flow and suspended sediment division at two highly asymmetric bifurcations in a river delta implications for channel stability the division of sediment at river bifurcations results from the complex interaction between three dimensional flow planform and channel bed morphology as well as the heterogeneity of the bed material sediment division processes cannot be incorporated in their full complexity in scale experiments and are difficult to reproduce with numerical models field measurements are thus necessary to advance our understanding of those processes in river deltas however such measurements are rare we present measurements of the flow and sediment division at two tidally influenced bifurcations of the kapuas river a large sand bedded suspended load dominated river in west kalimantan indonesia at both bifurcations a smaller channel branches off from the side of the main river which makes the planform strongly asymmetric the planform of both bifurcations has been stable at least since the end of the nineteenth century when the region was mapped for the first time based on our measurements we explore possible factors that stabilize the bifurcations we measure the flow velocities with a boat mounted acoustic velocity profiler and determine the sediment concentration from acoustic backscatter calibrated against water samples the side branch of the first bifurcation receives a proportionally lower fraction of sediment than water in contrast the side branch at the second bifurcation receives a proportionally higher fraction of sediment than water a comparison of flow velocity and suspended sand concentration indicates that the bed material sorting strongly influences the division of sediment in particular at one of the bifurcations which is situated in a meander bend

the late quaternary palaeoecology of a floristic kingdom the southwestern cape south africa abstract the southwestern cape province of south africa is the core region of the cape floristic kingdom one of six such large scale phytogeographical units and by far the smallest the region possesses one of the richest plant communities in the world when measured in terms of plant species numbers per unit area and many different hypotheses have been advanced in order to explain this prolific diversity historical processes and evolutionary development through time are frequently invoked as explanations but remarkably little is known of the historical biogeography of the region and fossil bearing sediments have yielded only a fragmented picture of the temporal development of the flora late quaternary terrestrial sediments offer an opportunity to investigate the more recent history of the flora although here too there exist several constraints which limit the precision of palaeoecological and palaeoenvironmental reconstructions for the moment knowledge of late quaternary vegetation history is restricted to upland sites of organic sediment accumulation although an increasingly detailed picture of vegetation development against a backdrop of environmental changes is emerging this paper reviews the evidence from several fossil pollen bearing sediments in the mountains of the southwestern cape and presents a vegetation history for the last 14 500 years we draw the somewhat surprising conclusion of relative vegetation stability during the time of marked climatic and environmental dynamism elsewhere the hypothesis that assumed limited vegetation change could be a consequence of high species richness coupled with low pollen identification resolution is tested using multiple discriminant analysis it is argued that apparently subtle shifts in vegetation community patterns did indeed prevail during the late quaternary this possibly has some important implications for the evolution and maintenance of high plant species diversity in the cape floristic region

four pleistocene formations in southwest british columbia their implications for patterns of sedimentation of possible sangamonian to early wisconsinan age semiahmoo and dashwood drifts were deposited during the penultimate glaciation and highbury sediments and muir point formation during the last interglacial these sediments are defined and described and stratotypes are established for them based on arboreal pollen assemblages lithologic similarity and relative stratigraphic position semiahmoo drift of the fraser lowland is tentatively correlated with dashwood drift of vancouver island stone provenance till fabrics glaciotectonic structures cross bedding and stone imbrication indicate that regional ice and meltwater flowed out of major fiords in the coast mountains and into the ancestral strait of georgia during the penultimate glaciation ice flowed southeastward against the east coast of vancouver island and down the axis of the strait pollen and molluscs in semiahmoo and dashwood fossiliferous muds give evidence of transition from glaciomarine to marine conditions at the close of the penultimate glaciation highbury sediments of the fraser lo

a high resolution 15 600 year pollen and microcharcoal record from the cederberg mountains south africa abstract the cederberg mountains western cape province south africa are located within the fynbos biome which exhibits some of the highest levels of species richness and endemism in the world the region s post glacial vegetation history however remains largely unknown presented here are high resolution pollen and microcharcoal records spanning the last 15 600 years obtained from the de rif rock hyrax midden from the driehoek valley of the central cederberg in this region previous pollen studies have shown muted variability in vegetation community composition during periods of globally marked climatic variability e g the last glacial interglacial transition in our record however significant changes in vegetation composition are apparent most notably they indicate a shift from ericaceous restioid fynbos present from 15 600 to 13 300 cal yr bp to a brief but prominent development of proteoid fynbos at the beginning of the holocene around 11 200 cal yr bp this vegetation shift is associated with increased moisture at the site and coincides with reduced fire frequency as indicated by the microcharcoal record at 10 400 cal yr bp there is a marked reduction in protea type pollen which is replaced by thicket characterised by dodonaea which became the dominant arboreal pollen type this shift was likely the result of a long relatively fire free period coupled with warmer and wetter climates spanning much of the early holocene a brief but marked decrease in water availability around 8500 8000 cal yr bp resulted in the strong decrease of dodonaea pollen the vegetation of the mid to late holocene is characterised by the increased occurrence of asteraceae and succulent taxa suggesting substantially drier conditions these data give unprecedented insight into the vegetation dynamics across a period of substantial rapid climate change and while they confirm the presence of fynbos elements throughout the last 15 600 years the results highlight significant fluctuations in the vegetation that were triggered by changes in both climate and fire regimes

the validity of the asteraceae poaceae fossil pollen ratio in discrimination of the southern african summer and winter rainfall zones abstract situated at the transition between the mid latitudes and the sub tropics southern africa has a climatic dichotomy between winter and summer rainfall zones wrz and srz the latitudinal extent of the winter rainfall zone during the late pleistocene remains contentiously debated within the regional palaeoscience literature one method posited to reflect the seasonality of rainfall at a given location throughout late pleistocene records for south africa is the ratio of fossil pollen asteraceae to poaceae although adopted for a range of southern african locations the veracity of this method has not been tested this study explores the extent to which this ratio can discriminate between the srz and wrz and the extent of the region subject to fluctuation during the late quaternary the ratio is found to successfully discriminate regions which are and would have remained during the past 20 000 cal yr bp in the srz and wrz exclusively on the basis of these statistics it appears that wrz conditions can be inferred from ratio scores 0 6 and srz conditions from scores

flow fields bed shear stresses and suspended bed sediment dynamics in bifurcations of a large river 1 channel bifurcations associated with bars and islands are important nodes in braided rivers and may control flow partitioning and thus affect downstream confluences as well as the formation and dynamics of bars however the morphodynamic processes associated with bar formation are poorly understood and previous studies have largely concerned laboratory experiments small natural streams or numerical analyses with large froude numbers high slopes and low shields stresses in these cases the morphologic changes at bifurcations are relatively rapid with predominant bed load transport and the suspended load playing a minor role in this paper the evolution of the flow structure and suspended bed sediment transport along four expansion diffluence units in the rio parana argentina are described the rio parana is a large multichannel river with a bed composed of medium and fine sands and possesses low froude numbers and high suspended bed material transport primary and secondary flow velocity components were measured with an acoustic doppler current profiler adcp along the expansion diffluence units and the backscatter signal of the adcp was calibrated to allow simultaneous measurements of suspended bed sediment concentrations the interactions between these variables show that the cores of primary flow velocity and suspended bed sediment concentration do not necessarily follow the thalweg at the bifurcation and that inertial effects on the suspended bed sediment may influence the morphodynamics of bar formation it is suggested that changes in flow stage as well as the presence of vegetation may further increase the deposition of suspended bed sediment at the bar head this study suggests that the ratio of suspended bed material to bed load is an important factor controlling the morphodynamics of bifurcations in large sand bed braided rivers

