explosive volcanism during the holocene in the upper limay river basin the effects of ashfalls on human societies northern patagonia argentina abstract in order to understand human response to holocene ashfall events tephra layers found in archaeological sites along the upper limay river basin northern patagonia argentina were bracketed with radiocarbon dates and correlated with tephra from a lacustrine sediment core and from outcrops and the archaeological evidence was analyzed a dark tephra associated with seismic activity was identified in the el trebol rockshelter filling interstices between fallen blocks together with remains of human activity and bones of extinct fauna marking a seismo volcanic event occurring between 11 758 and 12 866 cal yr bp this same seismic event affected the cuyin manzano site where the roof collapse made the site uninhabitable for a time a white tephra present in epullan grande epullan chica and traful i caves is correlated with nahuel huapi tephra nht equivalent to laya s rio blanco and rio pereyra members rio pireco formation nht is considered to have been derived from the same eruptive event with dates ranging between ca 1950 2500 cal yr bp a dark tephra from ortega s cave and several tephra from puerto tranquilo i rockshelter ranging between 521 and 2069 cal yr bp show how the mobile hunters gatherers of northern patagonia were able to cope with the changing circumstances

flow transmission along an arid zone anastomosing river cooper creek australia cooper creek is characterized by an exceptionally wide floodplain and a complex system of anastomosing channels which together can accommodate a large range of highly variable flows in consequence flood frequency curves are unusually steep with minimal tributary contribution over the two study lengths currareva nappa merrie 400 km and nappa merrie innamincka 32 km downstream output is largely controlled by upstream input so that similar flow regimes and hydrograph forms can be maintained despite transmission losses and the variable activation of channels input output relationships defined in terms of total flow volume and peak discharge suggest that above a threshold flow of about 25 duration transmission losses exceed 75 on average over the currareva nappa merrie length the large difference in absorption between primary channels on the one hand and subsidiary channels and the floodplain surface on the other means that transmission losses vary non linearly with stage they are relatively low when flow is confined to the primary channels but increase at higher stages as lesser channels and the floodplain are activated early levels do not recur until flows of 0 5 duration are attained when output input ratios increase rapidly with discharge the influences of seasonality and background flow on output discharge are small relative to that of input discharge despite the long river distance peak flows at currareva and nappa merrie are well correlated however over the shorter nappa merrie innamincka length with its more confined system of channels correlations are better and transmission losses less they vary from 60 just above the threshold discharge to lo at flows below 20 duration over this length outflow hydrographs can be effectively estimated by the three parameter muskingum procedure at least on an event by event basis but further application must await additional records to understand fully how the parameters vary in an environment where streamflow is transmitted through a highly variable system of channels

particle path characteristics at the large gravel bed river danube results from a tracer study and numerical modelling a tracer study performed on a 3 km long reach of the danube river in austria is presented forty artificial stones of three different sizes intermediate b axis 25 mm 40 mm 70 mm were produced and a coded radio acoustic transmitter was implanted the measurement system had to be improved to be applicable to large rivers with water depths up to 12 m the positions of the stones were observed approximately once a week depending on hydrology over a period of at least one year by radio tracking from a boat including a 15 year flood event transport paths and velocities as well as the incipient motion of bedload transport could be monitored for the first time on a large gravel bed river the particle paths were found to be mostly bankline parallel even though the stones passed a 30 river bend the median of the transverse particle displacement was found to be 4 of the longitudinal displacement calculations considering both transverse slope and transverse flow velocities showed transverse transport to be 6 6 of the longitudinal transport indicating that marginal lateral transport is mainly influenced by morphology a three dimensional 3d numerical model using a stochastic particle tracing approach was validated with the data indicating that the observed positions are well reproduced by the model within the observation period 74 of all stones passed the reach with more than 1000 detections particle transport could be characterized by a mean travel velocity of about 10 m per day variable for the different grain sizes single tracer stones were transported up to 1000 m during a single flood event size selective behaviour could be shown and the incipient motion of the large 70 mm gravel was detected at lower discharges than predicted by commonly used uniform bedload transport formulae copyright 2012 john wiley sons ltd

facies analysis of the kosi megafan deposits abstract the kosi river shows distinct changes in its channel pattern from chatra where the river enters the indogangetic plains to kursela where it joins the ganga river i e gravelly sandy braided zone 1 sandy braided zone 2 and straight zone 3 to meandering zone 4 facies analysis of the kosi channel deposits suggests that in zones 1 and 2 mid channel bars islands side bars 3 d dunes and various types of linguoid dunes are the most common features large linguoid dunes with lengths of up to 1 km and with superimposed smaller 3 d dunes are also common the deeper parts of the channel are probably characterized by longitudinal bars in zone 1 and sandwaves in zone 2 lateral accretion of sandy bars is a significant process in these zones zone 3 is characterized by side bars with deposition by lateral accretion as on point bars linguoid dunes may migrate up a side bar or move across it subparallel to the main channel thalweg the uppermost sedimentary sequence 8 10 m thick of the kosi megafan consists of mainly sands and minor gravels in the proximal northern half and sand and mud 45 vol in the distal southern half of the megafan this downfan trend reflects the change from braided to straight channel pattern numerous spring fed streams rework the surficial sediments of the megafan and produce a fining upward sequence up to 4 7 m thick on top of braided straight channel deposits of the megafan

holocene development of the river lippe valley germany a case study of anthropogenic influence the characteristics of the holocene valley floor of the river lippe germany are atypical for a river in central europe the valley floor consists of three terrace levels which are not always clearly separated from each other analysis of the sediments making up the terraces indicates that they accumulated during the course of the entire holocene although there is insufficient information available to allow detailed determination of phases of fluvial change and stability responsible for terrace formation two of the terraces exist only in the lower reaches of the valley where they converge and diverge with the third the lowest terrace consists only of a narrow strip running parallel to the river channel the configuration of the valley floor may be explained by a series of anthropogenic influences the earliest human impact probably occurred about 2000 years ago when during their campaign against german tribes the romans built a towpath and may have changed the channel planform from its natural anabranching pattern to a meandering form by building small dams on local distributary forks implementation of artificial meander cut offs to improve navigation on the river began in medieval times the morphological response to these human interventions was primarily degradational in the 19th century artificial lateral fill was used to narrow the channel and the towpath was renewed several times the trace of the most recent towpath is still discernible as a narrow strip parallel to the river channel and it constitutes the lowest terrace level comparison between the bankfull discharge of a 4000 year old abandoned channel and the formative flow for the modern channel supports the premise that prior to anthropomorphic influence the natural planform was anabranched copyright 2000 john wiley sons ltd

stable isotopes reveal differential patterns of holocene environmental change among tuco tucos rodentia ctenomyidae ctenomys from patagonia abstract to explore the role of past environmental changes in shaping interspecific differences in genetic structure among tuco tucos rodentia ctenomyidae ctenomys from the limay valley of northern patagonia we compared 13c and 18o values for modern and historical tooth enamel from the colonial tuco tuco c sociabilis and the parapatric patagonian tuco tuco c haigi specifically we sought to test the hypothesis that a mid holocene loss of genetic variation in c sociabilis was associated with environmental change for both species the 13c composition of tooth enamel differed significantly 5 1 difference in mean 13c values between modern and holocene 0 5 12 kyr b p samples suggesting marked temporal changes in the vegetation consumed for c sociabilis no statistically significant differences in the 13c composition of teeth were found among individuals from the early middle and late holocene in contrast significant differences in 13c values were found between middle and late holocene samples from c haigi none of our analyses revealed statistically significant differences in 18o values thus although the diets of both study species have changed from the holocene to the present only c haigi experienced significant dietary changes during the holocene we suggest that the ecologically more specialized c sociabilis continued to consume the same food resources even as changing holocene conditions caused preferred habitats to disappear in contrast the more generalized c haigi altered its diet allowing populations of this species to remain widespread despite changing holocene environments

