fluvial response to paleo equatorial climate fluctuations during the late paleozoic ice age the maritimes basin of atlantic canada preserves a 12 km thick stratigraphic succession deposited within a low latitude predominantly continental setting during the late paleozoic this paper examines the utility of using such successions for paleoclimate analysis with specific reference to changes in fluvial style modern rivers in the subhumid and semiarid tropics preserve a suite of features that are distinct from those in humid and arid environments these features include an abundance of sedimentary structures formed under froude transcritical to supercritical flow conditions and the presence of in situ vegetation growing within the channel environment this fluvial style is indicative of prolonged low flow conditions punctuated by intense precipitation events in a strongly seasonal climatic setting fluvial channel bodies within the maritimes basin preserve a remarkably similar style compared with these modern systems suggesting that a subhumid strongly seasonal paleoclimate was the dominant imprint on parts of the preserved successions strongly seasonal deposits are concentrated within four discrete stratigraphic intervals e1 e4 ranging in duration from 2 to 6 m y these intervals are correlated across the entire basin and between basins characterized by markedly different tectonic regimes which suggests that a coherent regional climate signal is recorded intervals are separated by deposits characteristic of humid and arid river systems and the alternation between these different fluvial styles implies that several pronounced long term changes in precipitation and runoff regimes occurred during the carboniferous within the region furthermore these intervals broadly coincide with major periods of southern hemisphere glaciation which suggests that glaciation had a profound and controlling effect on paleotropical climate

sediment provenance in a tropical fluvial and marine context by magnetic fingerprinting of transportable sand fractions the sources and fluxes of sediment to the great barrier reef lagoon from northeastern australian rivers have been the subject of much concern and study with the large catchments of the burdekin and fitzroy rivers thought to be the key sources at present here the utility of newly developed magnetic fingerprinting methods for identifying sediment provenance both onshore and offshore and in association with individual large flood events is investigated within the burdekin catchment sediments are mobilized from different subcatchments by runoff generated by intense localized rainfall events magnetic measurements were made on untreated and acid treated samples of river channel sediments within the burdekin river subcatchments and from the estuarine and inner shelf depocentres of burdekin river sediments the acid treatment removes all discrete magnetic particles and coatings and leaves magnetic inclusions protected within host silicate grains as the basis of the measured magnetic signature of a sample the magnetic properties of the acid treated samples display statistically distinct sediment provenance groupings sand samples from the upper burdekin river appear magnetically distinct from samples from tributaries of the burdekin e g hann creek fanning river and also from nearby coastal rivers including the haughton suspended sand samples from a burdekin flood event in 2000 appear to have a different source compared with those from floods in 1998 and 1999 comparisons of the terrestrial acid treated sand fractions with the same acid treated sand size fractions from transects taken offshore suggest that the surface sediments in upstart bay and bowling green bay have different sources some of these sources are as yet unidentified but may represent the unsampled lower discharge south western burdekin subcatchments and or along shore drift of sand from the south perhaps even from the fitzroy river over millennial timescales of cyclone pumping the magnetic inclusion method precludes any obfuscation or confounding of sediment source which might arise from hydraulic sorting and or post depositional magnetic diagenesis or authigenesis

caesium 137 in sandy sediments of the river loire france assessment of an alluvial island evolving over the last 50 years recent sedimentological and morphological evolution of an island in the river loire france was investigated using the 137cs method this study describes the morphological adjustment of the island in the last 50 years which corresponds to the increased bed incision of this sandy multiple channel environment because of among other things the increase in sediment extraction up to 1995 the results show that some 137cs can be retained by sandy particles potentially in clay minerals forming weathering features included in detrital sand grains from a morphological perspective significant lateral erosion can be observed in the upstream part of the island while a weak lateral accretion occurs in its downstream section data about 137cs and aerial photographs show that the morphology of the island margins has undergone significant changes leading to a lateral migration while the centre of the island has remained relatively stable or is slowly eroding the migration of the island depends on 1 the withdrawal of inherited preincision morphological units such as levees or the development of new units such as a channel shelf 2 water and sediment supply from surrounding channels during flood events 3 preferential sediment trapping 20 mm year 1 from the presence of riparian vegetation on the bank of the secondary channel that is subject to narrowing the sedimentological and morphological response of the island in the context of incision of the loire river bed is expressed mainly by lateral migration and secondarily by a low vertical adjustment

