have combined the results of recent empirical and theoretical studies with a multiple grain size sediment transport model midas to test the sensitivity of downstream fining trends to those controlling mechanisms our results demonstrate that subsidence and sediment feed rate are the most important mechanisms controlling downstream fining trends in a foreland basin if the evolution of hydraulic geometry is known to illustrate the application of this result we have replicated the textural trends of the mississippian upper pocono formation and burgoon sandstone of pennsylvania this methodology can be applied to constrain plausible values of accumulation and subsidence rate for ancient alluvial deposits and enhances our ability to interpret paleohydraulic conditions from such facies in a foreland basin

a comparison of mercurian reflectance and spectral quantities with those of the moon we have examined previously published earth based telescopic spectra of mercury with the goal of obtaining clues to the planet s surface composition the study has three parts first because of suggestions that mercury s crust may be anorthositic we compare near infrared spectra of mercury with spectra of lunar anorthosites the spectral slopes and lack of strong mineralogic absorption features make the mercurian spectra similar to spectra of lunar anorthosites second we compare albedo quantities for mercury and the moon computed from telescopically derived photometric parameters with the laboratory albedos of returned lunar samples the albedos calculated from the photometric parameters are much lower than those of lunar samples measured in the laboratory third the mercurian spectra are interpreted in terms of the current model for the reflectance of an iron bearing silicate regolith on an airless body recent steps toward quantifying this model permit estimates of the feo content and opaque mineral abundance of the surface to be made we find that the mercurian spectra least affected by telluric absorptions imply feo tio2 4

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

grain size along two gravel bed rivers statistical variation spatial pattern and sedimentary links a new set of field data facilitates a detailed analysis of variations in bed material grain size within two confluent gravel bed rivers in northeastern british columbia canada a preliminary assessment of grain size variability establishes a basis for examination of the spatial pattern of grain size change standard anova techniques are inappropriate because individual samples have unequal variances and are not normally distributed alternative tests for homoscedasticity and comparison of means are therefore utilized within site between sample variability is not significant the grain size distributions that were obtained at individual sites are therefore representative of the depositional environments that were sampled in both rivers mean grain size does vary significantly between sites and there is therefore a basis for examining the data for spatial patterns such as downstream fining textural variations along the two rivers studied here are complex and show negligible overall fining in over 100 km this is the consequence of a large number of tributary inputs and non alluvial sediment sources which are the legacy of late pleistocene glaciation the identification of lateral sources like these is fundamental for understanding textural changes within rivers the sedimentary link a channel reach between significant lateral sediment inputs provides a means of isolating fluvial maturation processes abrasion and sorting from contingent lateral inputs strong fining trends are apparent in most links and classification of grain size measurements according to their location within particular links greatly improves the statistical explanation of textural variation identification of sedimentary links provides a means of applying models of fluvial fining processes so isolation of link networks will aid the development of basin scale models of textural variation 1998 john wiley sons ltd

identifying the controls over downstream fining of river gravels abstract a wide range of rates of downstream fining in gravel bed rivers has been reported as a consequence explanations of the phenomenon range from those that consider only abrasion to others that consider only selective sediment transport this apparent confusion results in part from inconsistent definitions of downstream fining analysis of results from a small gravel bed stream is used to show how the method of sediment sampling and the percentile used to characterize fining rates can affect the results when fining is adequately defined a theoretical relationship between fining rate and drainage basin area is derived from established alluvial fan relations this relationship is remarkably consistent with a range of studies by numerous authors using often different methods such results suggest that fining is achieved through the mechanism of selective transport but that the overall control on fining patterns is the volume of sediment supplied from within the drainage basin as such the rate of size decline in sedimentary deposits such as river terraces has the potential to be used to infer past changes in climate tectonics and or relative base level models of this are presented and these are shown to be broadly consistent with observations made in an ancient sandstone body full use of changing rates of fining as paleo environmental indicators requires further work on the rate of adjustment of fining profiles with respect to the rates of change in the forcing environmental conditions

surface based fractional transport rates mobilization thresholds and partial transport of a sand gravel sediment twenty eight coupled observations of flow transport and bed surface grain size distribution were made in a laboratory flume using a wide range of flows and a sediment with a very poorly sorted bimodal grain size distribution these observations permit the transport rates of individual size fractions to be scaled by the proportion of each size immediately available for transport on the bed surface the key to our observations is the use of a sediment in which each size fraction has been painted a different color which permits reliable repeatable and nondestructive measurement of the bed surface grain size distribution from photographs of the bed surface at a given flow the fractional transport rates may be divided into two parts a finer grained portion within which fractional transport rates are a function only of their proportion on the bed surface and total transport rate and a coarser grained portion for which fractional transport rates also depend on the proportion of individual grains within a fraction that remain essentially immobile throughout the experimental run we define the latter condition as one of partial transport and observe that the grain size separating partial and fully mobilized transport consistently increases with flow strength complete mobilization of a size fraction occurs at roughly twice the shear stress necessary for incipient motion of that fraction zones of partial and full mobility are quite distinct when fractional transport rates are scaled by the bed surface grain size distribution although a region of partial transport is evident when these data and other experimental and field observations are scaled by the bulk grain size distribution of the sediment bed critical shear stresses for the incipient motion of individual fractions in our experimental sediment vary over an order of magnitude a result strongly in contrast to many earlier observations but consistent with our observations of incipient motion in sediments with bimodal grain size distributions

