glacial lake invermere upper columbia river valley british columbia a paleogeographic reconstruction terraces of thick lacustrine silt and deltaic gravel flank parts of the valley floor of the rocky mountain trench between skookumchuck and donald british columbia these indicate the presence of f

longitudinal facies architectural changes of a middle holocene anastomosing distributary system rhine meuse delta central netherlands abstract longitudinal changes in channel planform and facies architecture occur over a 22 km long reach of an anastomosing middle holocene distributary system in the rhine meuse delta central netherlands a geological map and a sequence of cross sections show that the channel belt is characterized by sand body width thickness ratios which decrease within the study area from approximately 25 upstream to well below 10 downstream accompanied by a change in channel planform as indicated by a downstream increase of the number of crevasse splays the lithological composition of the sand body is relatively homogeneous but there is always a well developed fining upward sequence from sandy channel deposits to clayey natural levee deposits within channel vertical aggradation appears to have been restricted during most of the period of activity of the distributary system and since lateral accretion was also limited especially in the downstream part rapid accumulation of channel deposits seems to have occurred predominantly during the avulsion stage leaving a narrow and shallow residual channel as the last remnant the longitudinal change in channel pattern is mainly attributed to a downstream increase in the rate of ground water level rise that resulted from holocene sea level rise in the rhine meuse delta a rate of ground water level rise of 1 5 mm yr 1 is required for the creation of fully developed anastomosing fluvial systems ribbons with a width thickness ratio of

log jam effects on bed load mobility from experiments conducted in a small gravel bed forest ditch abstract bed load transport experiments have been conducted in a steep gravel bed open ditch this initially straight ditch has been neglected for many years and looks at present like a second order natural stream channel the channel flows through a spruce forest and several log jams have produced chutes and pools creating supplementary roughness the total shear stress has been evaluated using the slope hydraulic radius product and the ratio between grain and bed form shear stresses has been calculated using different methods the shear stress has also been evaluated from the shear velocities and this gives a good evaluation of the grain shear stress additional experiments have been conducted with marked pebbles to estimate particle mobility and to improve the motion equations equations such as c a di d50 b b defined by andrews apply in these cases but the values of a and b are lower than those produced by this author in a second stage of experimentation we have destroyed the log jams resulting in a diminution of the roughness and critical shear stress when the total shear stress is used an increase of the grain shear stress and thus greater bed particle mobility for the same discharge it emerges from these experiments that the log jams contribute to the reduction of bed load evacuation and explain the very weak bed load discharge measured by a bed load trap 0 3 t km 2 yr 1

morphology of red creek wyoming an arid region anastomosing channel system red creek in the red desert area of the great divide basin wyoming is an arid region anastomosing stream the narrow deep and sinuous main channel is flanked by anastomosing flood channels or anabranches most anabranches are initiated at meander bends the primary mechanism of anabranch initiation is avulsion during overbank floods anabranch enlargement occurs by headward erosion anabranches act as distributary channels during floods when water and sediment from overbank flows are transported to and deposited on the floodplain via the anabranches during periods of low discharges the anabranches act as tributaries to the main channel transporting runoff from the floodplain and surrounding hillslopes to the main channel of red creek aggradation is occurring in the main channel and on the floodplain throughout the study reach infilling of the main channel occurs primarily by lateral accretion while the floodplain accretes vertically through deposition of overbank sediment from the main channel and anabranches infilling of the main channel may cause avulsion of the main channel into an anabranch the abandoned main channel segment may then fill completely or act as an anabranch because lateral migration of channels is inhibited by the high cohesion of the silt and clay channel sediment periodic avulsion is the primary form of lateral mobility in the system

