surface based transport model for mixed size sediment we present a transport model for mixed sand gravel sediments fractional transport rates are referenced to the size distribution of the bed surface rather than subsurface making the model completely explicit and capable of predicting transient conditions the model is developed using a new data set of 48 coupled observations of flow transport and bed surface grain size using five different sediments the model incorporates a hiding function that resolves discrepancies observed among earlier hiding functions the model uses the full size distribution of the bed surface including sand and incorporates a nonlinear effect of sand content on gravel transport rate not included in previous models the model shares some common elements with two previous surface based transport models but differs in using the full surface size distribution and in that it is directly developed from a relatively comprehensive data set with unambiguous measurement of surface grain size over a range of flow transport rate and sediments

the kinematics of movements along the insubric line and the emplacement of the ivrea zone schmid sm zingg a and handy m 1987 the kinematics of movements along the insubric line and the emplacement of the ivrea zone in h j zwart m martens i van der molen c w passchier c spiers and r l m vissers editors tectonic and structural processes on a macro meso and micro scale tecta a hys 135 47 66 the insubric line west of locamo is characterized by a 1 km thick greenschist facies mylonite belt east of locamo these mylonites are overprinted by a discrete brittle fault the mylonites are derived from basement units of the central alps sesia zone and the southern alps ivrea zone as well as from the permo mesozoic cover of the southern alps canavese sense of shear criteria indicate that the mylonites accomm ated backt s g followed by dextral strike slip motion mylonitization during backtbrusting was synchronous with backfolding of the central alpine nappes under higher metamorphic conditions a horizontal temperature gradient resulted from the rapid juxtaposition of the warm central alpine block against the cold southern alpine block mylonites formed during the later dextral strike slip event are related to large transcurrent displacements in the central alps deduced from regional kinematic considerations thus both mylo t ation events are contempor eous with deformation to the north and south of the insubric line insubric phase extensively modifying the pre insubric crustal configuration of the alps the insubric phase post dates the bergell intrusion 30 m y the emplacement of the geophysical ivrea body is a combined effect of vertical uplift due to e w directed crustal thinning during the early jurassic and underplating by continental crust associated with late cretaceous compression a deep crustal normal fault pogallo line subsequently rotated during tertiary alpine orogenesis separates deeper parts of the southern alpine crust ivrea zone from intermediate crustal levels strona ceneri zone the rigid ivrea body localir ed large strains within the insubric mylonite belt and is responsible for the present curvature of the insubric line in uetion

the morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae experimental studies of subcritical unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves flow depth was 0 11 m mean flow velocities were 0 86 1 0 m s 1 shear velocities were 0 058 0 71 m s 1 and dimensionless shear stresses were 0 56 0 87 bedwaves are asymmetrical in profile and range from 0 75 to 11 mm in height mainly 2 6 mm from 0 7 to 1 3 m in wavelength and have mean celerities of 10 mm s 1 flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs high resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave

extrusion and radial spreading beyond a closing channel abstract the development of experimental models of an extruding spreading zone beyond a closing channel is studied and a possible natural example described the strain pattern folding and radial displacements within the extruding spreading zone are analysed folds developed in the closing channel are either passively rotated or refolded when carried into the spreading zone a simple theoretical model of the experiments demonstrates that the stretching lineation is radial and parallel to the displacement direction towards the base and the front of the extruding spreading zone but perpendicular towards the top an intermediate domain with no stretching lineation separates the radial and concentric domains and corresponds to a transition through a uniaxial oblate ellipsoid k 0

incipient granular mass flows at the base of sediment laden floods and the roles of flow competence and flow capacity in the deposition of stratified bouldery sands extraordinary floods commonly produce thick deposits as a result of the entrainment and transport often at high concentrations of large volumes of clastic detritus this paper describes the sedimentology of a stratified bouldery sand deposit generated by a 20 km3 breakout flood from intracaldera lake taupo following the 1 8 ka taupo eruption the proximal deposit is characterised by a strongly bimodal grain size distribution with downstream fining boulders enclosed in a poorly sorted and weakly bedded coarse sand to gravel matrix this combination of characteristics is inferred to reflect simultaneous deposition of small and large particles by two distinct mechanisms i accretion of the sandy matrix from an incipient basal granular mass flow as the flood lost capacity and ii progressive emplacement of the boulders as the flood lost the competence to roll them along the channel floor the first mechanism is a product of a nonlinear particle concentration gradient in the sediment laden flow which caused it to segregate into a basal incipient granular mass flow that formed the transient depositional system indicated by the lithofacies the second mechanism is a function of the superjacent turbulent turbid flow that acted as the primary transporting system this pairing of transport and depositional systems is likely to characterise many sediment laden aqueous flows with clay mineral concentrations too low to develop non newtonian rheologies such as jokulhlaups and lahars that entrain unweathered pyroclastic material qualitative and quantitative differences in the way bedload is transported in the presence of this basal granular mass flow as compared to simple water make impossible accurate estimates of flow competence based on shields entrainment functions for beds of both uniform and mixed grain size distribution

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

cooling and uplift patterns in the lepontine alps south central switzerland and an age of vertical movement on the insubric fault line 96 new fission track ft apatite and zircon k ar and rb sr biotite and muscovite ages are presented for 19 samples mainly acid gneisses from a 40 km traverse through the lepontine alps in the maggia valley south central switzerland plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample the entire profile shows a fairly uniform late neogene recent mean uplift rate of 0 5 mm a confirmed by a gradient of ft apatite age with elevation cooling from higher temperatures occurred earlier in the south where uplift rates of 2 2 mm a in the steep belt root zone indicate 9 km early miocene uplift of the northern pennine block this uplift started before 23 ma and is interpreted as resulting from a major phase of backthrusting along the insubric line and as dating the formation of the mylonite belt estimated cooling rates constrain the timing of lepontine mid tertiary metamorphism 3 schematic models are proposed which also consider published rb sr k ar mica and hornblende and u pb monazite ages slow cooling differential initial heating and subsequent cooling of different parts of the central alps and post 38 ma cooling with syntectonic metamorphism at 27 ma are postulated as alternative interpretations of isotopic data and geologic evidence from extrapolation between k ar and rb sr mica ages and apatite ft ages 240 50 c is proposed as the closure temperature for the retention of fission tracks in zircon