obstacle marks produced by flow around stranded ice blocks during a glacier outburst flood jokulhlaup in west greenland the effects of glacier ice block grounding on the morphology and sedimentology of proglacial fluvial outwash were examined during a glacier outburst flood or jokulhlaup near sondre stromfjord west greenland observations made during and after the 1987 jokulhlaup both on the surface of an ice contact delta and within a confined valley sandur plain provided information about the formation of ice block obstacle marks and the significance of these bedforms for sandur morphology and sedimentology flow directions determined from obstacle mark morphology have been used successfully to chart flow direction changes on the falling limb of the jokulhlaup maximum flow depths for scour around stranded ice blocks may be given by 0 5 0 9 times the diameter of the ice block as estimated from the depth of scour the height of the obstacle shadow or the extent of ice block meltout sediments minimum flow depths can be represented by the height of the obstacle shadow above the mean bed level the internal composition of the shadow indicates the ability of the flow to transport various sizes of material into the lee of obstacles ice block obstacle marks within the distal portion of the sandur initiated waning stage channel change proximal and lateral erosion around stranded ice blocks extended downstream from the ice block forming chute channels which then captured waning stage flows resulting in significant bar incision with associated deposition of lobate or deltaic deposits it is suggested that ice block obstacle marks are important in terms of channel morphology channel morphological change and their usefulness as palaeohydrological indicators

the role of discharge variability in the formation and preservation of alluvial sediment bodies abstract extant planform based facies models for alluvial deposits are not fully fit for purpose because they over emphasise plan form whereas there is little in the alluvial rock record that is distinctive of any particular planform and because the planform of individual rivers vary in both time and space accordingly existing facies models have limited predictive capability in this paper we explore the role of inter annual peak discharge variability as a possible control on the character of the preserved alluvial record data from a suite of modern rivers for which long term gauging records are available and for which there are published descriptions of subsurface sedimentary architecture are analysed the selected rivers are categorized according to their variance in peak discharge or the coefficient of variation cvqp standard deviation of the annual peak flood discharge over the mean annual peak flood discharge this parameter ranges over the rivers studied between 0 18 and 1 22 allowing classification of rivers as having very low 0 90 annual peak discharge variance deposits of rivers with very low and low peak discharge variability are dominated by cross bedding on various scales and preserve macroform bedding structure allowing the interpretation of bar construction processes rivers with moderate values preserve mostly cross bedding but records of macroform processes are in places muted and considerably modified by reworking rivers with high and very high values of annual peak discharge variability show a wide range of bedding structures commonly including critical and supercritical flow structures abundant in situ trees and transported large woody debris and their deposits contain pedogenically modified mud partings and generally lack macroform structure such a facies assemblage is distinctively different from the conventional fluvial style recorded in published facies models but is widely developed both in modern and ancient alluvial deposits this high peak variance style is also distinctive of rivers that are undergoing contraction in discharge over time because of the gradual annexation of the channel belt by the establishment of woody vegetation we propose that discharge variability both inter annual peak variation and flashiness may be a more reliable basis for classifying the alluvial rock record than planform and we provide some examples of three classes of alluvial sediment bodies representing low intermediate and high very high discharge variability from the rock record that illustrate this point

bottomset architecture formed in the troughs of dunes and unit bars abstract bottomsets are formed in the troughs of migrating bedforms the flow and sediment dynamics downstream of bedforms differ and consequently the thickness grain size and internal structure of bottomsets vary greatly a classification of the controls on bottomset formation is proposed related to two and three dimensional processes flow and sediment unsteadiness and pre existing deposits structures and deposits created by flow and sediment unsteadiness as well as pre existing deposits are also considered in relation to changes they can induce in subsequent trough processes the classification is illustrated by observations from laboratory flumes the modern burdekin river australia and the rock record the roaches grit england and hawkesbury sandstone australia in bottomset examples described in this paper internal variability both vertically and laterally was much greater than that within the dominant foreset component complex compound structures were created by the interaction and predominance of different bottomset controls this combined with their high preservation potential makes them a highly useful paleoenvironmental indicator bottomset variation e g changes in the abundance of mud laterally will influence the permeability heterogeneity of fluvial reservoir rocks bottomsets formed under relatively steady conditions are likely to be laterally extensive and have similar characteristics over their entire length they may act as significant barriers to vertical flow conversely bottomsets generated in unsteady regimes tend to be highly variable potentially creating conduits between cross bed sets aiding inter set permeability