four large scale field aligned current systems in the dayside high latitude region a system of four current sheets of large scale field aligned currents facs was discovered in the data set of simultaneous viking and dmsp f7 crossings of the dayside high latitude region this paper reports four examples of this system that were observed in the prenoon sector the flow polarities of facs are upward downward upward and downward from equatorward to poleward the lowest latitude upward current is flowing mostly in the cps precipitation region often overlapping with the bps at its poleward edge and is interpreted as a region 2 current the pair of downward and upward facs in the middle of the structure are collocated with structured electron precipitation the precipitation of high energy 1 kev electrons is more intense in the lower latitude downward current sheet the highest latitude downward flowing current sheet is located in a weak low energy particle precipitation region suggesting that this current is flowing on open field lines simultaneous observations in the postnoon local time sector reveal the standard three sheet structure of facs sometimes described as region 2 region 1 and mantle referred to the midday region 0 currents a high correlation was found between the occurrence of the four fac sheet structure and negative interplanetary magnetic field imf by we discuss the fac structure in terms of three types of convection cells the merging viscous and lobe cells during strongly negative imf by two convection reversals exist in the prenoon sector one is inside the viscous cell and the other is between the viscous cell and the lobe cell this structure of convection flow is supported by the viking electric field and auroral uv image data based on the convection pattern the four fac sheet structure is interpreted as the latitudinal overlap of midday and morning fac systems we suggest that the four current sheet structure is common in a certain prenoon local time sector during strongly negative imf by

the geometry and organization of fluviodeltaic channel sandstones in the jurassic saltwick formation yorkshire england within the middle jurassic saltwick formation of yorkshire england 65 of the fluvial channel sandstones n 22 have width to thickness ratios in the range 17 30 more than 90 lie in the range 17 67 and are classified as narrow sheet channel sandstones a broad sheet channel sandstone c 15 m thick 3 km wide width to thickness ratio of 200 occurs at the base of the formation in the proximal part of the studied depositional system the thicknesses of the channel sandstones are in the range 4 m to more than 22 m with 90 in the range 4 12 m the widths of the channel sandstone sections are in the range 100 800 m with 80 between 100 and 400 m in plan view the channel sandstones display relatively constant thicknesses across the main part 80 of the channel body channel sandstones are usually multistorey the storeys are separated by major scour surfaces with depths of scour ranging from 2 m to 8 10 m the individual storeys are 4 13 m thick and transverse and oblique storey sections have lateral extents of 60 480 m vertical stacking of storeys is accompanied by a decrease in the width to thickness ratio of the channel sandstones whereas lateral stacking of storeys is accompanied by an increase mudstones and siltstones within channel sandstone bodies occur at the top of storeys and were in most cases deposited during channel abandonment the saltwick formation was deposited during increasing accommodation forming a transgressive systems tract the channel sandstone to interchannel sediment ratio is highest in the lower parts of the transgressive systems tract the quantitative geometry and architecture data from the saltwick formation may be useful for describing and modelling humid delta plain hydrocarbon reservoirs of transgressive systems tracts

a revised chronology for mississippi river subdeltas radiocarbon measurements by accelerator mass spectrometry relating to three of the four late holocene mississippi river subdeltas yielded consistent results and were found to differ by up to 2000 carbon 14 years from previously inferred ages these geological dataare in agreement with archaeological carbon 14 data and stratigraphic ages based on ceramic seriation and were used to develop a revised chronologic framework which has implications for prehistoric human settlement patterns coastal evolution and wetland loss and sequence stratigraphic interpretations