anabranching rivers their cause character and classification anabranching rivers consist of multiple channels separated by vegetated semi permanent alluvial islands excised from existing floodplain or formed by within channel or deltaic accretion these rivers occupy a wide range of environments from low to high energy however their existence has never been adequately explained they occur concurrently with other types of channel pattern although specific requirements include a flood dominated flow regime and banks that are resistant to erosion with some systems characterized by mechanisms to block or constrict channels thereby triggering avulsion the fundamental advantage of an anabranching river is that by constructing a semi permanent system of multiple channels it can concentrate stream flow and maximize bed sediment transport work per unit area of the bed under conditions where there is little or no opportunity to increase gradient on the basis of stream energy sediment size and morphological characteristics six types of anabranching river are recognized types 1 3 are lower energy and types 4 6 are higher energy systems type 1 are cohesive sediment rivers commonly termed anastomosing with low w d ratio channels that exhibit little or no lateral migration they are divisible into three subtypes based on vegetative and sedimentary environment type 2 are sand dominated island forming rivers and type 3 are mixed load laterally active meandering rivers type 4 are sand dominated ridge forming rivers characterized by long parallel channel dividing ridges type 5 are gravel dominated laterally active systems that interface between meandering and braiding in mountainous regions type 6 are gravel dominated stable systems that occur as non migrating channels in small relatively steep basins anabranching rivers represent a relatively uncommon but widespread and distinctive group that because of particular sedimentary energy gradient and other hydraulic conditions operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges

floodplain development based on selective preservation of sediments squamish river british columbia abstract in the conventional model of floodplain sediment accumulation mechanisms of floodplain growth are differentiated into lateral and vertical accretion processes in which within channel deposits are capped by overbank deposits in the high energy gravel based squamish river sediments laid down on bar surfaces are composed of trough and planar crossbedded coarse sands these sequences contrast incongruously with adjacent floodplain deposits which are composed in large part of vertically accreted fine sands atop coarse alluvial gravels using element analysis it is inferred that bar platform sediments are stripped away by chute channels which are subsequently infilled with lower energy deposits from this a model of floodplain growth based on selective preservation of bar platform sands and prefrential preservation of vertically accreted deposits is proposed this mechanism of sediment replacement occurs independent of channel planform type

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

an environmental gradient of vegetative controls upon channel planform in the source region of the yangtze and yellow rivers abstract alluvial reaches in the source region of the yangtze and yellow rivers located atop the qinghai tibetan plateau and mostly at elevations that are typically higher than 3500 m have similar slope and valley setting conditions however alluvial reaches in the source region of the yangtze river exhibit predominantly braided channel planforms while equivalent reaches in the source region of the yellow river have more diverse planform morphologies including anastomosing braided anabranching braided and anabranching anastomosing reaches the regional gradient of variation in riparian vegetation and interactions between vegetation development and hydrologic process are shown to be major factors influencing this difference in channel planforms a short period above freezing coincident with recurrent yet pulsed flows in the source region of the yangtze river suppress riparian vegetation growth promoting the development of braided channel morphologies in contrast a milder climate coincident with less variable flows in the source region of the yellow river permit the establishment of different forms of riparian vegetation thereby supporting the development of a wider range of channel planform morphologies

the development mechanism of gravel bars in rivers the development stability and growth direction of gravel mid channel bars in rivers were studied by analyzing sediment distribution and vegetation development along the bars field studies and sediment sampling were conducted in the source region of the yellow and lancang rivers on the qinghai plateau the results showed that the gravel bars in rivers always grow in the upstream direction because gravel and sand are deposited at the upstream end of the bars gravel bedload stopped at the bar head sand and silt moved as suspended load during the flood season and were deposited at the bar tail due to vegetation resistance consequently sediment size varied from gravel to sand and silt along the bars from the head to the tail on the bar surface dense vegetation coverage normally grew at the bar tail while only pioneer species occurred at the bar head the prevalent phenomena demonstrated the upstreamward growth of mid channel bars

assessing channel changes of the ganges padma river system in bangladesh using landsat and hydrological data abstract the ganga ganges 1 is an important river system in south asia which supports the life and livelihoods of millions of people both in india and bangladesh the system has a number of names throughout its length below its confluence with the brahmaputra at aricha it is known as the padma which in turn merges with the upper meghna at chandpur below which the channel is known as the lower meghna there is a growing concern about this large river system because its channels are subject to frequent migration threatening engineering structures and resulting in various environmental and social consequences which may be compounded by climatic variability land use change and agricultural intensification as the basin experiences rapid population growth concerns have been expressed that the construction of a barrage just upstream of the indo bangladesh border has adversely affected the ganges reach in bangladesh partly in order to investigate this the planform changes of the ganges and the padma within bangladesh was analysed over the period 1973 to 2011 using multitemporal landsat images and long term flow data in eight epochs with an average duration of 4 5 years the padma reach is less affected by the barrage and provides a useful control study areas of erosion and deposition were determined from sequential changes in the bankline positions mean channel width sinuosity and braiding index were analysed using a geographic information system gis flood frequency duration and magnitudes were studied using long term discharge records generally channel planform evaluation indicated that both the ganges and the padma experienced contraction expansion and readjustment in configuration over the last 38 years erosion and deposition statistics of the ganges indicate that 57 km2 of land was lost along the right bank whereas around 59 km2 has been gained along the left bank during the assessment period suggesting that the erosion and accretion of both banks is roughly balanced with a general movement towards the right bank the width of the ganges varied from a maximum of 5 36 to a minimum of 3 23 km during the observation period changes to sandbar area are in general much more radical than changes to the overall width and area of the channel measurement of areas of erosion and accretion showed that both banks of the padma experienced considerable loss of land the total net loss for left bank and right bank was 155 and 28 km2 respectively the padma is approximately twice the width of the ganges and the changes to its channel area are not as temporally dynamic as the ganges the relationship between bank curvature and erosion accretion of the river banks for both rivers was analysed and the results contradict established meander theory regression analysis between bank erosion rates annual average discharge and mean flood flow data showed that bank erosion was significantly correlated with annual average discharge for the padma r2 0 6283 and that the ganges bank erosion rate is influenced by mean flood flow r2 0 6738 the flood frequency shows generally good stability across the first eight of the nine epochs for the ganges but for the padma the frequency showed even greater stability we were unable to support the widely held belief that the upstream barrage has a deleterious effect on the ganges but note that there is a slight effect due to the periodic release of sediment through scour sluices

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

recent volcanism in the puyehue cordon caulle region southern andes chile 40 5 s petrogenesis of evolved lavas

holocene tephra succession of puyehue cordon caulle and antillanca casablanca volcanic complexes southern andes 40 41 s abstract puyehue cordon caulle and antillanca volcanic complexes are two of at least 50 active frontal arc volcanoes that define the 1400 km long southern volcanic zone of chile holocene tephra deposits in chile and argentina 40 41 s up to 100 km east of these volcanoes comprise at least five voluminous 1 to 8 km 3 pyroclastic fall layers that preceded several recently deposited cordon caulle pumice fallouts field observations of proximal medium and distal facies of the deposits in conjunction with geochronology and geochemistry of the volcanic complexes indicate that three fall layers are derived from puyehue volcano puyehue 1 and 2 and mil hojas whereas two are sourced from the antillanca complex playas blanca negra and nahuel huapi tephra 20 km to the south the oldest tephra calibrated 14 c age 10 49 0 12 ka 2 found only at medium distal facies is deposited directly on granitic moraine boulders and consists of deeply weathered orange dacitic pumice lapilli the next prominent tephra at 7 ka comprises dacitic pumice and its age is equivalent to a rhyodacitic dome exposed in the puyehue summit crater above these deposits there are phases of a complex eruption consisting of a conspicuous compositionally zoned tephra it also comprises a pyroclastic density current together with lithic rich and scoriaceous fallout deposits mineralogical geochemical and sr isotope evidence plus the isopach maps confirm that this sequence of eruptive events is sourced from antillanca at 1932 68 yrbp the total volume of this eruptive sequence exceeds 8 km 3 making it the largest holocene eruption from either volcanic complex this eruption was likely responsible for the destruction of an ancestral antillanca volcano and the formation of a 4 5 km diameter caldera a distinctive younger unit in the region is a voluminous rhyodacitic pumice fall calibrated 14 c age 1 11 0 07 ka above which a series of several alternating dark lithic and pumice lapilli beds accumulated correlation with proximal deposits indicates that the 1 11 ka eruption was derived from puyehue and destroyed 3 km 3 of rhyodacitic domes at this volcano summit historic explosive activity at the nascent casablanca volcano and along cordon caulle including the 2011 2012 eruption 1 km 3 of uncompacted pumice the largest from this fissural zone emphasizes an increased risk for volcanic hazards in central chile and argentina

