folsom drought and episodic drying on the southern high plains from 10 900 10 200 14c yr b p the paleoenvironments of late pleistocene and early holocene time on the southern high plains have been studied for decades but regionally extensive or long term easily recoverable proxy climate indicators are difficult to find the stratigraphy of valley fill and upland eolian deposits and stable carbon isotope data in addition to geographically limited paleontological data now provide clues to the environment during this time which includes the earliest or paleoindian period 11 200 8000 14c yr b p of human occupation during the clovis occupation 11 200 10 900 14c yr b p valleys contained perennial streams this was followed in folsom time 10 900 10 200 14c yr b p by an abrupt change to lakes and ponds with water levels fluctuating between several meters depth and no surface water and marshes and accumulation of sheet sands on uplands starting the earliest phase of construction of the regional dune fields these changing conditions indicate a shift from relatively wetter to relatively drier conditions with episodic drought stable c isotopes further indicate that warming characterized the clovis folsom transition during the rest of the paleoindian period the environment was relatively cool but fluctuated between wetter and drier conditions with an overall trend toward drying that resulted in further enlargement of the dune fields and culminated in the warm dry altithermal beginning 8000 14c yr b p clovis time probably was the wettest of any paleoindian period in terms of runoff and spring discharge the folsom period was drier and was the earliest episode of regional wind erosion and eolian deposition and may have been the warmest of paleoindian times evidence of a previously hypothesized clovis drought in this region is sparse

the lubbock lake landmark late pleistocene and early holocene paleoichthyofauna southern high plains of texas usa the lubbock lake landmark is located on the southern high plains of texas in yellowhouse draw that is part of the upper brazos river basin the late pleistocene and early holocene lubbock lake fish assemblages represent the first known quaternary paleoichthyofauna for the southern high plains none of the remains show evidence of cultural utilization their significance lies in their record and paleoenvironmental information the fish assemblages come from two strata from different locations within the landmark of the 845 elements recovered 184 22 are identifiable to family or lower level the elements overall represent seven families seven genera and six species the presence of most taxa is expected but two taxa have considerable range extensions and at least one was not known to inhabit the brazos river basin historically these disharmonious faunal elements have a retreat pattern to the southeast and northeast today the fish fauna reflects the overall vertebrate record at the landmark that indicates extensive regional changes occurring in the late quaternary climates and ecosystems that have led to modern conditions

analytical approach to river regime rational regime relationships for the width depth and slope of a channel in equilibrium are developed using the ackers and white sediment transport formulae and the white paris and bettess friction relationships together with the principle of maximum sediment transporting capacity favorable comparisons are made with data from sand and gravel channels and with existing empirical regime relationships

a comparison between flume and field bed load transport data and consequences for surface based bed load transport prediction 1 the ability of simple equations to predict bed load transport with limited knowledge of the bed surface material was investigated this was done using a data set consisting of 7 636 bed load transport values from the flume 1 317 data and from 84 river reaches 6 319 field data it was possible to collapse field and flume data by correcting the ratio between the shields number and its critical value with a very simple hiding function proposed as a power law of the d84 d50 ratio in so doing a surface based bed load transport formula was proposed it was successfully tested on an independent data set comprising sand and gravel bed rivers with slope in the range 0 0002 0 08 with 86 of the values predicted within a precision of 1 order of magnitude moreover the formula reproduced the low transport rates well contrary to the usual surface based formulas also tested and is particularly well suited for estimating low transport rates associated with near bankfull flow discharge this new formula is neither time consuming no fractionwise calculation nor data consuming the required parameters are the flow discharge the active width the slope and the surface grain diameters d50 and d84

experimental study of the transport of mixed sand and gravel we measured a wide range of transport rates for five different sand gravel mixtures in a laboratory flume each mixture used the same gravel and sand was added to produce mixtures containing 6 15 21 27 and 34 sand control of other variables allows us to isolate the effect of bed sand content on transport as sand content increases gravel transport rates increase by orders of magnitude even though the proportion of gravel in the bed decreases the increase in gravel transport rate is most rapid over the range of bed sand content between 15 and 27 the increase in transport rate is larger than predicted using standard scaling relations between transport rate and grain size indicating that models of transport and sorting and predictions of stream channel response to sand inputs need to be revised to account for the influence of sand content bed surface grain size was measured at the end of each run surface grain size varied with sand content but showed little or no coarsening with flow strength and transport rate this casts doubt on the idea that armor layers form at small flows and weaken or vanish with increasing flow and transport rate

theoretical regime equations for mobile gravel bed rivers with stable banks a system of rational regime equations is developed for gravel bed rivers with stable banks using the optimality theory ot the optimality theory is based on the premise that equilibrium river geometry is characterised by an optimum configuration defined here as maximum sediment transport efficiency theoretical dimensionless equations are derived for width depth slope width depth ratio and meandering braiding transition independent dimensionless variables comprise discharge sediment concentration and relative bank strength which is defined as the ratio of the critical shear stresses for the bank and bed sediments discharge exponents and general form of the equations agree well with previously developed empirical relations relative bank strength is used to parameterise the influence of riparian vegetation on bank strength and is evaluated by calibrating against observed width depth ratio once calibrated the hydraulic geometry of natural gravel rivers is well described by the theoretical equations including discrimination between meandering and braiding channels the results provide strong support for the assumption that equilibrium or regime river behavior is equivalent to an optimal state and underline the importance of bank strength and sediment load as controls on hydraulic geometry

