ground penetrating radar and its use in sedimentology principles problems and progress abstract ground penetrating radar gpr also referred to as ground probing radar surface penetrating radar subsurface radar georadar or impulse radar is a noninvasive geophysical technique that detects electrical discontinuities in the shallow subsurface

influence of hydraulic conditions over dunes on the distribution of the benthic macroinvertebrates in a large sand bed river 1 this study aims to relate the flow structure over mobile dunes recorded on the channel bed of the parana river argentina with the spatial distribution of the benthic macroinvertebrates the following main conclusions have been obtained 1 the dunes found in the active channel could be considered as hydraulic biotopes at a mesohabitat scale the bed forms in the thalweg region are subjected to higher shear stresses with low benthic densities 2 differences in benthic densities were also recorded at within dunes microhabitat scales the largest densities were found in the dune troughs where small bed shear stresses occur and minimum densities on the low stoss side of dunes where turbulent agitation near the bottom strongly disturb the bed particles 3 superimposed dunes on larger dunes may be considered as another microhabitat of still smaller dimensions summarizing multiscale approaches are needed if a comprehensive understanding linking hydrodynamics and morphodynamics processes with benthic ecology is intended

significant effect of sediment cohesion on delta morphology delta morphology is thought to be controlled by factors such as river discharge tides and waves numerical modelling shows that sediment cohesion also strongly influences the development of a delta s characteristics the morphologies of the world s deltas are thought to be determined by river discharge tidal range and wave action 1 more recently sea level rise 2 3 and human engineering4 have been shown to shape delta evolution the effects of factors such as sediment type and the overall amount of sediment carried by rivers are considered secondary4 5 6 in particular the role of sediment cohesion which is controlled by sediment size and type of vegetation is unclear here we use a numerical flow and transport model7 8 9 10 to show that sediment cohesiveness also strongly influences the morphology of deltas we find that holding all other factors constant highly cohesive sediments form bird s foot deltas with rugose shorelines and highly complex floodplains whereas less cohesive sediments result in fan like deltas with smooth shorelines and flat floodplains in our simulations sediment cohesiveness also controls the number of channels that form within the deltas and the average angle of bifurcation of those channels as vegetation generally acts as a cohesive agent we suggest that deltas that formed before the expansion of land plants in the devonian period should show fan like characteristics a finding consistent with the limited data from the sedimentological record11

scales and causes of heterogeneity in bars in a large multi channel river rio parana argentina to date published studies of alluvial bar architecture in large rivers have been restricted mostly to case studies of individual bars and single locations relatively little is known on how the depositional processes and sedimentary architecture of km scale bars vary within a multi km reach or over several 100s km downstream this study presents ground penetrating radar gpr and core data from 11 km scale bars from the rio parana argentina the investigated bars are located between 30 km upstream and 540 km downstream of the rio parana rio paraguay confluence where a significant volume of fine grained suspended sediment is introduced into the network bar scale cross stratified sets with lengths and widths up to 600 m and thicknesses up to 12 m enable the distinction of large river deposits from stacked deposits of smaller rivers but are only present in half the surface area of the bars up to 90 of bar scale sets are found on top of finer grained ripple laminated bar trough deposits bar scale sets make up as much as 58 of the volume of the deposits in small incipient mid channel bars but this proportion decreases significantly with increasing age and size of the bars contrary to what might be expected a significant proportion of the sedimentary structures found in the rio parana is similar in scale to those found in much smaller rivers in other words large river deposits are not always characterised by big structures that allow a simple interpretation of river scale however the large scale of the depositional units in big rivers causes small scale structures such as ripple sets to be grouped in thicker co sets which indicate river scale even when no obvious large scale sets are present the results also show that the composition of bars differs between the studied reaches upstream and downstream of the confluence with the rio paraguay relative to other controls on downstream fining the tributary input of fine grained suspended material from the rio paraguay causes a marked change in the composition of the bar deposits compared to the upstream reaches the sedimentary architecture of the downstream reaches in the top 5 m of mid channel bars shows i an increase in the abundance and thickness up to m scale of laterally extensive 100s of metres fine grained layers ii an increase in the percentage of deposits comprised of ripple sets to 40 in the upper bar deposits and iii an increase in bar trough deposits and a corresponding decrease in bar scale cross strata