impact of the 1960 major subduction earthquake in northern patagonia chile argentina abstract the recent sedimentation processes in four contrasting lacustrine and marine basins of northern patagonia are documented by high resolution seismic reflection profiling and short cores at selected sites in deep lacustrine basins the regional correlation of the cores is provided by the combination of 137cs dating in lakes puyehue chile and frias argentina and by the identification of cordon caulle 1921 22 and 1960 tephras in lakes puyehue and nahuel huapi argentina and in their catchment areas this event stratigraphy allows correlation of the formation of striking sedimentary events in these basins with the consequences of the may june 1960 earthquakes and the induced cordon caulle eruption along the liquine ofqui fault zone lofz in the andes while this catastrophe induced a major hyperpycnal flood deposit of ca 3 106 m3 in the proximal basin of lago puyehue it only triggered an unusual organic rich layer in the proximal basin of lago frias as well as destructive waves and a large sub aqueous slide in the distal basin of lago nahuel huapi a very recent mega turbidite in the two distal basins of reloncavi fjord located close to the lofz suggests that 1960 co seismic movements in this area may have triggered the remobilization of ca 187 106 m3 of marine sediments

channel geometry bed material and inferred flow conditions in ephemeral stream systems barrier range western n s w australia streams draining the barrier range uplands in arid western n s w are ephemeral and undergo rapid channel contraction away from the uplands as a consequence of transmission loss these processes are documented on the basis of surveys of channel form and bed material at sequential sites located along the contracting lowland reaches of two representative streams channel width and depth display considerable fluctuation downstream but cross sectional area and maximum bed surface grain size decline in a very regular manner flood discharge estimated from the channel capacity and predicted critical and mean flow velocities also declines regularly the trends are well fitted by log linear models and are reported in terms of a half distance the channel length in km over which the value of a morphometric parameter declines by 50 per cent rates of channel contraction and transmission loss are inferred to be inversely proportional to the volume of flow delivered from the upland section of the drainage basin unusual channel characteristics including extensive tree growth in the bed make application of ordinary flow equations difficult because friction factors cannot be estimated with confidence systematic changes in the nature of the bed surface in contracting streams such as those studied here also have implications for the application of critical velocity equations and highlight a need for further study of desert channels

the effects of a bypass dunefield on the stability of a headland bay beach a case study abstract this paper presents a case study on the modelling of a headland bay beach influenced by sediment input from migrating coastal dunes the study area is the region around the town of ingleses on santa catarina island santa catarina state south eastern brazil ingleses has been threatened by nature at two different fronts at the north eastern side of the town lies ingleses beach a headland bay beach which seems to be subject to persistent erosion in recent years on the south western side the town is threatened by a migrating dunefield which is encroaching onto houses and infrastructure but which has not reached the beach in the past 70 years a second dunefield in the area does not pose a direct threat to the town but passes it on the eastern side and forms an important influence on the development of the beach the sediment influx rate from this dunefield to the beach is 10 000 m3 year to investigate the effect of the dunefields on the morphodynamic system a numerical model has been created using the software packages swan and unibest and the static equilibrium bay shape was determined by the software package mepbay the result of the long term scenario as predicted by unibest agrees rather well with the static equilibrium bay shape found with mepbay this provides sounds basis for the credibility of both models consequently the predicted evolutional trend of the shoreline seems plausible

morphology and recent sediments of the lower anastomosing reaches of the attawapiskat river the attawapiskat river is one of the major low gradient rivers which cross the flat wetlands of the hudson bay lowland it has a nival regime leading to strong short duration spring floods during which considerable amount of sediment is carried for the remainder of the year only organic matter is transported in solution and suspension the land is isostatically rebounding after the pleistocene glaciations the river downcuts through the softer holocene estuarine basal clays and the pleistocene tills to the paleozoic carbonate bedrock the river develops an anastomosing pattern in the lower reaches and it acquires an irregular meandering character inland it does not construct a significantly thick delta as the river borne sediments are dispersed along the coasts by tides longshore currents and ice rafting the river is subjected to frequent ice jams in its lower reaches as warmer southern waters flow toward the still frozen mouth and sea during the jams secondary channels become active floodings develop and sedimentation occurs on the levees ice scours and ice rafted materials affect the river banks greatly the principal sedimentary environments within the river consist of 1 erosional shoals covered by boulder and pebble lags over till 2 sorted coarse to medium sized sands in junction bars downstream from islands these sands frequently develop gas bubbles and they contain characteristic fining upward sequences from boulder pavements at the base grading up into rippled sand with abundant organic matter grading up into laminated silt and sand and organic matter 3 secondary channels which develop thin silty and clay drapes on eroded hard substratum and are filled eventually by thick peats and 4 river banks which show well developed regressive sequences from pleistocene tills at the base overlain by estuarine sparsely fossiliferous clays capped by thin conglomeratic units with abundant reworked macoma balthica shells grading upward into upper tidal flat sand irregularly laminated sand and silt of marshes and regular interbedding of sand silt and organic layers of the levees at the top

impacts of pre vs postcolonial land use on floodplain sedimentation in temperate north america abstract this paper evaluates the relative importance of anthropogeomorphic sedimentation on floodplains in the mid latitudes of north america before and after the arrival of euroamericans geohistorical and geoarchaeological theories have emerged that have conflicts concerning the relative effectiveness of pre and postcolonial anthropogeomorphic change in north america many geoarchaeologists paleoecologists historians and cultural geographers have concluded that intensive precolonial land use resulted in substantially humanized landscapes prior to the arrival of colonists this presents a geomorphic paradox to fluvial geomorphologists and river restorationists who interpret buried floodplain soils as evidence of landscape stability evidence for these theories is reviewed from a geomorphic perspective while making distinctions between ecological and geomorphic change and recognizing spatial and temporal variability in some locations mid millennial land use generated erosion and floodplain sedimentation subsequent indigenous population declines and reductions in land use intensity may have been followed by precolonial geomorphic stability and pedogenesis an abandoned landscape hypothesis that floodplain soils developed on precolonial anthropogenic alluvium and were subsequently buried should be considered where indigenous populations and land use activities were high variability is also noted in postcolonial floodplain sedimentation many stratigraphic studies show that rates of sedimentation increased greatly after colonization in response to land clearance for agriculture timbering and mining an inventory of studies that document postcolonial floodplain sediment shows strong spatial patterns most studies of postcolonial sedimentation cluster in the eastern usa from the mid atlantic southward and in the mississippi valley from wisconsin to mississippi but many occurrences of mining sediment are scattered throughout the west few of the studies reviewed in the west document anthropogenic floodplain sediment from agriculture or logging although more thorough regional inventories could change that vast areas of temperate north america do not have historical deposits that are well represented in the scientific literature so assumptions that legacy sediment on floodplains is ubiquitous are ill founded spatial and temporal variations in pre and postcolonial floodplain sedimentation are complex and more research is needed to fill in the knowledge gaps

