paleogene synorogenic sedimentation in the altiplano plateau and implications for initial mountain building in the central andes sedimentologic data and palynological ages from paleogene clastic deposits of the northern and central altiplano plateau suggest foreland basin development in the central andes by mid paleocene time the nonmarine potoco formation 3000 6500 m thick constitutes the majority of cenozoic basin fill the potoco overlies the santa lucia formation 50 300 m thick previously dated as mid paleocene by mammal fossils and magnetostratigraphy new geochronologic data for the potoco formation include late eocene to oligocene palynomorph assemblages recovered throughout lower to upper stratigraphic levels these ages and published 40 ar 39 ar and k ar ages from overlying upper oligocene lower miocene volcaniclastic rocks indicate 1 nondeposition or greatly reduced deposition average sediment accumulation rates 10 m m y from mid paleocene to middle eocene time top santa lucia to lowermost potoco formation followed by 2 rapid deposition average sediment accumulation rates up to 500 m m y throughout late eocene and oligocene time majority of potoco formation lithofacies and paleocurrent data confirm both depositional phases a 20 100 mthick interval of superimposed paleosols in the basal potoco supports reduced sediment accumulation and low rates of subsidence during mid paleocene to middle eocene time the overlying main body of the potoco contains facies assemblages and stratigraphic architecture dominantly nonero

hydraulic and sedimentary processes causing anastomosing morphology of the upper columbia river british columbia canada abstract the upper columbia river british columbia canada shows typical anastomosing morphology multiple interconnected channels that enclose floodbasins and lateral channel stability we analysed field data on hydraulic and sedimentary processes and show that the anastomosing morphology of the upper columbia river is caused by sediment bedload transport inefficiency in combination with very limited potential for lateral bank erosion because of very low specific stream power 2 3 w m 2 and cohesive silty banks in a diagram of channel type in relation to flow energy and median grain size of the bed material data points for the straight upper columbia river channels cluster separately from the data points for braided and meandering channels measurements and calculations indicate that bedload transport in the anastomosing reach of the upper columbia river decreases downstream because of lateral channel stability no lateral storage capacity for bedload is created therefore the surplus of bedload leads to channel bed aggradation which outpaces levee accretion and causes avulsions because of loss of channel flow capacity this avulsion mechanism applies only to the main channel of the system which transports 87 of the water and 90 of the sediment in the cross valley transect studied because of very low sediment transport capacity the morphological evolution of most secondary channels is slow measurements and calculations indicate that much more bedload is sequestered in the relatively steep upper anastomosing reach of the upper columbia river than in the relatively gentle lower anastomosing reach with anastomosing morphology and related processes e g crevassing being best developed in the upper reach this confirms the notion of upstream rather than downstream control of upper columbia river anastomosis

avulsion processes at the terminus of low gradient semi arid fluvial systems lessons from the rio colorado altiplano endorheic basin bolivia abstract the rio colorado dryland river system in the southeast of the endorheic altiplano basin bolivia terminates on a very flat coastal plain at the edge of the salar de uyuni the world s largest salt pan with an area of ca 12 500 km 2 since the pleistocene the basin has experienced several lake expansion and contraction cycles in response to wetter and drier climate periods respectively at present the basin is in a dry climate period which results in a lake level lowstand and progradation of fluvial systems such as the rio colorado onto the former lake bottom the present field study of the terminus of the rio colorado shows that the river experiences a gradual downstream decrease of bankfull width and depth this bankfull decrease is caused by the combined effects of 1 extremely low gradient of the lake bottom and hence loss of flow energy and 2 downstream transmission losses due to high evaporation potential and river water percolation through the channel floor peak water discharge in seasonal short duration rain periods causes massive overbank flooding and floodplain inundation on satellite images the morphology of the river terminus has a divergent pattern and resembles a network of coeval sinuous distributary channels however field observations show that only one channel is active at low flow stage and at high flow stage an abandoned partially infilled channel may be active as well the active channel at its termination splits into narrow and shallow anastomosing streams before its demise on the lacustrine coastal plain the rest of the channels which form the divergent network are older sediment filled abandoned sinuous river courses with multiple random avulsion points these channel deposits together with extensive amalgamated crevasse splay deposits form an intricate network of fluvial sand deposits successive stages of progressively deeper crevasse channel incision into the floodplain are the result of waning stage return flow of floodwaters towards the main channel this results in increased crevasse channel width depth and length and redeposition of eroded lacustrine coastal plain sediment at the junction of the crevasse channel with the present day river a sedimentary model is proposed in which deep river bank incision by return flow in crevasse channels creates the preferential locus for river avulsion in a lowstand systems tract with little vertical accommodation increase the recurring avulsed river paths result in a thin but laterally extensive network of amalgamated channel fill point bar crevasse channel and crevasse splay deposits the resulting sedimentary architecture could easily be mistaken as produced by a distributary system of simultaneously active fluvial channels