variations in fluvial deposition on an alluvial plain an example from the tongue river member of the fort union formation paleocene southeastern powder river basin wyoming u s a abstract the tongue river member of the paleocene fort union formation is an important coal bearing sedimentary unit in the powder river basin of wyoming and montana we studied the depositional environments of a portion of this member at three sites 20 km apart in the southeastern part of the basin six lithofacies are recognized that we assign to five depositional facies categorized as either channel or interchannel wetlands environments 1 type a sandstone is cross stratified and occurs as lenticular bodies with concave upward basal surfaces these bodies are assigned to the channel facies interpreted to be the product of low sinuosity streams 2 type b sandstone occurs in parallel bedded units containing mudrock partings and fossil plant debris these units constitute the levee facies 3 type c sandstone typically lacks internal structure and occurs as tabular bodies separating finer grained deposits these bodies represent the crevasse splay facies 4 gray mudrock is generally nonlaminated and contains ironstone concretions these deposits constitute the floodplain facies 5 carbonaceous shale and coal are assigned to the swamp facies we recognize two styles of stream deposition in our study area laterally continuous complexes of single and multistoried channel bodies occur at our middle study site and we interpret these to be the deposits of sandy braided stream systems in the two adjacent study sites single and multistoried channel bodies are isolated in a matrix of finer grained interchannel sediment suggesting deposition by anastomosed streams a depositional model for our study area contains northwest trending braided stream systems avulsions of these systems created anastomosed streams that flowed into adjacent interchannel areas we propose that during late paleocene a broad alluvial plain existed on the southeastern flank of the powder river basin the braided streams that crossed this surface were tributaries to a northward flowing basin axis trunk stream that existed to the west

avulsion deposits in lower eocene alluvial rocks bighorn basin wyoming abstract eocene alluvial rocks in the northern bighorn basin can be subdivided into three major kinds fine grained deposits on which cumulative paleosols developed lithologically heterogeneous packages associated with immature paleosols and major sheet sandstone bodies the heterogeneous packages which alternate vertically with the cumulative paleosols are dominated by fine grained deposits and also contain ribbon sandstone bodies and thin 1 m sheet sandstone bodies most ribbon sandstones are less than 3 m thick and are more common in the middle and upper parts of the packages cut and fill deposits with fine grained fills are locally important because the lithologic packages are readily apparent in the field because of the immature or simple paleosols developed on the fine grain d facies they are referred to as simple paleosol packages the major sheet sandstones were deposited by meandering trunk rivers and locally overlie and truncate the other deposits contrary to classical models of meandering streams only a portion of the fine grained rocks in the study area were deposited by overbank flooding of a trunk channel on the basis of their stratigraphic position and few ribbon sandstones the upper parts of some simple paleosol packages are interpreted as ancient levee deposits associated with a trunk channel farther from the trunk channel overbank floods deposited alluvium on which the cumulative paleosols formed the greater parts of the simple paleosol packages including their fine grained deposits were deposited rapidly during channel avulsion ribbon sandstone bodies represent ancient splay channels on the avulsion belt simple paleosol packages in the willwood formation support the contention of smith et al 1989 that n many meandering river systems a significant portion of the fine grained deposits were deposited as the trunk channel avulsed deposits that resemble the simple paleosol packages may provide a means of recognizing avulsion episodes in other alluvial sequences

controls on the accumulation of coal and on the development of anastomosed fluvial systems in the cretaceous dakota formation of southern utah alluvial strata of the cretaceous dakota formation of southern utah are part of a transgressive systems tract associated with a foreland basin developed adjacent to the sevier orogenic belt these strata contain valley fill deposits anastomosed channel systems and widespread coals the coals constitute a relatively minor part of the dakota formation in terms of sediment volume but may represent a substantial amount of the time represented by the formation the coals are separated by clastic units up to 20 m thick the stratigraphically lowest clastic unit of the dakota formation lies above an unconformity cut into jurassic rocks incised valleys associated with the unconformity are up to 12 m deep two discrete episodes of valley fill sedimentation are recognized including a lower sandstone unit with conglomerate layers and an upper discontinuous coal bearing unit after the valleys filled the area became one of low relief where extensive mires formed peat accumulation was interrupted at least three times by deposition of clastic sediment derived from the west the clastic units consist of sandstone mudstone or heterolithic ribbon bodies stacked tabular sandstones and laminated mudstones and contain minor coal beds less than 0 35 m thick ribbon bodies are 1 9 m thick and 15 160 m wide have pronounced basal scours and are filled with both lateral and vertical accretion deposits an anastomosed channel complex is suggested by the large number of coeval channels of varying dimensions the variation in the structure and grain size of channel fills and the presence of abundant tabular sandstones interpreted as crevasse splays although some sandstone bodies have well developed lateral accretion surfaces the overall ribbon geometry indicates that mature meandering streams were not well developed this is in contrast to modern anastomosed systems which are commonly thought to be a transitional morphology caused by avulsion of a meander belt to a new position on its floodplain rather than being a transitional channel pattern related to river avulsion the anastomosed channels of the dakota formation may have formed part of a large inland delta that episodically invaded widespread mires the mires developed during periods when clastic influx was reduced either by high rates of subsidence close to the thrust belt or by deflection of rivers by emergent thrusts