quaternary stratigraphy geochronology and evolution of the magela creek catchment in the monsoon tropics of northern australia abstract magela creek a major tributary of the east alligator river in northern australia has left a detailed sedimentary record of a fluvial landscape dominated by climatic and eustatic changes associated with quaternary glacial interglacial cycles uranium series dates from young pisoliths in floodplain deposits indicate that ferruginisation is probably ongoing under present conditions while ferricretes in degraded terraces that flank the lower valley reveal a fluvial history extending back to early pleistocene or tertiary time inset within this older alluvium is a valley fill which from thermoluminescence dates was initiated about 300 kyr ago with each glacial climate change and associated fall in sea level distinct palaeochannels have been eroded into these floodplains infilling later with alluvium when climate and base level conditions were conducive to fluvial deposition radiocarbon dates show that the most recent palaeochannel beneath the modern magela creek last started to fill by downstream progradation and vertical accretion of bedload sand about 8 kyr the palaeochannel filled at an accelerating rate probably as a result of declining stream competence associated with drier conditions in the late holocene augmented by the backwater effects of sea level rise continued aggradation blocked the mouths of tributary valleys along magela creek forming alluvial dammed tributary lakes and deferred junction tributary streams from about 300 kyr cyclic episodes of channel incision and sediment evacuation in this tropical monsoon river valley have become less effective possibly because increasing aridity in the late quaternary has reduced the erosional effectiveness of australia s northern rivers reduced flow regime and rising sea level in the late holocene has resulted in the latest phase of alluvial accretion

mechanisms of anabranch avulsion within gravel bed braided rivers observations from a scaled physical model the physical modelling of gravel bed braided rivers using a 1 20 froude scale model permits analysis of the types and relative occurrence of different avulsion mechanisms anabranch avulsion within braided rivers involves three main mechanisms choking avulsion caused by blockage of one channel by a sediment lobe constriction avulsion produced by deflection confinement and subsequent diversion of the flow by a barform and apex avulsion following erosion at the outside of sinuous thalwegs and confined meander bends each avulsion mechanism is described and analysis of the abundance of each type illustrates choking avulsion to be predominant without braided rivers three factors are found to control anabranch avulsions namely flow discharge sediment flux and floodplain topography these are briefly discussed together with the implications of avulsion type for the alluvial architecture of gravelly braided alluvium

incremental aggradation on the okavango delta fan botswana abstract distributary channel switching is a relatively frequent event in the upper reaches of the okavango delta fan botswana this phenomenon was investigated by means of detailed topographic surveys and excavations along an abandoned channel and depth probing along an active channel system this work has confirmed that channel switching is the result of aggradation within the channel systems initially new channel systems are erosive but later in their evolution both channel bed and adjacent swamp peat areas begin to aggrade this phase leads to a change in channel gradient and causes the channel to become moribund abandonment follows with consequent desiccation of the peat burning of the peat completes the cycle of the channel evolution the entire process taking about one hundred years under present flow conditions the intial aggradation phase results in the accumulation of about 4 m of peat but net aggradation after collapse of the peat following a peat fire is 30 40 cm channel beds probably also experience about 4 m of aggradation as measured from the eroded channel floor after a peat fire the channel bed sands remain elevated by about 1 m relative to the surrounding burnt out peat as a result of the collapse of the peat the abandoned area becomes available for reflooding

late quaternary tephrostratigraphy of southern chile and argentina abstract the southern and austral volcanic zones of the andes comprise 74 volcanic centres with known post glacial activity at least 21 of these have had one or more large explosive eruptions in the late quaternary dispersing tephra over vast areas these tephra layers therefore have great potential as tephrochronological marker horizons in palaeoenvironmental studies in southern chile and argentina a region that is particularly useful to study climate dynamics of the southern hemisphere however to date tephrochronology has rarely been fully utilised in this region as a correlation and dating tool here we review the existing post glacial tephrostratigraphic record of the southern and austral volcanic zones and compile a database of known occurrences of tephra from these volcanoes in ice and lacustrine marine peat and cave sediment records we address the inconsistencies in and revisions of the tephrostratigraphies presented in prior literature and discuss the challenges in correlating tephras and the limitations of the tephrostratigraphic record in this area this study highlights the many gaps that still exist in our knowledge of the eruptive histories of these volcanoes but also reveals the largely under utilised potential of tephra as a correlation tool in this region this is exemplified by the severe lack of adequate geochemical analysis of tephra layers preserved in many lacustrine and peat sediment sections which are particularly important tephrostratigraphic records in southern chile and argentina due to the paucity of surface preservation

management responses to pulses of bedload sediment in rivers abstract rivers can experience sudden pulses of sediment from human and natural erosion processes that can accumulate in the bed abundant studies have examined the sources and dynamics of sediment pulses and problems caused by these pulses particularly flooding avulsions and habitat simplification much less has been written about what managers can do about sediment pulses and that is the purpose of this review the first option for managers is to do nothing and this decision can be informed by many case studies and by theory on the propagation and character of sediment pulses their diffusion translation and celerity doing nothing should be informed by the secondary effects of sediment pulses on channels including widening avulsions and tributary interactions if managers decide that something needs to be done about the sediment they have four options 1 reducing the sediment supply at source 2 trapping sediment in the channel 3 accelerating sediment transport through a reach and 4 directly extracting sediment the most common of these actions is undoubtedly to reduce the supply at source but there are few examples of the consequences of this for sediment pulses there are even fewer examples of trapping accelerating and extracting sediment all of these options have great potential for managing sediment pulses however they also have the potential to trigger incision of tributaries and of the channel behind the passing sediment wave overall the literature equips managers to understand the dynamics of sediment pulses but it does not yet equip them to confidently manage these geomorphic events

lateglacial changes in river morphologies of northwestern europe an example of a smooth response to climate forcing cher river france climatic oscillations occurring during the pleniglacial and lateglacial periods are generally associated with several changes in fluvial morphology and processes this study investigated two reaches of the cher river basin noyers sur cher and thenioux regarding morphological adjustments during the lateglacial period and the early holocene transitions from the pleniglacial to the lateglacial and from the lateglacial to the early holocene ca 14 700 11 500 cal bp are considered as interesting study cases because it consists of rapid and abrupt climate oscillations generally clearly recorded in current floodplains the sites studied are located in the cher river floodplain which is one of the main tributaries of the loire river several investigations have been conducted in the main valley but few of them specifically focused on geomorphological evolutions of medium sized catchments such as tributaries of the loire river therefore we examined this specific period through investigations recently conducted in the middle valley of the cher river this study aimed to 1 provide a geomorphological approach to fluvial adjustments during the lateglacial from cases located in lowland areas of a medium sized basin and 2 analyse the spatio temporal variability of fluvial responses assessment of geomorphological trajectories of the two reaches studied highlights that fluvial metamorphosis occurring during the lateglacial and early holocene seems to be characterised by lateral readjustments and a low signal of morphological instability shallow incision and aggradation processes slow and progressive change in channel morphology this observation is enhanced by the comparison with known geomorphological evolution patterns of adjacent basins the intrinsic components of the cher basin such as slope and valley width were considered as explanatory factors for the lack of morphogenetic processes in addition the geographical location of the reaches studied may also have led to a moderated expression of climate forcing indeed the elevation as well as the north south and east west gradients may contribute to the low level of geomorphological adjustments in response to these climatic oscillations this gradient could also have support this the fluvial response because of the weakly developed loess cover which may have limited the fine sediment supply and the aggradation of the valley bottom

formation of braided river floodplains waimakariri river new zealand floodplains along the braided gravel bed waimakariri river are discontinuous although generally extensive landforms composed predominantly of gravel bars capped with vertically accreted fines in order of importance three mechanisms lead to floodplain formation river bed abandonment by lateral migration of the braid train initiates the formation of the largest floodpiains which usually occur downstream of tributary fans and valley bedrock spurs in the headwater reaches localized river bed aggradation during high magnitude events leads to floodplain formation by producing a surface resistant to erosion by lesser events due to either the coarse nature of the deposit or by its elevation over the river bed or both the least important mechanism initiating floodplain development is localized river or channel incision a six stage model is proposed for the sequential development of floodplains on the waimakariri river from i active river bed ii stabilizing bar iii incipient floodplain iv established floodplain v mature floodplain and to vi terrace two mechanisms lateral migration by the braid train and reactivation of abandoned channels within floodplains operating separately or in combination are responsible for floodplain reworking and their relatively young age 250 years clearly braided rivers can construct substantial areas of well developed floodplain