variations in character and preservation potential of vegetation induced obstacle marks in the variable discharge burdekin river of north queensland australia abstract vegetation induced obstacle marks are described from dalrymple bend in the burdekin river of queensland australia and their preservation potential is discussed the bend is divided into a sand and gravel covered lower bar and a vegetated upper bar obstacle marks comprising an erosional scour and a depositional sediment tail are recognised on the lower bar and the lower margin of the upper bar two types of obstacle marks are dominant in the lower part of the lower bar sediment tail dominated obstacle marks associated with inclined trees of melaleuca argentea silver leaved paperbark and comparatively small obstacle marks around stranded driftwood obstacle marks in the upper part of the lower bar are formed around grass clumps armoured with mud and gravel at the lower margin of the upper bar scours and gravelly sediment tails are recognised around mature tall trees in these cases the scour is 4 m in width and covered with alternating beds of fine sand and mud among the obstacle marks described sediment tail dominated obstacle marks around groves of m argentea have the highest preservation potential obstacle marks in the lower margin of the upper bar are also considered to have high preservation potential obstacle marks around stranded driftwood have the lowest preservation potential obstacle marks around grass clumps survive more than 1 year but their long term preservation potential is nonetheless low implications for the stratigraphic record are also discussed

fossil trees in ancient fluvial channel deposits evidence of seasonal and longer term climatic variability it has been established that large numbers of certain trees can survive in the beds of rivers of northeastern australia where a strongly seasonal distribution of precipitation causes extreme variations in flow on both a yearly and longer term basis in these rivers minimal flow occurs throughout much of any year and for periods of up to several years allowing the trees to become established and to adapt their form in order to facilitate their survival in environments that experience periodic inundation by fast flowing debris laden water such trees notably paperbark trees of the angiosperm genus melaleuca adopt a reclined to prostrate downstream trailing habit have a multiple stemmed form modified crown with weeping foliage development of thick spongy bark anchoring of roots into firm to lithified substrates beneath the channel floor root regeneration and develop in flow parallel linear groves individuals from within flow parallel linear groves are preserved in situ within the alluvial deposit of the river following burial and death four examples of in situ tree fossils within alluvial channel deposits in the permian of eastern australia demonstrate that specialised riverbed plant communities also existed at times in the geological past these examples from the lower permian carmila beds upper permian moranbah coal measures and baralaba coal measures of central queensland and the upper permian newcastle coal measures of central new south wales show several of the characteristics of trees described from modern rivers in northeastern australia including preservation in closely spaced groups these properties together with independent sedimentological evidence suggest that the permian trees were adapted to an environment affected by highly variable runoff albeit in a more temperate climatic situation than the modem australian examples it is proposed that occurrences of fossil trees preserved in situ within alluvial channel deposits may be diagnostic of environments controlled by seasonal and longer term variability in fluvial runoff and hence may have value in interpreting aspects of palaeoclimate from ancient alluvial successions c 2001 elsevier science b v all rights reserved

geochemical systematics and precious metal content of the sedimentary horizons of lower gondwanas from the sattupalli coal field godavari valley india abstract major trace rare earth ree and platinum group elements pge have been analysed from the carbonaceous horizons of permian barakar formation of sattupalli coal field of godavari valley to understand the provenance and depositional environment barakar formation of coal field consists of sandstone siltstone carbonaceous shale and coal seams coal is enriched in trace elements compared with the clarke s background values ferromagnesian trace elements ni cr and co are similar in range in both coal and carbonaceous shales along with large ion lithophile elements lile rb sr cs and ba high field strength elements hfse nb ta zr and ti light ree lree la sm and ag archean upper crust auc normalized ree and multielement variation patterns of the coal show a flat ree with a slight depletion in ce and eu contents in most of the samples their multielement patterns exhibit enrichment of th and u negative ta zr hf and positive ti anomalies pronounced positive anomaly occurs at zr hf and ti that are combined with both positive and negative nb and ta anomalies in carbonaceous shales and clay sandstones have a high sio 2 50 80 wt low fe 2 o 3 al 2 o 3 and moderately high k 2 o contents auc normalized ree patterns are almost flat with slight negative to no eu anomalies followed by small negative ce anomalies ce ce 0 87 0 95 with one exception their multielement patterns exhibit positive u small negative nb ta a pronounced positive zr hf anomaly followed by negative ti and v anomalies enrichment of both zr and tio 2 in some samples reflects on the mafic and felsic sources at the source region the fractionated ree patterns hree enrichment and negative eu anomalies are consistent with contribution of zircon through felsic source nb ta troughs endorse their inheritance from schistose rocks of eastern ghats complex the high al 2 o 3 yb and th yb ratios along with negative eu anomalies reflect on the ttg felsic rocks of provenance based on the geochemical available petrological and paleocurrent analysis indicate the source rocks of the barakar formation were granite granitic gneiss high grade metamorphic rocks from eastern ghats and basic rocks the provenance is located west and south east i e partly peninsular gneiss and partly eastern ghat complex the au pge contents of sattupalli coalfield have been deposited through ne sw trending post depositional transverse faults that have affected all the gondwana formations during the early cretaceous the mafic rocks present within the basement could have been the source of these elements