fluvial archive of peculiar avulsive fluvial patterns in the largest quaternary intracratonic basin of tropical south america the bananal basin central brazil abstract the bananal basin is the most important intracratonic quaternary sedimentary basin of south america and preserves a good record of the paleohydrological conditions of central brazil at the cerrado amazon ecotone this area is currently a large tropical plain seasonally flooded by rainfall and water table saturation that can be classified as a seasonal wetland this paper presents a geological and geomorphological analysis supported with geochronological dating aiming to reconstruct the pleistocene environmental history of the bananal basin the basin was filled by the sediments sourced by the araguaia mortes fluvial system the deposits belong to the araguaia formation which covers 106 000 km 2 of the bananal basin tl and osl chronologies of the araguaia formation show that sedimentation started at least in the middle pleistocene mis 7 but aggradation and avulsion processes were important in the lower middle pleniglacial between 70 5 ka and 34 0 4 6 ka mis 4 and 3 and in the upper pleniglacial between 24 5 3 1 and 17 2 2 3 ka mis 2 river avulsions were more intense in the lower bananal basin bananal island region remarkably we did not record ages between 17 ka and 10 ka although avulsion seems to have been more active in the past the abandonment by the araguaia of a channel belt now occupied by the underfit javaes river indicates that avulsion is still active the avulsion processes are mainly related to palaeohydrological changes although present day seismic activity is also recorded additional data is needed to generate a more detailed scenario for the older middle pleistocene deposits the data is remarkable however because it is the first middle pleistocene record described in a large alluvial tropical system in brazil the fluvial record is temporally in agreement with records from other large brazilian fluvial systems such as the amazon and parana basins the bananal plain however is unique because of its particular style of sedimentation and geomorphologic processes that generated a mega scale anabranching like complex pattern of paleochannels presently occupied by underfit streams

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

causes of rapid mixing at a junction of two large rivers rio parana and rio paraguay argentina 1 airborne and satellite observations show that when large rivers join they can take hundreds of kilometers to mix completely but on occasion may mix very rapidly application of established semitheoretical analyses shows that long mixing lengths should be expected the first measurements of mixing processes at a large river junction rio parana and rio paraguay argentina combined width 2 8 km are presented at two occasions first when they mix in 400 km and second when mixing is complete in only 8 km downstream of the junction for the case of slower mixing at a point surveys showed that mixing driven by turbulent shear associated with a near vertical shear layer was restricted to close to the junction to 0 272 multiples of the postconfluence width downstream transect surveys showed penetration of more turbid water from the rio paraguay underneath the rio parana but this was insufficient to promote more rapid mixing there was no clear channel scale circulation present and slow mixing was compounded by reverse topographic forcing on the mainstream rio parana side of the river this kept more turbid water on the rio paraguay side of the river close to the bed in the case of rapid mixing we found clear channel scale circulation the momentum ratio between the combining flows reinforced the effects of the discordance in bed height between the tributaries at the confluence and allowed penetration of more turbid rio paraguay water further across the channel width deeper within the flow the importance of the interaction between momentum ratio and bed morphology at channel junctions makes mixing rates at the confluence dependent upon basin scale hydrological response which is more likely to differ between large confluent rivers than small rivers as a result of the different climatic topographic zones that they may capture

