prediction of alluvial channel pattern of perennial rivers abstract purely braided meandering and straight channels can be considered as end members of a continuum of alluvial channel patterns several researchers have succeeded in separating channel patterns in fields defined by flow related parameters however the discriminators of the principal channel patterns derived from these diagrams all require some a priori knowledge of the channel geometry in this paper a method is presented which enables prediction of the equilibrium conditions for the occurrence of braided and high sinuosity meandering rivers in unconfined alluvial floodplains the method is based on two almost channel pattern independent boundary conditions median grain size of the river bed material and a potential specific stream power parameter related to bankfull discharge or mean annual flood and valley gradient this can be regarded as a potential maximum of the available flow energy corresponding to the minimum sinuosity condition p 1 based on an analysis of 228 datasets of measurement sites along rivers from many parts of the world an independent discriminating function was found that separates the occurrence of braided rivers and meandering rivers with p 1 5 the function applies to equilibrium conditions of rivers that neither incise nor show rapid aggradation with a bankfull or mean annual flood discharge above 10 m3 s and a median bed material grain size between 0 1 and 100 mm

breccias and ores part 1 history organization and petrography of breccias abstract although the term breccia is one of the oldest rock names still in use and proliferating in its application a satisfactory definition remains problematic and an overall classification of breccias is non existent the names of breccia varieties several hundred please the heart rather than the computer and there is little interdisciplinary correlation among the more than fifty specialized schools of breccia study this contribution reviews briefly the history of breccia research and provides a list of the present study trends complete with references with emphasis on ore associated breccias wherever possible conventional breccia definitions and their shortcomings are discussed and a case is made for the introduction of the petrographic category of coarse fragmentites of which the typical sedimentary breccias and conglomerates as well as volcanic tectonic hydrothermal impact and other fragmental materials are members whether designated or not as breccia in the conventional literature this is followed by a proposed rational organization classification of coarse fragmentites and breccias based on the regime of formation and architecture fabric gefuge two fundamental end member sequences a b and a transitional suite c can be recognized a disaggregation fragmentites d breccias produced from a solid progenitor by increasing rate of fracturing expansion rotation mixing and attrition the transitional sequence is crackle mosaic rubble melange milled breccia b aggregation fragmentites a breccias generated by gathering aggregation and accumulation of originally dispersed fragments at the site of deposition these materials form a sequence ranging from tightly packed fragment supported matrix free fragmentites through matrix infilled matrix supported fragmentites to lonestone rocks c in transport fragmentites t breccias formed by redeposition of an aggregate gathered in an earlier phase of development as an a fragmentite or by transportation and redeposition of an advanced member i e rubble melange or milled fragmental aggregate of a d fragmentite examples of metallic mineralizations coeval with or superimposed on the fragmentite sequences are reviewed

forms and processes of two highly contrasting rivers in arid central australia and the implications for channel pattern discrimination and prediction tooth s nanson g c 2004 forms and processes of two highly contrasting rivers in arid central australia and the implications for channel pattern discrimination and prediction geological society of america bulletin 116 7 8 802 816 rae2008

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

measurements of two types of dilatational waves in an air filled unconsolidated sand this study consists of laboratory measurements of dilatational waves propagating through an air filled unconsolidated sand one excitation technique consists of a loudspeaker suspended in the air above the packing of sand a second excitation technique uses a mechanical shaker in contact with the sand the transmitted signals are received using microphones and geophones located at various depths within the sand an interpretation based on measured phase speeds indicates that the transmitted energy from the suspended loudspeaker source is partitioned primarily but not exclusively into the type ii dilatational wave this wave attenuates rapidly and is only detected at depths of less than about 15 cm for this particular sample at the deeper depths the detected signal is associated with the type i dilatational wave the mechanical shaker produces only a type i dilatational wave both the geophone and microphone sensors can detect both types of dilatational waves