flood behaviour of the burdekin river tropical north queensland australia strongly seasonal rainfall in north queensland australia causes extremely variable fluvial discharge and tropical cyclones commonly produce high magnitude short duration floods in the burdekin river catchment 129 500 km2 of eastern queensland rapid runoff and channel floodplain configuration lead to very fast flood wave speeds up to 4 34 m s 1 calculated the large catchment and varying tropical cyclone paths result in extreme variations in flood history at different sites changes in channel conditions bed elevation vegetation bedforms sediment surface character in successive events combined with complex rainfall patterns and highly unsteady flow make flood estimates extremely difficult because of the short duration of flow events and very rapid rate of change in discharge during individual events sediment transport rate bedforms and bed elevation change continuously for most of the year the channel is occupied by an underfit stream but this small perennial stream does not significantly rework the deposits or change channel form before the next major discharge event consequently there is little channel form recovery between major discharge events successive events have very variable size and consequently the bedforms and channel morphology are normally out of equilibrium with the flow and record the result of the most recent major events high magnitude floods transport coarse sediment onto the floodplain and the short duration of flooding and rapid runoff results in relatively low preservation of mud in the channel or overbank environment the holocene flood deposits include thick coarse sand and medium sand units and generally thin fine grained units

sedimentological effects and stratigraphic implications of a rare high stage flow in an evolving braided to anabranching stream with riparian woodland abstract low frequency 1 in 25 years high stage flows on the south platte and upper platte rivers in nebraska usa during september october 2013 produced sedimentary effects that are unaccounted for in depositional models of that fluvial system being superimposed on a historically recent trend of channel belt abandonment and encroachment of woody vegetation these outcomes are even more useful for comparison with the wider sedimentary record rooted vegetation particularly riparian trees played a major role in determining where erosion and sedimentation occurred outside of the main perennially flowing channels effects of the 2013 event include 1 small pebble to cobble sized gravel in various bar top and flood channel settings 2 common current or sediment shadows including streamlined sediment shadow bars several meters or more in length which formed downstream from rooted trees and logjams 3 sand lobes that prograded into densely wooded abandoned parts of the historical channel belt at angles of as much as 85 from average downstream flow 4 large scours some approaching 2 m in depth many of which formed behind trees near the diffluences of flood channels and the main channel 5 deposits of sand that were baffled by nonwoody or shrubby vegetation in the abandoned channel belt and 6 local accumulations of large woody debris in front of rooted trees as cross flood channel dams and rarely as walls alongside flood channels in an aggrading system these features should be preserved as a distinctive set of sedimentary structures which may be interpretable as evidence for episodic major flows in a system in which woody vegetation has encroached widely onto the channel belt

fluvial processes in a forested anastomosing river flood partitioning and changing flow patterns in an effort to further our understanding of multiple channel systems this paper presents data on the flood response of channels in one of the last wooded semi natural anastomosing systems in europe the gearagh ireland is characterized by hundreds of small islands separated by interconnected channels of low slope these include channels that cross islands at right angles to the main flow and blind anabranching channels islands are relatively stable and wooded with evidence of division by channel erosion and growth by in channel sedimentation four active zone cross profiles were surveyed each containing between seven and 13 channels velocities were measured in several channels before and during two separate floods from these observations channels have been categorised into three types fast shallow and trapezoidal slow deep and more irregular and flood channels during the floods interchannel flows were caused by variations in water surface elevations due to backing up behind debris dams and it is suggested that this is the origin of the anomalous cross island channels and one cause of island division another potential cause of island division blind anabranching channels is the result of concentrated bank scour between root masses biotic components such as debris dams tree root masses and tree throw pits play a key role in the partitioning of flow and cause variations in channel velocities and the overbank velocity distribution the implications of these observations for channel pattern maintenance are briefly discussed