a method for characterising longitudinal irregularity in river channels abstract a method for analysing longitudinal variations in cross sectional shape and channel slope is developed and demonstrated with an application to the wimmera river australia the method involves characterising the geometry and spatial location of individual cross sections using four parameters and then conducting statistical analyses of the parameters from a set of cross sections the method extends the general characterisation of natural channels allows examination of the spatial characteristics of stream channels allows stochastic generation of stream channel cross sections and can provide input into hydraulic and stream habitat analyses the application to the wimmera river identifies longitudinal trends and variability in cross section characteristics including quantification of differences in channel characteristics associated with a quasi periodic switching between two different channel plan forms single thread and anastomosing

dynamics of bedload transport in turkey brook a coarse grained alluvial channel automatic and continuously recording samplers are deployed in a hertfordshire gravel bed stream to show that bedload transport is related to stream power the pattern is similar to that already established for north american channels but because the record is so detailed it is possible to identify the cause of the considerable scatter that is normal in such relationships a major factor is the occurrence of rhythmic pulses in bedload discharge that are not matched by similar fluctuations in hydraulic variables it is suggested that these pulses reflect downstream differences in the concentration of mobile particles in a slow moving traction carpet and that they may be likened to kinematic waves the record also reveals that the threshold of sediment transport always presumed hithero to be associated with incipient motion is related to the cessation of bedload transport in a river flood indeed the mean value of stream power at the finish of bedload transport is only 20 percent of that prevailing at the moment of incipient sediment motion because of this there is an inevitably poor correlation between actual bedload transport rates and those predicted by bedload equations which rely upon a single traction threshold these new data show that the general inverse relationship between bedload discharge and water depth grain size ratio proposed by bagnold 1977 1980 is not universal transport efficiency for this gravel bed stream is typically 0 05 per cent of available stream power which compares with 1 6 per cent for a river moving both gravel and sand and 5 per cent for another channel where bedload is composed predominantly of sand sized particles it is argued that coarse and fine grained alluvial channels may need to be considered separately by allowing for differences in traction threshold at the beginning and end of bedload events and by averaging bedload discharge flood by flood in order to smooth out the effect of pulses it is possible to achieve a reasonably good prediction of average bedload transport rate in terms of stream power

stochastic variability of fluvial hydraulic geometry mississippi and red rivers major reasons for uncertainty in engineering predictions of river flows are the spatial variability and measurement error associated with depicting channel geometry this paper develops a descriptive spatial stochastic model of depth based hydraulic geometry in application to four dense data sets from the mississippi and red rivers to represent the irregularity in flow area and hydraulic radius as functions of flow depth at a station cross section geometry is defined using regressed power functions accompanied by random residuals more than 1600 surveyed cross sections are analyzed to describe trends in the spatial variance and gradual large scale and periodic medium scale trends in the spatial mean of cross sectional geometry parameters and channel bed slope also described are the probability distribution and correlation structure of each parameter s random component the relationships between spatial statistics of hydraulic geometry and bankfull width channel sinuosity meander patterns bed elevation and riffle pool spacing and bank composition are explored conclusions and implications are discussed regarding the variable nature of fluvial hydraulic geometry and regarding the incorporation of geometric parameter uncertainty into prescriptive models of river processes

role of near bed turbulence structure in bed load transport and bed form mechanics the interactions between turbulence events and sediment motions during bed load transport were studied by means of laser doppler velocimetry and high speed cinematography sweeps u 0 w 0 w 0 which contribute negatively to the bed shear stress and are relatively rare individually move as much sediment as sweeps of comparable magnitude and duration however and much more than bursts u 0 and inward interactions u 0 w 0 when the magnitude of the outward interactions increases relative to the other events therefore the sediment flux increases even though the bed shear stress decreases thus although bed shear stress can be used to estimate bed load transport by flows with well developed boundary layers in which the flow is steady and uniform and the turbulence statistics all scale with the shear velocity it is not accurate for flows with developing boundary layers such as those over sufficiently nonuniform topography or roughness in which significant spatial variations in the magnitudes and durations of the sweeps bursts outward interactions and inward interactions occur these variations produce significant peaks in bed load transport downstream of separation points thus supporting the hypothesis that flow separation plays a significant role in the development of bed forms

downstream hydraulic geometry of rivers in victoria australia abstract downstream variation of hydraulic geometry in rivers characterized by fine textured banks and low width to depth ratios 7 25 is investigated in victoria australia with the aim of developing predictive models of channel geometry for large scale spatial modeling applications a one dimensional hydraulic model is used to determine the mean bank full geometry and discharge q bf for 93 sites which are investigated in relation to discharge of fixed average recurrence interval ari the median bank full ari is estimated at 0 8 years with 75 of sites between 0 5 and 2 5 years exponents in the downstream hydraulic geometry relations for width depth and velocity are respectively 0 43 0 40 and 0 18 q q bf and 0 44 0 38 and 0 03 q q 2 i e 2 year ari falling near the mode of global values q 2 and slope explain 66 of variance in q bf while q 2 explains 73 and 69 of the variance in width and depth relations respectively q 2 provides a reliable substitute for q bf in spatial modeling applications spatial variation in hydraulic geometry relations within and between river basins remains largely unexplained the w d ratio characteristically decreases with increasing distance along the lower reaches of most rivers and this has contributed to the lower than expected value for the width exponent