architecture of channel belt deposits in an aggrading shallow sandbed braided river the lower niobrara river northeast nebraska abstract architecture of recent channel belt deposits of the niobrara river northeast nebraska usa records the response of a sandy braided river to rapid base level rise up to 3 m of aggradation has occurred within the lower 14 km of the niobrara river since the mid 1950s as a result of base level rise at the confluence of the niobrara and missouri rivers aerial photographs and channel surveys indicate that the lower niobrara has evolved from a relatively deep stable channel with large bank attached braid bars to a relatively shallow aggrading channel with braid bars and smaller secondary channels architecture of channel belt deposits associated with the recent aggradation has been defined using ground penetrating radar gpr and vibracores the channel belt deposits exhibit a series of amalgamated channel fills and braid bar complexes i e macroforms radar facies identified in the gpr data represent architectural elements of the braid bar complexes large and small bedforms two dimensional 2 d and three dimensional 3 d dunes and channels individual braid bars appear to consist of basal high flow and upper low flow components preservation of the complete high flow bar geometry is generally incomplete due to frequent migration of smaller scale secondary channels within the channel belt i e braided channel network at low discharges the large scale stratification of the braid bar deposits is dominated by cross channel and upstream accretion elements of downstream accretion are also recognized these accretion geometries have not been documented previously in similar sandy braided rivers braid bar deposits with low flow modification e g incision by secondary channels are recognized in the deeper portions of the deposits imaged by gpr preservation of braid bars with both high and low flow components is a result of the rapid base level rise and channel bed aggradation experienced by the niobrara river over the past 45 years recent avulsion of the river channel allowed preservation of the upper low flow component of the braid bar deposits i e bar top sequences the relative abundance and stratigraphic position of the amalgamated channels and braid bar complexes within the channel belt deposits constitute a signature of the recent base level rise

a sea level curve for the canadian beaufort shelf radiocarbon dated peat and peaty clay samples from geotechnical boreholes in the canadian beaufort continental shelf have been used to reconstruct a late quaternary relative sea level rsl curve the samples were carefully selected and evaluated using palynological techniques to ensure that reasonable age error limits could be given to each sample the dated samples were then related to the local geological setting using seismic profiles to determine the environment of deposition the resulting data show a rise of 140 m in rsl since 27 000 years bp a minor lowering of rsl at some time between 20 000 and 10 000 years bp is inferred from acoustic data contributions from basin subsidence sediment loading and consolidation account for 35 m of the total rsl rise the rsl curve is interpreted in the light of recent models of the isostatic and eustatic responses of the earth s crust at the laurentide ice sheet margin ice may have been more extensive during the middle wisconsinan than previously thought a

modelling hydrodynamics in the rio parana argentina an evaluation and inter comparison of reduced complexity and physics based models applied to a large sand bed river abstract depth averaged velocities and unit discharges within a 30 km reach of one of the world s largest rivers the rio parana argentina were simulated using three hydrodynamic models with different process representations a reduced complexity rc model that neglects most of the physics governing fluid flow a two dimensional model based on the shallow water equations and a three dimensional model based on the reynolds averaged navier stokes equations flow characteristics simulated using all three models were compared with data obtained by acoustic doppler current profiler surveys at four cross sections within the study reach this analysis demonstrates that surprisingly the performance of the rc model is generally equal to and in some instances better than that of the physics based models in terms of the statistical agreement between simulated and measured flow properties in addition in contrast to previous applications of rc models the present study demonstrates that the rc model can successfully predict measured flow velocities the strong performance of the rc model reflects in part the simplicity of the depth averaged mean flow patterns within the study reach and the dominant role of channel scale topographic features in controlling the flow dynamics moreover the very low water surface slopes that typify large sand bed rivers enable flow depths to be estimated reliably in the rc model using a simple fixed lid planar water surface approximation this approach overcomes a major problem encountered in the application of rc models in environments characterised by shallow flows and steep bed gradients the rc model is four orders of magnitude faster than the physics based models when performing steady state hydrodynamic calculations however the iterative nature of the rc model calculations implies a reduction in computational efficiency relative to some other rc models a further implication of this is that if used to simulate channel morphodynamics the present rc model may offer only a marginal advantage in terms of computational efficiency over approaches based on the shallow water equations these observations illustrate the trade off between model realism and efficiency that is a key consideration in rc modelling moreover this outcome highlights a need to rethink the use of rc morphodynamic models in fluvial geomorphology and to move away from existing grid based approaches such as the popular cellular automata ca models that remain essentially reductionist in nature in the case of the world s largest sand bed rivers this might be achieved by implementing the rc model outlined here as one element within a hierarchical modelling framework that would enable computationally efficient simulation of the morphodynamics of large rivers over millennial time scales