climatic buffering and anthropogenic degradation of a mediterranean type shrubland refugium at its semi arid boundary south africa the mega diverse mediterranean type fynbos biome may be vulnerable to future changes in climate and associated fire regimes in particular to increasing summer drought intensity and associated pot

kulshan caldera a quaternary subglacial caldera in the north cascades washington calderas that collapse during large pyroclastic eruptions are anomalously rare in the cascade arc recognition of the early pleistocene 4 5 8 km kulshan caldera filled with rhyodacite ignimbrite at the northeast foot of mount baker brings to only three the quaternary calderas identified in the cascades a near vertical ring fault cut in basement rocks of the north cascades encloses 30 km 2 of intracaldera ignimbrite and intermixed collapse breccia 1 km thick but with no floor exposed the lake tapps tephra in the puget lowland is the correlative fallout 200 km from the source it is as thick as 30 cm features of the distal ash fall and the intracaldera tuff suggest large scale phreatomagmatism during an eruption that may have started subglacially several advances of the cordilleran ice sheet subsequently obliterated the topographic rim removed every vestige of extracaldera ignimbrite and proximal fallout and stripped any precaldera extrusive rocks the former existence of which is suggested only by a few silicic intrusions that cut the circumcaldera basement although the caldera is not structurally resurgent several early intracaldera rhyodacite lavas intrude and rest directly on ignimbrite or on ashy caldera lake sediments reworked from the eruption products subsidence areas pumice compositions and volumes of magma erupted 50 km 3 are similar for the kulshan rockland and crater lake mazama events the three quaternary caldera forming eruptions now recognized in the cascades

evaluating competing hypotheses for the origin and dynamics of river anastomosis anastomosing rivers have multiple interconnected channels that enclose flood basins various theories potentially explain this pattern including an increased discharge conveyance and sediment transport capacity of multiple channels deltaic branching avulsion forced by base level rise or a tendency to avulse due to upstream sediment overloading the former two imply a stable anabranching channel pattern whereas the latter two imply disequilibrium and evolution towards a single channel pattern in the absence of avulsion our objective is to test these hypotheses on morphodynamic scenario modelling and data of a well documented case study the upper columbia river proportions of channel and floodplain sediments along the river valley were derived from surface mapping initial and boundary conditions for the modelling were derived from field data a 1d network model was built based on gradually varied flow equations sediment transport prediction mass conservation transverse slope and spiral meander flow effects at the bifurcations the number of channels and crevasse splays decreases in a downstream direction also measured sediment transport is higher at the upstream boundary than downstream these observations concur with bed sediment overloading from upstream which can have caused channel aggradation above the surrounding floodplain and subsequent avulsion the modelling also indicates that avulsion was likely caused by upstream overloading in the model multi channel systems inevitably evolve towards single channel systems within centuries the reasons are that symmetric channel bifurcations are inherently unstable while confluenced channels have relatively less friction than two parallel channels so that more discharge is conveyed through the path with more confluences and less friction furthermore the present longitudinal profile curvature of the valley could only be reproduced in the model by temporary overfeeding

chute channel dynamics in large sand bed meandering rivers meander bends of many large sand bed meandering rivers are partitioned by chute channels that convey permanent flow and co exist with the mainstem for decades as a first step toward understanding the dynamics and morphodynamic implications of these bifurcate meander bends this study applied binary logistic regression analysis to determine whether it is possible to predict chute initiation based on attributes of meander bend character and dynamics regression models developed for the strickland river papua new guinea the lower paraguay river paraguay argentina and the beni river bolivia revealed that the probability of chute initiation at a meander bend is a function of the bend extension rate the rate at which a bend elongates in a direction perpendicular to the valley axis trend image analyses of all rivers and field observations from the strickland suggest that the majority of chute channels form during scroll slough development rapid extension is shown to favour chute initiation by breaking the continuity of point bar deposition and vegetation encroachment at the inner bank resulting in widely spaced scrolls with intervening sloughs that are positively aligned with primary over bar flow the rivers plot in order of increasing chute activity on an empirical meandering braided pattern continuum defined by potential specific stream power pv and bedload calibre d50 increasing stream power is considered to result in higher bend extension rates with implications for chute initiation in addition chute stability is shown to depend on river sediment load relative to flow discharge qs q such that while the beni may plot in the region of highly braided rivers by virtue of a high potential specific stream power the formation of stable chute channels is suppressed by the high sediment load this tendency is consistent with previous experimental studies and results in a planform that is transitional between single thread meandering and braided copyright 2011 john wiley sons ltd

palynology and paleotemperature analysis of the whidbey formation puget lowland washington a series of pollen assemblages and the environments of deposition are shown by samples from 143 levels of interbedded sand silt clay and peaty and woody sediments in sections from the pleistocene whidbey formation at useless bay the type locality west beach point wilson and foulweather bluff deposition of the formation is interpreted to have taken place over a broad floodplain under fluvial conditions where drainage shifted between and into tracts of paludified ground of the four sections studied only useless bay and point wilson contain assemblages with analogs found in the modern pollen rain on the floodplains of the nisqually skokomish and chehalis rivers in western washington q mode factor analysis of pollen percentages in the whidbey samples extracted douglas fir pine alder grass and sedge as the principals for 93 of the data stepwise regression relating modern pollen rain and temperature resulted in a transfer function that when applied to the principals for the whidbey produced mean july temperatures for the intervals represented by the sections temperature curves for useless bay and point wilson show maxima comparable with the modern average of 15 8 c maximum values at west beach and foulweather bluff are approximately 2 lower these curves when compared with curves from late quaternary deposits and the interglacial puyallup and alderton formations indicate that sediments belonging to the whidbey are largely from an interglacial age comparable to the holocene or present interglaciation but also from cooler episodes similar to the late fraser glaciation