three dimensional model of alluvial stratigraphy theory and applications abstract a three dimensional model of alluvial stratigraphy has been developed to simulate the spatial distribution proportion and connectedness of coarse grained channel belt deposits in alluvial strata as a function of channel belt width floodplain width bankfull channel depth channel belt and overbank sedimentation rates avulsion location and period compaction and tectonism tilting and faulting in this model a floodplain surface of variable width and length is occupied by a single channel belt changes in floodplain topography are produced by spatial and temporal variation of channel belt and floodplain deposition rates and by compaction and local or regional tectonism the location and timing of avulsions are determined by local changes in floodplain slope relative to channel b lt slope and by flood magnitude and frequency the diverted channel belt follows the locus of maximum floodplain slope at the end of each simulation architectural parameters are calculated including channel deposit proportion and connectedness and the dimensions of channel belt sandstone bodies three dimensional perspective diagrams mesh surfaces and two dimensional stratigraphic sections can be plotted to illustrate depositional surfaces time planes and the location and geometry of coarse grained channel belt deposits within finer grained overbank deposits the model predicts that channel belt proportion and connectedness and dimensions of sandstone bodies vary as a function of distance from avulsion points and cross section orientation upstream from avulsion points sandstone bodies have low width thickness ratios because of aggradation in a fixed channel belt immediately downstream from avulsion points channel belts tend to be connected resulting in sandstone bodies with high width thickness ratios avulsion sequences develop where points of avulsion shift up valley with a progressive decrease in avulsion period such sequences may produce successions in which channel belt proportion and connectedness vary vertically with a cyclic period of 103 to 105 years down valley increases in aggradation rate or down valley decreases in floodplain slope for example associated with a rise in base level may result in an increase in channel belt proportion and connectedness because of high avulsion frequencies in down valley regions of the floodplain down valley decreases in aggradation rate as in alluvial fans foreland basins and during base level fall may result in high avulsion frequencies in up valley parts of the floodplain tectonic tilting and faulting locally increase avulsion probabilities and channel belts generally shift toward areas of maximum subsidence under certain conditions however depositional topography may cause channels to shift away from areas of maximum subsidence channel deposit proportion and connectedness are gen rally high near downthrown areas of the floodplain but distribution clustering of channel belts may not be a reliable indicator of fault geometry or displacement models of alluvial architecture that consider only sediment accumulation rate as the main controlling factor are oversimplified the three dimensional model presented here predicts many of the features of channel behavior observed in modern rivers but there is a pressing need for better models and adequate natural data to test them

middle and late holocene avulsion history of the river rhine rhine meuse delta netherlands at present little evidence is available on avulsion rates avulsion frequencies and interavulsion periods of aggrading fluvial systems over time scales of millenia the large number of high quality 14c ages from the holocene rhine meuse delta central netherlands enables a detailed assessment of such avulsion parameters and an evaluation of the factors controlling avulsion frequency 14c histogram analysis indicates that instantaneous avulsion coexistence of successive channel belts for less than a few centuries has dominated in this area interavulsion periods were typically 1000 yr although values of 500 or 5000 yr have also been found avulsion frequency was especially large prior to 4300 cal yr b p indicating a strong control by aggradation rate in turn controlled by sea level rise in addition periods with high avulsion frequency appear to correlate with time intervals characterized by large scale transgressive tendencies the time interval with the largest number of coexisting distributaries corresponds to the period with dominance of anastomosing fluvial style