alluvial architecture paleohydrology and provenance tracking from the neoproterozoic banganapalle formation kurnool group india an example of continental sedimentation before land plants abstract an integrated study involving facies association analysis architectural element analysis paleohydrology and detrital zircon geochronology allowed detail insight of sedimentology and paleogeography of neoproterozoic banganapalle formation bf kurnool group detailed process based facies and paleoenvironmental analysis have led to the identification of fourteen facies types grouped under four different facies associations which record paleoenvironmental settings ranging between mid alluvial fan to distal fluvial plain whereas mass flows hyperconcentrated flows streamlets and sheet floods dominate the depositional process on the bf fan middle to distal surface streams of varied geometry viz braided and ephemeral meandering occupied the proximal and distal bf alluvial plain our study supports the emerging view that even without the buffering effect of land plants channels developed sinuous planform in pre vegetation time subject to low gradient and discharge of alluvial plain and adequate supply of fine detritus from occurrence of boulder gravelly mass flows boundary pebbly streamlet thalweg and pebbly granular sheet flood deposits in the alluvial fan thick silty muddy stable flood plain deposits in the distal alluvial plain and unequivocal southward paleocurrent irrespective of facies association it is inferred that the bf alluvial basin received mixed sediment from long term beveled source in the north and northwest from two u pb detrital zircon geochronology age clusters viz paleo to mesoarchean 3410 ma 3075 ma and neoarchean to late paleoproterozoic 2750 ma 2500 ma provenance for the bf is tracked in the north and northwest of the kurnool basin i e the ttg suite of the peninsular gneiss and volcanics hosted greenstone belts in the central and western dharwar craton a mixing of sediments from two different provenances is inferred a consistent low water discharge with marginal downstream increase is suggested for the bf alluvial system from paleohydraulic analysis except for some occasional spurts of high discharge in mid fan at times of intense rainfall in the hinterland

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

hornblende k ar ages and the climax of tertiary metamorphism in the lepontine alps south central switzerland an old problem reassessed abstract twenty five new k ar ages for amphiboles from the tertiary barrovian type metamorphic terrain of the central alps were determined these results lead to the conclusion that the meso alpine metamorphism reached its peak condition and maximum expanse about 26 3 m y ago along a profile of increasing grade of alpine metamorphism in both basement and cover rocks the following observations can be made 1 pre alpine hornblendes in the zone of greenschist facies show ages ranging from 639 to 152 m y the implied open system behavior may be related to the observed mineralogical instability of these hornblendes during the alpine overprint in contrast two greenschist facies hornblendes which grew in pennine mesozoic rocks during the alpine metamorphism yield ages of 23 2 and 29 3 m y 2 on both sides of the boundary marking the first occurrence of alpine staurolite pre or early alpine hornblende k ar ages are not fully reset unless the rocks were completely recrystallized for one rock specimen which clearly exhibits two generations of hornblende an age two times greater was obtained for the smaller crystals oriented parallel to the foliation than for the structurally younger hornblende porphyroblasts which overgrew the preexisting texture 3 in the high grade area of alpine metamorphism only young alpine ages between 23 and 28 m y were found for hornblendes which grew syn to postkinematically with respect to the last penetrative deformation one amphibolitic inclusion not affected by this deformation yielded a significantly older hornblende age of 32 1 m y these results suggest that temperature controlled ar diffusion is not the main factor governing the k ar system of metamorphic hornblendes we conclude that these hornblende ages should not be interpreted as cooling ages from a structurally ill defined earlier event instead they indicate that the high grade metamorphism responsible for the well known isograd pattern in the lepontine area persisted until some 23 m y ago

sediment flux dynamics as fingerprints of catchment rehabilitation the case of western rift valley escarpment of northern ethiopia variability in supply and entrainment of stream bedload particles during storm events is an important hydrogeomorphologic response of steep mountain streams to land management in their catchments because of difficulties in its measurement and absence of accurate measuring equipment bedload transport in mountain streams is however poorly understood this study focuses on analyzing bedload supply and entrainment in 11 streams of sloping 27 65 catchments 0 4 25 km2 in northern ethiopia which have experienced severe deforestation and degradation processes until the first half of the 1980s and considerable rehabilitation thereafter field measurements of the median diameter of the 10 coarsest bedload particles max10 moved in each event n 332 and stream bed particle sampling were carried out in three rainy seasons 2012 2014 event peak discharges were calculated from daily measurements by 11 crest stage gauges using manning s equation percentages of land cover classes in the catchments were detected from high resolution 0 6 m google earth imagery 1 february 2014 morphometric characteristics of the catchments were computed from an aster digital elevation model and topographic maps hydraulic competence analysis for entrainment of the average max10 was carried out using peak discharge stream power and critical shear stress approaches the supply of max10 was positively related to scar density on the surrounding slopes r2 0 50 p 0 05 and catchment area r2 0 36 p 0 05 while negatively related with total forest cover r2 0 63 p 0 05 or vegetation cover r2 0 58 p 0 05 a multiple regression analysis showed that 98 of the variability in max10 is explained by scar density and catchment area entrainment of max10 was positively related to peak discharge qp r2 0 60 p 0 01 stream power r2 0 71 p 0 01 strickler s roughness sn r2 0 86 p 0 01 and critical shear stress c with reference to d50 r50 r2 0 96 p 0 01 and with reference to d84 r84 r2 0 93 p 0 01 the relationships between event max10 and event qp in each catchment were also used to extrapolate the peak discharges that would have prevailed in the first half of the 1980s and the results showed huge reductions in peak discharges p 0 01 as a result of the reforestation interventions overall this study demonstrates that i the supply of bedload in steep tropical mountains is determined negatively by forest or vegetation cover and positively by the presence of scar networks in the sloping parts of the catchments and catchment size ii entrainment of the coarsest bedload particles is determined positively by peak discharge stream power stream bed roughness and most strongly by critical shear stress and iii in steep mountain streams where direct measurement of bedload movement is difficult yet simple in situ measurement of the coarsest bedload particles max10 supplied and entrained in each event helps for understanding the dynamics of sediment flux in relation to catchment rehabilitation