identification of a kulshan caldera correlative tephra in the palouse loess of washington state northwest usa abstract the kulshan caldera formed at 1 15 ma on the present day site of mt baker washington state northwest usa and erupted a compositionally zoned dacite rhyolite magma and a correlative eruptive the lake tapps tephra this tephra has previously been described but only from the puget lowland of nw washington here an occurrence of a kulshan caldera correlative tephra is described from the quaternary palouse loess at the washtucna site wa 3 site wa 3 is located in east central washington 340 km southeast of the kulshan caldera and 300 km east southeast of the lake tapps occurrence in the puget lowland major and trace element chemistry and location of the deposit at washtucna within reversed polarity sediments indicates that it is not correlative with the mesa falls rockland bishop ash lava creek b or huckleberry ridge tephras instead the washtucna deposit is related to the lake tapps tephra by fractional crystallisation but is chemically distinct a consequence of its eruption from a compositionally zoned magma chamber the correlation of the washtucna occurrence to the kulshan caldera forming eruption indicates that it had an eruptive volume exceeding 100 km3 and that its tephra could provide a valuable early pleistocene chronostratigraphic marker in the pacific northwest

stability morphology and surface grain size patterns of channel bifurcation in gravel cobble bedded anabranching rivers this study presents the first detailed field based analysis of the morphology of bifurcations within anabranching cobble gravel rivers bifurcations divide the flow of water and sediment into downstream anabranches thereby influencing the characteristics of the anabranches and the longevity of river islands the history morphology bed grain size and flow vectors at five bifurcations on the renous river new brunswick canada were studied in detail the angles of bifurcations within five anabranching rivers in the miramichi basin were investigated the average bifurcation angle was 47 within the range of values cited for braided river bifurcations bifurcation angle decreased when anabranches were of similar length shields stresses in channels upstream of bifurcations were lower than reported values for braided rivers stable bifurcations displayed lower shields stresses than unstable bifurcations contrary to experimental results from braided river bifurcations bifurcations in anabranching rivers are stabilized by vegetation that slows channel migration and helps to maintain a uniform upstream flow field the morphology of stable bifurcations enhances their stability a large bar shaped like a shallow ramp that increases in elevation to floodplain level forms at stable bifurcations floodplains at stable bifurcations accrete upstream at rates between 0 9 and 2 5 m a 1 bars may also form within the entrance of an anabranch downstream of the bifurcation node these bars are associated with bifurcation instability forming after a period of stability or an avulsion channel abandonment occurs when a bar completely blocks the entrance to one anabranch the stability of channels upstream of bifurcations and the location of bars at bifurcations influence bifurcation stability and the maintenance of river anabranching in the long term copyright 2006 john wiley sons ltd

bifurcation dynamics and avulsion duration in meandering rivers by one dimensional and three dimensional models at river bifurcations water and sediment are divided over two branches the dynamics of the bifurcation determine the long term evolution centuries of the downstream branches potentially leading to avulsion but the dynamics are poorly understood the long term evolution can only be studied by one dimensional models because of computational costs for such models a relation describing the sediment division is necessary but only few relations are available and these remain poorly tested so far we study the division of sediment and the morphodynamics on a timescale of decades to centuries by idealized three dimensional modeling of bifurcations with upstream meanders and dominantly bed load transport an upstream meander favors one bifurcate with more sediment and the other with more water leading to destabilization the bifurcations commonly attain a highly asymmetrical division of discharge and sediment after a few decades to a few centuries depending on combinations of the relevant parameters although past work on avulsions focused on slope advantage we found that bifurcations can be quasibalanced by opposing factors such as a bifurcate connected to the inner bend with a downstream slope advantage nearly balanced bifurcations develop much slower than unbalanced bifurcations which explains the observed variation in avulsion duration in natural systems which branch becomes dominant and the timescale to attain model equilibrium are determined by the length of the downstream bifurcates the radius of the upstream bend a possible gradient advantage for one bifurcate and notably the width depth ratio the latter determines the character of the bars which may result in overdeepening and unstable bars the distance between the beginning of the upstream bend and the bifurcation determines the location of such bars and pools which may switch the dominant bifurcate in fact when the bifurcation is quasibalanced by opposing factors any minor disturbance or a different choice of roughness or sediment transport predictor may switch the dominant bifurcate the division of sediment is nearly the same as the division of flow discharge in most runs until the discharge division becomes very asymmetrical so that a bifurcate does not close off entirely this partly explains the sustained existence of residual channels and existence of anastomosing rivers and the potential for reoccupation of old channel courses we develop a new relation for sediment division at bifurcations in one dimensional models incorporating the effect of meandering the flow and sediment divisions predicted by two existing relations and the new relation for one dimensional models are in qualitative agreement with the three dimensional model these one dimensional relations are however of limited value for wider rivers because they lack the highly three dimensional bar dynamics that may switch the direction of bifurcation evolution the potential effects of bed sediment sorting bank erosion and levee formation on bifurcation stability and avulsion duration are discussed

late wisconsinan glaciomarine deposition and isostatic rebound northern puget lowland washington the distribution and age of glaciomarine and marine sediment in the northern puget lowland washington demonstrate that rapidretreatofcontinentalice theeverson marineincursion andhighratesofisostatic reboundoccurredbetweenabout13 600and 11 300 14 c yr b p 11 3 ka glaciomarine and marine deposits are thickest in zones where retreating ice lobes grounded in the northeast puget lowland and near large drainages glaciomarine sediment was depositedmainlyfrom 1 submarineoutwash in ice proximal zones 2 turbid underflows dispersed melt water icebergs and resedimentation in transitional zones and 3 dispersedmeltwaterandcurrentsinicedistal zones marine estuarine and emergence intertidalandbeach faciesaccumulated in areas more than 10 km from ice margins particularly near major rivers molluscan and foraminiferal assemblages intheglaciomarineandmarinedepositsindicate that turbid cool brackish water covered much of the puget lowland during the everson interval water was generally shallower 30 m in the southern part of the area and deeper 15 60 m to the north mineralogy and geochemical properties such as boron or sodium content of the gravel free fraction do not clearly distinguish glaciomarine and marine deposits from terrestrial deposits isostatic rebound rapidly lifted the glaciomarine and marine deposits through sea level between about 13 5 and 11 3 ka the presentaltitudesofradiocarbon datedshell and the marine limit show that initial rates of isostatic rebound exceeded 10 cm yr 21 in thenorthernpugetlowland butdroppedto 2cmyr 21 before 11 ka the uplift gradient is about 0 6 m km 21 to the north and steepens locally to at least 1 3 m km 21 the pattern of emergence in the northern puget lowland is anomalous locally perhaps as a result of complex isostatic effects near the glacier margin rapid rise of sea level or tectonic deformation

late quaternary dynamics of southern africa s winter rainfall zone abstract variations in the nature and extent of southern africa s winter rainfall zone wrz have the potential to provide important information concerning the nature of long term climate change at both regional and hemispheric scales positioned at the interface between tropical and temperate systems southern africa s climate is influenced by shifts in the intertropical convergence zone the westerlies and the development and position of continental and oceanic anticyclones over the last glacial interglacial cycle substantial changes in the amount and seasonality of precipitation across the subcontinent have been linked to the relative dominance of these systems central to this discussion has been the extent to which the region s glacial climates would have been affected by expansions of antarctic sea ice equatorward migrations of the westerlies more frequent intense winter storms and an expanded wrz this paper reviews the developing body of evidence pertaining to shifts in the wrz and the evolution of ideas that have been presented to explain the patterns observed dividing the region into three separate axes along the western and southern margins of the continent and across the interior into the karoo and the kalahari a range of evidence from both terrestrial sites and marine cores is considered and potential expansions of the wrz expansions are explored despite the limitations of many of the region s proxy records a coherent pattern has begun to develop of a significantly expanded wrz during phases of the last glacial period with the best documented being between 32 17 ka while more detailed inferences will require the recovery and analysis of longer and better dated records this synthesis provides a new baseline for further research in this key region