anastomosing river deposits sedimentation rates and basin subsidence magdalena river northwestern colombia south america situated in a tectonically active foreland basin the magdalena river consists of vertically accreting levee confined channels and adjacent extensive wetlands which are interpreted as an anastomosing river sedimentary system equivalent rates of basin filling and subsidence average 3 8 mm yr 1 based on 18 14c dates from five bore holes drilled to depths of 55 m and sediment transport budgets from 35 years of measurement located in a savanna tropical climate anastomosing river deposits of the magdalena are remarkably similar to the anastomosing deposits of the upper columbia river in a temperate cold climate in western canada suggesting that climate is not a controlling factor of anastomosis the geometry of anastomosing channel fills in the magdalena consists of stratigraphically non uniform low sinuous narrow stringers of sand up to 30 m thick by 600 m wide a width depth ratio of 20 thin 1 2 m off channel crevasse splay sand sheets extend laterally up to 10 km distance when buried both sand deposits become encased by lacustrine or marsh mud to form stratigraphic traps while there are few modern anastomosing river systems as compared to braiding and meandering there may be a disproportionately large number of ancient anastomosed fluvial rock sequences due to the rapid rate of vertical accretion such a different depositional style and geometry of sand bodies have considerable significance in the interpretation of some ancient fluvial rock sequences because it provides an alternative to the meandered and braided river deposition models

origin of reversible cementation and brittleness of the fragipan horizon in albeluvisols of the carpathian foothills poland abstract fragipan horizon is a natural subsurface soil layer exhibiting higher bulk density and lower porosity in comparison with the overlying horizons significantly restricting infiltration of water and root penetration in contrast to hard and cemented horizons e g duripan clods of the fragipan horizon btx slake or fracture immediately when placed in water the most interesting issue concerning genesis and properties of the fragipan horizon is its reversible cementation i e very hard consistence in dry state and brittle consistence in moist state the main objective of the present study was to explain and improve the understanding of the specific consistence of the fragipan horizon in moderately humid climate on the example of albeluvisols of the carpathian foothills in poland basing on mineral and chemical composition of bridges linking coarse soil material quantitative x ray diffraction shows that the studied fragipan horizon contains high concentration of swelling clay minerals soil material from the fragipan horizon exhibits numerous clay and iron clay coatings and clay infillings indicating translocation of colloidal clay and iron oxides from overlying horizons in addition bridges between coarse grains of the soil material occur the fragipan cementing material is composed mainly of sio2 al2o3 and fe2o3 with lower concentration of k2o mgo cao and na2o the chemical composition of bridges within the pan is similar to that of clay coatings and clay infillings suggesting that clay minerals are major bonding agents and are responsible for brittleness and more friable consistence of the pan in the moist state additionally free iron aluminium and silica may play some role in specific cementation of the investigated fragipan horizon but these elements are not main bonding agents

floodplain sedimentation quantities patterns and processes this paper presents the result of measurements of floodplain sedimentation using sediment traps the study was carried out on two embanked floodplains along the rivers rhine and meuse in the netherlands during a 3 day flood in january 1993 raster maps of sediment accumulation were made by interpolating the measurements from the traps using block kriging the sediment maps show clear patterns in sediment accumulation together with the estimated interpolation errors average sediment accumulation ranges between 0 57 and 1 0 kg m 2 high sediment accumulation is found on the levees 4 kg m 2 or more and on low lying areas 1 6 kg m 2 sediment accumulation decreases with distance from the main channel the sedimentation patterns are related to floodplain topography and sediment transporting mechanisms sediment transport by turbulent diffusion as well as by convection can be recognized also flood duration and the process of sediment settling out in ponding water in closed depressions are important the applied method allows comparison of the results with raster based sedimentation models