preservation of planar laminae due to migration of low relief bed waves over aggrading upper stage plane beds comparison of experimental data with theory experimental studies of the formation of planar laminae by migration of low relief bed waves over aggrading upper stage plane beds show that the average thickness of laminae at a point increases with both aggradation rate and the variance of the heights of bed waves passing that point in general the preserved laminae represent only a small proportion generally less than 50 of the height of the largest bed waves in the population the theory developed by paola borgman 1991 relating the probability density function of stratal thickness to that of bed wave height for the case of no net aggradation was adapted for aggrading conditions and shown to agree well with data these results suggest that the theory can be used to estimate from the distribution of thickness of planar laminae either the distribution of bed wave heights mean aggradation rate mean bed wave length or mean bed wave celerity provided the other parameters can be estimated

rolling structures at large shear strain abstract at large shear strain shear criteria are often obliterated or become ambiguous from examination of both natural examples and experimental models we describe a new criterion called rolling structure widely represented in sheared rocks a typical rolling structure is composed of a rigid or competent object e g porphyroclast boudin fossil etc with two tails asymmetrically disposed around it in most cases tails are of the same material as that of the rotating object and result from strain softening and grain size reduction at the object periphery forming a mantle z and s asymmetries of rolling structures represent dextral and sinistral senses of shearing respectively tails must not be confused with pressure shadows which usually present an opposite asymmetry for a given shear sense besides the determination of the sense of shear the occurrence of these structures allows a minimum estimate of the strain intensity in strongly sheared rocks since rolling structure length is proportional to shear strain

geomorphological impacts of an extreme flood in se spain abstract long term field studies in semiarid ephemeral streams are rare these geomorphic data are essential for understanding the nature of the processes in order to develop modelling for risk assessments and management an extreme flood event on 28 september 2012 affected the murcia region of se spain including long tem monitoring sites on two fluvial systems in the guadalentin basin the nogalte and torrealvilla detailed morphological data were collected before and immediately after the event and the amount of morphological change erosion and deposition have been related to peak flow conditions at the sites on the nogalte channel peak flow reached 2500 m3 s 1 at the downstream end of the catchment in less than 1 h the event had a recurrence interval of 50 years based on rainfall records and damage to old irrigation structures the major effect in the braided gravel channel of the nogalte was net aggradation with massive deposition in large flat bars the measured changes in bankfull capacity were highly correlated with most hydraulic variables net changes in cut and fill in cross sections on the nogalte were highly related to peak discharge and stream power but much less so to measures of hydraulic force velocity shear stress unit stream power relationships of amount of erosion to hydraulic variables were much weaker than for amount of deposition which was largely scaled to channel size and flow energy changes on the torrealvilla were much less than on the nogalte and net erosion occurred at all sites sites on the nogalte channel in schist exhibited higher deposition than those of the torrealvilla sites on marl for the same hydraulic values overall less morphological change took place in the extreme event on the nogalte than predicted from some published hydraulic relations probably reflecting the high sediment supply and the hydrological characteristics of the event the results demonstrate the high degree of adjustment of these channels to the occasional high magnitude flash flood events and that such events need to be allowed for in management the detailed quantitative evidence produced by these long term monitoring sites provide valuable rare data for modelling morphological response to flood events in ephemeral channels

a note on cluster bedform behaviour in a gravel bed river abstract the behaviour of cluster bedforms was investigated in the farma r central italy a study reach was surveyed and the three components of the pebble clusters occuring on the surface of a gravel bar were painted using different colours before and after each flood all the clusters were photographed and a comparison was made this simple methodology allowed the evolution of the painted clusters to be followed and also allowed a survey of the dislodgment of the clusters particles and the addition of new ones the experiment shows that the mobility of the wake grains is higher than that of the stoss particles a pebble cluster generally loses the wake grain first followed by the stoss pebbles the obstacel clast is commonly the last to be entrained and acts as a single particle after the wake and stoss particles have been dislodged different explanations of this unexpected behaviour are postulated and the interaction of pebble with bed armouring and bedload size distribution is discussed

shear criteria in the grenville province ontario canada abstract the grenville province of canada is a major proterozoic orogenic belt mapping and structural analysis shows that the central ontario segment of this belt is composed of a stack of imbricated thrust nappes separated by mylonite zones the mylonite zones were generated at high metamorphic grades and contain a distinctive assemblage of mesoscopic and microscopic asymmetric structures these structures include rotated tectonic inclusions rotated boudins and pinch and swell structures sheath folds rotated single crystal porphyroclasts single crystal fish and shear band foliation the shear asymmetry indicated by these structures is highly consistent between members of the assemblage and over a large area of the grenville province and shows that the thrust nappes were transported to the northwest the consistency of the structures is thought to be due to cyclical dynamic recrystallization leading to steady state foliations in the mylonites

critical flow constrains flow hydraulics in mobile bed streams a new hypothesis a new hypothesis predicts that in mobile bed river channels interactions between the channel hydraulics and bed configuration prevent the froude number fr from exceeding 1 for more than short distances or periods of time flow conditions in many steep competent streams appear to be close to critical froude numbers of steep slope 0 01 sand bed streams with considerable freedom to adjust boundaries oscillate between 0 7 and 1 3 over 20 to 30 s cycles with an average of 1 0 at the channel thalweg criticalflow in these streams is maintained by the interaction between the mobile bed and free water surface at highfr which results in a cyclical pattern of creation and destruction of bed forms field observations support that a similar mechanism offlow bed form interaction constrains fr 1 in active bed braided gravel rivers step pool streams laboratory rills lahar runout channels and even some bedrock channels empirical and analytical results show that as slope increases competentflows tend to asymptotically approach criticalflow an assumption of criticalflow would dramatically simplify paleohydraulicflow reconstructions and modeling offlow hydraulics in high gradient streams