pumice in the interglacial whidbey formation at blowers bluff central whidbey island wa usa abstract a new 40ar 39ar age of 128 9 ka and chemical analyses of pumice layers from interglacial alluvium at blowers bluff whidbey island wa show that the deposits are part of the whidbey formation a widespread mainly subsurface unit glass chemistry of the dated dacitic pumice does not match any analyzed northern cascade source but upper pleistocene dacites from glacier peak and early pleistocene silicic rocks from the kulshan caldera are chemically similar the chemistry of pumiceous dacite in younger units including the latest pleistocene partridge gravel is similar to that of the dated material the deep troughs of the modern northern puget lowland must have been filled during deposition of the whidbey formation allowing volcanic rich sediment to reach what is now whidbey island topographic analysis of lidar images demonstrates that extensive erosion occurred during latest pleistocene ice retreat the partridge gravel likely records subglacial fluvial erosion along an ice tunnel and ice marginal deposition into adjacent marine waters pumice in the partridge gravel probably was reworked from stratigraphically and topographically lower deposits including those at blowers bluff

late quaternary peat accumulation in southern africa abstract it is suggested that the accumulation of peat deposits and other organic sediment may be a clue to the timing of moister periods in the late quaternary climatic history of southern africa data presented from 26 sites across the sub continent point to two periods during which organic sedimentation commenced suggesting a moister late pleistocene and a period during the mid late holocene when conditions also became more conducive to peat formation owing to the confusing and conflicting nature of other palaeoenvironmental information organic sediment accumulation chronologies seem to provide additional insight into the environmental history of southern africa and the implications of this are discussed

evolution of a bifurcation in a meandering river with adjustable channel widths rhine delta apex the netherlands rivers may dramatically change course on a fluvial plain such an avulsion temporarily leads to two active channels connected at a bifurcation here we study the effect of dynamic meandering at the bifurcation and the effect of channel width adjustment to changing discharge in both downstream branches on the evolution of a bifurcation and coexisting channels as an example we reconstructed the last major avulsion at the rhine delta apex we combined historical and geological data to reconstruct a slowly developing avulsion process spanning 2000 years and involving channel width adjustment and meandering at the bifurcation based on earlier idealised models we developed a one dimensional model for long term morphodynamic prediction of upstream channel and bifurcates connected at the bifurcation node the model predicts flow and sediment partitioning at the node including the effect of migrating meanders at the bifurcation and channel width adjustment bifurcate channel width adaptation to changing discharge partitioning dramatically slows the pacing of bifurcation evolution because the sediment balance for width adjustment and bed evolution are coupled the model further shows that meandering at the bifurcation modulates channel abandonment or enlargement periodically this explains hitherto unrecognised reactivation signals in the sedimentary record of the studied bifurcation meander belts newly identified in our geological reconstruction historical maps show that bifurcation migration due to meander bend dynamics increases the bifurcation angle which increases the rate of closure of one bifurcate the combination of model and reconstruction identifies the relevant timescales for bifurcation evolution and avulsion duration these are the time required to fill one downstream channel over one backwater length the time to translate one meander wavelength downstream and for strong river banks the adaptation timescale to adjust channel width the findings have relevance for all avulsions where channel width can adjust to changing discharge and where meandering occurs copyright 2011 john wiley sons ltd

splitting rivers at their seams bifurcations and avulsion river bifurcations are critical but poorly understood elements of many geomorphological systems they are integralelements of alluvial fans braided rivers fluvial lowland plains and deltas and control the partitioning of water and sediment throughthese systems bifurcations are commonly unstable but their lifespan varies greatly in braided rivers bars and channels migrate splitand merge at annual or shorter timescales thereby creating and abandoning bifurcations this behaviour has been studied mainly bygeomorphologists and fluid dynamicists bifurcations also exist during avulsion the process of a river changing course on afloodplain or in a delta which may take 102 103years and has been studied mainly by sedimentologists this review synthesizesour current understanding of bifurcations and brings together insights from different research communities and different environmen tal settings we consider the causes and initiation of bifurcations and avulsion the physical mechanisms controlling bifurcation andavulsion evolution mathematical and numerical modelling of these processes and the possibility of stable bifurcations we end thereview with some open questions copyright 2012 john wiley sons ltd

high precision 40ar 39ar dating of oligocene rhyolites from the mogollon datil volcanic field using a continuous laser system replicate 40ar 39ar analyses of sanidine from seven oligocene 28 2 ma rhyolite flows using a continuous laser dating system routinely yield analytical precisions of less than 0 45 1s on samples smaller than 0 5 mg this is as good or better than has been obtained by any other method even though the sample mass is several orders of magnitude smaller the system has the potential of resolving differences in apparent age of as little as 0 2 for high k ca samples as young as a few ma the analyses suggest that the seven volcanic units analyzed were erupted in an interval of about 0 1 m y or less

eruptive history and geochronology of the mount baker volcanic field washington mount baker a steaming ice mantled andesitic stratovolcano is the most conspicuous component of a multivent quaternary volcanic field active almost continuously since 1 3 ma more than 70 packages of lava flows and 110 dikes have been mapped 500 samples chemically analyzed and 80 k ar and 40ar 39ar ages determined principal components are 1 the ignimbrite filled kulshan caldera 1 15 ma and its precaldera and postcaldera rhyodacite lavas and dikes 1 29 0 99 ma 2 60 intracaldera hydrothermally altered andesite dacite dikes and pods remnants of a substantial early postcaldera volcanic center 1 1 0 6 ma 3 unaltered intracaldera andesite lavas and dikes including those capping ptarmigan and lasiocarpa ridges and table mountain 0 5 0 2 ma 4 the long lived chowder ridge focus 1 29 0 1 ma an andesite to rhyodacite eruptive complex now glacially reduced to 50 dikes and remnants of 10 lava flows 5 black buttes stratocone basaltic to dacitic and several contemporaneous peripheral volcanoes 0 5 0 2 ma and 6 mount baker stratocone and contemporaneous peripheral volcanoes 0 1 ma to holocene glacial ice has influenced eruptions and amplified erosion throughout the lifetime of the volcanic field although more than half the material erupted has been eroded liberal and conservative volume estimates for 77 increments of known age yield cumulative curves of volume erupted vs time that indicate eruption rates in the range 0 17 0 43 km3 k y for major episodes and long term background rates of 0 02 0 07 km3 k y andesite and rhyodacite each make up nearly half of the 161 56 km3 of products erupted whereas basalt and dacite represent only a few cubic kilometers each representing 1 3 the total during the past 4 m y the principal magmatic focus has migrated stepwise 25 km southwestward from the edge of the chilliwack batholith to present day mount baker