avulsion the relative roles of autogenic and allogenic processes abstract avulsion the abandonment of all or part of a channel belt in favor of a new course is controlled by both autogenic intrabasinal and allogenic extrabasinal processes however their relative roles are not known well the rhine meuse delta is without doubt the best studied delta regarding holocene avulsion processes based on studies of this delta an overview is given of 1 allogenic and autogenic processes influencing different aspects of avulsion 2 the relative importance of these processes in time and space and 3 observations that can be used to differentiate autogenic and allogenic causes of avulsion the following avulsion parameters are quantified and their allogenic and or autogenic nature are discussed period of activity of channel belts period between beginning and end of overbank sedimentation by the river channel interavulsion period time period between successive avulsions of a channel belt avulsion frequency number of avulsions per time interval in a given area and avulsion location in the rhine meuse delta the period of activity of the holocene channel belts varies considerably but shows no significant trend over time whereas extrabasinal factors climate base level and tectonics changed this suggests that this parameter is mainly autogenic controlled the average interavulsion period increased from 8000 cal yr bp to 2800 cal yr bp and decreased since then significant fluctuations occurred on a shorter time scale a maximum variability in the interavulsion period occurred between 3200 and 1800 cal yr bp this was a time when large channel belts gradually came into existence during this period the avulsion frequency also reached a maximum the long term increasing trend in interavulsion period is related to the decreasing rate of sea level rise the decreasing interavulsion period since 2800 cal yr bp can be explained by increased discharge and sedimentation resulting in an increase in the number of avulsions seven avulsion sequences as predicted by the mackey and bridge mackey s d bridge j s 1995 three dimensional model of alluvial stratigraphy theory and application journal of sedimentary research b65 1 pp 7 31 model may be recorded in the holocene dataset of the rhine meuse suggesting a periodicity of 500 600 yr each avulsion sequence coincides with a peak in the avulsion frequency the periodicity of 500 yr in the avulsion frequency is proposed to be autogenic however there is also a long term trend in the avulsion frequency that can be related to sea level rise and after 3000 14c yr bp to climate change and human influence avulsion locations are not randomly distributed over the delta and are related to the allogenic factors relative sea level rise local tectonics and changes in discharge and sediment load

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

development of anastomosing channels in south central indiana abstract numerous streams in south central indiana tributary to the ohio river exhibit anastomosed channel patterns the anastomosing patterns are characterized by a primary channel and one or more auxiliary channels anabranches that are often abandoned or receive flow only during flood development of the anastomosed pattern involves avulsion and proceeds in a sequential manner where 1 aggradation significantly reduces channel depth 2 overbank flow occurs more frequently as channel capacity is reduced 3 floodplain deposits are incised by sediment deficient overbank flow producing a new channel gully having a lower gradient and elevation than the primary channel and 4 the gully created by overbank flow extends upstream and captures the drainage system converting the previous trunk channel into an anabranch avulsion is typically associated with aggrading fluvial systems but the investigated streams are undergoing an episode of degradation avulsion occurs because of localized deposition along concave up channel segments the concave up channel reaches are spatially controlled by the occurrence of non resistant strata that separate beds of resistant sandstone thus the location of avulsion and anastomosing channels is predetermined by the lithology and stratigraphy of the underlying bedrock

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

combining historical and process perspectives to infer ranges of geomorphic variability and inform river restoration in a wandering gravel bed river restoration approaches such as dam removal and channel reconstruction have moved beyond the realm of small streams and are being applied to larger rivers this development has substantial economic and ecological implications but may test gaps in our understanding of larger river systems and of restoration science we examine how information about historical ranges of geomorphic variability can inform stream restoration in the context of the clark fork river montana focusing on a study reach where one of the largest restoration projects to date was implemented upstream of the recently removed milltown dam analysis of historical sources and aerial photographs of the clark fork river s pre mining mining and more recent history suggest that a wandering channel pattern has persisted despite variations in sediment supply and transport capacity predictive metrics for channel pattern also suggest a wandering pattern transitional between braided and meandering in this geomorphic setting these analyses suggest that the creation of a single thread meandering channel which incorporates structures to limit erosion and channel movement is inconsistent with the historical range of variability in this reach the perils of restoring channels to a condition different than the historical range of variability for their geomorphic setting were illustrated on the clark fork by flood induced avulsions of the restored channel that occurred soon after project construction application of an experimental approach to restoration founded on the method of multiple working hypotheses provides a means for embracing uncertainty can maximize the potential for site specific restoration success and can foster advances in restoration science copyright 2012 john wiley sons ltd

postglacial vegetation climate and fire history along the east side of the andes lat 41 42 5 s argentina abstract the history of the low elevation forest and forest steppe ecotone on the east side of the andes is revealed in pollen and charcoal records obtained from mid latitude lakes prior to 15 000 cal yr bp the vegetation was characterized by steppe vegetation with isolated stands of nothofagus the climate was generally dry and the sparse vegetation apparently lacked sufficient fuels to burn extensively after 15 000 cal yr bp a mixture of nothofagus forest and shrubland steppe developed fire activity increased between 13 250 and 11 400 cal yr bp contemporaneous with a regionally defined cold dry period huelmo mascardi cold reversal the early holocene period was characterized by an open nothofagus forest shrubland mosaic and fire frequency was high in dry sites and low in wet sites the data suggest a sharp decrease in moisture eastward from the andes a shift to a surface fire regime occurred at 7500 cal yr bp at the wet site and at 4400 cal yr bp at the dry site preceding the expansion of austrocedrus by 1000 1500 yr the spread of austrocedrus is explained by a shift towards a cooler and wetter climate in the middle and late holocene the change to a surface fire regime is consistent with increased interannual climate variability and the onset or strengthening of enso the present day mixed forest dominated by nothofagus and austrocedrus was established in the last few millennia

the sedimentary dynamics of active fluvial channels on the okavango fan botswana the 1800 km2 okavango alluvial fan of northern botswana represents an unusual depositional setting in which peat forming perennial swamps 6000 km2 occur in a region of aeolian and semi arid sedimentation within an incipient graben of the east african rift a channel system distributes water and sediment on the fan surface but cannot contain seasonal flood water which spreads laterally from the channels through permeable channel margins sustaining the flanking swamps all sediment introduced is deposited on the fan a detailed study of sediment movement and associated hydrological conditions in the channels was undertaken to examine sediment dispersal bedload greatly exceeds suspended load at least by a factor of four vegetation and peat form permeable levees which confine the channels in the upper reaches two way exchange of water occurs between channel and swamp depending on the season but on the fan itself channels lose water to the swamp bedload measurements reveal that the channel system is in a state of grade disequilibrium with interspersed depositional and erosional reaches deposition of most of the incoming bedload occurs on the upper portion of the fan in a meandering and anastomosed channel system but on the midfan deposition of bedload occurs by channel bed aggradation at a rate of up to 5 cm yr 1 further down slope the channel enters a large lake where all remaining bedload is deposited the presently observed sedimentation patterns may be due to a recent disturbance of the fluvial system either by avulsion or neotectonics

alluvial river response to neotectonic deformation in louisiana and mississippi repeat geodetic surveys show uplift of the monroe and wiggins anticlines in louisiana and mississippi there are deformed quaternary terraces which indicate long term deformation in the valleys of the alluvial rivers that cross these structures and there are floodplain and channel convexities that provide evidence of modern deformation in addition the channels show significant variations of morphology sinuosity gradient and depth and behavior appropriate to reaches of increased and decreased valley slope these alluvial rivers are adjusting to modern deformation and their adjustment confirms two geodetic leveling anomalies