a 130 ka reconstruction of rainfall on the bolivian altiplano new efforts to link climate reconstructions from shoreline deposits and sediment cores yield an improved and more detailed lake history from the bolivian altiplano on the southern altiplano 10 lake oscillations have been identified from this new unified chronology each coincident with north atlantic cold events such as heinrich events h5 h2 h1 and the younger dryas by coupling this new lake history to a hydrologic budget model we are able to evaluate precipitation variability on the southern bolivian altiplano over the last 130 ka these modeling efforts underscore the relative aridity of the altiplano during the rare and small lake cycles occurring between 80 and 20 ka when colder temperatures combined with little or no change in rainfall produced smaller paleolakes relative aridity between 80 and 20 ka contrasts with the immense tauca lake cycle 18 1 14 1 ka which was six times larger than modern lake titicaca and coincided with heinrich event 1 this improved paleolake record from the southern altiplano reveals a strong link between central andean climate and atlantic sea surface temperature gradients during the late pleistocene even though today rainfall variability is driven mostly by pacific sea surface temperature anomalies associated with el nino southern oscillation however not all heinrich events appear to result in lake expansions most conspicuously during the global cold interval between 80 and 20 ka when the altiplano and amazon basin were relatively arid

river channel adjustment to meander cutoffs on the river bollin and river dane northwest england morphological adjustment of a channel after cutoff should be expected but rarely has the mode and timescale of adjustment in natural channels been measured four natural cutoffs which have occurred since 1980 on two rivers in northwest england have been monitored from pre cutoff stage two of the cutoffs were neck cutoffs of large loops two were chute cutoffs in which significant bend curvature remained all the cutoffs were the terminal result of progressive bank erosion and or scour of the floodplain rather than overflow and all the cutoffs took place in peak flow events though two of these were of relatively low magnitude high frequency rapid widening and sedimentation took place within the straight channels produced by neck cutoffs immediately after breaching adjustment was mainly by formation of multiple riffles and bars producing a variable morphology in the first 2 4 years subsequently the morphology became more regular and in one case had stabilised within about eight years progressive steepening and acceleration of bank erosion appears to have been propagated upstream in one case but in the other cases erosion was very localised rates of vertical accretion at the entrances to the old channel were much higher than most quoted in the literature rates of change in all morphological sedimentary and biotic components exhibit an exponential decline with time but with slightly differing timescales on these active gravel bed streams major adjustment within the new channels may be completed within 6 12 years though lakes persist in the abandoned channels for much longer

geoscience of ancient mediterranean harbours abstract although much has been written on the subject of ancient mediterranean harbours the relatively new area of harbour geoarchaeology remains dispersed in the geoscience and archaeological literature over a decade of research has amassed rich and varied datasets of anthropogenically forced coastal evolution with a remarkable number of between site analogies this new research field also shows the rich potential of geoscience to reconcile important archaeological questions no single publication however has yet drawn on these geological patterns to yield a detailed overview suitable for geoscientists and environmental archaeologists the aim of this review article is to 1 discuss how ancient harbours have come to be preserved in the geological record 2 expound the basic principles and palaeoenvironmental tools underpinning ancient harbour geoarchaeology 3 outline some of the most significant research advances made and 4 discuss a new chrono stratigraphic model applicable to harbour sequences

insights into river bank erosion processes derived from analysis of negative erosion pin recordings observations from three recent uk studies recent studies of river bank erosion in three catchments in the uk have been characterized by the persistent occurrence of negative erosion pin results the cause of these negative recordings is considered with reference to field data from the afon trannon nant tanllwyth and river arrow and to a laboratory study of freeze thaw and desiccation processes it seems that there is potential for and in some cases evidence of a number of different circumstances that generate negative results but none of these alone is sufficient to explain all incidents factors considered include deposition of sediment during high flows soil fall from the upper parts of the bank on to lower erosion pins loosening of the soil surface and expansion contraction of the soil mass with fluctuations in temperature and moisture content movement of the erosion pin within the bank and human interference each has its own implications for the use of erosion pins further issues arise when including negative data in subsequent data analysis and it is demonstrated that attempts to correlate erosion rates with hydro meteorological data in order to ascertain causes of erosion will be influenced by the way in which negative data are handled it is thus suggested that any study of river bank erosion using erosion pins should state whether or not negative data were obtained and if so how they were included in data analysis failure to include this information could lead to comparison of mean erosion rates that reflect bank processes very differently the studies presented here offer a clear example of the value of anomalous field data results which do not appear to fit expected patterns can reveal as much about the processes in operation as those that do