morphology and flow fields of three dimensional dunes rio parana argentina results from simultaneous multibeam echo sounding and acoustic doppler current profiling 1 most past studies of river dune dynamics have concentrated on two dimensional 2 d bed forms with constant heights and straight crest lines transverse to the flow and their associated turbulent flow structure this morphological simplification imposes inherent limitations on the interpretation and understanding of dune form and flow dynamics in natural channels where dune form is predominantly three dimensional for example studies over 2 d forms neglect the significant influence that lateral flows and secondary circulation may have on the flow structure and thus dune morphology this paper details a field study of a swath of 3 d dunes in the rio parana argentina a large 0 35 km wide 1 2 km long area of dunes was surveyed using a multibeam echo sounder mbes that provided high resolution 3 d detail of the river bed simultaneous with the mbes survey 3 d flow information was obtained with an acoustic doppler current profiler adcp revealing a complicated pattern of dune morphology and associated flow structure within the swath dune three dimensionality appears intimately connected to the morphology of the upstream dune with changes in crest line curvature and crest line bifurcations junctions significantly influencing the downstream dune form dunes with lobe or saddle shaped crest lines were found to have larger more structured regions of vertical velocity with smaller separation zones than more 2 d straight crested dunes these results represent the first integrated study of 3 d dune form and mean flow structure from the field and show several similarities to recent laboratory models of flow over 3 d dunes

application of a roughness length representation to parameterize energy loss in 3 d numerical simulations of large rivers 1 recent technological advances in remote sensing have enabled investigation of the morphodynamics and hydrodynamics of large rivers however measuring topography and flow in these very large rivers is time consuming and thus often constrains the spatial resolution and reach length scales that can be monitored similar constraints exist for computational fluid dynamics cfd studies of large rivers requiring maximization of mesh or grid cell dimensions and implying a reduction in the representation of bedform roughness elements that are of the order of a model grid cell or less even if they are represented in available topographic data these subgrid elements must be parameterized and this paper applies and considers the impact of roughness length treatments that include the effect of bed roughness due to unmeasured topography cfd predictions were found to be sensitive to the roughness length specification model optimization was based on acoustic doppler current profiler measurements and estimates of the water surface slope for a variety of roughness lengths this proved difficult as the metrics used to assess optimal model performance diverged due to the effects of large bedforms that are not well parameterized in roughness length treatments however the general spatial flow patterns are effectively predicted by the model changes in roughness length were shown to have a major impact upon flow routing at the channel scale the results also indicate an absence of secondary flow circulation cells in the reached studied and suggest simpler two dimensional models may have great utility in the investigation of flow within large rivers

on the relationship between flow and suspended sediment transport over the crest of a sand dune rio parana argentina the links between large scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades with past studies illustrating the origin of such turbulence and its influence on flow resistance sediment transport and bedform morphology in this study of turbulence and sediment suspension over large sand dunes in the rio parana argentina time series of three dimensional velocity and at a point suspended sediment concentration and particle size were measured with an acoustic doppler current profiler and laser in situ scattering transmissometer respectively these time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment the analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections regions of high suspended sediment concentration are found to correlate well with such events the frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations such a finding suggests that a combination and interaction of vortex shedding and wake flapping changing length of the lee side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures including information on the topology of dune related turbulent flow structures at the flow stage investigated the turbulent flow events and their associated high suspended sediment concentrations are seen to grow with height above the bed until a threshold height ca 0 45 flow depth is reached above which they begin to decay and dissipate