response of reach scale bankfull channel geometry to the altered flow and sediment regime in the lower yellow river abstract the channel geometry in an alluvial river usually varies in response to the altered flow and sediment regime from upstream damming bankfull channel dimensions have significantly adjusted along the lower yellow river lyr because of recent channel degradation caused by the operation of the xiaolangdi reservoir which has led to longitudinal variability in the cross sectional geometry therefore describing the bankfull channel geometry of the lyr is more representative using reach averaged variables in this study an integrated approach is proposed for calculating reach scale bankfull channel dimensions based on the measured section scale bankfull geometry in the lyr and the proposed approach integrates a geometric mean based on the log transformation with a weighted average based on the spacing between two consecutive sections the post flood reach scale bankfull channel dimensions in three different channel pattern reaches of the lyr were then calculated annually during the period from 1999 to 2012 using the proposed approach and the surveyed post flood profiles at 91 sedimentation sections calculated results indicate that the channel geometry changed in the components of bankfull width and depth in the braided reach with an increase of about 400 m in the reach scale bankfull width over the past 13 years and the channel evolution occurred mainly in the component of bankfull depth in the transitional and meandering reaches with the increased reach scale bankfull depths of 2 4 and 1 8 m respectively finally empirical relationships for the three reaches of the lyr were developed between the reach scale bankfull channel dimensions and the previous four year average discharge and incoming sediment coefficient during flood seasons with higher correlations being obtained when calibrated by the measurements from 1999 to 2010 furthermore the developed relations were also verified by the observed data in 2011 and 2012 with encouraging results

comment on long waves in erodible channels and morphodynamic influence by stefano lanzoni et al 1 enhanced understanding of information propagation by waves in rivers is essential in the context of fluvial hydraulics and morphodynamics lanzoni et al 2006 are complimented for their interesting contribution which would help to understand the long waves in fluvial flows with sediment transport and morphological evolution the investigation is formulated on the basis of a set of hyperbolic equations under the framework of shallow water hydrodynamics however the continuity equation of the fluid phase features a formal flaw this comment presents the correct continuity equations identifies the flaw and briefly addresses its potential impact on the analysis of fluvial processes

downstream changes in the channel geometry of a large gravel bed river 1 field data obtained on a nearly contiguous segment of the colorado river in western colorado and eastern utah are used to examine the mechanisms driving downstream changes in channel geometry measurements characterizing the bank full hydraulic geometry bed material grain size and average channel gradient were made at closely spaced intervals in 10 alluvial and quasi alluvial reaches covering 260 km of the river these data indicate that changes in surface and subsurface grain sizes are small in relation to the change in channel slope over the full length of the study area the median grain size of the surface sediment decreases by a factor of a little more than 2 whereas the average channel slope decreases by a factor of about 5 the decreases in slope and median grain size are offset by a large increase in bank full depth relative to width such that the bank full shields stress b is constant downstream for the reach as a whole b averages 0 049 which is roughly 50 higher than the threshold for bed load transport

coupled and decoupled numerical modeling of flow and morphological evolution in alluvial rivers existing numerical river models are mostly built upon asynchronous solution of simplified governing equations the strong coupling between water flow sediment transport and morphological evolution is thus ignored to a certain extent an earlier study led to the development of a fully coupled model and identified the impacts of simplifications in the water sediment mixture and global bed material continuity equations as well as of the asynchronous solution procedure for aggradation processes this paper presents the results of an extended study along this line highlighting the impacts on both aggradation and degradation processes simplifications in the continuity equations for the water sediment mixture and bed material are found to have negligible effects on degradation this is however in contrast to aggradation processes in which the errors purely due to simplified continuity equations can be significant transiently the asynchronous solution procedure is found to entail appreciable inaccuracy for both aggradation and degradation processes further the asynchronous solution procedure can render the physical problem mathematically ill posed by invoking an extra upstream boundary condition in the supercritical flow regime finally the impacts of simplified continuity equations and an asynchronous solution procedure are shown to be comparable with those of largely tuned friction factors indicating their significance in calibrating numerical river models it is concluded that the coupled system of complete governing equations needs to be synchronously solved for refined modeling of alluvial rivers