the role of discharge variability in the formation and preservation of alluvial sediment bodies abstract extant planform based facies models for alluvial deposits are not fully fit for purpose because they over emphasise plan form whereas there is little in the alluvial rock record that is distinctive of any particular planform and because the planform of individual rivers vary in both time and space accordingly existing facies models have limited predictive capability in this paper we explore the role of inter annual peak discharge variability as a possible control on the character of the preserved alluvial record data from a suite of modern rivers for which long term gauging records are available and for which there are published descriptions of subsurface sedimentary architecture are analysed the selected rivers are categorized according to their variance in peak discharge or the coefficient of variation cvqp standard deviation of the annual peak flood discharge over the mean annual peak flood discharge this parameter ranges over the rivers studied between 0 18 and 1 22 allowing classification of rivers as having very low 0 90 annual peak discharge variance deposits of rivers with very low and low peak discharge variability are dominated by cross bedding on various scales and preserve macroform bedding structure allowing the interpretation of bar construction processes rivers with moderate values preserve mostly cross bedding but records of macroform processes are in places muted and considerably modified by reworking rivers with high and very high values of annual peak discharge variability show a wide range of bedding structures commonly including critical and supercritical flow structures abundant in situ trees and transported large woody debris and their deposits contain pedogenically modified mud partings and generally lack macroform structure such a facies assemblage is distinctively different from the conventional fluvial style recorded in published facies models but is widely developed both in modern and ancient alluvial deposits this high peak variance style is also distinctive of rivers that are undergoing contraction in discharge over time because of the gradual annexation of the channel belt by the establishment of woody vegetation we propose that discharge variability both inter annual peak variation and flashiness may be a more reliable basis for classifying the alluvial rock record than planform and we provide some examples of three classes of alluvial sediment bodies representing low intermediate and high very high discharge variability from the rock record that illustrate this point

interchannel hydraulic geometry and hydraulic efficiency of the anastomosing columbia river southeastern british columbia canada the morphodynamics of the anastomosing channel system of upper columbia river in southeastern british columbia canada is examined using an adaptation of conventional hydraulic geometry termed interchannel hydraulic geometry interchannel hydraulic geometry has some of the characteristics of downstream hydraulic geometry but differs in that it describes the general bankfull channel form and hydraulics of primary and secondary channels in the anastomosing channel system interchannel hydraulic geometry generalizes these relationships and as such becomes a model of the geomorphology of channel division and combination interchannel hydraulic geometry of upper columbia river based on eld measurements of ow velocity and channel form at 16 test sections is described well by simple power functions wbf 3 24qbf0 64 dbf 1 04qbf0 19 vbf 0 30qbf0 17 these results with other related measurements of ow resistance imply that channel splitting leads to hydraulic inef ciency higher ow resistance on the anastomosing columbia river because these ndings differ from those reported in studies elsewhere we conclude that hydraulic ef ciency does not provide a general explanation for anabranching in river channels copyright 2003 john wiley sons ltd

a comparison of srtm and high resolution digital elevation models and their use in catchment geomorphology and hydrology australian examples the recently released shuttle radar topography mission srtm 3 arc second digital elevation data set provides a complete global coverage of the earth s land surface in this paper we examine the srtm data for three catchments in australia over a range of climates geology and resultant geomorphology to test this new data set the srtm data are compared with high resolution digital elevation models we use basic hydrological and geomorphological statistics and descriptors such as the area slope relationship cumulative area distribution and hypsometric curve along with strahler and networking statistics the above measures describe the surface morphology of a catchment therefore integrating catchment geology climate and vegetation the srtm data were also assessed as input into the siberia landscape evolution and soil erosion model as were runoff properties using a wetness index the results demonstrate that the 90 m srtm data provide a poor catchment representation hillslopes appear as a linked set of facets and display little of the complex curvature that is observed in high resolution data while catchment area slope and area elevation hypsometry properties are largely correct catchment area relief and shape as measured by the width function are poorly captured by the srtm data also results in working statistics are also variable the large grid size of the srtm data also results in incorrect drainage network patterns and different runoff properties consequently care must be used for quantitative assessment of catchment hydrology and geomorphology as in all cases srtm derived catchment area is incorrect and smaller digital elevation grid sizes are required for accurate catchment wide assessment while only a limited number of catchments have been examined we believe our findings are applicable to other areas