decimeter scale in situ mapping of modern cross bedded dune deposits using parametric echo sounding a new method for linking river processes and their deposits 1 collecting high resolution data to quantify the sedimentary architecture within contemporary alluvial channels remains one of the outstanding challenges in fluvial sedimentology here we present data collected using a new geophysical method the parametric echo sounder pes which can meet this challenge from surveys over a field of sand dunes in the rio parana argentina we demonstrate the unique ability of pes to image the subsurface structure within active channels at a decimetric resolution these data reveal the bounding surfaces between bars and dunes as well as the foresets and reactivation surfaces within them this provides quantitative in situ data for recent work that suggests cross strata preserved by dunes may be more related to flow depths less than the commonly assumed bankfull level these surveys demonstrate that pes can provide hitherto unobtainable data from alluvial channels and presents significant opportunities for more detailed coupled studies of fluvial processes and their deposits

suspended sediment transport and deposition over a dune rio parana argentina field data from the rio parana argentina are used to examine patterns of suspended sediment transport over a sand dune measurements of three dimensional velocity are made with an acoustic doppler current profiler whilst suspended sediment concentration and particle size have been quantified using a laser in situ sediment scattering transmissometer suspended sediment concentration and streamwise and vertical sediment flux are highest close to the bed with an upward vertical flux over the stoss side of the dune and downward flux over the lee side suspended sediment concentrations are higher over the crest compared with the trough and suspended sediment is coarsest near the bed about 17 of the suspended load transported over the crest is deposited in the lee side before it reaches the trough most of this deposited sand is coarser sediment that originates close to the bed over the crest a result consistent with simulations based on the model of mohrig and smith water resources research 1996 32 3207 3217 for the excursion lengths of sediment dispersed in the lee side of a dune copyright 2009 john wiley sons ltd

how do we identify big rivers and how big is big abstract big rivers are the trunk rivers that carry the water and sediment load from major orogens or that drain large areas of a continent identifying such rivers in the ancient record is a challenge some guidance may be provided by tectonic setting and sedimentological evidence including the scale of architectural elements and clues from provenance studies but such data are not infallible guides to river magnitude the scale of depositional elements is the most obvious clue to channel size but evidence is typically sparse and inadequate and may be misleading for example thick fining upward successions may be tectonic cyclothems two examples of the analysis of large ancient river systems are discussed here in order to highlight problems of methodology and interpretation the hawkesbury sandstone triassic of the sydney basin australia is commonly cited as the deposit of a large river on the basis of abundant very large scale crossbedding an examination of very large outcrops of this unit including a coastal cliff section 6 km long near sydney showed that even with 100 exposure there are ambiguities in the determination of channel scale it was concluded in this case that the channel dimensions of the hawkesbury rivers were about half the size of the modern brahmaputra river the tectonic setting of a major ancient fluvial system is commonly not a useful clue to river scale the hawkesbury sandstone is a system draining transversely from a cratonic source into a foreland basin whereas most large rivers in foreland basins flow axially and are derived mainly from the orogenic uplifts e g the large tidally influenced rivers of the athabasca oil sands alberta epeirogenic tilting of a continent by the dynamic topography process may generate drainages in unexpected directions for example analyses of detrital zircons in upper paleozoic mesozoic nonmarine successions in the sw united states suggests significant derivation from the appalachian orogen many large rivers in se asia flow along structural trends generated during the himalayan orogeny e g brahmaputra jamuna irrawaddy mekong red but evidence for large channel systems is not always present where it might be expected for example in the gulf of thailand up to 8 km of cenozoic nonmarine sediments have been recorded in fault bounded basins but seismic time slice images of the pliocene holocene section indicate rivers of modest dimensions the existing major trunk river the chao phraya which empties into the gulf at bangkok may be but a remnant of the mekong which was tectonically diverted to its present course at some point during the late cenozoic