the lepontine alps as an evolving metamorphic core complex during a type subduction evidence from heat flow mineral cooling ages and tectonic modeling new surface heat flow data are presented for the valle maggia region within the high grade metamorphic core of the lepontine alps the steady state values range from 44 4 to 55 2 mw m2 and are anomalously low compared with those measured around the periphery of the lepontine these differences are considered to reflect the combined effects of differential uplift and erosion superimposed on a transient low heat flow regime which has existed since the miocene the uplift denudation history of the lepontine is modeled by using various mineral geochronometers two major pulses of accelerated uplift are recognized at circa 22 m y b p and circa 10 m y b p respectively these events directly coincide with times of dominant crustal convergence in the helvetic and jura stages of a subduction from known pmax tmax conditions of oligocene metamorphism estimates of low mantle heat flow and experimentally determined values of crustal heat production best fit thermal models are presented to explain surface heat flow and mineral cooling age data for the metamorphic core a two stage underplating model is shown to be consistent with the excavation history of the lepontine and with geological and geophysical data from more external parts of the central alps this information is integrated to explain the establishment of a crustal scale duplex in the metamorphic core complex contemporaneous with thin skinned tectonics in the foreland

channel reach morphology in mountain drainage basins a classification of channel reach morphology in mountain drainage basins synthesizes stream morphologies into seven distinct reach types colluvial bedrock and five alluvial channel types cascade step pool plane bed pool rime and dune ripple coupling reach level channel processes with the spatial arrangement of reach morphologies their links to hillslope processes and external forcing by confinement ripar ian vegetation and woody debris defines a process based framework within which to assess channel condition and response potential in mountain drainage basins field investigations demonstrate character istic slope grain size shear stress and roughness ranges for different reach types observations consistent with our hypothesis that alluvial channel morphologies reflect specific roughness configurations ad justed to the relative magnitudes of sediment supply and transport ca pacity steep alluvial channels cascade and step pool have high ratios of transport capacity to sediment supply and are resilient to changes in discharge and sediment supply whereas low gradient alluvial channels pool rime and dune ripple have lower transport capacity to supply ra tios and thus exhibit significant and prolonged response to changes in sediment supply and discharge general differences in the ratio of transport capacity to supply between channel types allow aggregation of reaches into source transport and response segments the spatial distribution of which provides a watershed level conceptual model linking reach morphology and channel processes these two scales of channel network classification define a framework within which to in vestigate spatial and temporal patterns of channel response in moun tain drainage basins

environmental change geomorphic processes and land degradation in tropical highlands abstract the international symposium highland2006 held in mekelle ethiopia from 21 to 25 september 2006 was the second time that ethiopia could host a major international scientific event on geomorphological processes highland2006 created a forum for those conducting research on the impacts of environmental change on geomorphic processes and land degradation in east african highlands as well as in similar regions around the globe discussions of results and exchange of experiences during indoor sessions as well as excursions have resulted in this distinctive set of scientific papers the two main questions addressed in this special issue through case studies in tropical mountain areas on all continents are 1 how have environmental changes impacted the type and intensity of geomorphic processes in tropical mountains since late pleistocene times the answer to this question is strongly linked to our knowledge on changes in climate vegetation cover human activities hydrology hillslope and fluvial processes for such environments 2 which factors control the intensity of land degradation its on site and off site impacts in tropical mountains this question addresses the role of natural and anthropogenic factors in the degradation of the vegetation cover and in controlling the intensity of hydrological processes soil erosion landsliding reservoir sedimentation and flooding in parallel with this special catena issue a number of selected papers on land degradation and its mitigation in tropical highlands are presented in a special issue of soil tillage research

paleohydraulic reconstruction of flash flood peaks from boulder deposits in the colorado front range nine watersheds in the colorado front range with steep bedrock channels were used to test the accuracy of paleohydraulic reconstruction of large flash floods using boulder deposits the nine basins consist of eight small ungauged basins ranging in size from 1 6 to 29 km2 and the big thompson river at the mouth of the big thompson canyon draining 790 km2 between 1923 and 1976 all nine basins had had one catastrophic flash flood the magnitude of which has been estimated by the conventional slope area method in each basin coarse boulder deposits of the large flash floods were identified and three axes of the five largest boulders were measured along with at least two profiles of the valley cross section a simple arithmetic average of two theoretical and two empirical relationships was used to estimate average flood velocity using boulder size and shape average depth was estimated as the arithmetic average of four values computed from the manning equation a regression equation for boulder size and unit stream power a relative smoothness equation and a modified shields relationship the appropriate flood width for the estimated average depth was found by iteration using the valley cross sections the paleohydraulic discharges thus computed generally underestimate conventional slope area discharge estimates on small streams by as much as 75 although the average amount is only 28 too low and the reconstructed discharge in one stream was 31 too large the big thompson river flood of 1976 was overestimated by 76 reasons for discrepancy in reconstructed peaks could include 1 the possibility that floods may have been able to move boulders larger than those available to be moved 2 overestimation of the slope area discharge because high water marks were set prior to erosion of the channel 3 underestimation of original roughness coefficients and 4 macroturbulent effects during fast deep flows the paleohydraulic technique is applied to two other streams in colorado with sedimentological evidence of large flash floods but no conventional indirect discharge estimates a small tributary to the big thompson river draining 1 8 km2 has a paleohydraulic reconstructed flood peak of about 60 m3 s from a flood in 1976 using boulders excavated from a foundation site in holocene alluvium along boulder creek in boulder colorado a paleohydraulic reconstructed flood peak of between 860 and 1 512 m3 s is calculated this is 1 4 to 2 4 times the magnitude of the estimated 500 yr flood