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

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

the role of vegetation in the formation of anabranching channels in an ephemeral river northern plains arid central australia as the distribution and abundance of vegetation in drylands is often controlled by the greater availability of water along river channels riparian vegetation has the potential to influence significantly dryland river form process and behaviour this paper demonstrates how a small indigenous shrub the inland teatree melaleuca glomerata influences the formation and maintenance of anabranching channels in a reach of the ephemeral marshall river northern plains arid central australia here the marshall is characterized by ridge form anabranching where water and sediment are routed through subparallel multiple channels of variable size which occur within a typically straight channel train channels are separated by channel train ridges narrow flow aligned vegetated features or by wider islands by providing a substantial element of boundary roughness dense stands of teatrees growing on channel beds or atop the ridges and islands influence flow velocities flow depths and sediment transport resulting in flow diversion bank and floodplain erosion and especially sediment deposition ridges and islands represent a continuum of forms and their formation and development can be divided into a three stage sequence involving teatree growth and alluvial sedimentation 1 teatrees colonize a flat sandy channel bed initiating the formation of ridges by lee side accretion individual ridges grow laterally vertically and longitudinally and maintain a geometrically similar streamlined lemniscate form that presents minimum drag 2 individual ridges grow in size and interact with neighbouring ridges causing the lemniscate forms to become distorted ridges in the lee of other ridges tend to be protected from the erosive effects of floods and survive whereas individual teatrees or small ridges exposed to flow concentrated between larger ridges tend to be removed 3 ridges lengthen and coalesce with downstream ridges eventually subdividing the channel train into well defined anabranches this sequence turns a channel initially obstructed with dense and chaotic stands of teatrees into a well organized system of ridge form anabranches in the moderate to low gradient marshall river which is colonized by an abundance of within channel vegetation and subject to declining downstream discharges this helps to minimize flow resistance thereby maintaining an efficient water and sediment flux copyright 2000 john wiley sons ltd

the development of an automated correction procedure for digital photogrammetry for the study of wide shallow gravel bed rivers this paper develops an automated correction procedure for dealing with point errors associated with through water photogrammetry for application in the study of clear water shallow gravel bed rivers the procedure involves combining digital photogrammetry and image analysis techniques to i correct for the effects of refraction at an air water interface and ii eliminate and reinterpolate points where the bed has not been seen the correction procedure was applied to raw digital elevation models dems generated using digital photogrammetry from 1 3000 scale aerial photography of a small reach of the north ashburton river new zealand the accuracy of corrected and uncorrected dems is evaluated using an independent data set a measure of geomorphological usefulness as well as dem external reliability is obtained from calculations of water depth distributions and mean bed level results show that digital photogrammetry used in conjunction with image analysis techniques can successfully be used for extracting high resolution dems of gravel river beds in exposed areas errors are small and random tending to cancel out over large numbers of points where water is shallow and following correction point elevation errors are statistically no different from those for exposed zones in deeper water despite an improvement following application of the correction procedure elevation errors scale with water depth the geomorphological potential of photogrammetric survey of large gravel river beds is demonstrated by the ease and accuracy of calculations of water depth distribution important for the assessment of a river s ecological and recreational characteristics and mean bed level important for the calculation of reach scale sediment volumes copyright 2000 john wiley sons ltd