channel planform as a non controlling factor in fluvial sedimentology the case of the squamish river floodplain british columbia abstract channel planform styles demonstrate a continuum of variability whereas end member situations may exhibit diagnostic sedimentologic characteristics non end member situations may be exceedingly difficult to reliably differentiate the relevance of channel planform based sedimentologic differentiation of fluvial depositional units has been questioned for some time e g jackson 1978 bridge 1985 brierley 1989a despite reservations outlined in these papers facies models and interpretations of depositional suites in the rock record continue to be applied in terms of channel planform style this paper describes a test of this principle in one particular depositional setting the squamish river in southwestern british columbia is a high energy gravel based river in a fjord setting in a 20 km reach this river displays a distinct downstream gradation in channel planform type from braided through wandering gravel bed to meandering assemblages of bedform scale facies units on bar surfaces in this river do not differ by planform type brierley 1989a rather facies associations relate directly to the pattern of morphostratigraphic units on bar surfaces brierley 1991a when analysed in floodplain exposures sediment sequences which are equivalent to morphostratigraphic units are termed elements these elements are defined in terms of their geometry bounding surface and sediment characteristics detailed analysis of floodplain deposits in the braided wandering gravel bed and meandering reaches of the squamish river demonstrates that just as assemblages of bedform scale facies cannot be differentiated by planform style neither can assemblages of floodplain elements indeed for the squamish river the type character and spatial association of these elements cannot be differentiated reliably at the channel planform scale this indicates the limited relevance of planform differentiation of sedimentary environments under transitional planform situations such as those of high energy gravel bed rivers in confined valleys in such situations sediment inventory at the element scale provides greater insight into mechanisms of floodplain evolution than can be provided by interpretation at either the channel bedform or channel planform scales

patterns of alluvial rivers the pattern planform of a river can be considered at vastly different scales depending upon both the size of the river and the part of the fluvial system that is under consideration figure 1 for example in the broadest sense river patterns comprise a drainage network dendritic parallel trellis etc figure la the type of pattern is of interest to geomorphologists and geologists who interpret geologic conditions from aerial photographs at another scale a river reach which in figure lb is meandering is of interest to the geomorphologist who is interested in what that pattern reveals about river history and behavior and to the engineer who is charged with maintaining navigation and preventing major instability when a single meander is examined figure 1 c the hydraulics offtow the sediment transport and the potential for bank erosion are of concern in addition the sedimentologist is interested in the distribution of sediment within the bend bed forms within the channel figure ld and sedimentary structures figure ie which also establish a component of roughness for the hydraulic engineer finally the individual grains figure if provide geologic information on the sediment sources the nature of sediment loads and the feasibility of dredging for gravel there is an interaction of hydrology hydraulics geology and geomorphology at all scales which emphasizes the point that the fluvial system as a whole cannot be ignored even though only a component of the system is to be studied in this review only the patterns or planforms of alluvial rivers are discussed although it is apparent that the hydrologic and sediment yield characteristics of the drainage basin figure la as well as its geologic history cannot be ignored in the explanation of the pattern of any river

control on sediment and organic carbon delivery to the arctic ocean revealed with space borne synthetic aperture radar ob river siberia an important control on river biogeochemistry and sediment load is the process of water exchange between primary channels and the flood plain particularly in low r elief areas containing lakes ephemeral channels and other aquatic ecosystems flood plain exchange may be a dominant process on the lowland rivers of arctic russia which are among the world s largest in water discharge yet are strikingly deficient in their delivery of sediment to the arctic shelf temporal synthetic aperture radar sar amplitude and interferometric images of the ob river siberia reveal a time varying limnological network controlling water sediment and nutrient exchange between flood plain wetlands and the main channel the amount of hydrologic exchange decreases by one order of magnitude from june to september enhancing sedimentation over as much as 90 of the flood plain and enriching channel waters with colloidal organic carbon this observation combined with russian field measurements of water discharge and sediment load indicates that a major sediment sink on the lower ob flood plain may be responsible for the low amount of sediment delivery by the ob river to its estuary and the kara sea

a typology of pleistocene cold based rivers abstract a classification of periglacial rivers is proposed it is based on the generally valid laws relating the ratio of stream power to bed load with fluvial processes and morphology however river patterns may be modified by the special characteristics of the periglacial environment snowmelt especially in areas of continuous permafrost and river ice especially in zones of discontinuous permafrost determine the respective hydrological regimes the frozen state of the subsoil as well as the density of the vegetation cover contributes significantly to the bank cohesion and the sediment load of the rivers these specific periglacial characteristics have a complex influence on the thresholds in river activity resulting in a high diversity of periglacial river types

a 100 ka record of fluvial activity in the fitzroy river basin tropical northeastern australia this study reports the nature and timing of quaternary fluvial activity in the fitzroy river basin which drains a diverse 143 000 km 2 area in northeastern queensland before discharging into the

sedimentary structures exposed in bar tops in the brahmaputra river bangladesh the brahmaputra river is one of the world s largest sand bed braided rivers with a channel belt up to 15 km wide a mean channel depth of 5 m and maximum scour depths of up to 40 m the recorded discharge varies by around 60 000 cumecs every year following the annual monsoon producing dramatic stage fluctuations with the water level falling between 7 and 8 m from bankfull to low flow stage in the dry season large areas of bar top and channel bed are exposed and low flow channels cut natural sections through bar tops exposing the internal stratification elements of upstream downstream and lateral accretion are identified although the high width depth ratio of channels up to 500 1 results in extremely low depositional dips on bed bounding surfaces within the accretionary elements vertical and lateral changes in sedimentary structures are ubiquitous and varied but some patterns can be discerned channel deposits are dominated by sinuous crested dunes and trough cross stratification upper stage plane bed lamination often with a very low angle depositional dip has been observed in beds up to 4 m thick and is generally found at the base of the exposed bar top sections trough cross stratification in bar tops occurs in sets from 0 1 m to 3 m thick with rapid vertical and lateral changes in set thickness very large sets of trough cross stratification 3 m thick do not occur in the bar top sections current ripple lamination is most abundant on the tops of bars with very high rates of climb associated with high rates of sediment deposition from these observations a generalised vertical sequence of bar top origin might include trough cross stratification overlain by upper stage plane bed lamination isolated sets of trough cross stratification truncated and capped by current ripple lamination this sequence which is considered to represent the deposits of a single flood may occur within the upstream downstream or lateral accretion elements however it should be noted that vertical and lateral changes in facies are extremely abundant with many changes in sedimentary structures and reactivation surfaces within each depositional episode

simulating unsteady flow anabranching and hyporheic dynamics in a glacial meltwater stream using a coupled surface water routing and groundwater flow model 1 flooding affects ecosystems by transporting water and solutes across aquatic terrestrial interfaces removing nutrient and organic substrate limitations and spurring biogeochemical activity few studies have considered the influence of flooding on surface water groundwater interactions this research examines the temporally variable water storage and exchange in a stream in the mcmurdo dry valleys mdv of antarctica where diel flood pulses occur due to glacial melt several mdv streams display truncated discharge peaks suggesting water storage between the source glacier and the gauging station we tested the hypothesis that stream braids and subsurface water storage contribute to the difference between glacial melt and stream outflow hydrographs by constructing a coupled surface water routing and subsurface water flow model this model routes water into stream braids at high flows and allows this water to infiltrate and return to the stream via subsurface flow paths as flows recede our simulation demonstrates the importance of surface subsurface water interactions in controlling the hydrograph shape maximum simulated discharge was sensitive to storage parameters including aquifer depth and the flooding threshold while minimum discharge was sensitive to hydraulic conductivity subsurface storage volume varied by 38 over a diel cycle and stream subsurface exchange rates varied from 0 to 0 19 m3 h 1 m 1 with exchange from the stream to the subsurface during high flows and vice versa at low flows these results underscore how unsteady flow can increase hyporheic interactions and ecosystem productivity and provide support for maintaining natural stream morphology and flow regimes