a worldwide correlation for exponential bed particle size variation in subaerial aqueous flows the particle size of the bed sediments in or on many natural streams alluvial fans laboratory flumes irrigation canals and mine waste deltas varies exponentially with distance along the stream a plot of the available worldwide exponential bed particle size diminution coefficient data against stream length is presented which shows that all the data lie within a single narrow band extending over virtually the whole range of stream lengths and bed sediment particle sizes found on earth this correlation applies to both natural and artificial flows with both sand and gravel beds irrespective of either the solids concentration or whether normal or reverse sorting occurs this strongly suggests that there are common mechanisms underlying the exponential diminution of bed particles in subaerial aqueous flows of all kinds thus existing models of sorting and abrasion applicable to some such flows may be applicable to others a comparison of exponential laboratory abrasion and field diminution coefficients suggests that abrasion is unlikely to be significant in gravel and sand bed streams shorter than about 10 km to 100 km and about 500 km respectively copyright c 1999 john wiley sons ltd

rock type controls on downstream changes in clast parameters in sandur systems in southeast iceland abstract the role of rock type in controlling downstream variation in clast parameters in non jokulhlaup affected oraefi sandar in iceland was evaluated clast changes in the svinafellsa are simple but the effects of tributary sources complicate the clast parameter distance relationships in the morsardalur kjos system the behavior of individual rock types must be studied to understand the changes both selective sorting and abrasion are taking place and the relative importance of these processes is evaluated the volcanic rocks are rounding relatively rapidly with variation among individual rock types but there are no significant changes in sphericity and clast form with distance the compressive strength of the volcanic suite is variable and significant weathering changes must be takin place during transport from rock face to distal sandur the influence of such weathering changes on changes in clast parameters needs further research

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

slope induced changes in channel character along a gravel bed stream the allt dubhaig scotland changes in channel character along a small river in the scottish highlands are described using measurements in seven reaches over a 3 km length with no significant tributaries but a decline in slope from 0 02 to o ooo15 because of local baselevel control this decline in slope is associated with rapid downstream fining of the gravel bed followed by an abrupt transition to a sand bed the channel pattern alters progressively rather than abruptly in the sequence 1 near braided 2 meandering with active point bar chutes 3 meandering with active outer bank talweg 4 stable equiwidth sinuous the changes in channel pattern and hydraulic geometry are predicted better by rational approaches based on critical shear stress or other physical cdncepts than by purely empirical discriminant or trend equations measurements in five reaches confirm a downstream decrease in shear stress and the amount and calibre of bedload it is argued that the downstream changes in channel character in this stream are induced by profile concavity inherited from deglacial conditions are typical of many streams in mountainous areas and can be understood in terms of slope induced changes in hydraulic properties

the nahal yatir bedload database sediment dynamics in a gravel bed ephemeral stream the nahal yatir bedload transport database arises from the northern negev river sediment monitoring programme and represents the first body of bedload information to be collected during flash floods in desert gravel bed streams bedload flux was established automatically with three slot samplers of the birkbeck type this was complemented by hydraulic measurements that allow sediment transport to be rated against channel average shear stress and specific stream power bed material and bedload grain size distributions are also provided the database is given in both printed and electronic tabular format

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

how round is round a new approach to the topic roundness by fourier grain shape analysis numerous methods for roundness measurement have been developed none however has been generally accepted because of conceptual and practical deficiencies modern image processing and fourier grain shape analysis have eliminated the practical shortcomings but the conceptual ones remained single higher harmonics of the fourier series for example fail to serve as reliable equivalents for roundness evaluation the concept outlined in this paper recognizes three criteria for the evaluation of roundness 1 all curvatures convex as well as concave or plane elements must be considered 2 the positions of morphological elements are significant because salient parts of a particle are more easily abraded than protected ones consequently the curvatures have to be weighted by their relative position on the particle 3 positions and curvatures of morphological elements have to be compared with the particle s ultimate abraded shape which is assumed to be an ellipsoid the ellipsoid reflects the aspect of form or sphericity the distinction between sphericity and roundness is retained because there is no evidence that sphericity changes significantly during transport the measurement is based on the outline of a particle s maximum projection plane which is transformed to a fourier series roundness data are derived from the complete amplitude spectrum the aspect of sphericity is eliminated by subtracting the amplitude spectrum of the best approximating ellipse from the spectrum of the empirical shape the residual amplitudes are normalized and summed in a final step the resulting values are rescaled this guarantees reasonable boundaries and a normal distribution of roundness values the procedure is automated and its efficiency permits the calculation of large samples tests on fluvial and coastal gravel populations demonstrate that the method is sensitive to abrasional wear during all stages of roundness