river channel changes of the subansiri in assam india abstract the sequential changes in the position of banklines as a result of bank erosion as well as various changes in the channel have been studied in the subansiri river in assam india the study was carried out from available topographic maps of 1920 1970 and satellite imagery of 1990 the types of changes taking place in the subansiri river channel as observed from the above mentioned maps and imagery are grouped into four categories 1 alteration of the direction of flow due to neck cut off 2 widening of a channel in response to bar development 3 development and subsequent abandonment of anabranches and 4 progressive shifting of meander bends the river channel has become substantially wider during the period of 1970 1990 there has also been a progressive increase in the number and area of mid channel bars with a corresponding decrease in the number and area of point bars during 1920 1990 these periodic and progressive changes are responsible for a change in channel pattern of the river a study made on nature and amount of change in the banklines of the subansiri river by dividing the channel 84 km in 1970 into 10 equal segments reveals that the types of change are varied and in some cases chaotic in nature study of the bankline shift of the subansiri determined separately for the periods 1920 1970 1970 1990 and 1920 1990 at 20 evenly spaced transverse sections reveals that there has been a substantial westward shift of both the banklines up to a maximum of 6 km in the three upstream sections during 1920 1970 however shifting of banklines in both directions and the unequal amount of shifting in the same direction have caused a marked increase in channel width during 1970 1990 it is found that due to bankline migration the total amount of area eroded along the entire course under study during 1920 1970 and 1970 1990 are 107 90 and 57 50 km2 respectively the overall sinuosity which was 1 51 in 1920 has decreased to 1 30 in 1970 while the corresponding braiding intensity has increased from 0 79 to 1 32 the river channel seems to make a remarkable change from a meandering pattern in 1920 towards a braided pattern by 1990 as seen from gradual increase in overall braiding intensity

fluvial dynamics of an anabranching river system in himalayan foreland basin baghmati river north bihar plains india abstract anabranching river systems are now regarded as a separate class in river classifications owing to their distinctive morphological hydrological characteristics and fluvial processes a better understanding of anabranching rivers still needs detailed data from different environmental and geographical settings this paper presents a detailed account of an anabranching river system from the himalayan foreland basin the baghmati river system from north bihar plains eastern india provides a typical example of an anabranching river system located in the interfan area between the kosi and the gandak megafans the river system is braided in upstream reaches and meandering in downstream reaches but the midstream anabranching reach is characterized by low width depth ratio 11 16 gentle gradient 0 00018 0 00015 variable peak discharge frequent flooding and high sediment load the anabranching in the midstream reaches is a response to its inability to transport high sediment load due to gentle channel slope and dominance of aggradation process the development of anabranches is related to rapid and frequent avulsions of the river channels with eight major avulsions observed in the 30 km wide floodplain in the last 230 years the decadal scale avulsion history of the baghmati river system makes it hyperavulsive and the major causative factors for such channel instability are sedimentological readjustments and active tectonics in the basin area

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

plateau style accumulation of deformation southern altiplano 1 employing surface mapping of syntectonic sediments interpretation of industry reflection seismic profiles gravity data and isotopic age dating we reconstruct the tectonic evolution of the southern altiplano 20 22 s between the cordilleras defining its margins the southern altiplano crust was deformed between the late oligocene and the late miocene with two main shortening stages in the oligocene 33 27 ma and middle late miocene 19 8 ma that succeeded eocene onset of shortening at the protoplateau margins shortening rates in the southern altiplano ranged between 1 and 4 7 mm yr with maximum rates in the late miocene summing rates for the southern altiplano and the eastern cordillera we observe an increase from eocene times to the late oligocene to some 8 mm yr followed by fluctuation around this value during the miocene prior to shutoff of deformation at 7 8 ma and transfer of active shortening to the sub andean fold and thrust belt shortening inverted early tertiary graben and half graben systems and was partitioned in three fault systems in the western central and eastern altiplano respectively the east vergent fault systems of the western and central altiplano were synchronously active with the west vergent altiplano west flank fault system from these data and from section balancing we infer a kinematically linked western altiplano thrust belt that accumulated a minimum of 65 km shortening evolution of this belt contrasts with the eastern cordillera which reached peak shortening rates 8 mm yr in between the above two stages hence local shortening rates fluctuated across the plateau superimposed on a general trend of increasing bulk rate with no trend of lateral propagation this observation is repeated at the shorter length and time scales of individual growth structures that show evidence for periods of enhanced local rates at a timescale of 1 3 myr we interpret this irregular pattern of deformation to reflect a plateau style of shortening related to a self organized state of a weak crust in the central south american back arc with a fault network that fluctuated around the critical state of mechanical failure tuning of this state may have occurred by changes in plate kinematics during the paleogene initially reactivating crustal weak zones and by thermal weakening of the crust with active magmatism mainly in the neogene stage