current crescents formed by marine algae valonia sp a new record of obstacle marks lacking preservation of obstacles obstacle marks formed around non permanent tools commonly occur on the beach faces of lower long sand island and sagar island two tidal islands in the ganga river estuary india the marine globose algae valonia sp which bloom profusely in the waters of the bay of bengal during winter are deposited on beach faces after being carried by wave swash and then act as impermeable obstacles in the path of wave backwash this creates around the globose algal bodies u shaped or crescentic marks 2 3 cm in width with arms 1 3 cm in length the globose vesicles are soon broken up by external forces or by forces of surface tension leaving crescentic marks around a circular filamentous impression the crescents always open toward the direction of wave backwash these structures may prove useful not only for interpreting the palaeoslope and palaeocurrent directions but also for recognizing the palaeoecology

maximum clear water scour around circular piers the potential predictors of the maximum clear water scour are compared with the available experimental data the range of flow parameters for which these formulas either overpredicted or underpredicted are delineated comparison indicates that the scour formula by laurson and toch is the best predictor among those examined for it encompasses all data and overpredicts less than the other formulas however the laursen and toch formula predicts that the scour depth is independent of sediment size a new formula for the maximum clear water scour is proposed it is very similar to that of laursen and touch but includes the effect of sediment size on scour depth

gravel antidunes in the tropical burdekin river queensland australia the geological record is punctuated by the deposits of extreme event phenomena the identification and interpretation of which are hindered by a lack of data on contemporary examples it is impossible to directly observe sedimentary bedforms and grain fabrics forming under natural particle transporting high velocity currents and therefore their characteristics are poorly documented the deposits of such flows are exposed however in the dry bed of the burdekin river queensland australia following tropical cyclone induced floods long wave length up to 19 m gravel antidunes develop during short days high discharge flows in the upper burdekin river maximum recorded discharge near the study reach over 25 600 m s in february 1927 flood water levels fall quickly metres in a day and flow is diverted away from raised areas of the river bed into subchannels exposing many of the high stage bedforms with little reworking by falling stage currents gravel bedforms were observed on the dry river bed after the moderate flows of february 1994 max 7700 m s and january 1996 max 3200 m s the bedforms had wave lengths in the range 8 19 m amplitudes of up to 1 m with steeper stoss than lee faces and crest lines generally transverse to local peak discharge flow direction the gravel fabric and size sorting change systematically up the stoss and down the lee faces the antidune deposits form erosive based lenses of sandy gravel with low angle downstream dipping lamination and generally steep upstream dipping a b planes the internal form and fabric of the antidune gravel lenses are distinctly different from those of dune lee gravel lenses the erosive based lenses of low angle cross bedded gravel with steep upstream dipping a b planes are relatively easy to recognize and may be diagnostic of downstream migrating antidunes the antidune gravel lenses are associated with thick to 1 m high angle cross bed sets ancient antidune gravel lenses may be diagnostic of episodic high discharge conditions and particularly when they are associated with high angle cross bedded gravelly sand they may be useful for palaeoenvironmental interpretation

plant material deposition in the tropical burdekin river australia implications for ancient fluvial sediments abstract in the deposits of the burdekin river north queensland australia plant material character and directional fabric together with the sedimentary facies character are generally diagnostic of depositional environment plant material is selectively entrained transported and deposited in all flow conditions ranging from dry season minimal flow