modelling landscape evolution in the waipaoa catchment new zealand a phenomenalogical approach landscape evolution models require detailed strength measurements which are difficult to obtain for large areas we offer an alternative phenomenalogical approach to modelling landscape evolution one that requires few input parameters the phenomena are hills channels floodplains knickpoints and coastal floodplain the model is demonstrated on the highly erodible waipaoa catchment in new zealand a digital elevation model is simulated every thousand years from 63000 bp through to present day there is good agreement between predicted and observed terrace heights and also good agreement between predicted and observed knickpoint locations suggesting the phenomenalogical approach is indeed representing the main drivers of landscape evolution in the waipaoa catchment

structure and evolution of the rockeskyllerkopf volcanic complex west eifel volcanic field germany the rockeskyllerkopf volcanic complex rvc comprises three overlapping monogenetic volcanic centers southeast lammersdorf sel mauseberg m and rockeskyllerkopf rkk each volcanic center comprises proximal wall deposits with a well defined crater wall unconformity and crater fill deposits that partially to completely cover the outer crater wall the sel center is a phreatomagmatic tuff ring composed of lithic rich tephra deposited by pyroclastic falls and surges the second center mauseberg with its crater to the northwest of the sel center is predominantly magmatic topographic and outcrop patterns suggest that this center may have formed a series of overlapping scoria cones along a n s trending fissure the youngest center rkk which lies on a poorly developed palaeosol within the earlier mauseberg deposits comprises a well developed proximal crater wall sequence this sequence of magmatic likely strombolian fall and grain avalanche deposits passes upward into a crater fill sequence that comprises variably welded bombs the final eruptions in the center were massive lava flows that were ponded within the rkk crater ar ar age dating of reequilibrated fragments of phlogopite megacrysts in the sel lavas indicates volcanic activity began at 474 39 ka literature k ar dates for the youngest lava flows in the rkk center give ages of 360 60 to 470 ka our interpretation of the age data and the presence of the poorly developed palaeosol between the mauseberg and rkk centers indicates that volcanism in the rvc began around 470 ka with the eruption of the sel and mauseberg centers followed a few thousand years later by the eruption of the rkk center

the nature and controls on downstream fining within sedimentary links abstract at scales of 100 to 102 km grain size change in gravelly alluvial systems is characterized by punctuated downstream fining significant lateral sediment inputs define a sequence of sedimentary links within which fluvial processes produce fining trends using a new set of surface grain size data eighteen such links along two canadian gravel bed rivers are examined negative exponential functions are found to be most appropriate for describing within link grain size changes development of a pragmatic means of predicting grain size is then pursued by searching for a means to relate model diminution coefficients to link attributes consideration of lithologically differentiated fining trends indicates that lithology plays an insignificant role in controlling fining rate examination of within link longitudinal profiles indicates a reasonable relation between fining rate and the rate of change of channel slope the predictive value of this relation is limited by the regional nature of the data upon which it is based and the relative difficulty of obtaining accurate gradient information however as previously noted for alluvial fans although for different reasons channel gradient is correlated with link length and a reasonable predictive tool is apparent in a relation between link length and diminution rate the validity of the derived relation cannot be tested for want of a suitable data set several lines of evidence indicate that abrasion is unimportant as a fining mechanism on these rivers and it is argued that this may be typical of sedimentary links fining rate must then be a function of the efficiency with which sorting processes operate and it is suggested that this efficiency is dependent on aggradation rates associated with the lateral sediment influxes at the head and terminus of individual links

which tributaries disrupt downstream fining along gravel bed rivers tributaries and other lateral sediment sources can have a significant impact on river bed sediment texture and in turn on channel form sufficiently voluminous or distinct sediment inputs redefine the mainstem grain size distribution punctuating downstream maturation and isolating a sequence of discrete sedimentary links within these links fining processes usually dominate such that models of sorting and abrasion when applied to individual links provide reasonable predictions of grain size change links represent the fundamental natural unit within which fining models can be tested developed and applied identification of significant lateral sources is therefore important yet beyond vague references to relative tributary size sediment load and sediment calibre no criteria exist for the a priori discrimination of such sources in this paper a procedure for identifying significant lateral tributary sources without the benefit of grain size information is outlined a high resolution characterisation of bed material texture along two canadian gravel bed rivers facilitated classification of all their perennial tributaries as either significant or insignificant three absolute tributary basin parameters and their relative counterparts chosen to reflect the likely controls on tributary significance are then used to develop a discriminant function which isolates a large proportion of significant tributaries while minimising incorrect classifications examination of consistently misclassified anomalous tributaries reveals the importance of lateral source spacing and of inconsistencies in the geomorphic history of the contributing basins in turn a general tributary categorisation procedure is suggested which includes a logistic regression model for attaching probability statements to individual classifications the generality of the discriminant and logistic functions cannot be assessed because of the lack of other suitable data sets