quantification of braided river channel change using archival digital image analysis historical archives of grey scale river channel imagery are extensive here we present and test a methodology to extract detailed quantitative topographic date from such imagery of sand bed rivers extracting elevation information from rivers is difficult as they are characterized by a low relative relief less than 4 m the area of interest may be spatially extensive e g active channel widths 500 m in large braided rivers the rate of change of surface elevation is generally low except in the vicinity of individual channel banks where the rate of change is very high there is the complication that comes from innundation and there may be an added complication caused by blockage of the field of view by vegetation here we couple archival photogrammetric techniques with image processing methods and test these for quantification of sand bed braided river dynamics illustrated for a 500 m wide 3 km long reach of the spouth sasketchewan river canada digitial photogrammetry was used to quantify dry areas and water edge elevations a methodology was then used to calibrate the special signature of inundated areas by combining established two media digital photogrammetric methods and image matching this allowed determination of detailed depth maps for inundated area and when combined with dry area data creation of depths detectable from sequential digital elevation models the result was a series of elevation models that demonstrate the potential for acquiring detailed and precise elevation data from any historical aerial imagery of rivers without needing associated calibration data provided that imagery is of the necessary scale to capture the features of interest we use these data to highlight several aspects of channel change on the south saskatchewan river including bar movement bank erosion and channel infilling

anastamosing channels in the lower neches river valley texas active and semi active anastomosing holocene channels upstream of the delta in the lower valley of the meandering neches river in southeast texas represent several morphologically distinct and hydrologically independent channel systems these appear to have a common origin as multi thread crevasse channels strongly influenced by antecedent morphology levee breaching leads to steeper cross valley flows toward floodplain basins associated with pleistocene meander scars creating multi thread channels that persist due to additional tributary contributions and ground water inputs results are consistent with the notion of plural systems where main channels tributaries and sub channels may have different morphologies and hydrogeomorphic functions the adjacent trinity and sabine rivers have similar environmental controls yet the trinity lacks evidence of extensive anastomosing channels on its floodplain and those of the sabine appear to be of different origin the paper highlights the effects of geographical and historical contingency and hydrological idiosyncrasy copyright 2014 john wiley sons ltd

paleohydrology of the laguna lejia north chilean altiplano and climatic implications for late glacial times paleoenvironmental and sedimentological data from laguna lejia 23 30 s 67 42 w 4325 m in the high altiplano of the chilean atacama desert indicate that climatic conditions during late glacial times were significantly wetter than today a water and energy budget model was used to simulate climatic scenarios that would have resulted in the observed changes in the water levels water surfaces water volumes and salinity during the time interval different climatic scenarios for the laguna lejia catchment include precipitation increases of over 100 up to 400 500 mm yr today 200 mm yr to account for the late glacial lake levels with shorelines 15 25 m higher than at present and a lake surface of 9 11 km2 compared to the present 2 km2 during the lake maximum about 13 500 11 300 yr b p finely laminated sediments consisting of mg calcite diatoms and sometimes gypsum were deposited the paleoenvironmental evidence indicates that north easterly wind direction prevailed and the summer rainfall resulted from a seasonal poleward shift of the tropical circulation the lake history of laguna lejia is representative for the altiplano area between 21 and 24 s

rivers turned to rock late quaternary alluvial induration influencing the behaviour and morphology of an anabranching river in the australian monsoon tropics abstract late quaternary alluvial induration has greatly influenced contemporary channel morphology on the anabranching gilbert river in the monsoon tropics of the gulf of carpentaria the gilbert one of a number of rivers in this region has contributed to an extensive system of coalescing low gradient and partly indurated riverine plains extensive channel sands were deposited by enhanced flow conditions during marine oxygen isotope oi stage 5 subsequent flow declined probably associated with increased aridity however enhanced runoff recurred again in oi stages 4 3 65 50 ka aridity then capped these plains with 4 7 m of mud a widespread network of sandy distributary channels was incised into this muddy surface from sometime after the last glacial maximum lgm to the mid holocene during a fluvial episode more active than the present but less so than those of oi stages 5 and 3 this network is still partly active but with channel avulsion and abandonment now occurring largely proximal to the main gilbert flow path a tropical climate and reactive catchment lithology have enhanced chemical weathering and lithification of alluvium along the river resulting in the formation of small rapids waterfalls and inset gorges features characteristic more of bedrock than alluvial systems thermoluminescence tl and comparative optically stimulated luminescence osl ages of the sediments are presented along with u th ages of pedogenic calcrete and fe mn oxyhydroxide oxide accumulations they show that calcrete precipitated during the late quaternary at times similar to those that favoured ferricrete formation possibly because of an alternating wet dry climate intense chemical alteration of the alluvium leading to induration appears to have prevailed for much of the late quaternary but probably due to exceptional dryness not during the lgm the result has been restricted channel migration and a reduced capacity for the channel to adjust and accommodate sudden changes in bedload consequent avulsions have caused local stream powers to increase by an order of magnitude inducing knickpoint erosion local incision and the sudden influx of additional bedload that has triggered further avulsions the gilbert river while less energetic than its pleistocene ancestors is clearly an avulsive system and emphasizes the importance in some tropical rivers of alluvial induration for reinforcing the banks generating nickpoints reworking sediment and thereby developing and maintaining an indurated and anabranching river style