anabranching rivers ridge form alluvial channels in tropical northern australia sand dominated ridge form anabranching rivers on the kimberley plateau in northwestern australia are a new type of alluvial channel system not previously described or explained in detail this paper examines the morphology sedimentology and stratigraphy of this type of river and proposes mechanisms for their formation and maintenance alternating with relatively narrow bedrock reaches of valley are wider depositional alluvial reaches that support roughly parallel very elongated steep sided and treed sandy ridges of approximately floodplain height these ridges subdivide the total flow system into remarkably straight canal like anabranching channels in this seasonally arid environment there is an abundance of riparian vegetation in places growing chaotically over wider sections of stream bed it is proposed here that the formation of ridges results in a reduction of flow resistance with an increase in depth the ridges concentrate the flow and compensate for the less efficient flow conditions associated with these well vegetated alluvial reaches trees generally do not survive on the bed of the higher energy channels enhanced velocities and increased bed shear act to maintain or increase water and sediment flux in these alluvial reaches the formation of the channels and intervening ridges may be aided by the development of double flow helices in each anabranch similar to that described for much smaller ridge features observed in laboratory flumes anabranches are also observed having formed by channel avulsion scouring new channels into adjacent floodplains particularly in widened sections of valley near tributary junctions

coarse grained floodplain deposits in the seasonal tropics towards a better facies model abstract floodplain deposits in semiarid and subhumid settings with erratic rainfall may contain relatively high proportions of sand because of climatically controlled patterns of discharge and vegetation cover ancient formations with abundant floodplain sands may record fluvial systems with extreme discharge variability of this type but are difficult to interpret because of the paucity of published modern analogues the lower burdekin and haughton river alluvial and delta plains of north queensland australia are used here to illustrate this type of fluvial architecture the modern burdekin and haughton channels have gravelly sand beds and transport a mixed load in the wet season the channels have low relief levees that decrease in amplitude downstream across the low gradient floodplains tropical cyclone activity results in high magnitude short duration sediment charged floods that inundate large areas of the coastal lowland bankfull discharge is infrequently exceeded return interval 25 years because the channels are enlarged by major cyclone induced runoff events and between these events flows are too small to modify the channel form significantly suspended sediment loads and the proportions of these that are sand are particularly high when intense rainfall occurs after prolonged drought when vegetation cover is reduced as a consequence of these factors most of the deposition on the levees and floodbasins occurs in very large discharge events with significant suspended sand load these floods carry a lot of sand out of the channel and efficiently in suspension over the floodplains because of deep inundation boreholes drilled in these floodplains encountered sharply bounded and gradationally bounded sand beds that fine upward or show no vertical grain size trend these are interbedded with thin silt clay and minor gravel beds holocene paleochannels make up about 30 of the floodplain area and consist predominantly of coarse sand that is similar to modern channel bed deposits the overbank sand in contrast is predominantly fine medium grained and beds have distinctly different granulometry from each other and from channel floor deposits efficient sediment partitioning in the runoff and extreme variation in flood magnitude explain variations in sand bed granulometry in the alluvial valley and upper delta plain individual sand beds are difficult to correlate between adjacent boreholes even over very short distances tens of meters in contrast on the lower delta plain overbank sands have a more sheet like geometry this is explained by the lower lying landscape negligible levee development greater inundation depths and reduced surface roughness in addition the channel bodies are thinner and wider and may be connected to overbank sand wings

fluvial islands first stage of development from nonmigrating forced bars and woody vegetation interactions abstract fluvial islands can develop from the channel bed by interactions between pioneer trees and bars although vegetation recruitment and survival is possible on all bar types it is easier for trees to survive on nonmigrating bars developed from a change in channel geometry or to the presence of a steady perturbation this field study details the first stages of development of a vegetated mid channel nonmigrating or forced bar and its evolution toward an island form over six years analysis of bed topographical changes vegetation density and roughness scour and fill depths sediment grain size and architecture and excess bed shear stress highlighted a specific signature of trees on topography and grain size segregation two depositional processes combining the formation of obstacle marks and upstream shifting deposition of sediments led to the vertical accretion of the vegetated bar during the first stage of the bar accretion bedload sediment supply coming from surrounding channels during floods was identified as a key process modulated by the presence of woody vegetation and a deflection effect induced by the preexisting topography grain size segregation between vegetated and bare areas was also highlighted and interpreted as an important process affecting the development of surrounding channels and the degree of disconnection and hence the speed of development of a growing island the heterogeneity of bedload supply can explain why sediment deposition and density of trees are not strictly related a general conceptual model detailing the first stages of evolution from a bar to an established island is proposed for relatively large lowland rivers