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

wandering miramichi rivers new brunswick canada abstract six rivers within the miramichi basin of new brunswick display long sections with multiple channels that surround semi permanent islands linked by single channels this type of anabranching river may be termed wandering displaying a similar planimetric pattern to wandering rivers in western canada this research addresses three objectives 1 to provide a detailed description of the location pattern characteristics and boundary conditions of wandering miramichi rivers 2 to compare the pattern characteristics among wandering rivers within the miramichi basin and to other wandering meandering and braided rivers and 3 to investigate formative processes for wandering miramichi rivers wandering rivers within the miramichi region occur in larger valleys located downstream of an abrupt change in bedrock from resistive to more erosive bedrock lithology larger rivers display greater braid indices than smaller rivers wandering miramichi rivers display more similarities to meandering rivers than braided rivers exhibit an anabranching pattern yet have characteristics markedly different from anastomosed rivers energy levels mid channel bar formation migration rates and width depth ratios are all within the meandering range lower than braided and higher than anastomosed once anabranches are created larger rivers apparently maintain a greater number of anabranches because of greater discharge wandering miramichi rivers may represent another anabranching river pattern type different from anastomosed and previously described wandering rivers

synchronisation of sedimentary records using tephra a postglacial tephrochronological model for the chilean lake district abstract well characterised tephra horizons deposited in various sedimentary environments provide a means of synchronising sedimentary archives the use of tephra as a chronological tool is however still widely underutilised in southern chile and argentina in this study we develop a postglacial tephrochronological model for the chilean lake district ca 38 to 42 s by integrating terrestrial and lacustrine records tephra deposits preserved in lake sediments record discrete events even if they do not correspond to primary fallout by combining terrestrial with lacustrine records we obtain the most complete tephrostratigraphic record for the area to date we present glass geochemical and chronological data for key marker horizons that may be used to synchronise sedimentary archives used for palaeoenvironmental palaeoclimatological and palaeoseismological purposes most volcanoes in the studied segment of the southern volcanic zone between llaima and calbuco have produced at least one regional marker deposit resulting from a large explosive eruption magnitude 4 some of which now have a significantly improved age estimate e g the 10 5 ka llaima pumice eruption from llaima volcano others including several units from puyehue cordon caulle are newly described here we also find tephra related to the cha1 eruption from chaiten volcano in lake sediments up to 400 km north from source several clear marker horizons are now identified that should help refine age model reconstructions for various sedimentary archives our chronological model suggests three distinct phases of eruptive activity impacting the area with an early to mid holocene period of relative quiescence extending our tephrochronological framework further south into patagonia will allow a more detailed evaluation of the controls on the occurrence and magnitude of explosive eruptions throughout the postglacial

modelling streamflow in a large anastomosing river of the arid zone diamantina river australia anastomosing rivers form a subset of the anabranching family of river types and provide considerable challenges to modelling of their streamflow because of complex flow patterns across greatly varying floodplain widths estimates of distributed flow data are required for catchment management purposes and ecological studies of these rivers but are hindered by a paucity of measured discharge data a grid based semi distributed conceptual model structure is applied to a 330 km reach of the arid zone diamantina river of central australia model complexity is constrained by data availability with only a single gauging station located at the downstream end of the reach to provide discharge data for model calibration the model uses a simple conceptual bucket structure and accounts for exceptionally high transmission losses as well as flow patterns and wave speeds that vary with discharge within the reach the intricate flow patterns across the floodplains widths of up to 50 km are simulated using a grid based structure that required the following features i cell connections that are explicitly defined using a code that allows for multi directional flow from a cell and ii each cell having a binary flow pattern with the second connection pattern being triggered when the surface storage of the cell exceeds a calibrated level for a given land type satellite images were used to define the flow paths and hence cell connection patterns utilised by various sized floods the model was able to provide acceptable simulation of large floods but with decreasing model performance in the simulation of small to medium sized floods simulation suggested that incorrectly defined flow paths for the smaller floods were a major factor in this decreased performance the capability of the model would be improved by further detailed mapping using satellite imagery of spatial patterns of inundation as discharge varies

recognition of the sedimentary architecture of dryland anabranching anastomosing rivers abstract the application of rigorous criteria for identifying fluvial style from sedimentary deposits is crucial if environmental reconstructions from stratigraphy are to be reliable from modern observations it appears that anabranching anastomosing rivers can be divided into separate humid and dryland classes each class has dissimilar characteristics to the other necessitating different criteria for their recognition in the rock record the criteria being used by sedimentologists to recognize deposits of anabranching rivers are derived solely from modern examples in humid regions heavily biased to a few specific cases in canada these criteria are dominated by the concept of ribbon sandstone bodies encased in fine grained floodplain sediment with an overall low sand to mud ratio in the succession there has been a general failure to appreciate that dryland anabranching rivers have characteristics significantly different from these and almost certainly leave a sedimentary record quite dissimilar to that previously expected avulsion or obtrusion the gradual formation of an alternative channel are the characteristic dynamic of anabranching rivers forming and maintaining coexisting multiple anabranch channels avulsion or obtrusion require stream banks to have a high resistance to erosion usually achieved by cohesive bank sediments or the reinforcement of banks by vegetative root mats in some drylands riparian vegetation can be sparse and bank resistance may depend in large part on a high clay content here channel fills are mud dominated and floodplains have a uniform fine grained character which is usually structureless because of intense pedoturbation and some bioturbation the evidence from such present day dryland anabranching rivers suggests that their sedimentary record is likely to be one of laterally extensive mud rich beds with weak or enigmatic fluvial indicators elsewhere dryland anabranching systems are sand dominated their banks reinforced with riparian gallery forests sustained by infrequent flow and a shallow water table though the stratigraphy of these has barely been investigated they are clearly different from humid region anabranching systems for their channels and islands are predominantly sandy anabranching rivers with prominent channel sandbodies separated by muddy islands have never been described from contemporary drylands the interpretation of a dryland origin for ancient stratigraphic successions with this form is made therefore without evidence that they can actually exist in the light of this such interpretations should be reevaluated

avulsion mechanisms on the okavango fan botswana the control of a fluvial system by vegetation a study of the avulsion of a major distributory channel on the alluvial fan 22 000 km2 in area of the okavango river in northern botswana has revealed that channels serve as arterial systems distributing water which sustains large areas of permanent swamp the channels are vegetatively confined a primary channel defined here as a channel which receives water and sediment directly from the fan apex aggrades vertically as a result of bedload deposition the rate of aggradation increases downchannel and may exceed 5 cm yr 1 in the distal reaches rapid aggradation is associated with a decline in flow velocity this initiates a series of feedback mechanisms involving invasion of the channel by aquatic plants which trap floating plant debris further reducing flow rate and causing the channel water surface to become elevated thereby increasing rate of water loss from the channel accelerating blockage and aggradation the channel ultimately fails enhanced water loss from the channel promotes the growth of flanking swamp vegetation which confines the failing channel increased flow through the swamp erodes pre existing hippopotamus trails producing a secondary channel system which overlaps but does not connect directly to the failing reach of the primary channel the region of failure of the primary channel migrates upstream accompanied by headward propagation of the secondary channel system the swamp distal to the failed primary channel dessicates and is destroyed by peat fires secondary channels are stable and not prone to blockage comparison with avulsions described in other river systems indicates that the influence of plants in the okavango river system is exceptionally strong

anastomosis and the continuum of channel pattern anastomosing rivers are characterized by multiple channels separated by islands excised from the floodplain their status relative to the continuum concept of channel pattern is assessed with channel pattern defined in terms of three variables flow strength bank erodibility and relative sediment supply using an ordinal scaling l ow m oderate h igh the traditional forms of straight meandering and braided have respective representations of l l l m l m l m and h h m h in terms of those variables the anastomosing pattern is on average represented by l l m h but not so definitively as other forms specification of the third element sediment supply is particularly hampered by the paucity of data but aggradation a characteristic of many anastomosing rivers can be thought of as symptomatic of a moderately high rate of supply relative to the ability for onward transport a sufficiently high rate of supply to a channel with low flow strength and resistant banks would induce shoaling and or lateral constriction that locally forces flow out of the main channel and ultimately leads to the cutting of anabranches a flow regime characterized by concentrated floods of relatively large magnitude is also regarded as highly conducive to the formation of new channels where low bank erodibility constrains channel capacity anastomosis may in certain cases represent a transitional form of channel pattern but there is no denying the longevity of some anastomosing systems