dryland anabranching river morphodynamics rio capilla salar de uyuni bolivia the dryland anabranching river rio capilla is characterized by nonvegetated and vegetated reaches with prominent channel morphology to identify the morphodynamics of such dryland anabranching systems and their controls we investigated the rio capilla of the southern altiplano plateau using high resolution satellite imagery and field measurements comparison of high resolution satellite data reveals that erosion exceeds deposition for the main channel accompanied by changes in channel planform such as meander and channel morphology on site surveys combined with high precision gps and high resolution satellite imagery show that channels are characterized by shallowness and poor development of levees the study area of the rio capilla is divided into two zones of different slopes zone 1 with a high slope and zone 2 with a low slope zone 1 has a relatively straight main channel with few anabranches and grass covered banks that are stable despite the high gradient whereas zone 2 is typified by more anabranches with nonvegetated banks and the main channel experiences prominent bank accretion and erosion excavations show that point bar deposits are fine sand dominated in two reaches and that river banks primarily consist of silt and clay the limited vegetation cover and abundance of desiccation cracks and macropores make the river bank more erodible leading to pronounced lateral migration in this low gradient dryland river system shallow channels and poor development of levees in combination with in channel accretionary benches result in frequent overbank flooding which results in a high density of crevasse splays over unconsolidated river banks and accretionary benches connection of headcuts and crevasse channels together with lateral migration and chute channels and reactivation of partially abandoned meanders produces an anabranching pattern in such dryland river systems

monitoring river channel and flume surfaces with digital photogrammetry this paper describes and illustrates a technique for high resolution monitoring of the surface morphology of water worked sediments the monitoring uses close range digital photogrammetry while photogrammetry is a long established technique more recent developments in digital photogrammetry allow application in fluvial research to be highly cost effective in both flume and natural river channel studies results are presented that involve two scales of laboratory flume a smaller scale application associated with sediment sorting processes in a straight channel and a larger scale application involving sediment transport and bed material feedbacks in a meandering channel subject to overbank flows a preliminary assessment of data quality is undertaken with encouraging results the precision of elevation estimates corresponds to the scale of the imagery acquired and hence may be controlled by design of the image acquisition process

anabranching rivers on the northern plains of arid central australia anabranching rivers are a widespread feature of the northern plains in the alice springs region of central australia but their unusual characteristics previously have not been described on the northern plains anabranching occurs on rivers transporting bedloads of coarse sand and gravel and is characterised by channels of variable size and shape which occur within a broader typically well defined channel train channels are separated by channel train ridges narrow flow aligned vegetated features or by wider islands ridges and islands are either depositional features formed in situ by accretionary processes or erosional features formed by excision from once continuous areas of floodplain vegetation plays a key role in the initiation survival and growth of depositional forms through its influence on flow sediment transport and ridge and island stability anabranching is also related to the influence of tributaries for some large rivers alternate from single thread to anabranching along their length in response to tributary inputs of water and sediment tributary inputs occur during flow events that are either independent from or in concert with floods in the trunk channel ridges and islands form in association with tributaries as a result of various hydrological depositional and erosional processes including irrigation of enhanced numbers of in channel trees and resulting lee side sediment accretion floodplain scour and the formation and maintenance of deferred junction tributaries the change from single thread to anabranching downstream of tributary junctions occurs in the absence of any significant change in channel gradient or degree of channel confinement on the northern plains anabranching appears to be a stable river pattern that helps to maintain the throughput of relatively coarse sediment in low gradient typically 0 0005 0 002 channels characterised by an abundance of within channel vegetation and subject to declining downstream discharges