in channel woody vegetation controls on sedimentary processes and the sedimentary record within alluvial environments a modern example of an anabranch of the river loire france a multidisciplinary approach was used to analyse the role of pioneer trees populus nigra on bar formation in the modern fluvial system of the river loire france data were collected on seven plots located on a secondary channel chosen for their vegetation cover and their sedimentary context physical features of vegetation topographical evolution sediment grain size and flow velocity were investigated during and after floods between 2001 and 2003 woody vegetation responds morphologically to fast flowing waters and sediment deposition these adaptations influence the role played by trees on local sedimentary processes some morphological types of trees exert a strong influence during flood events by deflecting reducing or increasing the flow energy as a consequence the evolution of bedforms distribution of sediment grain size and slopes are controlled by the flood type the morphological context i e high or low energy zones the morphology of the vegetated bar and the characteristics of vegetation conceptual models are proposed to account for local processes and evolution of tree groves the first model which describes the behaviour of groves during a single flood underlines the sediment deposition downstream of the trees and the trapping of bedload sediments during the falling stage of the flood the reorientation of current streams constrained by both local bed topography and woody vegetation induces transverse sediment fluxes for these water levels the control exerted by trees on small scale sedimentary processes becomes significant the second model shows the evolution of vegetation bands over several flood events in particular it describes the influence of vegetation on the fixing and reworking of bedload sediments during the falling limb of the hydrograph the coalescence of vegetation induced bedforms and the resulting morphological changes increase the deflection power of the vegetated bar inducing the deposition of finer sediments

sedimentology of the upper burdekin river of north queensland australia an example of a tropical variable discharge river the burdekin river is an example of a class of tropical streams which experience two to four orders of magnitude variation in discharge in response to seasonal but erratic monsoonal rainfall floods of the burdekin rise abruptly reaching peak discharges of up to 40 000 m s in less than 24 h maintain peak flow for up to a few days and recede exponentially the geomorphology and deposits of these rivers reflect the extreme discharge fluctuations and have not previously been described a stretch of the upper burdekin river comprising four bends and one straight reach was examined near the town of charters towers the river bed is largely exposed for most of any year with a small misfit perennial channel carrying low stage flow major geomorphic elements of bends include point bars with ridge and swale topography three distinct types of chute channels avalanche slipfaces up to 5 m or more high around the downstream edges of bars and on the outer part of one point bar an elevated vegetated ridge straight reaches are flat or gently inclined sand and gravel covered surfaces much of the river bed is covered by well sorted in places gravelly coarse to very coarse grained sand with local accumulations of pebble to boulder gravel lower parts of the river bed are periodically draped by mud which is desiccated on exposure dunes and plane beds are the most commonly occurring bedforms with local development of gravelly antidunes most bank tops and upper vegetated bars are covered by silt and fine grained sand the river bed also hosts a low diversity but locally high abundance flood tolerant flora dominated by the paperbark tree melaleuca argentea which plays an important role in controlling the distribution of sediment the gross geomorphology of the river bed and most of the sedimentary features are interpreted as having formed during major bankfull or near bankfull flows which have a recurrence of about 18 years based on 65 years hydrographic data the initial rapid drop in discharge following flood peaks appears to preserve flood peak features on upper bars more or less intact whereas lower areas are subjected to variable degrees of modification during falling stage and by more frequent non bankfull discharge events

early holocene hydrology and environments of the ner river poland abstract the ner river valley central poland underwent substantial transformation during the weichselian holocene transition as a result of fluvial processes and climate changes resulting in the establishment of its present shape in the holocene a multiproxy study based on organic deposits from a palaeochannel fill lutomiersk koziowki shows that after the channel was cut off during the late glacial termination it became a shallow oxbow fed by local springs in the boreal period the oxbow lake was also fed by precipitation and became a telmatic environment overgrown by rush and swamp vegetation finally it was covered by overbank deposits the first flooding phase 9900 9600 cal bp was followed by the accumulation of overbank sediments after 9500 cal bp and flooding increased after ca 9300 9000 cal bp pollen data provide information on the regional vegetation context for local and regional changes in the atlantic period an increase in both summer and winter temperatures is inferred from the pollen data corresponding to an expansion of thermophilous deciduous forests while in general flooding phases of the early holocene are poorly recognised in eastern europe the lutomiersk koziowki site may be considered as one of the reference points for this phenomenon in the region