mid holocene glacier peak and mount st helens we tephra layers detected in lake sediments from southern british columbia using high resolution techniques abstract a glacier peak tephra has been found in the mid holocene sediment records of two subalpine lakes frozen lake in the southern coast mountains and mount barr cirque lake in the north cascade mountains of british columbia canada the age depth relationship for each lake suggests an age of 5000 5080 14 c yr b p 5500 5900 cal yr b p for the eruption which closely approximates the estimated age 5100 5500 14 c yr b p of the dusty creek tephra assemblage found near glacier peak the tephra layer which has not been reported previously from distal sites and was not readily visible in the sediments was located using contiguous sampling magnetic susceptibility measurements wet sieving and light microscopy the composition of the glass in pumice fragments was determined by electron microprobe analysis and used to confirm the probable source of this mid holocene tephra layer using the same methods the a d 1481 1482 mount st helens we tephra layer was identified in sediments from dog lake in southeastern british columbia suggesting the plume drifted further north than previously thought this high resolution method for identifying tephra layers in lake sediments which has worldwide application in tephrachronologic paleoenvironmental studies has furthered our knowledge of the timing and airfall distribution of holocene tephras from two important cascade volcanoes

simulated climate and biomes of africa during the late quaternary comparison with pollen and lake status data new compilations of african pollen and lake data are compared with climate ccm1 ncar boulder and vegetation biome 1 2 gsg lund simulations for the last glacial maximum lgm and early to mid holocene emh the simulated lgm climate was ca 4 c colder and drier than present with maximum reduction in precipitation in semi arid regions biome simulations show lowering of montane vegetation belts and expansion of southern xerophytic associations but no change in the distribution of deserts and tropical rain forests the lakes show lgm conditions similar or drier than present throughout northern and tropical africa pollen data indicate lowering of montane vegetation belts the stability of the sahara and a reduction of rain forest the paleoenvironmental data are consistent with the simulated changes in temperature and moisture budgets although they suggest the climate model underestimates equatorial aridity emh simulations show temperatures slightly less than present and increased monsoonal precipitation in the eastern sahara and east africa biome simulations show an upward shift of montane vegetation belts fragmentation of xerophytic vegetation in southern africa and a major northward shift of the southern margin of the eastern sahara the lakes indicate conditions wetter than present across northern africa pollen data show an upward shift of the montane forests the northward shift of the southern margin of the sahara and a major extension of tropical rain forest the lake and pollen data confirm monsoon expansion in eastern africa but the climate model fails to simulate the wet conditions in western africa

life of a bifurcation in a gravel bed braided river channel bifurcation is a key element in braided rivers determining the water and sediment distribution and hence controlling the morphological evolution recent theoretical and experimental findings as well as field observations showed that bifurcations in gravel bed braided rivers are often asymmetrical and highly unstable in this paper field data are presented on a bifurcation in the tagliamento river northeast italy the planform configuration of the bifurcation and its temporal evolution was monitored by an automatic digital camera during a series of seven floods with different magnitudes this remote sensing technique allowed a high temporal resolution pictures were acquired every hour that was proved to be essential in a highly dynamic system as the one considered here digitized maps of the channels provided information on the location of the bifurcation the width of the anabranches the angle between them along with the occurrence and migration of sediment bars data were acquired at two different water levels giving the possibility to compare low and high flow conditions the monitored bifurcation is largely unstable and shows sudden changes in the water distribution mainly driven by the bar migrating in the upstream channel and entering the distributaries a relationship between width asymmetry and flood magnitude was observed confirming previous analyses moreover recent theoretical findings were applied in order to test the possibility to estimate general trends in bifurcation evolution the analysis pointed out the relevance of a correct assessment of the characteristic temporal scales as the bifurcation evolves on a timescale similar to that of bar migration and flood duration understanding the interactions between these processes is therefore crucial in order to increase the ability to model and predict the morphological evolution of a braided network copyright 2012 john wiley sons ltd

a 19 5 kyr vegetation history from the central cederberg mountains south africa palynological evidence from rock hyrax middens abstract in arid and semi arid areas such as southern africa rock hyrax procavia capensis middens represent an exceptionally valuable source of late quaternary palaeoenvironmental information pollen and stable isotope data derived from two rock hyrax middens extracted from de rif in the cederberg mountains of the southwestern cape casts new light on climatic changes that occurred across the last glacial interglacial transition lgit and the holocene 19 5 0 7 cal kbp and how the region s vegetation responded to these changes significant changes in vegetation community composition would be expected in response to environmental changes characteristic of the contrast between glacial and interglacial climates however the pollen assemblages in general indicate that mountain fynbos remained dominant throughout the record and that most of the pollen taxa exhibited only muted frequency variations this finding could be considered to be inconsistent with indications of marked climatic variations recorded in the stable isotope records for the same midden an analysis of the geological setting and the bioclimatic affinities of the taxa suggest that the explanation for this inconsistency may lie in the dominating influence of sandstone substrates and the relative resilience of mountain fynbos vegetation to climate change

coeval brittle and ductile deformation beneath the late wisconsinan puget lobe washington state usa late wisconsinan glacial sediments exposed on whidbey island and camano island puget sound washington state usa were deposited in a proglacial shallow marine outwash environment during northward retreat of the puget lobe of the cordilleran ice sheet sediments mainly comprise massive and cross bedded sand and gravels and rhythmically bedded clay and silt fine sand couplets interbedded with diamictons that were deposited by a range of mass flows of different viscosities although sediment stratigraphy and ice advance retreat patterns are well established for the puget lobe brittle and ductile deformation structures within and separating these sediment units are less well understood these structures record the nature of ice bed interactions taking place in subglacial and proglacial environments this study examines evidence for these processes and environments key deformation structures identified include open to overturned folds normal and reverse faults clastic dikes and hydrofractures and passive loading structures evidence for coeval development of ductile and brittle deformation structures shows the close relationship between porewater changes sediment rheology and sediment system responses to changes in strain caused by ice bed interactions

radiocarbon age constraints on rates of advance and retreat of the puget lobe of the cordilleran ice sheet during the last glaciation abstract calibrated radiocarbon dates of organic matter below and above till of the last fraser glaciation provide limiting ages that constrain the chronology and duration of the last advance retreat cycle of the puget lobe in the central and southeastern puget lowland seven dates for wood near the top of a thick proglacial delta have a weighted mean age of 17 420 90 cal yr b p which is the closest limiting age for arrival of the glacier near the latitude of seattle a time distance curve constructed along a flowline extending south from southwestern british columbia to the central puget lowland implies an average glacier advance rate of ca 135 m yr the glacier terminus reached its southernmost limit ca 16 950 yr ago and likely remained there for ca 100 yr in the vicinity of seattle where the glacier reached a maximum thickness of 1000 m ice covered the landscape for ca 1020 yr postglacial dates constraining the timing of ice retreat in the central lowland are as old as 16 420 cal yr b p and show that the terminus had retreated to the northern limit of the lowland within three to four centuries after the glacial maximum the average rate of retreat was about twice the rate of advance and was enhanced by rapid calving recession along flowline sectors where the glacier front crossed deep proglacial lakes