fine grained splay deposition in the avulsion belt of the lower saskatchewan river canada abstract in or about 1883 a northward avulsion of the meandering lower saskatchewan river at the cumberland marshes initiated a broad belt of alluvial sedimentation that has continued to evolve to the present and now covers some 500 km2 the avulsion belt comprises a complex alluvial terrain dominated by individual and coalesced splays and associated wetlands connected by networks of active and abandoned channels of various sizes the windy lake splay a predominantly fine grained feature typical of large parts of the avulsion belt was examined to investigate the relationships between splay evolution and facies development the splay now largely inactive mostly formed over a 35 year period depositing nearly 2 m of sediment over a 7 2 km2 area the splay surface is ch racterized by stable well defined anastomosed distributary channels that separate interchannel wetland basins heights of channel levees diminish downstream and the levees merge with channel mouth bars whose progradation forms the locus of lakeward channel extension and the base for subsequent subaerial levee deposition the anastomosed channel pattern was created when the mouth bars of separate distributary channels merged during progradation vibracore and soil probe profiles show the splay to be dominated by upward coarsening lacustrine prodeltaic mouth bar facies sequences capped by levees and organic rich interchannel wetland deposits channels now containing sand only sporadically in narrowed or slightly migrated reaches will eventually fill with fine sediment when abandonment s complete the windy lake splay is typical of stage iii splays smith et al 1989 whose deposits and associated lacustrine fills form at least half of the avulsion belt thin widespread predominantly upward coarsening sequences capped by either nondepositional horizons or true overbank fines may be characteristic of many avulsive suites in the ancient alluvial record

late weichselian and holocene palaeogeography of the rhine meuse delta the netherlands abstract a large number of lithological borehole descriptions 14c dates archaeological artefacts and gradients of palaeochannels were used to reconstruct the holocene development of the rhine meuse delta data were stored in a geographical information system database that enables generation of palaeogeographical maps for any time during the holocene the time resolution of the palaeogeographic reconstruction is about 200 years during the holocene avulsion was an important process resulting in frequent shifts of areas of clastic sedimentation palaeogeographic evolution of the rhine meuse delta is mainly governed by complex interactions among several factors these are 1 location and shape of the late weichselian palaeovalley 2 sea level rise which resulted in back filling of the palaeovalley 3 peat formation which was most extensive in the western part of the back barrier area especially between 4000 and 3000 14c yr bp this more or less fixed the river pattern at that time and resulted in few avulsions 4 neotectonics differential tectonic movements of the peel horst and roer valley graben seem to have influenced river behaviour formation of an asymmetrical meander belt location of avulsion nodes especially from 4500 to 2800 14c yr bp when the rate of sea level rise had decreased after 2800 14c yr bp the sea level rise further decreased and the tectonic influence still may have influenced avulsions but from then on other factors became dominant 5 increased discharge sediment load and or within channel sedimentation after 2800 14c yr bp river meanders of rhine distributaries as well as the single channel of the meuse show remarkable increases in wavelength interpreted as a result of increased bankfull discharge and sediment load increased discharge may initially have been caused by a higher precipitation after 2000 14c yr bp both discharge and sediment load seem to have increased as a result of human influence alternatively decreasing gradients as a result of sea level rise may have caused an increased within channel sedimentation and channel widening which would also lead to increased meander wavelengths 6 composition of the river banks meandering river channels tend to adhere to the sandy margins of the late weichselian palaeovalley and a high channel sinuosity is found in areas where river banks consist of sand 7 marine ingressions e g the ad 1421 st elizabeth s flood caused large scale erosion in the southwestern part of the fluvial deltaic plain 8 human influence since about ad 1100 human influence dominated the palaeogeographic evolution rivers were embanked and dredged meanders were cut off and for the meuse a new distributary canal the bergsche maas was dug in ad 1904 discharge distribution over the various channels is strictly controlled nowadays an unusual feature of the delta is the prolonged existence of the oude rijn channel belt