spatial disaggregation and aggregation procedures for characterizing fluvial features at the network scale application to the rhone basin france network scale heterogeneity in fluvial forms and processes has been analyzed for decades through the field based characterization of a reduced number of sites spatially continuous data sets are increasingly available at a national scale e g digital elevation models or aerial orthophotographs and open up new possibilities to investigate spatial structures in stream networks the paper introduces a gis methodological framework to support and automate the large scale characterization of the fluvial system based on spatial disaggregation and aggregation procedures the spatial disaggregation consists in the longitudinal discretisation of geographical objects into relatively high resolution spatial units to characterize fluvial features continuously along stream networks because rivers are heterogeneous and hierarchically organized the spatial aggregation aims to delineate statistically pattern or process based spatial units to answer questions at appropriate scales the paper develops the gis implementation of the disaggregation and aggregation procedures for characterizing stream networks in a geomorphic perspective we illustrate the approach on the rhone network focusing on the characterization of three geographical objects commonly used in classification schemes the streamline the valley bottom and the active channel the latter on a limited area the spatial aggregation is automated using the threshold test of pettitt 1979 to detect change points in univariate data series and delineate homogeneous spatial units throughout the stream network we provide three simple application examples to illustrate the potentialities of the spatial aggregation to investigate spatial heterogeneity in stream networks finally insights limitations and future challenging issues emerging from the application of the methodological framework are discussed

a catchment scale assessment of anabranching in the 143 000 km2 fitzroy river catchment north eastern australia this paper presents a catchment scale investigation of anabranching in the moderately large 143 000 km2 fitzroy river catchment in north eastern australia the primary aim is to determine whether mapped and remotely sensed data can provide useful information about the characteristics of anabranching in a catchment of this scale anabranching comprises 6 of the total channel network by length and 24 of higher order channels those with catchment areas over 100 km2 three anabranching planform morphologies are described which occur in geographically distinct regions and add to previous descriptions of anabranching rivers sinuosity and link length are calculated for all channels of the mapped channel network a link is a stretch of river between two stream junctions slope mean floodplain width mean annual rainfall and underlying and catchment geology parameters are calculated for links of a stream network derived from a digital elevation model dem in which each link has either a single channel or anabranching morphology anabranching and single channel links do not occupy different ranges of attribute values and a logistic regression analysis was unable to predict anabranching however slope catchment area mean floodplain width length and sinuosity parameters all have significantly different means when comparing single channel with anabranching links although it is shown that the difference in mean floodplain widths is the result of its correlation with catchment area anabranching channels have a tendency towards shorter link lengths lower sinuosities and lower valley slopes and occur at larger catchment areas than single channels these differences are discussed in the context of published hypotheses regarding the cause of anabranching however the spatial resolution and precision of our data limit our ability to investigate controls on anabranching which will require detailed measurement of variables at a reach scale

drought related changes to geomorphologic processes in central mali geomorphologic evidence exists for repetitive drought conditions in the african sahel within this framework of broad climatic changes through time the 1968 1985 drought episodes are not abnormal the impact of recent drought on the economies and environments of sahelian nations has been substantial however and the recovery capabilities off severely damaged lands are not well known study of the geomorphology and surface processes across a portion of mali provides some constraints on the responses of desert fringe fluvial systems to changing environmental conditions multi temporal orbital image data were used in combination with field investigation to map drought affected soils and to document changes to both fluvial and aeolian processes across the region of study a combination of statistical methods yields consistent evidence of net albedo increases associated with particular landforms and surface processes over a nine year interval although aeolian processes are a significant transport mechanism for removal and redistribution of soil materials both orbital data and field study indicate that fluvial erosion is responsible for much of the primary topsoil loss and landform modification in this portion of the sahel

evaluation of part of the mekong river using satellite imagery abstract large rivers of the world are difficult to investigate this is essentially due to the logistical problems involved when attempting to study rivers that are about a thousand kilometres or more in length and drain basins of appropriate dimensions this creates difficulties in river management and also in determining the possible impact on the river system of specific development projects or basin land use changes we demonstrate the capability of satellite images a huge number of which are currently archived in various centres around the world to provide at least a coarse scale solution to these problems a significant amount of work can be done even without the application of any special remote sensing technique about 14 000 km 2 of the middle mekong basin in lao pdr has been studied the mekong is the 12th longest river in the world and is ranked eighth in terms of mean discharge the mekong is worth investigating for several reasons 1 it has a fascinating and complicated physiography 2 little published information exists for the basin at least for the part selected for the case study 3 various development plans have been drawn up for the mekong with possible environmental impact on the basin and the river the images are used in conjunction with the limited geomorphic and hydrologic data available in order to 1 provide a concise account of the local geomorphology 2 map environmental degradation and sediment transfer in parts of the basin and 3 determine the nature of possible environmental impact associated with certain proposed development projects on the river this is carried out by searching through the spot satellite images archived at the centre for remote imaging sensing and processing national university of singapore selecting and studying clear scenes and comparing the 1996 1998 and 1999 images of the study area to record changes over time primarily the study is a demonstration in combining remote sensing and geomorphology for environmental management of a large river system it is also a plea for transforming the satellite images accumulating in various archives to useful geomorphological databases given the scale of large rivers satellite images are excellent tools for their investigation