correlation of fluvial terraces within the hikurangi margin new zealand implications for climate and baselevel controls abstract a correlation of fluvial terraces is presented for eight non glacial catchments of the eastern north island new zealand within the actively uplifting hikurangi margin using a combination of loess and tephra coverbed stratigraphy and radiocarbon and osl dating of fluvial deposits and loess coverbeds we demonstrate correlation of four fill terraces t1 t4 the available age constraints suggest t1 15 30 ka t2 31 50 ka t3 50 70 ka and t4 115 ka but the association and temporal link with loess deposits suggest correlation with cold periods and thus the refinement of t2 late mis 3 31 40 ka t3 mis 4 55 70 ka t4 mi substage 5b 90 ka 5d 110 ka or mis 6 140 160 ka the ability to correlate terraces between catchments plus the lack of independent evidence for tectonic triggering events indicates terraces have probably formed in response to either climate terrestrial or baselevel sea level control climate control is indicated by the temporal link of post glacial incision with re establishment of forest cover and of lgm aggradation with limited grass and shrub cover and periglacial processes aggradation due to increased sediment supply under reduced vegetation is dramatically demonstrated by formation of the taupo pumice alluvium terrace in response to inundation by volcanic deposits unwelded ignimbrite following the 1 8 ka taupo eruption and the response to post settlement

selective sorting and abrasion of river gravel ii applications most gravel rivers show a tendency for characteristic grain size to decrease in the downstream direction over scales of tens or hundreds of kilometers it has been surmised that this downstream fin

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

the river it goes right on post glacial landscape evolution in the upper waipaoa river basin eastern north island new zealand abstract post glacial incision created 50 to 120 m of relief in the headwaters of the waipaoa river basin but it is not known how the signal of incision propagated along the trunk streams we addressed this issue by imposing previously determined long term rates of rock uplift that vary between 0 6 and 4 mm year 1 on the preaggradation longitudinal profile acting over the past 27 1 2 cal ka bp these rates are sufficient to passively elevate the waipaoa 1 terrace to its present level but during the last glacial maximum rates of vertical incision failed to match those of the uplifting rocks thus by the time favorable conditions for incision were reestablished after the antarctic cold new zealand late glacial reversal we estimate that the channel would have been elevated 55 m above the datum the river should otherwise have maintained the channel incised rapidly after the transition from glacial to interglacial conditions and reacquired its steady state form in the mid holocene 5 5 2 cal ka bp thereafter we suggest rates of incision matched rates of uplift no knickpoints are found on the upper waipaoa and mangatu rivers because incision commenced simultaneously along the entire length of the trunk streams and the pattern of incision is consistent with the downstream decline in the rate of rock uplift we also used hypsometric analysis to show how post glacial incision affected the surface morphology of low order 0 1 4 3 km 2 drainages in the headwaters of the waipaoa river basin values of the hypsometric integral are higher for proximal basins which the incision signal did not permeate than for low order basins bordering the upper waipaoa river and the lower reaches of tributaries that possessed enough erosive capacity to keep pace with incision along the mainstem hillslope adjustments were focused on the lower and mid sections of these basins so that the most pronounced changes to the hypsometric curve occur in the vicinity of the toe whereas the entire form of the hypsometric curve changes once mass wasting processes encroach onto hillslopes in the middle and upper sections of a basin

downstream fining through selective particle sorting in an equilibrium drainage network the phenomenon of downstream fining has been attributed to both particle abrasion and selective particle sorting the latter is generally considered to play the dominant role within resistant lithologies it has been recognized that tributaries can disrupt fining patterns however few downstream fining studies have considered the entire fluvial network structure here we combine a theory for selective transport with a model of river basin evolution in order to simulate the dynamics of selective sorting throughout a drainage network previous numerical modeling studies of single thread or braided channels have treated downstream fining as a phenomenon driven by differential deposition rates we show however that in an eroding drainage network downstream fining emerges as a natural dynamic adjustment to variable water sediment and energy inputs even under conditions of uniform size distribution in sediment flux thus although selective deposition and abrasion clearly can and do play a role in some fluvial systems neither is necessary to produce downstream fining within a drainage network

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