implications for paleomobility studies of the effects of quaternary volcanism on bioavailable strontium a test case in north patagonia argentina abstract strontium isotopes 87sr 86sr are used as geochemical tracers for paleomobility studies because they display predictable and stable patterns in ecosystems primarily controlled by the underlying geological regimes while bedrock geology is stable over thousands of years geomorphological processes can influence the 87sr 86sr in ecosystems over archeologically relevant timescales among these geomorphological processes the deposition and reworking of volcanic sediments over quaternary timescales are little studied but could be an important control of 87sr 86sr variations in many archeological regions north patagonia is a key archeological region to address animal and human movements and an ideal location to test the influence of quaternary volcanism on 87sr 86sr variation as it is located downwind of major volcanic centers in this study we aim to assess the main environmental and geological controls of bioavailable 87sr 86sr and to build a high resolution isoscape using a machine learning regression framework for forthcoming paleomobility studies we sampled several locations and analyzed different types of samples n 94 the ratios show a limited range of variation which is not related to the bedrock geology rather bioavailable 87sr 86sr variations display a progressive increase going eastward away from the andes following dust aerosol deposition and elevation variations r2 0 71 rmse 0 00041 we argue that this trend relates to the deposition and reworking of unradiogenic volcanic sediments by aeolian fluvial and glacial erosion during the quaternary as most of this sediment reworking occurred during glacial periods the current bioavailable 87sr 86sr variations across the study area likely represent a long term average that varied little during the holocene consequently our isoscape provides a solid base for holocene paleomobility studies in north patagonia and underlines the importance of quaternary volcanism processes for interpreting 87sr 86sr data in paleomobility studies in volcanic regions

hydraulic geometry of secondary channels of lower fraser river british columbia from acoustic doppler profiling 1 the hydraulics and morphology of secondary channels within the lower fraser river gravel reach were examined using data collected with an acoustic doppler profiler during the large 2002 freshet data were collected over an area of channel subreach for a range of subreach morphologies upstream mid and downstream as suggested by visual evidence at a station hydraulic geometry of subreach types stratified along gradients of width depth and velocity fish habitat is more abundant and more persistently available in the wide deep downstream subreaches but higher velocities preferred by some species occur in the mid stream and upstream subreaches additional high flow data used to develop bank full scaling relations classical downstream hydraulic geometry conformed well to a simple power law up to and including data points from the main channel however width and depth exponents deviated from classical results investigation suggests that the relations observed in this study approach expected relations for constant slope and channel boundary materials

impacts of flow regulation on geomorphic adjustment and riparian vegetation succession along an anabranching reach of the upper yellow river abstract closure of longyangxia dam in 1986 the uppermost large dam along the trunk stream of the upper yellow river in western china permanently altered the flow regime in guide reach an anabranching section of river located 41 km downstream of the dam two smaller hydropower structures nina and laxiwa dams were constructed between longyangxia dam and guide in 2003 and 2009 respectively but they had a minor influence on the flow regime this study uses the indicators of hydrologic alteration model satellite imagery and field analyses to assess how the magnitude and duration of changes to the flow regime impacted upon geomorphic adjustments and riparian vegetation succession from 1987 to 2015 flow impoundment resulted in near instantaneous reduction in the width of the active channel zone human activities have manipulated the additional floodplain areas created by these changes aggregation of mid channel bars into larger compound features decreased channel multiplicity negligible bed incision likely reflects local supply of sediment from tributary sources downstream of the dam significant correlations are shown between changes to the balance between channel depositional and erosional processes induced by flow regulation and the functional composition of vegetation succession patterns maintenance of a higher water table for longer periods of time is inferred to have altered mutual interactions between hydrogeomorphic processes and riparian vegetation in this semi arid setting

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

tephrostratigraphy of a deep sea sediment sequence off the south chilean margin new insight into the hudson volcanic activity since the last glacial period abstract a detailed tephrostratigraphy study supported by accelerator mass spectrometry ams 14 c dating stable isotope 18 o and geochemical analyses has been carried out from a deep sea core collected offshore the chonos taitao archipelago southern chile close to the main andean volcanic centres of the southern volcanic zone svz counting of glass shards in the drill core revealed the occurrence of twelve ash layers over the last 22 ka bp their provenance was determined by comparing the 14 c age and major element geochemistry of glass shards with tephra recovered in terrestrial fallout deposits from the southern south volcanic zone ssvz four cryptotephra layers in the uppermost part of the studied core 1 9 16 1 ka bp can be confidently correlated to the corresponding cerro hudson volcanic eruptive events hw7 hw3 hw2 and hw1 for the eight remaining tephra layers deposited between 16 1 ka and 20 ka bp no direct correlation is possible with onland volcanic products however the major element geochemistry of the glass shards strongly suggests the cerro hudson as the volcanic source for these tephra on the other hand the lack of two of the largest hudson holocene eruptions 3 6 and 7 5 ka bp in the uppermost part of deep sea core may be linked to changes in the position of the southern westerlies wind belt sww as a response to the climate variability in northern patagonia during the holocene

human subsistence and environmental stability during the last 2200 years in epullan chica cave northwestern patagonia argentina a perspective from the zooarchaeological record abstract the faunal content from epullan chica archaeological site ech thereafter a small cave located in northwestern patagonia argentina was studied from taphonomic and paleoenvironmental points of view this cave is placed in the ecotone between monte desert and the patagonian steppe in the middle limay river basin was occupied since the end of the late holocene zooarchaeological evidence retrieved from ech is diverse including fresh water mollusk shells diplodon chilensis eggshell and bone fragments of rheidae bones and teeth of large lama guanicoe medium e g chaetophractus villosus conepatus chinga and micro sized several species of sigmodontine and caviomorph rodents mammals most of the recorded taxa were the result of human exploitation however owl pellets preserved in the sediments and other taphonomic signatures clearly indicate that avian predators are responsible for part of the micromammal record micromammal abundances during the last 2 2 ka bp are suggestive of a relative environmental stability around ech at least until the beginning of the last century in this sense late holocene landscapes were dominated by open steppe areas and large rocky outcrops with minor changes in humidity and temperature during this period

holocene stability and recent dramatic changes in micromammalian communities of northwestern patagonia abstract micromammalian fossil sequences of two classical archaeological sites cueva traful i cti and cueva epullan grande ll both in the province of neuquen northwestern patagonia argentina were studied these sequences cover the entire holocene with assemblages mainly composed by rodent and marsupial remains accumulated by owls both sites are located near the ecotone fringes of the major vegetation units currently present in patagonia forest patagonian steppe cti and patagonian steppe monte desert ll comparisons between fossil and recent owl pellet samples in a regional approach indicated that micromammal communities remained stable over the major part of the last 10 ka with some minor expansion of desert adapted taxa to the west since the middle holocene and a dramatic change in the assemblages during the last 100 150 years two alternative but not contradictory explanations can be presented to interpret the stable period first climate environmental changes were not of sufficient magnitude to produce a clear reorganization of micromammal communities forest did not invade cti surroundings and monte did not reach around ll second the basic structure of northwestern patagonian steppe micromammal communities was resilient to minor or moderate climate environmental changes the strong shift depicted in the studied sequences in very recent times 100 150 years can be linked to human impact on the regional environment this change involves a deep restructuration of small mammal communities ranging from virtual extirpations of some species to dramatic increments of opportunistic taxa a century of sheep overgrazing coupled with the introduction of exotic shrubs was the main factor driving the configuration of present small mammal assemblages in northwestern patagonia

landslide induced river channel avulsions in mountain catchments of southwest new zealand pulsed or chronic supply of landslide debris to valley floors has historically caused substantial aggradation and channel instability in several alpine catchments of sw new zealand in this regional investigation of landslide impacts on river morphology three types of landslide induced channel avulsion are discerned i upstream backwater avulsions ii contact avulsions and iii downstream loading avulsions the basis for this qualitative geomorphic process response framework is the principal direction of fluvial response with respect to its position relative to the causative landslide emplacement site downstream avulsions have the highest damage potential to land use and infrastructure on unconfined mountain fringe alluvial fans in the wake of such events catastrophic aggradation may obliterate up to several km2 of mature floodplain forests by burial under several metres within a few decades estimates of mean aggradation rates are high 220 mm year 1 and exceed long term 103 year trends of fluvial degradation by an order of magnitude future potential avulsion routeways may be detected by geomorphic mapping of abandoned channels which are preferentially reactivated in the wake of landslide induced sediment waves