a multiresolution index of valley bottom flatness for mapping depositional areas 1 valley bottoms function as hydrological buffers that significantly affect runoff behavior distinguishing valley bottoms from hillslopes is an important first step in identifying and characterizing sediment deposits for hydrologic and geomorphic purposes valley bottoms occur at a range of scales from a few meters to hundreds of kilometers in extent this paper describes an algorithm for using digital elevation models to identify valley bottoms based on their topographic signature as flat low lying areas the algorithm operates at a range of scales and combines the results at different scales into a single multiresolution index this index classifies degrees of valley bottom flatness which may be related to depth of deposit the index can also be used to identify groundwater constrictions and to delineate hydrologic and geomorphic units

anabranching and maximum flow efficiency in magela creek northern australia 1 anabranching is the prevailing river pattern found along alluvial tracts of the world s largest rivers hydraulic geometry and bed material discharge are compared between single channel and anabranching reaches up to 4 times bank full discharge in magela creek northern australia the anabranching channels exhibit greater sediment transporting capacity per unit available stream power i e maximum flow efficiency mfe simple flume experiments corroborate our field results showing the flow efficiency gains associated with anabranching and highlight the prospect of a dominant anabranch which is found in many anabranching rivers these results demonstrate that anabranching can constitute a stable river pattern in dynamic equilibrium under circumstances in which a continuous single channel would be unable to maintain sediment conveyance we propose the existence of a flow efficiency continuum that embraces dynamic equilibrium and disequilibrium vertically accreting anabranching rivers

least action principle equilibrium states iterative adjustment and the stability of alluvial channels the energy based least action principle lap has proven to be very successful for explaining natural phenomena in both classical and modern physics this paper briefly reviews its historical development and details how in three ways it governs the behaviour and stability of alluvial rivers first the lap embodies the special stationary equilibrium state of motion and so its incorporation with the principle of energy conservation explains why so many optimizing hypotheses have been proposed in fluvial geomorphology second the variational approach underlying the lap provides a more straightforward and simpler fuzzy object orientated method for solving river regime problems than do the various complex newtonian formulations third it is shown that in fluvial systems with surplus energy the surplus can be expended with slope and or channel geometry adjustments with the degree of channel geometry adjustment quantified by the dimensionless numbers f for depth dominated adjustment and h for width depth dominated adjustment different planforms are preferred at different energy levels with h providing a quantitative measure of the flow s efficiency for moving sediment in rivers with insufficient energy the interactions of endogenous and exogenous factors are shown to be capable in certain circumstances of achieving a stationary equilibrium condition which acts as the attractor state importantly this study describes how iterative changes enable systems to achieve such a stable equilibrium copyright 2007 john wiley sons ltd

methodological sensitivity of morphometric estimates of coarse fluvial sediment transport abstract the estimation of fluvial sediment transport rate from measurements of morphological change has received growing recent interest the revival of the morphological method reflects continuing concern over traditional methods of rate determination but also the availability of new survey methods capable of high precision high resolution topographic monitoring remote sensing of river channels through aerial digital photogrammetry is a potentially attractive alternative to labour intensive ground surveys however while photogrammetry presents the opportunity to acquire survey data over large areas data precision and accuracy particularly in the vertical dimension are lower than in traditional ground survey methods this paper presents results of recent research in which digital elevation models dems have been developed for a reach of a large braided gravel bed river in scotland using both digital photogrammetry and high resolution rtk gps ground surveys a statistical level of change detection is assessed by comparing surfaces with independent check points the methodological sensitivity of the annual channel sediment budget 1999 2000 to the threshold is presented results suggest that while the remote survey methods employed here can be used to develop qualitatively convincing moderate precision dems of channel topography rmse 0 21 m the remaining errors imply significant limits on reliable change detection which lead to important information losses tests at a 95 confidence interval for change detection show that over 60 of channel deposition and 40 of erosion may be obscured by the lower level of precision associated with photogrammetric monitoring when compared to ground survey measurements this bias reflects the difficulty of detecting the topographic signature of widespread but shallow deposition on bar tops