the 1870s avulsion of the saskatchewan river several major avulsions of the saskatchewan river have occurred in the cumberland marshes east central saskatchewan during the past few thousand years the most recent avulsion occurred in about the 1870s converting over 500 km2 of floodplain into a belt of anastomosing channels splay complexes and small lakes a region that is still evolving today the avulsion began near the tip of a large meander bend by following a small outflowing creek sturgeon river which in turn followed an abandoned former channel of the saskatchewan river flow began to permanently divert out of the saskatchewan when a narrow strip of floodplain separating the sturgeon river from the nearby torch river became breached diversion into the connected sturgeon torch began to increase sometime in the 1870s and probably culminated around 1882 the triggering event for the avulsion may have been a chute cutoff of the meander bend shown by numerical modeling experiments to have significantly raised water surface elevations at th

experimental study of avulsion frequency and rate of deposition in existing models of alluvial architecture it is typically assumed that mean avulsion frequency is independent of sedimentation rate however if avulsion is driven by superelevation of a river bed above its surrounding flood plain one might expect avulsion rate to increase with sedimentation rate we have carried out a series of experiments with laboratory scale fluvial fans in which we measured the frequency of apical avulsions as a function of mean sedimentation rate on the fan avulsion frequency increased strongly with increasing sedimentation rate and then stabilized as mass flows began to influence deposition in the regime of increasing avulsion frequency the added volume of sediment needed to trigger an avulsion decreased with increasing sedimentation rate although our experimental results cannot simply be scaled up to natural rivers they suggest the possibility of coupling between avulsion frequency and sedimentation rate that would be strong enough to change qualitatively the results of existing models of alluvial architecture these models should be applied with caution until avulsion mechanics are better understood

avulsions channel evolution and floodplain sedimentation rates of the anastomosing upper columbia river british columbia canada ages of channels of the anastomosing upper columbia river south eastern british columbia canada were investigated in a cross valley transect by c 14 dating of subsurface floodplain organic material from beneath levees the avulsion history within the transect was deduced from these data and morphological stages in channel development were recognized additionally floodplain sedimentation rates were established the new data demonstrate that the upper columbia river is a long lived dynamic anastomosing system results show that anastomosis at the study location has persisted since at least 2700 cal years bp with avulsions occurring frequently at least nine channels have formed in the studied cross valley transect within the past 3000 years channel lifetimes from formation to abandonment appear to be highly variable ranging from approximately 800 to 3000 years log jams provoking avulsions and or silting up of old channels are proposed as reasons for this variability long term average floodplain sedimentation rates appear to be significantly lower than previously proposed by smith 1983 int assoc sedimentol spec publ 6 155 168 a long term 4550 years average of 1 75 mm per year after compaction was based on c 14 dates whilea short term sedimentation rate of 0 8 mm was determined for a single relatively small seasonal flood in 1994 using sediment traps however short term sedimentation rates vary considerably over the floodplain with levees aggrading up to four times faster than floodbasins channels of the upper columbia river anastomosed reach follow a consistent pattern in their development with each stage being characterized by different morphology and processes channel evolution comprises the following succession 1 avulsion stage in which a crevasse splay channel deepens by scour and levee sedimentation 2 widening and deepening stage in which bank slumping and bed scouring dominates 3 infilling stage in which either channel narrowing bank accretion or channel shallowing bed accretion takes place and 4 abandonment stage in which the residual remnant channel is filled exclusively by silt clay and organic material vertical stacking super imposition of active channels on recent channel fill sand bodies is a notable feature of the upper columbia river which suggests that reoccupation of residual channels is a common process