defining phases of bedload transport using piecewise regression differences in the transport rate and size of bedload exist for varying levels of flow in coarse grained channels for gravel bed rivers at least two phases of bedload transport with notably differing qualities have been described in the literature phase i consists primarily of sand and small gravel moving at relatively low rates over a stable channel surface transport rates during phase ii are considerably greater than phase i and more coarse grains are moved including material from both the channel surface and subsurface transition from phase i to phase ii indicates initiation and transport of grains comprising the coarse surface layer common in steep mountain channels while the existence of different phases of transport is generally acknowledged the threshold between them is often poorly defined we present the results of the application of a piecewise regression analysis to data on bedload transport collected at 12 gravel bed channels in colorado and wyoming usa the piecewise regression recognizes the existence of different linear relationships over different ranges of discharge the inflection where the fitted functions intersect is interpreted as the point of transition from phase i to phase ii transport this is termed breakpoint a comparison of grain sizes moved during the two phases shows that coarse gravel is rarely trapped in the samplers during phase i transport indicating negligible movement of grains in this size range gravel larger than about d16 of the channel surface is more consistently trapped during phase ii transport the persistence of coarse gravel in bedload samples provides good evidence that conditions suitable for coarse grain transport have been reached even though the size of the sediment approaches the size limits of the sampler 76 mm in all cases a relative breakpoint rbr was defined by the ratio between the discharge at the breakpoint and the 1 5 year flow a surrogate for bankfull discharge expressed as a percentage the median value of rbr was about 80 percent suggesting that phase ii begins at about 80 percent of the bankfull discharge though the observed values of rbr ranged from about 60 to 100 percent variation in this value appears to be independent of drainage area median grain size sorting of bed materials and channel gradient at least for the range of parameters measured in 12 gravel bed channels data archive http dx doi org 10 2737 rds 2007 0003

incipient sediment motion at high slopes in uniform flow condition 1 this paper reviews the problem of the incipient motion of cohesionless particles in a high sloping water stream a theoretical analysis of the equilibrium of a gravel particle is developed by a deterministic approach with the introduction of the relative degree of exposure of the particle itself to the stream flow the parameter relative degree of exposure relates the positioning of a particle with the upstream one the problem of velocity distribution and the definition of drag coefficient is also revised to better account for low submergence the results of the theory are compared with a series of ad hoc experiments performed by c gregoretti

unequal mobility of gravel and sand in weakly bimodal river sediments bed load was trapped during flood events over a 20 month period at the lower end of the allt dubhaig a small river in scotland with rapid downstream fining of gravel bed material on a slowly aggrading concave long profile the channel bed near the trap is predominantly gravel with a secondary sand mode total transport in each event depended mainly on peak shear stress rather than duration over a threshold bed load was mainly sand in smaller events bimodal in intermediate events and mainly gravel in the biggest floods mean and maximum grain diameter both increased with peak shear stress but in different ways analysis of fractional transport rates and maximum grain size in relation to peak shear stress suggests that gravel transport is slightly size selective but sand transport is close to equal mobility the slight selectivity in gravel transport is consistent with previous field studies of near equilibrium unimodal beds and supports assumptions made in the numerical model of hoey and ferguson 1994 which successfully simulates the observed amount of downstream fining over the 2 5 km upstream of the bed load trap

more on the evolution of bed material waves in alluvial rivers sediment waves or pulses can form in rivers following variations in input from landslides debris ows and other sources the question as to how rivers cope with such sediment inputs is of considerable practical interest experimental numerical and eld evidence assembled by the authors suggests that in mountain gravel bed streams such pulses show relatively little translation instead mostly dispersing in place this research has recently been the subject of discussion in particular it has been suggested that a the equations of ow and sediment mass balance used in the analyses and in most other morphodynamic analyses require correction b the dominance of dispersion appears only because the hyperbolic nature of the governing equations has not been adequately considered and c the sediment transport equation used in the analyses does not lead to generalizable results here we suggest that a the relations for mass balance do not require the indicated correction b the hyperbolic nature of the governing equations does not preclude the result of dispersion dominating translation in mountain streams and c the general behaviour of an appropriate hyperbolic model of sediment waves pulses includes the relative roles of dispersion and translation and is not affected by the precise choice of a sediment transport relation as long as the choice is reasonable for the case in question copyright 2005 john wiley sons ltd

fully coupled unsteady mobile boundary flow model a fully coupled one dimensional river model capable of predicting sediment transport and bed level changes under unsteady flow conditions is described the governing equations for mass and momentum balance of sediment water mixture under unsteady flow conditions in natural rivers with irregular geometries are solved simultaneously using the preissmann four point linear implicit scheme with weighting factors for space and time coordinates the resulting system of algebraic equations is solved using a solution procedure that bridges upstream and downstream boundary nodal values by successive substitutions the friction slope is expressed in a consistent form that allows the consideration of the changing alluvial roughness effect the system of governing equations is explicitly coupled by introducing the term representing the rate of change of bed level the fully coupled unsteady mobile boundary flow model fcm is applied to the classical problem of sediment deposition upstream of a dam with an arbitrar

form drag in rivers due to small scale natural topographic features 2 irregular sequences 1 the size shape and spacing of small scale topographic features found on the boundaries of natural streams rivers and floodplains can be quite variable consequently a procedure for determining the form drag on irregular sequences of different sized topographic features is essential for calculating near boundary flows and sediment transport a method for carrying out such calculations is developed in this paper this method builds on the work of kean and smith 2006 which describes the flow field for the simpler case of a regular sequence of identical topographic features both approaches model topographic features as two dimensional elements with gaussian shaped cross sections defined in terms of three parameters field measurements of bank topography are used to show that 1 the magnitude of these shape parameters can vary greatly between adjacent topographic features and 2 the variability of these shape parameters follows a lognormal distribution simulations using an irregular set of topographic roughness elements show that the drag on an individual element is primarily controlled by the size and shape of the feature immediately upstream and that the spatial average of the boundary shear stress over a large set of randomly ordered elements is relatively insensitive to the sequence of the elements in addition a method to transform the topography of irregular surfaces into an equivalently rough surface of regularly spaced identical topographic elements also is given the methods described in this paper can be used to improve predictions of flow resistance in rivers as well as quantify bank roughness