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

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

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

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

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

anabranching rivers ridge form alluvial channels in tropical northern australia sand dominated ridge form anabranching rivers on the kimberley plateau in northwestern australia are a new type of alluvial channel system not previously described or explained in detail this paper examines the morphology sedimentology and stratigraphy of this type of river and proposes mechanisms for their formation and maintenance alternating with relatively narrow bedrock reaches of valley are wider depositional alluvial reaches that support roughly parallel very elongated steep sided and treed sandy ridges of approximately floodplain height these ridges subdivide the total flow system into remarkably straight canal like anabranching channels in this seasonally arid environment there is an abundance of riparian vegetation in places growing chaotically over wider sections of stream bed it is proposed here that the formation of ridges results in a reduction of flow resistance with an increase in depth the ridges concentrate the flow and compensate for the less efficient flow conditions associated with these well vegetated alluvial reaches trees generally do not survive on the bed of the higher energy channels enhanced velocities and increased bed shear act to maintain or increase water and sediment flux in these alluvial reaches the formation of the channels and intervening ridges may be aided by the development of double flow helices in each anabranch similar to that described for much smaller ridge features observed in laboratory flumes anabranches are also observed having formed by channel avulsion scouring new channels into adjacent floodplains particularly in widened sections of valley near tributary junctions

effects of digital elevation model resolution on derived stream network positions 1 this study examines the effect of digital elevation model dem resolution on the positional accuracy of derived hydrologic networks and quantitatively confirms that dem resolution should be greater than the average hillslope length when used for hydrologic modeling seven hundred kilometers of mapped stream networks are compared to stream networks derived from 17 dems with resolutions ranging from 30 m to 3 km comparison between predicted and mapped streams reveals that accuracy of predicted stream locations decays quickly beyond a dem resolution of 180 m a new application of gyasi agyei et al s 1995 dem resolution suitability test based on average slope and vertical resolution indicates an average hillslope length of between 150 and 180 m tarboton et al s 1991 method of determining average hillslope length based on slope and accumulation areas reconfirms the length to be 150 m and verifies the link between network accuracy and hillslope scale two algorithms are used to derive stream networks the d algorithm tarboton 1997 which allows for flow dispersion and the d8 algorithm o callaghan and mark 1984 which does not a comparison between the d8 and d algorithms shows that modeling flow dispersion is not necessary in steep terrain as both algorithms performed equally well however the d algorithm is found to be less susceptible to modeling erroneous hillside flow convergence making it preferable to the d8 algorithm when used by tarboton et al s 1991 method of determining the average hillslope lengths finally field observation indicates that predicted first and second order streams tend to exist when predicted however mapped streams have a higher positional accuracy

the mekong from satellite imagery a quick look at a large river abstract physical datasets on rivers thousands of kilometres long can be efficiently and rapidly acquired through satellite imagery as large rivers commonly cross several tectonic and climatic zones their channels are composed of a number of units each with characteristic morphology and behaviour separating a large river into such units provides a framework for understanding the geomorphology of the river concerned allowing acquisition of information for environmental impact analysis and river management about 2500 km of the mekong river from the china border to the sea has been examined with spot satellite images with a selection of modis and ikonos scenes in support hydrological data of the mekong river commission hydrographic maps of the commission and field observations have been used to verify and extend the findings from the satellite imagery the technique provides a rapid and holistic conceptualisation of forms and functions along the derived eight unit classification of the mekong river such a framework is valuable for 1 determining selected aspects of the geomorphological behaviour of a large river system 2 rapidly analysing project related environmental impacts and 3 examining the geological evolution of the river investigating large rivers is difficult and resource consuming but satellite imagery provides an easy and rapid tool the cost of the images however may be high as a number of them are required for this type of analysis

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

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