late quaternary climate history of the bolivian altiplano abstract sediment cores and outcrops from the bolivian altiplano are used to interpret late pleistocene holocene paleoclimates and lake evolution based on a multi proxy approach ostracod content palynology sedimentology and radiocarbon dating despite the different sensitivity of the lacustrine basins to environmental changes interpreting the records in terms of paleohydrology climate and more especially timing of events is difficult notwithstanding these problems lake titicaca in the north and lake pocoyu lake poopo salars of coipasa and uyuni in the south reveal a similar evolution in general trends through the course of time during the minchin phase until 30 26 kyr bp the altiplano was wetter than present the last glacial maximum ca 26 14 kyr bp marked by cooler drier conditions was followed by a return to a wetter climate interrupted by short arid events between ca 14 and 10 5 kyr bp another cycle of aridity between 10 5 and 8 kyr bp took place abruptly just after the humid tauca phase the mid holocene is interpreted as climatically unstable with an alternation of humid and dry episodes since 3 9 kyr bp wetter conditions have persisted and intensified to the present the data are interpreted in terms of changes in inter tropical convergence zone extension summer precipitation and in polar air mass intensity winter precipitation

crevasse splay morphodynamics in a dryland river terminus rio colorado in salar de uyuni bolivia high resolution satellite imagery in combination with field investigation enables the analysis of splay morphological developments in time and space at the low gradient terminus of the modern rio colorado dryland river system in the salar de uyuni bolivia two sets 2004 2005 2010 2011 of satellite images are used to visualize splay morphological developments and three types of crevasse splays are identified based on temporal changes 1 new crevasse splays ncs refers to those splays that are not observed in the first set of images but can be observed on the second sets of images 2 changing crevasse splays ccs representing those splays that are found on the first set of images but their geometry has changed between two satellite observation periods and 3 inactive crevasse splays ics referred to those splays that are found on the first set of images but on the second set of images they remain the same or are partially overlapped by adjacent expanding crevasse splays the density or frequency of these three types of crevasse splays has no correlation with the distance to the starting point of the river or the local slope the cross sectional area of the studied river steadily decreases downstream and frequency analysis shows a downstream increase in the number of crevasse splays and suggests an exponential relationship with the river cross sectional area development of crevasse splays is often attributed to topographic lows between adjacent crevasse splays and they fill in the depression by compensational stacking e g li et al 2014 these amalgamated splays are capable of forming a large area of sheet sands and potentially provide an analogue for thin bedded hydrocarbon reservoirs

structure from motion photogrammetry a low cost effective tool for geoscience applications abstract high resolution topographic surveying is traditionally associated with high capital and logistical costs so that data acquisition is often passed on to specialist third party organisations the high costs of data collection are for many applications in the earth sciences exacerbated by the remoteness and inaccessibility of many field sites rendering cheaper more portable surveying platforms i e terrestrial laser scanning or gps impractical this paper outlines a revolutionary low cost user friendly photogrammetric technique for obtaining high resolution datasets at a range of scales termed structure from motion sfm traditional softcopy photogrammetric methods require the 3 d location and pose of the camera s or the 3 d location of ground control points to be known to facilitate scene triangulation and reconstruction in contrast the sfm method solves the camera pose and scene geometry simultaneously and automatically using a highly redundant bundle adjustment based on matching features in multiple overlapping offset images a comprehensive introduction to the technique is presented followed by an outline of the methods used to create high resolution digital elevation models dems from extensive photosets obtained using a consumer grade digital camera as an initial appraisal of the technique an sfm derived dem is compared directly with a similar model obtained using terrestrial laser scanning this intercomparison reveals that decimetre scale vertical accuracy can be achieved using sfm even for sites with complex topography and a range of land covers example applications of sfm are presented for three contrasting landforms across a range of scales including an exposed rocky coastal cliff a breached moraine dam complex and a glacially sculpted bedrock ridge the sfm technique represents a major advancement in the field of photogrammetry for geoscience applications our results and experiences indicate sfm is an inexpensive effective and flexible approach to capturing complex topography