the dominance of dispersion in the evolution of bed material waves in gravel bed rivers bed material waves are temporary zones of sediment accumulation created by large sediment inputs recent theoretical experimental and field studies examine factors influencing dispersion and translation of bed material waves in quasi uniform gravel bed channels exchanges of sediment between a channel and its floodplain are neglected within these constraints two factors influence relative rates of dispersion and translation 1 interactions between wave topography flow and bed load transport and 2 particle size differences between wave material and original bed material our results indicate that dispersion dominates the evolution of bed material waves in gravel bed channels significant translation requires a low froude number which is uncharacteristic of gravel bed channels and low wave amplitude which for a large wave can be achieved only after substantial dispersion wave material of small particle size can promote translation but it primarily increases bed load transport rate and thereby accelerates wave evolution copyright 2001 john wiley sons ltd

theoretical development on the effects of changing flow hydraulics on incipient bed load motion 1 several decades of flume and field measurements have indicated that in rough turbulent flows the critical shields stress increases with increasing slope and associated decreasing relative depth this result contradicts the usual consideration of a decreased critical shields value on very steep slopes because of increased gravitational effects however recent studies have demonstrated that these experimental results could be reproduced with a force balance model if the classical logarithmic velocity profile was replaced with a velocity profile that was more compatible with available velocity measurements over gravel beds these measurements indicate the existence of a roughness layer that is a zone of almost constant velocity close to the bed whose properties mean velocity and turbulence depended on the flow s relative depth unfortunately velocity profile measurements for low relative depth associated with steep slopes are scarce and it is still difficult to include such flow properties in a force balance model flow resistance data on the basis of depth average velocity measurements are very common and cover a wide range of slopes and relative depths in this paper these data are used to fit a velocity profile including a roughness layer when used in a force balance model for incipient motion it adequately reproduced a data set composed of 270 critical shields values measured in a flume with near uniform sediments the relevance of this research to field problems is discussed using a data set composed of 92 critical shields stresses obtained from field measurements finally a model is proposed for field applications taking into account the slope effect

the role of vegetation and bed level fluctuations in the process of channel narrowing abstract a catastrophic flood ire 1965 on plum creek a perennial sandbed stream in the western great plains removed most of the bottomland vegetation and transformed the single thalweg stream into a wider braided channel following eight years of further widening associated with minor high flows a process of channel narrowing began in 1973 narrowing continues today the history of channel narrowing was reconstructed by counting the annual rings of 129 trees and shrubs along a 5 km reach of plum creek near louviers colorado sixty three of these plants were excavated in order to determine the age and elevation of the germination point the reconstructed record of channel change was verified from historical aerial photographs and then compared to sediment stratigraphy and records of discharge and bed elevation from a streamflow gaging station in the study reach channel narrowing at plum creel occurs in two ways first during periods of high flow sand and fine gravel are delivered to the channel temporarily raising the general bed level subsequently several years of uninterrupted low flows incise a narrower channel second during years of low flow vegetation becomes established on the subaerial part of the present channel bed in both cases surfaces stabilize as a result of vegetation growth and vertical accretion of sediment

width adjustment in experimental gravel bed channels in response to overbank flows 1 we conducted a series of flume experiments to investigate the response of self formed gravel bed channels to floods of varying magnitude and duration floods were generated by increasing the discharge into a channel created in sand and gravel sized sediment with a median grain size of 2 mm flooding increased the shields stress along the channel perimeter causing bank erosion and rapid channel widening the sediment introduced to the channel by bank erosion was not necessarily deposited on the channel bed but was rather transported downstream a process likely facilitated by transient fining of the bed surface at the end of each experiment bank sediments were no longer in motion partial bed load transport characterized the flat bed portion of the channel and the shields stress approached a constant value of 0 056 about 1 2 times the critical shields stress for incipient motion furthermore the discharge was entirely accommodated by flow within the channel the creation of a stable channel entirely eliminated overbank flows we speculate that similar processes may occur in nature but only where bank sediments are non cohesive and where channel narrowing processes cannot counteract bank erosion during overbank flows we also demonstrate that a simple model of lateral bed load transport can reproduce observed channel widening rates suggesting that simple methods may be appropriate for predicting width increases in channels with non cohesive unvegetated banks even during overbank flows last we present a model for predicting the equilibrium width and depth of a stable gravel bed channel with a known channel forming shields stress