the southern whidbey island fault an active structure in the puget lowland washington information from seismic reflection profiles outcrops boreholes and potential field surveys is used to interpret the structure and history of the southern whidbey island fault in the puget lowland of western washington this northwest trending fault comprises a broad as wide as 6 11 km steep northeast dipping zone that includes several splays with inferred strike slip reverse and thrust displacement transpressional deformation along the southern whidbey island fault is indicated by along strike variations in structural style and geometry positive flower structure local unconformities out of plane displacements and juxtaposition of correlative sedimentary units with different histories the southern whidbey island fault represents a segment of a boundary between two major crustal blocks the cascade block to the northeast is floored by diverse assemblages of pre tertiary rocks the coast range block to the southwest is floored by lower eocene marine basaltic rocks of the crescent formation the fault probably originated during the early eocene as a dextral strike slip fault along the eastern side of a continental margin rift bending of the fault and transpressional deformation began during the late middle eocene and continues to the present oblique convergence and clockwise rotation along the continental margin are the inferred driving forces for ongoing deformation evidence for quaternary movement on the southern whidbey island fault includes 1 offset and disrupted upper quaternary strata imaged on seismic reflection profiles 2 borehole data that suggests as much as 420 m of structural relief on the tertiary quaternary boundary in the fault zone 3 several meters of displacement along exposed faults in upper quaternary sediments 4 late quaternary folds with limb dips of as much as 9 5 large scale liquefaction features in upper quaternary sediments within the fault zone and 6 minor historical seismicity the southern whidbey island fault should be considered capable of generating large earthquakes m s 7 and represents a potential seismic hazard to residents of the puget lowland

tidal impacts on the subtidal flow division at the main bifurcation in the yangtze river delta flow division at bifurcations in the yangtze estuary has received ample attention since it may control the pathways of terrestrial sediments over downstream river branches including the 12 5 m deepwater navigation channel while some efforts have been made to interpret flow division at the bifurcations of the yangtze estuary little attention has been paid to the role of tides flow division at estuarine bifurcations is made complicated by tides that propagate from the outlet of the tidal channels into the delta to quantify the tidal influence on the distribution of river discharge and more generally to understand the mechanisms governing the subtidal flow division at the tidally affected bifurcation in the yangtze river delta a two dimensional hydrodynamic model is employed in this model the landward boundary is chosen beyond the tidal limit where the tidal motion has faded out entirely the seaward boundary is chosen such that the river discharge does not influence the water level subtidal discharges are decomposed using the method of factor separation to distinguish between the effects of tides river discharge and river tide interactions on the subtidal flow division results indicate that tides modify the river discharge distribution over distributary channels in the yangtze river delta particularly in the dry season a significant difference in the subtidal flow division during spring tide and neap tide shows that the tidally averaged flow division over the distributaries in the delta greatly depends on tidal amplitude by varying the river discharge at the landward boundary and amplitudes and phases of the principal tidal constituents at the seaward boundary of the established model the sensitivities of the subtidal flow division to the river discharge and tidal amplitude variation were investigated in detail generally the tidal impacts on the subtidal flow division are around 12 to 22 with river discharge varying from 30 000 m3s 1 to 20 000 m3s 1 this effect on the flow distribution can even overwhelm the effects induced by river discharge based on geometry only when the flow discharge is lowest furthermore the fortnightly tidal cycle plays an important role in enhancing the inequality of the subtidal flow division caused by the m2 tidal component solely at the tidal bifurcation in the yangtze river delta during low flow

lake tapps tephra an early pleistocene stratigraphic marker in the puget lowland washington abstract the rhyolitic lake tapps tephra was deposited about 1 0 myr ago shortly after culmination of the early phase of the salmon springs glaciation in the puget lowland it is contained within sediments that were deposited in ponds or lakes in front of the reteating glacier an herb dominated tundra existed in the southern puget lowland at that time lake tapps tephra is most likely the product of an eruption that in part was phreatomagmatic it forms an early pleistocene stratigraphic marker across the southern sector of the puget lowland and provides a link between puget lobe sediments of the cordilleran ice sheet and sediments deposited by olympic alpine glaciers

bifurcation modelling in a meandering gravel sand bed river the rhine bifurcation at pannerden forms the major distribution point for water supply in the netherlands distributing not only water and sediment but also flooding risks and navigability its morphological stability has been a concern for centuries we present experiences from more than two decades of numerical morphological modelling of this bifurcation with a gravel sand bed and a meandering planform successive computations have shown the importance of upstream approach conditions the necessity to include physical mechanisms for grain sorting and alluvial roughness and the need to assume a thicker active layer of the river bed than is suggested by laboratory flume experiments using a constant discharge the active layer must be thicker in the model to account for river bed variations due to higher frequency discharge variations that are filtered out in morphological modelling we discuss limitations in calibration and verification but argue that notwithstanding these limitations 2d and 3d morphological models are valuable tools not only for pragmatic applications to engineering problems but also for revealing the limitations of established knowledge and understanding of the relevant physical processes the application of numerical models to the pannerden bifurcation appeared to reveal shortcomings in established model formulations that do not pose particular problems in other cases this application is therefore particularly useful for setting the agenda for further research copyright 2012 john wiley sons ltd

late holocene palaeolimnological and climate dynamics at princessvlei south africa evidence from diatoms the cape flats region situated in the winter rainfall zone of south africa is a low lying tombolo underlain by recent fluvial and aeolian sands and characterised by numerous small lakes and wetlands one of these is princessvlei a eutrophic freshwater coastal lake the lake lies in an inter dunal depression encroached on in the more recent past by high density residential industrial and agricultural land uses a 210 cm core extracted from the lake periphery yielded high diatom fossil concentrations for the upper 174 cm princessvlei appears to oscillate between two ecologically stable states namely a state characterised by clear water oligotrophic benthic communities and a turbid state dominated by eutrophic planktonic species the two stable ecological states are interpreted to be a function of the relative dominance of catchment precipitation or groundwater influx which augments the open water conditions from 2600 to 1500 cal bp 173 135 cm the system is predominantly turbid with greater