hydraulic and morphological characteristics of middle and upper reaches of the parana river argentina and brazil the parana river basin is a 2 600 000 km2 south american fluvial system most of which is in brazil and argentina the channel is about 4000 km long the mean annual discharge is 17 000 m3 s 1 and the annual sediment load is more than 150 000 000 tons the basin of the paraguay river its main tributary covers about 38 of the parana basin this paper compares the upper and middle channels of the parana hydraulic morphological and sedimentological features of the parana river upstream and downstream of the confluence with the paraguay were analyzed in order to evaluate the changes in these variables the effect of the paraguay on water and suspended sediment discharges was examined the paraguay river contribution explains most of the variation in channel conditions upstream and downstream of the confluence the channel pattern however remains braided the high bed load of the parana river in proportion to the total sediment discharge 21 in the upper course 25 in the middle course determines the similarity in drainage

facies and architectural analysis of channel sandy macroforms in the upper parana river abstract the upper parana river in the area under analysis has a multichanneled pattern with many islands and channel bars channel bars are sandy macroforms classified as central and lateral bars this paper presents a sedimentological analysis of these bars according to their textures facies and facies architecture central bars are unstable deposits situated in the highest flow energy sites of the main channel with intensive processes of deposition and erosion they are composed of cross stratified medium to coarse sand sp and st facies lateral bars are very stable macroforms situated in the lower flow energy areas with no or very few erosional processes where flaser and climbing rippled fine sand to mud sr so fl and fm facies are the most common sediments

the geochemistry during management of lake acidification caused by the rewetting of sulfuric ph 4 acid sulfate soils abstract understanding the geochemistry and kinetics of acidification events arising from acid sulfate soils is important to enable effective management and risk assessment large scale exposure and oxidation of acid sulfate soils occurred during a drought in the lower lakes murray darling basin of south australia we examined the geochemical changes that occurred in one region boggy lake that experienced surface water acidification and was subsequently neutralised via aerial limestone caco3 dosing and dilution via natural lake refill very low ph

tributary distributary and other fluvial patterns what really represents the norm in the continental rock record abstract this paper evaluates the recent claim made by weissmann et al 2010 that deposits of distributive fluvial systems dfs may form the bulk of the continental fluvial record weissmann et al 2010 define dfs as a pattern of channel and floodplain deposits that radiate outward from an apex that is located where the river enters the sedimentary basin as such dfs are fan like systems on which multiple channels coexist and distribute water and sediment across the fan published criteria for the recognition of dfs weissmann et al 2010 are 1 a radial pattern of channels from the dfs apex 2 downslope decreases in channel size 3 down dfs grain size decreases and 4 a lack of lateral channel confinement however in weissmann et al 2010 and subsequent papers only the first of these criteria is applied rigorously thus allowing a variety of types of fluvial system including avulsive incised anabranching and coastal plain distributary systems to potentially be classified uncritically as dfs an ancient succession formed by dfs should preserve mostly small channel bodies with a limited range of dimensions and no outsized channel bodies channel bodies in dfs dominated successions should also decrease in size in a down palaeoslope direction and show palaeocurrent relationships indicative of radial dispersal in order for the term dfs to be applied these characteristics should also be established within stratigraphic bodies that formed coevally however very few examples have yet been published that satisfy these criteria if the claim that dfs dominate the alluvial stratigraphic record is valid then few if any sedimentary bodies formed by large channels should be preserved a review of published research demonstrates however that the alluvial stratigraphic record contains a broad diversity of fluvial network styles including abundant representatives of tributary avulsive anabranching and incised in addition to distributary types it is also clear that the deposits of large main stem rivers that are comparable to the big rivers of the modern world are abundantly preserved in the stratigraphic record an analysis of the planimetric area of modern tributary and distributary fluvial systems is presented demonstrating that tributary systems are also likely to represent a far larger proportion of the ancient sedimentary record than dfs the body of evidence presented herein from modern and ancient alluvial systems thus suggests that the hypothesis of weissmann et al 2010 should be wholly rejected dfs do not dominate the continental stratigraphic record