calcite mylonites in the central alpine root zone abstract north of the insubric line in the central alpine root zone carbonate rocks are concentrated in very narrow zones and have been metamorphosed under amphibolite facies conditions by the tertiary lepontine metamorphism grain size 1 mm post metamorphic deformation under greenschist facies conditions produced calcite mylonite bands a few millimeters to tens of meters wide in these marble zones microstructural development begins with twin formation bending of twin boundaries grain and twin boundary migration and recrystallization in high stress regions progressive mylonitization by dynamic recrystallization results in a microstructure with elongated calcite crystals long axis 20 50 m axial ration 1 4 in this fine grained matrix porphyroclasts of calcite quartz white mica biotite diopside tremolite scapolite and plagioclase are preserved ultra mylonite bands in pure calcite rocks show an even finer grain size of 5 10 m lattice preferred orientation is not present in the undeformed marbles but it develops during mylonitization the c axis orientation in the mylonites forms an asymmetric point maximum in the ultra mylonite no preferred orientation is left it is concluded from microstructural and textural aspects that during mylonitization dislocation creep accompanied by dynamic recrystallization were the most important processes whilst grain boundary sliding was the dominant mechanism during the formation of the ultra mylonites shear sense determinations indicate a horizontal right lateral strike slip shear system this is in good agreement with evidence regarding other movements along the insubric line which can be observed in ductile and brittle shear zones

a multi technique evaluation of hydrothermal hematite u pb isotope systematics implications for ore deposit geochronology abstract hematite fe2o3 is a common iron oxide mineral known to incorporate u into its crystal lattice at up to wt concentrations and has been previously used to date ore formation within iron oxide copper gold and banded iron formation deposits however there has been no detailed evaluation of the potential challenges this novel mineral geochronometer may present for accurate temporal interpretation we report a multi technique u pb geochronological study comprising laser ablation inductively coupled plasma mass spectrometry la icp ms sensitive high resolution ion microprobe shrimp and the first isotope dilution thermal ionization mass spectrometry id tims procedure and analyses using an in situ grain micro sampling technique we assess intra and inter sample data reproducibility and examine the compatibility of spatial and analytical resolutions to texturally and compositionally diverse samples five samples of hydrothermal primary oscillatory zoned hematite were analysed from distinct lithologies throughout the 1 59 ga olympic dam cu u au ag deposit with typical u concentrations ranging between 20 and 1000 ppm la icp ms mapping of isotope distributions across mm sized grains allows pinpointing of u rich and relatively u pb isotopically homogeneous domains micro domains were extracted via laser cut square shaped trenches 100 100 m u pb datum range in complexity with concordant age domains measured by id tims indicating that hematite can retain a near closed u pb system over 1 6 ga while also demonstrating that younger pb loss events may be recorded significant reverse discordance is common in the data likely to be a real feature due to both internal decoupling and mobility of pb within a single grain and u pb fractionation during microbeam analysis these effects will obscure the interpretation of real ages for proterozoic samples where non zero age pb loss events occur and restricts the evaluation of common pb mixing components as a result microbeam analyses may be biased younger or older in terms of 207pb 206pb dates in samples with complex histories microbeam datum compare favourably with high precision id tims ages although reproducible id tims u pb dates were generally not obtained in the best case example a 207pb 206pb weighted mean age of 1589 91 0 91 ma mswd 1 3 n 5 within 0 2 of the olympic dam granite host rock age 1593 87 0 21 ma was generated all five hematite samples dated by la icp ms and shrimp yield weighted mean 207pb 206pb dates within 2 of the host granite age after small degrees of data rejection the results demonstrate that with careful sample petrography screening and data interpretation hematite can be considered a very useful u pb mineral geochronometer with potential application to all u bearing fe oxide rich mineral systems

continuous monitoring of bedload discharge in a small steep sandy channel this paper reports on bedload flux and texture monitored in a natural steep sandy ephemeral channel draining a small gullied sandy watershed the barranca de los pinos 1 32 ha spain bedload flux was continuously monitored with two independent reid type slot samplers bedload texture was determined from the sediment collected in the samplers channel morphology was surveyed with a high spatial resolution with a terrestrial laser scanner the monitored instantaneous bedload fluxes are among the highest measured in natural rivers characterized by high temporal and spatial variability related to the presence of bedforms shallow bars and sand sheets and to the reworking of the dry bed between and at the end of individual flow events the grain size distribution of the bedload indicates equal mobility but bedload texture fluctuates depicting the transport of coarser bar surfaces and of finer grained anabranch surfaces as well as of the overall bed subsurface

sequence and style of major post nappe structures simplon pennine alps abstract in the upper pennine nappe complex of the simplon pennine alps switzerland and italy at least three phases of major post nappe folding in places associated with thrusting can be distinguished these are superimposed on an earlier formed partly chaotic complex of tectonic units including the bernhard and monte rosa continental flakes and the zermatt saas and antrona ophiolite complexes the earliest post nappe folds were essentially isoclinal throughout the whole region and were accompanied by a strong schistosity which is the main foliation in most areas later two successive phases of back folding led to the present overall structure both phases typically show rapid variations in style from open folds lacking axial planar schistosity to very tight structures with complete foliation transposition this has been demonstrated by systematically mapping the major axial traces over the whole region successively removing the major structures in reverse order shows that the ophiolite complexes were originally part of a continuous unit marking an important suture between the bernhard and monte rosa nappes