regional bankfull geometry relationships for southern california mountain streams and hydrologic applications abstract this study develops and intercompares regional relationships for bankfull channel width hydraulic depth and cross sectional area for southern california mountain streams based on several data sources surveyed streams us geological survey stream survey reports and existing literature although considerable uncertainty exists in estimating bankfull conditions the relationships developed from the varying data sources show significant agreement for small watersheds with drainage area ranging from 15 to 2000 km 2 the estimates of bankfull top width ranged from 7 2 to 44 5 m and hydraulic depth estimates ranged from 0 35 to 1 15 m the utility of the developed bankfull geometry regional curves is demonstrated for southern california catchments through a the computation of the bankfull discharge and b the estimation of the surface runoff response necessary to produce bankfull conditions in the streams at the outlet of these catchments for selected locations with instantaneous flow records the occurrence frequency of events exceeding bankfull flow was examined for the available 10 15 year span of observational records bankfull discharge estimates for all small watersheds in the region ranged from 1 3 to 74 m 3 s while the range at the selected gauged stream locations was from 2 6 to 16 4 m 3 s stream locations along the transverse mountains of southern california showed an average occurrence frequency of less than 1 year whereas along the peninsular mountains the average return period tended to be greater than 1 year the application of the regional curves to the estimation of the surface runoff response necessary to produce bankfull conditions at the channel outlets of small catchments may be used as an index for conditions of minor flooding with saturated soils this surface runoff response index ranges from 2 0 to 5 5 mm for a 3 hour rainfall duration for southern california watersheds greater than 15 km 2 in area differences between the values for the peninsular and transverse mountain ranges are linked to geological climatic and geomorphologic differences the developed regional geometry relationships are suitable for use in various hydrologic modeling applications including distributed modeling with high resolution pertinent to flash flood forecasting

lubbock lake late quaternary cultural and environmental change on the southern high plains usa lubbock lake southern high plains of texas contains a cultural faunal and floral record within a virtually complete geological record spanning the past 11 100 years more than 88 archaeological occurrences have been excavated from five major stratigraphic units the paleoindian record 11 500 6500yr bp begins with clovis age occupation ca 11 100yr bp found within fluvial deposits stratum 1 subsequent paleoindian occupations are found in lake and marsh sediments stratum 2 archaic occupations 8500 2000yr bp are contained within aeolian and marsh deposits strata 3 and 4 ceramic occupations 2000 500yr bp are found on a soil developed in stratum 4 in marsh sediments strata 4 and 5 and in slopewash and aeolian sediments stratum 5 the protohistoric 500 300yr bp and historic 300 100yr bp remains are in slopewash aeolian and marsh sediments stratum 5 and associated soils the southern high plains remained a grasslands throughout the last 11 500 years and neither man nor bison abandoned the region the successive local faunas reflect changing ecosystems under pluvial to arid to more mesic to semiarid conditions the occupation of lubbock lake through time appears to have been by small groups of people for both economic and short term residential uses these hunter gatherer peoples underwent adaptive change brought about by climatic stress and alterations to food resources

critical specific stream power in gravel bed rivers experiments with marked pebbles were carried out on different sized rivers of the belgian ardenne catchment areas varying from less than 1 km2 to 2700 km2 specific stream power required to cause bedload movement was evaluated and critical values were obtained three types of relationship between critical specific stream power 0 and grain size d were established the values for 0 in the largest river the ourthe were the lowest and were close to the values obtained for mountainous rivers carrying large boulders in medium sized rivers catchment area between 40 and 500 km2 the critical unit stream power was higher it is likely that it is due to the bedform s greater resistance this resistance would use up some of the energy that can cause movement and transport of bedload the amount of resistance of the bedform can be expressed as bedform shear stress determined by the relationship between grain shear stress that determines movement and transport of the bedload and the total shear stress this ratio varies between 0 4 and 0 5 in the medium sized rivers compared to 0 7 in the ourthe in headwater streams less than 20 km2 there is greater loss of energy due to bedform resistance 0 3 critical specific stream power is higher in this third type of river than in the other two

effect of bank stability on geometry of gravel rivers a bank stability analysis is incorporated into an analytical procedure for modeling the hydraulic geometry of an alluvial gravel bed channel the new analysis includes a procedure that calculates the mean bed and bank shear stress as well as assessing the bank stability the bank stability criterion accounts for the increased stability of the channel banks due to consolidation of the bank sediment cementing by fines and binding of the sediment by root masses the two key parameters in the bank stability analysis are the median grain diameter of the bank sediment d50bank and the modified friction angle of the bank sediment a sensitivity analysis of the two parameters indicate that the bank stability can exert a large influence on the channel geometry this is supported by testing the theory on published field data the bank sediment size was assumed to equal the bed sediment size and the variation of the friction angle was investigated the estimated values are seen to increase systematically

geomorphic response of the jingjiang reach to the three gorges project operation upstream damming often causes significant downstream geomorphic adjustments remarkable channel changes have occurred in the jingjiang reach of the middle yangtze river since the onset of the three gorges project tgp therefore it is important to investigate the variations in different fluvial variables for better understanding of the channel evolution characteristics as an example of the jingjiang reach recent geomorphic adjustments in the study reach have been investigated quantitatively including variations in sediment rating curve fluvial erosion intensity channel deformation volume and bankfull channel geometry these fluvial variables adjusted in varying degrees in response to the altered flow and sediment regime caused by the tgp operation a focus of this study has been especially paid on the variation in the bankfull channel geometry calculated bankfull dimensions at section and reach scale indicate that i there were significant bank erosion processes in local regions without bank protection engineering with empirical relations being developed to reproduce the variation in bankfull widths at four typical sections ii the variation in the reach scale channel geometry occurred mainly in the component of bankfull depth owing to the construction of large scale bank revetment works with the depth increasing from 13 7 m in 2002 to 15 0 m in 2014 and with an increase in the corresponding bankfull area of about 11 and iii the reach scale bankfull channel dimensions responded to the previous five year average fluvial erosion intensity during flood seasons at zhicheng with higher correlations for the depth and area being obtained when calibrated by the measurements in 2002 2012 furthermore these developed relations for the section and reach scale bankfull channel geometry were also verified by the observed data in 2013 2014 with encouraging results being obtained this article is protected by copyright all rights reserved