sediment characteristics and transportation dynamics of the ganga river abstract understanding of river systems that have experienced various forcing mechanisms such as climate tectonics sea level fluctuations and their linkages is a major concern for fluvial scientists the 2525 km long ganga river derives its fluvial flux from northern part of the indian subcontinent and drops in the ganga brahmaputra delta and the bengal fan regions this paper presents a study of the ganga river sediments for their textural properties grainsize characteristics and transportation dynamics a suite of recently deposited sediments 189 bedload samples and 27 suspended load samples of the river and its tributaries was collected from 63 locations dry and wet sieve methods of grainsize analysis were performed and folk and ward s parameters were calculated transportation dynamics of the sediment load was assessed by means of channel hydrology flow sediment rating curves bedform mechanics grainsize images and cumulative curves textural properties of the bedload sediments of the ganga river tributaries originating from the himalaya orogenic belt the northern indian craton and the ganga alluvial plain regions are characterised by the predominance of fine to very fine sand medium to fine sand and very fine sand to clay respectively downstream textural variations in the bedload and suspended load sediments of the ganga river are therefore complex and are strongly influenced by lateral sediment inputs by the tributaries and channel slope at the base of the himalaya a very sharp gravel sand transition is present in which median grainsize of bedload sediments decreases from over 0 16 to 2 46 within a distance of 35 km downstream decline in mean grainsize of bedload sediments in the upper ganga river within the alluvial plain can be expressed by an exponential formula as mean grainsize in 0 0024 distance in kilometres from the himalayan front 1 29 it is a result of selective transport phenomena rather than of abrasion the condition attributed to channel competence with low gradient of 70 to 25 cm km coarsening of the bedload sediments in the middle segment of the river develops due to the coarse bedload sediments mean grainsize 1 8 2 2 supply from the tributaries of the northern indian craton region and increased unit stream power the ganga river sediments show a wide range of grainsize parameters bedload sediments mean grainsize 1 5 3 8 standard deviation 0 3 0 7 skewness 0 2 0 5 and kurtosis 0 8 2 4 suspended load sediments mean grainsize 2 9 5 0 standard deviation 0 6 1 6 skewness 0 6 0 4 and kurtosis 0 7 4 6 mean grainsize of the bedload sediments is 2 6 which is 1 3 units coarser than the suspended load sediments the cumulative grainsize curves of the bedload sediments show a distinct break in slope representing nearly 80 of the bedload sediment transported as graded suspension during the peak monsoon discharge most of the bedload sediments are transported as the discontinuous fine sandy graded suspension by the unidirectional unsteady shooting flow at a high transport rate about 8 106 tons d a model of sedimentation transportation at river channel is proposed the ganga river mobilises a total of 729 106 tons of sediments annually through a narrow zone within its river valley under the present hydrogeological conditions the river sedimentation is climatically controlled and is predicted to produce a 2000 km long 2 to 40 km wide and 25 to 50 m thick ribbon shaped well sorted symmetrically skewed fine sand body

the late quaternary evolution of the negro river amazon brazil implications for island and floodplain formation in large anabranching tropical systems abstract the rio negro has responded significantly in the late pleistocene and holocene to lagged environmental changes largely associated with activity during the last glacial in the amazon basin on the basis of geological structure the rio negro can be divided into six distinct reaches that each reflects very marked differential processes and geomorphological styles no deposits of the upper pleniglacial were recognized in the field the oldest recognizable late pleistocene alluvial unit is the upper terrace of middle pleniglacial age ca 65 25 ka reach i tentatively correlated with the oldest terrace identified on the left bank of reach iii at that time the river was mainly an aggradational bed load system carrying abundant quartz sand a product of more seasonal conditions in the upper catchment the late glacial 14 10 ka is represented by a lower finer grained terrace along the upper basin reach i which was recognized in the tiquie curicuriari and vaupes rivers at that time the river carried abundant suspended load as a response to climatic changes associated with deglaciation since about 14 ka the river has behaved as a progradational system infilling in downstream series a sequence of structurally controlled sedimentary basins or compartments creating alluvial floodplains and associated anabranching channel systems reach ii was the first to be filled then reach iii both accumulating mainly sand fine deposits increase downstream in reach iii and become predominant in some anabranch islands of the distal reach the lowermost reaches of the negro v and vi have been greatly affected by a rising base level and associated backwater effect from aggradation of the amazon during late glacial and recent times reach v has acted almost entirely as a fine sediment trap the remarkable anavilhanas archipelago is the product of holocene deposition in the upper part of this sedimentary basin however suspended sediment load declined about 1 5 ka prior to the lower part of this basin becoming infilled the progradational behavior of the rio negro filling tectonic basins as successive sediment traps with sand in the upper basins and fines in the downstream ones illustrates how a large river system responses to profound changes in late quaternary base level and sediment supply the most stable equilibrium conditions have been achieved in the holocene in reaches iib and iiia where an anabranching channel and erosional relictual island system relatively efficiently convey water and sediment downstream reaches iiib and v never achieved equilibrium conditions during the holocene characterised as they are today with incomplete floodplains and open water