fault sourced alluvial fans and their interaction with axial fluvial drainage an example from the plio pleistocene upper valdarno basin tuscany italy abstract the present study deals with the fault sourced alluvial fan deposits of the plio pleistocene upper valdarno basin northern apennines italy different phases of alluvial fan aggradation progradation and backstep are discussed as possible effects of the interaction among fault generated accommodation space sediment supply and discharge variations affecting the axial fluvial drainage the upper valdarno basin located about 35 km se of florence is filled with 550 m palustrine lacustrine and alluvial deposits forming four main unconformity bounded units i e synthems the study alluvial fan deposits belong to the two uppermost synthems montevarchi and torrente ciuffenna synthems and are early to middle pleistocene in age these deposits are sourced from the fault bounded ne margin of the basin and interfinger with axial fluvial deposits alluvial fan deposits of the montevarchi synthem consist of three main intervals i a lower interval which lacks any evidence of a depositional trend and testify balance between the subsidence rate i e fault activity and the amount of sediment provided from the margin ii a coarsening upward middle interval pointing to a decrease in subsidence rate associated with an augment in sediment supply iii a fining upward upper interval locally preserved documenting a phase of tectonic quiescence associated with a progressive re equilibration of the tectonically induced morphological profile the basin scale unconformity which separates the montevarchi and torrente ciuffenna synthems was due to the entrance of the arno river into the basin as consequence of a piracy this event caused a dramatic increase in water discharge of the axial fluvial system and its consequent embanking such an erosional surface started to develop in the axial areas and propagated along the main tributaries triggering erosion of the alluvial fan deposits alluvial fan deposits of the torrente ciuffenna synthem accumulated above the unconformity during a phase of tectonic quiescence and show a fining upward depositional trend this trend was generated by a progressive decrease in sediment supply stemming out from upstream migration of the knickpoints developed during the embanking of the axial system

on traction carpet sedimentation traction carpets are highly concentrated bedload layers that are developed beneath and driven by turbulent overlying flows they have a convex up velocity profile and comprise a lower frictional and an upper collisional region the frictional region having a particle concentration more than 80 of the packed bed is characterized by nearly continuous grain contacts low strain rate and hampered grain segregation on the other hand the collisional region has a particle concentration between 15 and 80 of the packed bed and is characterized by active grain collisions and higher strain rate deposition from the traction carpets occurs via progressive aggradation of the bed rather than via en masse freezing while the downward grain flux from the overlying flow maintains them the thickness of a traction carpet stratum is therefore determined by the cumulative amount of sediment settled during the lifespan of a traction carpet and can be much larger than the thickness of a moving traction carpet inverse grading can be produced in the collisional region by the vertical gradient of dispersive pressure which is related to nonuniform distribution of particles when a thick frictional region develops however the inverse size distribution in the collisional region is poorly recorded in the deposits depositional features of traction carpet strata are therefore determined by the duration of a traction carpet and the thickness ratio of collisional to frictional regions the thickness ratio is further controlled by the applied shear stress sediment fallout rate and grain size generally a collisional region is better developed beneath a highly competent and coarse grained gravelly flow whereas a thick frictional region is developed beneath a heavily sediment laden fine grained sandy flow this explains the more common occurrence of inverse grading in coarse grained deposits

traction carpet stratification in turbidites fact or fiction abstract the spaced stratification that characterizes some thick deep water sandstones consists of a stack of layers 5 10 cm thick that are inversely graded at their base and structureless at their top these layers have been attributed to deposition from a succession of collapsed inertia layers traction carpets prior to collapse each layer was believed to have been maintained by shear from the overriding turbidity current this depositional model is based on the mistaken assumption that the distribution of shear stress in such a traction carpet is indistinguishable from the distribution of shear stress in a grain flow on a steep slope instead shear stress in an externally sheared traction carpet should be approximately constant from the top to the base of the layer and deposition sho ld be a gradual process leaving no record of sequentially active inertia layers inertia layers that do form under large turbidity currents are probably never as thick as 5 10 cm based on available data from shear cell experiments instead inertia layers are probably about 1 cm thick similar to the thickness of the inversely graded basal part of each layer in field examples of spaced stratification the rest of each inversely graded stratum probably forms due to rapid fallout from suspension the basal inversely graded 1 cm is believed to record strong shear that may accompany impingement on the bed of large turbulent eddies hence spaced stratification may not be a structure produced beneath steady flows but may record strong y fluctuating hydrodynamic conditions and vigorous burst sweep cycles in large turbidity currents

the transport of gravel in an ephemeral sandbed river the channel dynamics of an ephemeral sandbed river the metsemotlhaba in southeast botswana were studied over a five year period using tagged magnetic particles of pebble and cobble size scour chains and topographic cross sections with particular emphasis on the three dimensional dispersal of gravels patterns of scour and fill and depth of active layer two major flow events of equivalent magnitude occurred moving the tagged particles in december 1987 a mean distance of 837 m at a mean burial depth of 0 40 m and in march 1991 a mean distance of 263 m at a mean burial depth of 0 39 m the volume of mobile sediment based on scour depth and distance of travel was 2 7 times greater in the december 1987 event in which the mean scour depth was almost twice the mean burial depth of the tagged particles the distribution of distance of movement was asymmetrical in this first flood when the tracer started from a surface location but was monotonic thereafter intervening small to medium events yielded limited tracer movement with a mean burial depth equivalent to depth of scour the tracer moved in the low and transitional flow regimes burial depth distribution followed the gamma model field data confirm that longitudinal transport is independent of particle size and shape and strongly skewed with respect to distance whilst depths of scour in excess of 1 m for high magnitude events suggest that scour values predicted from the empirical equation of leopold et al underestimate by an order of magnitude copyright 1999 john wiley sons ltd

kyanite eclogite to amphibolite fades evolution of hydrous mafic and pelitic rocks adula nappe central alps in the southern adula nappe central alps two stages of regional metamorphism have affected mafic and pelitic rocks earlier eclogite facies with a regional zonation from glaucophane eclogites to kyanite hornblende eclogites was followed by a tertiary overprint which varied from greenschist to high grade amphibolite facies despite a common metamorphic history contrasting equilibration conditions are often recorded by high pressure mafic eclogite and adjacent predominantly lower pressure pelite assemblages this pressure contrast may be explained by different overprinting rates of the two bulk compositions during unloading the rates are controlled by a mechanism in which dehydrating metapelites provide the h2o required for simultaneous overprinting of enclosed mafic eclogites by hydration