bankfull discharge in mountain streams theory and practice the results are presented of an investigation of bankfull discharge in two polish carpathian streams skawica and krzyworzeka existing definitions of river bankfull were reviewed and applied in tests carried out on selected cross sections of the streams the woodyer method was given special attention with a correspondingly detailed survey of plants characterizing river benches riley s bench index method and the methods of williams wolman schumm and brown and woloszyn were tested the report concludes that bankfull discharge value for a mountain stream should not be reported as a single number but rather as a range of discharges within which one could expect the bankfull value to lie copyright 2002 john wiley sons ltd

channel stability in bed load dominated streams with nonerodible banks inferences from experiments in a sinuous flume 1 experimental and computational evidence shows that bed state adjustments can accommodate a twofold to fourfold change in sediment supply without otherwise affecting the morphology of a sinuous gravel bed stream channel with nonerodible banks in contrast previously published experiments of the same kind but with erodible channel banks suggest that changes in sediment supply are taken up almost exclusively by changes in channel sinuosity and hence the reach average water surface gradient it appears that stream channel response to changes in sediment supply is strongly conditioned by the nature of the stream boundary and that determines which of several potential adjustments are possible

is the critical shields stress for incipient sediment motion dependent on channel bed slope data from laboratory flumes and natural streams show that the critical shields stress for initial sediment motion increases with channel slope which indicates that particles of the same size are more stable on steeper slopes this observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force processes that might explain this discrepancy are explored using a simple force balance model including increased drag from channel walls and bed morphology variable friction angles grain emergence flow aeration and changes to the local flow velocity and turbulent fluctuations surprisingly increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical shields stress are similar for flume experiments and natural streams and significant variations in bed morphology in flumes is unlikely instead grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed roughness scale to flow depth i e relative roughness a model for the local velocity within the grain roughness layer is proposed based on a 1 d eddy viscosity with wake mixing in addition the magnitude of near bed turbulent fluctuations is shown to depend on the depth averaged flow velocity and the relative roughness extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes

downstream variation in bankfull width of wadeable streams across the conterminous united states abstract bankfull channel width is a fundamental measure of stream size and a key parameter of interest for many applications in hydrology fluvial geomorphology and stream ecology we developed downstream hydraulic geometry relationships for bankfull channel width w as a function of drainage area a w a dhg wa for nine aggregate ecoregions comprising the conterminous united states using 1588 sites from the u s environmental protection agency s national wadeable streams assessment wsa including 1152 sites from a randomized probability survey sample sampled stream reaches ranged from 1 to 75 m in bankfull width and 1 to 10 000 km 2 in drainage area the dhg wa exponent which expresses the rate at which bankfull stream width scales with drainage area fell into three distinct clusters ranging from 0 22 to 0 38 width increases more rapidly with basin area in the humid eastern highlands encompassing the northern and southern appalachians and the ozark mountains and the upper midwest great lakes region than for the west both mountainous and xeric areas the southeastern coastal plain and the northern plains the dakotas and montana stream width increases least rapidly with basin area in the temperate plains cornbelt and southern plains great prairies in the heartland the coefficient of determination r 2 was least in the noncoastal plains 0 36 0 41 and greatest in the appalachians and upper midwest 0 68 0 77 dhg wa equations differed between streams with dominantly fine bed material silt sand and those with dominantly coarse bed material gravel cobble boulder in six of the nine analysis regions where dhg wa equations varied by sediment size fine bedded streams were consistently narrower than coarse bedded streams within the western mountains ecoregion where there were sufficient sites to develop dhg wa relationships at a finer spatial scale and ranged from 1 23 to 3 79 and 0 23 to 0 40 respectively with r 2 0 50 for 10 of 13 subregions range 0 36 to 0 92 enhanced dhg equations incorporating additional data for three landscape variables that can be derived from gis mean annual precipitation elevation and mean reach slope significantly improved equation fit and predictive value in several regions most notably the western mountains and the temperate plains channel width was also related to human disturbance we examined the influence of human disturbance on channel width using several indices of local and basinwide disturbance contrary to our expectations the data suggest that the dominant response of channel width to human disturbance in the united states is a reduction in bankfull width in streams with greater disturbance particularly in the western mountains where population density road density agricultural land use and local riparian disturbance were all negatively related to channel width and in the appalachians and new england where urban and agricultural land cover and riparian disturbance were all negatively associated with channel width

eileen johnson s legacy of research the discussion begins where her career on a national and international basis began at lubbock lake that starting point lead to significant broadening of her research both intellectually and geographically and was reflected in the work of her collaborators and students in paleoindian archaeology in all other archaeological time periods and traditions on the southern plains in the method and theory of zooarchaeology and quaternary vertebrate paleontology work beyond the high plains and sustained long term research programs both on and beyond the southern plains