preservation of in situ arborescent vegetation and fluvial bar construction in the burdekin river of north queensland australia abstract in sub humid parts of north queensland ne australia certain types of trees are well adapted to living in river bed habitats the bed of the tropical variable discharge upper burdekin river hosts a community dominated by the paperbark melaleuca argentea trees grow preferentially in flow parallel linear groves and engineer their own environment by deflecting currents building sand and gravel bars and stabising banks this is the first study to document in channel bar development resulting from vegetation growth rather than the reverse which has been inferred by previous workers in the burdekin river study site individual melaleuca range from seedlings to mature trees over 100 years old these trees survive regular partial to total submergence and impact damage during wet season runoff events often reaching over 20 000 m3 s 1 at peak discharge partly by adopting structural and growth modifications these modifications include a reclined downstream trailing habit multiple stemmed form modified crown with weeping foliage development of thick spongy bark root regeneration and group strategies notably development of flow parallel linear groves following death in situ remains of trees are preserved within the mainly coarse sand to gravel channel fill either as reclined stems trunks stripped of branches and foliage or as more upright trunks snapped at a height of typically 1 2 m above base both with roots the morphological adapations and styles of preservations of in situ vegetation within the burdekin river are considered distinctive of variable discharge rivers and may be useful in the identification of facies formed in such environments in the rock record particularly when associated with bar development

flow and sediment dynamics in the vegetated secondary channels of an anabranching river the loire river france this study investigates the hydrological and sedimentological mechanisms occurring in the vegetated secondary channels of an anabranching river affected by incision the loire river france during and after flood events that occurred between 2000 and 2003 observations and measurements were performed on a vegetated secondary channel located in the study site of brehemont 790 km downstream the source morphological changes and sediment dynamics were analysed using low elevation airborne photographs topographic and bathymetric surveys and scour chains the hydraulic behaviour of the channel was also analysed by measurements performed on flow velocity and direction during different flood stages in order to quantify the influence of woody vegetation on flow resistance the roughness of bands of trees was determined from measurements performed on the field the impact of the disruption of armour layers on bedload pulses the variation of sedimentary processes during a single flood event and the fixation of bedforms by vegetation are all identified as key processes influencing the behaviour of the study channel topographic surveys demonstrate that sediment dynamics is substantial in the upstream part of the channel and that sediment budgets are different according to the temporal scale considered moreover an asymmetrical behaviour of the secondary channel is demonstrated reduced quantities of sediment deposited and preserved in the vegetated zones contrast with material by passing observed in the third order channels flow velocity and direction measurements indicate that these parameters vary according to the water level and to the morphological units of the channel pools riffles vegetated areas during low flows scouring and export of particles from the secondary channel are a consequence of reduced sediment supply from the main channel of the loire river for these water levels sedimentation occurs in pools where velocity and turbulence decrease whereas third order channels are subjected to erosion during high discharges large quantities of sediment available in the main channel feed the temporary stores formed by riffles and bars in the secondary channel the vegetated area located in the downstream part of the secondary channel deflects current trickles at low discharges and decreases flow velocity during high water levels the sedimentary accretion observed in this area exerts a feedback on flow and sedimentary processes d 2005 elsevier b v all rights reserved

unit bar architecture in a highly variable fluvial discharge regime examples from the burdekin river australia unit bars are relatively large bedforms that develop in rivers over a wide range of climatic regimes unit bars formed within the highly variable discharge burdekin river in queensland australia were examined over three field campaigns between 2015 and 2017 these bars had complex internal structures dominated by co sets of cross stratified and planar stratified sets the cross stratified sets tended to down climb the development of complex internal structures was primarily a result of three processes i superimposed bedforms reworking the unit bar avalanche face ii variable discharge triggering reactivation surfaces and iii changes in bar growth direction induced by stage change internal structures varied along the length and across the width of unit bars for the former down climbing cross stratified sets tended to pass into single planar cross stratified deposits at the downstream end of emergent bars such variation related to changes in fluvial conditions whilst bars were active a hierarchy of six categories of fluvial unsteadiness is proposed with these discussed in relation to their effects on unit bar and dune internal structure across deposit variation was caused by changes in superimposed bedform and bar character along bar crests such changes related to the three dimensionality of the channel and bar geometry when bars were active variation in internal structure is likely to be more pronounced in unit bar deposits than in smaller bedform for example dune deposits formed in the same river this is because smaller bedforms are more easily washed out or modified by changing discharge conditions and their smaller dimensions restrict the variation in flow conditions that occur over their width in regimes where unit bar deposits are well preserved their architectural variability is a potential aid to their identification this complex architecture also allows greater resolution in interpreting the conditions before and during bar initiation and development