bedforms and sediment transport processes in the ephemeral streams of kobo basin northern ethiopia abstract bedforms and channel deposits architecture were studied in the ephemeral streams of the structural basin of kobo in northern ethiopia the froude numbers calculated at bankfull flow for a few of the study reaches are in agreement with the extensive occurrence of horizontal planar lamination small bedforms not previously reported in the literature are found and described as produced mainly by scouring rather than accretion processes equations to predict particle entrainment proved that large individual boulders standing scattered on the stream bed can be transported at bankfull flow for long distances direct inspection of the old alluvium and the modern channel deposits allow to identify a typical arrangement within a single bed consisting of divisions with specific characteristics this model of division association is explained in terms of vertical distribution of shear stress and compared with hyperconcentrated flow and traction carpet models in contrast with recent theories of flat asymmetrical bed waves progradation at froude numbers close to unity

threshold of coarse sediment transport in broad and narrow natural streams the threshold of coarse sediment transport has been examined in natural streambeds in an upland pennine u k area threshold values of the total boundary shear stress t0 for a given grain size in a narrow natural stream w d 11 efficiency in the entrainment process is related not only to the overall channel geometry but also varies as a function of discharge in channels characterized by compound roughness empirical curves relating t0 and a mean grain size d5 are presented but are limited in application to streams of similar physical and hydraulic characteristics as the ones examined in this investigation considerable divergence is noted between these empirical functions and a summary empirical function for general application obtained from a published source the reasons for this divergence are discussed the influence of grain shape was found not to be important in the initiation of motion criterion this conclusion may reflect the limited range of natural grain shapes in the study streams but might reasonably apply to other field investigations of similar streams modifications of the shields and yalin diagrams are suggested for practical applications in shallow streams with poorly graded bed material the shields parameter may be regarded as an inverse function of the relative protrusion of individual grains in the shallow flow depth d5 d the increased importance of augmented drag forces in the entrainment process in shallow flows is suggested as the physical explanation for the reduced values of the shields parameter however the relationships presented should not be applied to laboratory experiments concerned with well graded sediments therefore beds with little deviation in level in which the shields parameter may be regarded as constant at high reynolds grain numbers consistent estimated field values of a threshold sediment transport parameter might be used to compare field data to threshold values derived from statistical arguments and laboratory experiments reported in the literature

strain patterns in an ice cap and implications for strain variations in shear zones glacier flow is a form of gravity tectonics in thin sheets characterized by the development of large strains foliation and folds strain is difficult to measure in ice but numerical methods provide a good means of estimating cumulative strains in simple terms flow in an ice cap involves vertical flattening at the center with bottom parallel shear increasing in intensity downwards and outwards in steady state cumulative strains will be plane if flow is two dimensional as in parts of many valley glaciers and some ice caps and of flattening type if lateral extension occurs normal to the flow direction as in many ice sheets in both radial and plane flow strain gradients will be large vertically and low horizontally outwards cumulative strain magnitude can be extremely large near the base and at the margin z is everywhere sub vertical if flow is unsteady more complex strain patterns develop even if changes in the flow field are slight over bedrock ridges flow perturbations can lead locally to rotations greater than those of the quasi simple shear acting near the base in steady state flow this may cause the x direction of the cumulative strain to rotate through the shear plane in the neighborhood of the perturbation in which case strain magnitudes subsequently diminish leading to low or zero strains in plane flow and to constrictional strains in radial flow in the latter case x lies horizontal and perpendicular to the flow direction and we have local constrictional strain in an overall flattening field the zones of perturbed strain tend to be associated with the inverted limbs of recumbent folds shear zones in rocks are expected to show similar patterns of strain perturbation

morphology and sediment dynamics of ephemeral stream terminal distributary systems in the kobo basin northern welo ethiopia channel morphology bedforms and sedimentary structures of the distributary systems of ephemeral streams in the structural basin of kobo northern ethiopia are analysed the basin is characterised by a semi arid monsoon affected climate most of the rivers originate in the western margin range and dry up in the middle of the basin where they form large distributary systems they show characteristics different from those of the terminal fans and floodouts previously reported in the literature no flow data are available but bankfull discharge for several study reaches has been obtained by field survey of a number of cross sections hydraulic geometry streambed gradient and bed material grain size distribution in both the main stems and the distributary channels two different types of terminal distributary systems have been identified bird foot and lobate the first type by far predominates in the kobo basin bird foot distributary systems were recognized to consist of five distinctive reach units while the lobate splays have a simpler morphology and are thought to evolve into the former more complex type a description of channel morphology bedforms and sedimentary structure distribution is given and these characteristics are compared to those of other similar depositional systems reported in the literature other features such as the lack of scour pockets around boulder particles scattered on the streambed surface and the interaction of individual large particles with massive or horizontal laminated sand on top of typical upward fining channel sequences are also highlighted

a sem cathodoluminescence study on weathering of quartz grains in faults central japan summary cathodoluminescence cl of quartz grains or particles was observed using a scanning electron microscope the quartz grains were collected from fault gouge and fault breccia within the atotsugawa fault central japan and the quartz particles were obtained by crushing parent rocks of the fault such as granite and gneiss in an iron bowl in comparison with particles from the parent rocks those from the fault showed the following differences cl inherited from the parent rock cl caused by faulting and cl modified by weathering or alterations of the fault these differences are attributed to the faulting and subsequent weathering the cl observations were in general agreement with secondary electron observations and it is recommended that both methods should be used in parallel

kinematics of the northern part of the simplon line central alps abstract structural studies were carried out along the northern part of the simplon line in order to constrain the kinematic history of the line during tertiary times the ductile strain accumulated within both basement units and mesozoic cover reveals the upper units sheared towards the west southwest the simplon line can be described as a major shear zone along which the main movement occurred perpendicular to the line this shearing developed during retrograde metamorphic conditions and postdates earlier deformations associated with the high temperature lepontine episode finally it was followed by the formation of the berisal syncline back folding episode which generated some brittle movements towards the south localized at the contact between the lower and upper pennine units i e on the simplon line