channel networks carved by subglacial water observations and reconstruction in the eastern puget lowland of washington channels of inferred subglacial fluvial origin channelways are conspicuous across the recently deglaciated eastern puget lowland of washington state exploiting the unusually well defined geometry of the former puget lobe ice sheet we have reconstructed the subglacial hydraulic head in this region to better understand the location and orientation of these channels we used established principles of channelized water flow under static ice because a three dimensional linear viscous ice model shows that sliding induced pressure variations can be ignored at all but the smallest scales of individual landforms two types of predicted subglacial flow paths emerge one drained continuously from the interior toward the ice sheet margin the other diverted water into a series of valleys in the flanking cascade range forming ice marginal lakes that discharged episodically and probably catastrophically these reconstructed flow paths correlate very well with the independently mapped distribution of channelways and also of ice marginal lakes throughout this area our study suggests that the formation and preservation of these landforms require both the concentration of subglacial water notably near ablation zone ice margins and favorable preglacial bed rock topography where these elements occur together landforms of subglacial fluvial origin should be recognizable across the now exposed beds of other former ice sheets as well

the use of new elevation data srtm aster for the detection and morphometric quantification of pleistocene megadunes draa in the eastern sahara and the southern namib abstract new freely available digital elevation data derived from the shuttle radar topography mission srtm 3 and the advanced spaceborne thermal emission and reflection radiometer aster allow for the first time the use of area wide high resolution elevation models they are useful for various investigations especially in regions that are devoid of detailed topographic maps in this contribution the geomorphological sedimentological geochronological and archaeological results of the cologne collaborative research centre acacia are presented showing that the southern great sand sea of egypt eastern sahara is composed of a complex dune pattern of pleistocene megadunes draa and holocene dunes a conceptual model is presented suggesting draa formed by a strong unimodal trade wind regime that acted most probably as helical roll vortices the draa differ conspicuously in their dimensions and in their evolution from the smaller longitudinal dunes which arise from the recent bimodal wind regime first results of comparative investigations in the southern namib erg namibia yielded similar conditions and a synchronous landscape development based on these results the second part of the paper evaluates the elevation data for both regions with respect to their availability for the determination of draa and dunes and the morphometric quantification of the draa due to their different resolution srtm 3 90 m aster 30 m and their different sensor technology srtm 3 radar aster stereo data both models have assets and drawbacks but can complement each other with the calculation of geomorphometric parameters elevation slope the aster digital elevation model allows the location of the draa of the great sand sea of egypt the quantification of their spatial distribution size volume and consequently the holocene dune movement in conclusion the resolution of the aster elevation model is applicable for quantitative analyses of well defined morphological elements with altitudinal variations 40 50 m the volume estimation presented provides a tool for a realistic assessment of the sand mass balance of sand seas this is not possible with the srtm 3 model and generally impossible with both models if the altitudinal variation is

late quaternary palaeoenvironments of the winter rainfall zone of southern africa palynological and sedimentological evidence from the agulhas plain abstract south africa s winter rainfall zone is a climatically sensitive region lying at the interface of the earth s temperate and sub tropical climatic systems its seasonally arid climate has generally prevented the preservation of long and organic rich sedimentary deposits producing a spatially limited and temporally biased quaternary record this paper seeks to address this issue further and develop a fuller understanding of the wider climatic changes in this region during the late quaternary modern climatic data does not support the definitions of the winter rainfall zone presented in previous syntheses and this factor amongst others may explain difficulties in resolving the palaeoenvironmental record in this region we present palaeoenvironmental evidence from the agulhas plain an area marginal to the modern winter rainfall zone cored sedimentological and palynological records from two pans are integrated with previously reported records of aeolian activity providing evidence for phases of enhanced aridity and humidity during the late quaternary the record is fragmentary which may reflect the cycling of sediment between the pans and their accompanying lunette dunes lacustrine sedimentary sequences dating to c 33 ka 47 ka provide evidence for enhanced humidity consistent with evidence in the winter and year round rainfall zones increased humidity is also recorded from c 2 ka following drier conditions than the present c 2 7 2 5 ka palynological evidence supports the sedimentological interpretations of pan water status and offers rare insights into the nature of the region s unique fynbos vegetation during the late pleistocene

late holocene environments at verlorenvlei western cape province south africa abstract evidence on late quaternary palaeoenvironments of the western and southwestern cape of south africa has facilitated detailed reconstructions mainly for the mountains where organic sediment accumulation has been favoured by relatively humid climates on the lowlands particularly those of the west coast region the search for sites traditionally regarded as suitable for the accumulation of sediments containing preserved fossil pollen has commenced only more recently this area of the western cape province has however provided quaternary scientists with a rich archaeological record that has yet to be tested against independent lines of palaeoecological evidence a sequence of sediments which have accumulated in what is today a large freshwater coastal lake verlorenvlei has been sampled at grootdrift 15 km inland of the coast a series of seven sediment cores was extracted during 1991 and three of these have now been sub sampled radiocarbon dated and subjected to a range of palaeoenvironmental techniques in particular pollen analysis sedimentology and geochemistry three pollen diagrams are presented which reveal the vegetation history of both the immediate grootdrift environment and the wider catchment during several periods over the last 5500 years a mid holocene higher sea level is evident in two of the cores which were examined for pollen throughout their length the surrounding area appears at this time to have been somewhat more arid than today following a hiatus in sedimentation some time after 4300 bp the marine conditions at the site disappear and are replaced by fresh water as the dominant hydrological and ecological influence around the time of colonial occupation of the region some 300 years ago lacustrine conditions prevailed at grootdrift and greater moisture availability is indicated in the catchment during the last 300 years the picture revealed by high resolution palynology of the third core is one of progressive levels of human disturbance both in and around the verlorenvlei evidence from a textural and geochemical analysis of the sediments is consistent with the interpretation based on fossil pollen the implications of this information are examined against the archaeology and palaeoecology of several cave sites in the vicinity of the vlei the consistency apparent in the range of types of evidence suggests that some reliance can be placed on the mid late holocene palaeoenvironmental reconstruction at verlorenvlei the paper thus provides insight into the nature of the complex interactions between late holocene climate change sea level fluctuation vegetation change vlei hydrology and human activity in the western cape coastal region

sediment transport of fine sand to fine gravel on transverse bed slopes in rotating annular flume experiments large scale morphology in particular meander bend depth bar dimensions and bifurcation dynamics are greatly affected by the deflection of sediment transport on transverse bed slopes due to gravity and by secondary flows overestimating the transverse bed slope effect in morphodynamic models leads to flattening of the morphology while underestimating leads to unrealistically steep bars and banks and a higher braiding index downstream however existing transverse bed slope predictors are based on a small set of experiments with a minor range of flow conditions and sediment sizes and in practice models are calibrated on measured morphology the objective of this research is to experimentally quantify the transverse bed slope effect for a large range of near bed flow conditions with varying secondary flow intensity sediment sizes 0 17 4 mm sediment transport mode and bed state to test existing predictors we conducted over 200 experiments in a rotating annular flume with counterrotating floor which allows control of the secondary flow intensity separate from the streamwise flow velocity flow velocity vectors were determined with a calibrated analytical model accounting for rough bed conditions we isolated separate effects of all important parameters on the transverse slope resulting equilibrium transverse slopes show a clear trend with varying sediment mobilities and secondary flow intensities that deviate from known predictors depending on shields number and strongly depend on bed state and sediment transport mode fitted functions are provided for application in morphodynamic modeling