downstream fining in large sand bed rivers abstract downstream fining of bed sediments is a well known phenomenon in gravel bed rivers but also occurs in large sand bed rivers the underlying processes however are not necessarily the same due to the difference in sediment mobility grain size distribution width and geographical setting downstream fining studies focusing on large sand bed rivers are scarce nevertheless in this paper the processes that affect downstream fining in gravel bed rivers are reviewed and it is evaluated to what extent they are relevant for large sand bed rivers furthermore several new downstream fining processes are discussed ending in a conceptual model of downstream fining in large sand bed rivers in deriving the conceptual model downstream fining processes are divided into three categories abrasion selective transport and sediment addition extraction abrasion rates in large sand bed rivers are small due to the durable lithologies the small grain size the high degree of rounding the dominance of grinding as abrasion process and the dominant suspended transport mode of the bed sediment only in the upstream part of large sand bed rivers abrasion rates may be somewhat higher selective transport rates in large sand bed rivers everywhere overrun abrasion rates but are small compared to selective transport rates in gravel bed rivers because the unimodal sediments in sand bed rivers exhibit stronger hiding exposure effects during threshold of motion conditions but especially because many sand bed rivers though not all are in state of fully mobilised transport making differences in threshold of motion between coarse and fine grains relatively unimportant selective transport in these rivers is probably mainly the result of the presence of suspended load transport in combination with the effects of dune and perhaps bend sorting which are all processes that act on a relatively large scale within sand bed rivers the degree of selective transport probably decreases downstream due to the change in mixture bimodality and sorting efficiency there are indications that coarse grains may sometimes even be more mobile than fine grains in the downstream part of sand bed rivers note that in contrast to abrasion selective transport only produces a stable downstream fining pattern if the river profile is concave generally a stronger profile concavity leads to a higher aggradation rate and a stronger downstream fining trend sediment addition and extraction processes can obscure the effects of abrasion and selective transport in sand bed rivers overbank sedimentation for instance may significantly decrease the downstream fining rate in large sand bed rivers because especially fine grains are removed from the bed dredging can also strongly affect the downstream fining trend depending on the volume and the grain size of the sediment involved meander migration may cause a nett loss of coarse grains from the channel in aggrading circumstances increasing the downstream fining rate river bifurcations can cause a discontinuity in the downstream fining pattern because bend sorting upstream of the bifurcation causes the river branch that originates in the outer bend of the main channel to be much coarser than the other branch all these addition and extraction processes not only have a direct effect on the grain size composition of the river bed but also indirectly influence downstream fining by their effect on the longitudinal bed profile other sediment addition and extraction processes such as tributary confluences non alluvial sediment sources and dumping are less common in large sand bed rivers but can locally have a great effect a full determination of the relative importance of all downstream fining processes in large sand bed rivers requires extensive field measurements the construction of detailed fractionwise sediment balances and the development of numerical downstream fining models that incorporate all processes described above