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

effect of sand supply on transport rates in a gravel bed channel in a series of flume experiments using constant discharge flow depth and gravel feed rate sand feed rates were varied from 0 16 to 6 1 times that of gravel the bed slope decreased with increasing sand supply indicating that the gravel could be transported at the same rate along with increasing amounts of sand at smaller shear stresses prediction of river response to an increase in sediment supply requires prediction of mutual changes in bed composition and transport and therefore a transport model defined in terms of the grain size of the bed surface a recent model provides satisfactory prediction of the experimental observations and indicates the general response of gravel beds to increased sand supply an increase in sand supply may increase the sand content of the river bed and the mobility of gravel fractions which can lead to bed degradation and preferential evacuation of these sediments from the river

genesis of the batinah melange above the semail ophiolite oman abstract the batinah melange which overlies the late cretaceous semail ophiolite in the northern oman mountains comprises mostly sedimentary rocks of deep water facies alkalic lavas and intrusives all of continental margin affinities together with smaller volumes of semail ophiolitic and metamorphic rocks four intergradational textural types of melange can be recognized sheet melange has large 1 km intact sheets either with little intervening matrix or set in other melange types and with an organised sheet orientation fabric slab melange is finer textured 100 m and more disrupted block melange has smaller m blocks with some matrix and a weak to random block fabric clast melange is matrix supported rudite with a weak depositional clast fabric structural relationships particularly the absence of tectonic fabrics the decreasing strength of fragment fabrics with increasing fragmentation and the abundance of brittle fragmentation suggest that these melange types formed by either gravity driven sedimentary processes or superficial sliding or thrusting of individual rock slabs in the slab melange long sequences can be pieced together passing up from upper triassic mafic sub marine extrusives and sediments into radiolarian cherts hemipelagic and redeposited limestones and terminating in non calcareous radiolarities with mn deposits of early cretaceous age mafic sills are numerous these sequences can be matched with sub ophiolite rocks now exposed in fault corridors through the semail these sequences become progressively disrupted upwards in the corridors and can be traced continuously into overlying melange which then thins away from the corridors we argue that during late cretaceous emplacement over the arabian margin active fault corridors split the semail slab and acted as conduits up which sub ophiolite rocks were supplied to the ophiolite surface there the rocks were redisributed by superficial processes

quartz textures from the simplon fault zone southwest switzerland and north italy quartz textures measured from monomineralic veins within the simplon fault zone show very similar patterns over a strike length of some 30 km the c axes form asymmetric kinked single girdles with variably developed vestiges of crossed girdle limbs the kinking of the girdle may be controlled by the two c axis orientations where double rhomb a slip is possible a strong a maximum lies between 20 and 30 to the lineation in the xz plane which seems too large to be the angle to the bulk shear plane straight single girdle patterns may be possible for strict simple shear in natural fault zones where deformation is in general heterogeneous on all scales kinked single girdles and type i crossed girdle textures are much more common perhaps because such textures provide more freedom to accommodate a general two dimensional strain

dynamic interactions between lahars and stream flow a case study from ruapehu volcano new zealand three lahars were sampled in the whan gaehu river on the eastern flank of ruapehu volcano new zealand at 23 5 and 42 km from the source as they were flowing on september 27 and 29 and october 6 1995 the lahars were generated by water explosively ejected from the highly mineralized crater lake we used the chemical contrast between the lahars and resident stream water in their paths to describe a four phase model of a noncohesive lahar wave in a river channel 1 ambient stream water pushed ahead of the lahar in a process of miscible displacement due to hydrodynamic dispersion 2 a zone of mixing between the stream water and the lahar that increases in length with distance from source 3 a remnant of the original lahar least diluted by stream water that decreases in length and dilutes downstream 4 the tail of the lahar surge peak discharge occurs at the end of phase 1 as water is pushed ahead of the lahar peak sediment concentration occurs at the end of phase 2 where debris entrainment by the flow is at its greatest i e in front of the lahar proper deposits record only phases 2 and 3 of the lahar wave phase 1 flow left only a tide line of organic debris and phase 4 deposits were rapidly eroded by later streamflow downstream dilution by stream water eventually caused transformation of phases 2 and 3 of the lahars from debris flow to hyperconcentrated streamflow and then to normal streamflow as the flows became progressively finer grained and more turbulent

downstream changes in dryland river channels the northern plains of arid central australia abstract many dryland rivers undergo marked downstream changes owing to factors such as infrequent floods flow transmission losses and typically few tributary inflows beyond the headwaters along the sandover bundey sandover bundey and woodforde rivers on the northern plains of arid central australia downstream channel changes are broadly similar in upland zones small rocky channels transporting sand and gravel gradually increase in size before entering piedmont zones where channels and narrow floodplains are confined by bedrock alluvial terraces or aeolian dunes in lower gradient lowland zones channels and floodplains remain confined and in the absence of tributary inflows channel cross sectional areas and discharges decrease downstream confining landforms are not present in floodout zones which results in splay formation increased floodplain widths and marked overall downstream decreases in cross sectional areas eventually channelised flow and bedload transport terminate although occasional large floods continue across extensive unchanneled alluvial surfaces termed floodouts these broad similarities apart downstream changes along the three rivers differ in detail the sandover is largely a single thread channel whereas many reaches of the sandover bundey and woodforde are anabranched on the small woodforde river downstream decreases in parameters such as cross sectional area and width are roughly linear on the larger sandover and sandover bundey downstream changes are more irregular particularly through the floodout zones where there are marked fluctuations in widths depths and bed slopes the irregular downstream changes typical of the lower reaches of these large rivers may be due to the reduced influence of vegetation on bankline stability and width adjustment relative to that of smaller rivers on the northern plains as in other drylands complex interactions between discharge sediment transport slope patterns of tributary drainage bank sediment type and vegetation result in variable patterns of downstream channel change