influence of tropical easterlies in southern africa s winter rainfall zone during the holocene abstract south africa s southwestern cape occupies a critical transition zone between southern hemisphere temperate winter and tropical summer moisture bearing systems in the recent geological past it has been proposed that the relative influence of these systems may have changed substantially but little reliable evidence regarding regional hydroclimates and rainfall seasonality exists to refine or substantiate the understanding of long term dynamics in this paper we present a mid to late holocene multi proxy record of environmental change from a rock hyrax midden from katbakkies pass located along the modern boundary between the winter and summer rainfall zones derived from stable carbon and nitrogen isotopes fossil pollen and microcharcoal these data provide a high resolution record of changes in humidity and insight into changes in rainfall seasonality whereas previous work concluded that the site had generally experienced only subtle environmental change during the holocene our records indicate that significant abrupt changes have occurred in the region over the last 7000 years contrary to expectations based on the site s location these data indicate that the primary determinant of changes in humidity is summer rather than winter rainfall variability and its influence on drought season intensity and or length these findings are consistent with independent records of upwelling along the southern and western coasts which indicate that periods of increased humidity are related to increased tropical easterly flow this substantially refines our understanding of the nature of temperate and tropical circulation system dynamics in sw africa and how changes in their relative dominance have impacted regional environments during the holocene

interaction between migrating bars and bifurcations in gravel bed rivers 1 in the present work we investigate the interaction between migrating alternate bars and the dynamics of river bifurcations laboratory experiments are carried out to study a y shaped bifurcation with fixed banks and erodible bed composed of well sorted sand the problem is also analyzed by developing a theoretical one dimensional model results show the occurrence of regular fluctuations in the discharge distribution at the bifurcation node which are strictly related to bar migration the effectiveness of bars in conditioning the bifurcation behavior increases with bar amplitude and decreases with bar migration speed four qualitatively different behaviors of the system are observed as the controlling parameters of the flow are varied within a range significant for gravel bed rivers the theoretical predictions are in good qualitative agreement with the experimental observations

vegetation history inferred from pollen in late quaternary faecal deposits hyraceum in the cape winter rain region and its bearing on past climates in south africa abstract pollen analysis shed light on vegetation community structure over the last 23 000 year in the western cape the pollen from dated hyrax faecal accumulations hyraceum relates to the evolution of climate and contributes to proxy records in southern africa principal components analyses of the pollen data and 13c values of the hyrax dung samples show millennial and shorter scale temperature moisture and seasonality variations in the winter rain region the moisture availability at times do not parallel that in previously studied proxy records in the summer rain region showing an asynchronous moist event in the early holocene and drier conditions in the middle holocene anomalies in climate between the two regions may depend on the degree of northward or southward shifting of winter and summer rain circulation systems scenarios with winter rain or cool growing seasons mostly typify the dung sequence but do not exclude the possibility of southward displacement of the westerly belt under precessional strength with slight summer rain increases during the last glacial maximum limited southward displacement is also possible during the mid holocene

thermoluminescence dating tests for lacustrine glaciomarine and floodplain sediments from western washington and british columbia to help further develop reliable procedures for accurate thermoluminescence tl dating of quaternary waterlaid sediments we tested tl dating procedures on sediment types rarely examined six glaciolacustrine samples three samples of glaciomarine drift and eight samples of floodplain deposits we used the partial bleach r beta gamma technique applied to fine silt polymineral grains results from our younger known age glaciolacustrine sediments confirm earlier observations that only the clayey laminae are generally suitable for tl dating a clayey lamina older than ca 140 150 ka produced no age underestimation and this result ca 300 ka suggests that such older lake sediments are probably suitable for tl dating two proximal samples of glaciomarine drift produced large tl age overestimates whereas the single distal glaciomarine drift sample yielded an expected age 177 38 ka suggesting that follow up studies are warranted our results for eight flood plain samples confirm that zeroing of li

flow structures at an idealized bifurcation a numerical experiment river bifurcations are key nodes within braided river systems controlling the flow and sediment partitioning and therefore the dynamics of the river braiding process recent research has shown that certain geometrical configurations induce instabilities that lead to downstream mid channel bar formation and the formation of bifurcations however we currently have a poor understanding of the flow division process within bifurcations and the flow dynamics in the downstream bifurcates both of which are needed to understand bifurcation stability this paper presents results of a numerical sensitivity experiment undertaken using computational fluid dynamics cfd with the purpose of understanding the flow dynamics of a series of idealized bifurcations a geometric sensitivity analysis is undertaken for a range of channel slopes 0 005 to 0 03 bifurcation angles 22 to 42 and a restricted set of inflow conditions based upon simulating flow through meander bends with different curvature on the flow field dynamics through the bifurcation the results demonstrate that the overall slope of the bifurcation affects the velocity of flow through the bifurcation and when slope asymmetry is introduced the flow structures in the bifurcation are modified in terms of bifurcation evolution the most important observation appears to be that once slope asymmetry is greater than 0 2 the flow within the steep bifurcate shows potential instability and the potential for alternate channel bar formation bifurcation angle also defines the flow structures within the bifurcation with an increase in bifurcation angle increasing the flow velocity down both bifurcates however redistributive effects of secondary circulation caused by upstream curvature can very easily counter the effects of local bifurcation characteristics copyright 2011 john wiley sons ltd

a unified framework for stability of channel bifurcations in gravel and sand fluvial systems bifurcating rivers shape natural landscapes by distributing water and sediments on fluvial plains and in deltas symmetrical bifurcations were often found to be unstable so that one branch downstream of the bifurcation enlarged while the other dwindled a unified theory able to predict bifurcation stability in both gravel bed and sand bed rivers is still lacking here we develop a new theory for the stability of bifurcations for the entire range of gravel bed to sand bed rivers the theory indicates opposite behavior of gravel bed and sand bed rivers we predict that symmetrical bifurcations are inherently stable for intermediate shields stresses but are inherently unstable for the low and high shields stresses found in the majority of rivers on earth in the latter conditions asymmetrical bifurcations are stable these predictions are corroborated by observations and have ramifications for many environmental problems in fluviodeltaic settings

incipient motion of sand particles on side slopes several criteria have been theoretically proposed for the critical shear stress of noncohesive sediment on side slopes however no laboratory work has ever been done on this problem a theoretical study of the incipient motion of sediment particles on side slopes is presented including the effects of viscous sublayer turbulence and lift force as a special case of level beds the theory describes the experimental values reported by several workers a specially designed rotary wind tunnel permits the evaluation of the critical shear stress on side slopes and the experiment supports the theory these works reveal that the lift force can be neglected in estimating the critical shear stress using the proposed theory if the averaged experimental values are concerned but inclusion of the lift force in the theory is recommended for safety and for describing the lower limit of the possibility of incipient motion