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

the measurement of river bank erosion and lateral channel change a review a detailed review and chronological survey is presented of the various techniques which have been used for the measurement of river bank erosion and channel change the techniques are classified according to the time scales involved long intermediate and short and each is discussed with respect to accuracy and repeatability the methods covered include sedimentological evidence botanical evidence historical sources planimetric resurvey repeated cross profiling erosion pins and terrestrial photogrammetry prospects for future developments are also discussed

downstream channel changes on a contracting anabranching river the lachlan southeastern australia abstract downstream trends in flow channel morphology and sediments were investigated over 220 km of the middle and upper alluvial lachlan river in southeastern australia the lachlan is a perennial river that rises from temperate humid highlands and flows inland into a semiarid low gradient plain lacking perennial tributaries long daily stage records were used to calculate flow parameters and flood frequencies in single and anabranching channel reaches of the river morphological trends were based on cross section and height data compiled from new and water authority data and from particle size trends constructed from bed and bank sediment samples mean and peak flows channel size and bed particle size all diminish in a downstream direction in the partially confined piedmont zone channel and floodplain morphology feature large flood effects and large channel dimensions that reflect extreme flow variability on the alluvial plains the unconfined single channel contracts with distance downvalley and anabranches develop at high flow stages the development of a full anabranching pattern is associated with large flood volume losses diminishing flood volumes mean flows and channel dimensions is attributed to storage of flood waters in lakes floodways and lagoons and through transmission losses during overbank flows the frequency of bankfull discharge consistently decreases downstream and may reflect the long duration and lower frequency of floods in the lower reaches of the river particle size of bed sediment fines downstream at rates consistent with those reported from sand bed rivers elsewhere this is attributed to a combination of hydraulic sorting and low rates of sediment supply from the catchment

riparian vegetation and the late holocene development of an anabranching river magela creek northern australia many anabranching rivers are characterized by dynamic interactions between fluvial processes and riparian vegetation but uncertainties surround the processes and time scales of anabranch development we use geomorphological investigations and optically stimulated luminescence osl dating to determine spatial and temporal trends in the development of anabranching along a 6 5 km long reach of magela creek in the seasonal tropics of northern australia many trees and shrubs that survive the wet season floods establish on the sandy beds and lower banks such that anabranches divide and rejoin around numerous ridges and islands that are formed mainly by accretion in the lee of in channel vegetation and less commonly by excision from formerly continuous island or flood plain surfaces once ridges and islands form colonizing vegetation maintains their stability by increasing sediment cohesion and decreasing flow erosivity over the holocene magela creek has vertically aggraded and extended in length by delta progradation into madjinbardi billabong resulting in a time sequence of anabranches and associated ridges and islands from older upstream to younger downstream osl ages for islands in the upstream and middle reaches are ca 1 6 ka and older and the narrow deep anabranches width depth w d typically 10 30 have few in channel obstructions farther downstream island osl ages are ca 0 7 ka and younger anabranches tend to be wider and shallower w d 30 with more obstructions and splays and locally scoured island and floodplain surfaces are more common based on these findings previous flow and sediment transport measurements and theoretical analyses we posit that there is a decline in anabranch efficiency from an upstream equilibrium system in mass flux balance to a downstream disequilibrium system characterized by bed aggradation and localized island and floodplain erosion in the downstream reaches inefficient high w d and obstructed anabranches do not persist because they either aggrade and are abandoned or they are subdivided into more efficient lower w d and less obstructed anabranches as a result of the interactions between in channel vegetation growth and ridge and island accretion or local excision consequently a more efficient anabranching system gradually develops with characteristics similar to those in the upstream reaches this enhances downstream sediment transfer which enables ongoing delta progradation and provides fresh sediment surfaces for vegetation to colonize and initiate new anabranches the osl ages from magela creek demonstrate that a recognizable but relatively inefficient anabranching system can develop within a few centuries while adjustment to a more efficient system occurs over a few millennia

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

landsat imagery based visualization of the geomorphological development at the terminus of a dryland river system abstract in this remote sensing based study we present the analysis of the geomorphological development at the low gradient terminus of the modern rio colorado dryland river system in the endorheic altiplano basin bolivia changes in the river morphology occur after short periods of catastrophic peak discharge which result in the expansion of existing crevasse splays formation of new crevasse splays and in river path avulsion episodic peak discharge events in the study area were pinpointed and quantified by combining daily precipitation records from gauging stations in the vicinity with catchment area analysis from aster global dem gdem remote sensing data a time series of landsat imagery for the period 1975 2001 was then used to analyze the river morphology changes after major peak discharge events compensational stacking of crevasse splays in combination with river avulsion produced a thin but aerially extensive connected sand sheet at the terminus of the fluvial system