revisiting the drivers of at a station hydraulic geometry in stream reaches abstract knowledge of temporal variations in river hydraulic characteristics i e width depth velocity across stream networks is a key element for catchment management because hydraulics influence physical habitats and biodiversity water temperature nutrient fluxes sediment transport and associated ecosystem services reach scale at a station hydraulic geometry describes the variation of mean water depth and width with discharge within a stream reach the ability to understand and predict the form of these hydraulic geometry relationships across river networks therefore represents a powerful tool for river managers although variations of these relationships between rivers have been widely documented they are still largely unexplained this study provides an analysis of a unique data set of hydraulic geometry collected in 492 stream reaches in france relationships between the hydraulic geometry parameters and many variables describing catchment and reach scale characteristics of reaches were analyzed using stepwise linear regression and discriminant analysis results show that width discharge relationships across reaches are well predicted by longitudinal and lateral descriptors of reaches the ratio of median to bankfull width the median froude number the width to depth ratio the active channel relative to the catchment area and the fluvial pattern braided or not fluvial pattern reach averaged active channel width and bankfull width can be accurately estimated using aerial imagery making application of our models over river networks possible fewer relationships significantly explained depth discharge relationships despite a weak influence of reach froude number flow resistance i e relative grain size relative bedform submergence and width to depth ratio unavailable descriptors of channel resistance would probably improve these models e g describing bank stability spill and form resistance despite the uncertainties of hydraulic geometry relationships this study shows that predictions of hydraulic geometry exponents across river networks are possible offering a large range of potential applications

integrating hydraulic equivalent sections into a hydraulic geometry study abstract hydraulic geometry hg is an important geomorphic concept that has played an indispensable role in hydrological analyses physical studies of streams ecosystem and aquatic habitat studies and sedimentology research more than 60 years after leopold and maddock 1953 first introduced the concept of hg researchers have still not uncovered the physical principles underlying hg behavior one impediment is the complexity of the natural river cross section the current study presents a new way to simplify the cross section namely the hydraulic equivalent section which is generalized from the cross section in the gradually varied flow of an alluvial river gvfar and features hydrodynamic properties and bed building laws similar to those of the gvfar energy balance was used to derive the stage z discharge q relationship in the gvfar the gvfar in the songhua river and the yangtze river were selected as examples the data including measured discharge river width water stage water depth wet area and cross section were collected from the hydrological yearbooks of typical hydrological stations on the songhua river and the yangtze river from 1955 to 1987 the relationships between stage z discharge q and cross sectional area a stage z at various stations were analyzed and at a station hydraulic geometry ahg relationships were obtained in power law forms based on derived results and observational data analysis the z q and z a relationships of ahg were similar to rectangular weir flows thus the cross section of the gvfar was generalized as a compound rectangular hydraulic equivalent cross section as to bed building characteristics the bankfull discharge method and the stage discharge relation method were used to calculate the dominant variables of the alluvial river this hydraulic equivalent section has the same z q relation z a relation dominant discharge dominant river width and dominant water depth as the cross section in the gvfar with the hydraulic equivalent section the relationships between the discharge and river width river depth and width to depth ratio are easier to depict in dhg analysis replacement of the cross section in the gvfar with the hydraulic equivalent section is expected to promote further development of hg

a critical perspective on 1 d modeling of river processes gravel load and aggradation in lower fraser river 1 we investigate how well a width averaged morphodynamic model can simulate gravel transport and aggradation along a highly irregular 38 km reach of lower fraser river and discuss critical issues in this type of modeling bed load equations with plausible parameter values predict a gravel input consistent with direct measurements and a sediment budget simulations using spatially varying channel width and forced by dominant discharge or a 20 year hydrograph match the observed downstream fining well they reproduce major qualitative features of a 47 year sediment budget and match maximum local rates of aggradation and degradation to well within a factor of 2 locations and rates of aggradation and degradation are influenced by channel constrictions runs for single years of unusually high or low peak discharge suggest that the main transfers of gravel occur in different parts of the reach in different years one dimensional morphodynamic modeling of highly nonuniform rivers has serious conceptual limitations but may be a valuable complement to empirical approaches

non linearity of reach hydraulic geometry relations summary reach average hydraulic geometry rhg defines the relations between the average hydraulic variables estimated at river reach water depth velocity channel width and the discharge single log linear models are classically used by fluvial geomorphologists hydrologists and ecologists to represent these relations in this paper we show that these single log linear models are not relevant in most of cases and so can lead to significant errors rhg estimated on 15 alluvial river reaches the catchment sizes vary from 10 km 2 to 1700 km 2 were found non linear for two thirds of the river reaches when considering discharges ranging from low to bankfull flows so we propose using two power functions or log linear to model these relations i e the log piecewise linear model this model can detect a discharge qbreak i e a physical transition or break point between both relations from very low flows about 30 of the median discharge to qbreak a first relation is comparable to the rhg found in the literature for aquatic habitat analysis the second relation is relevant for flows ranging from qbreak to bankfull discharge and therefore may be more useful for sediment transport and flood routing analysis averaging along a reach including several pool riffle units makes it difficult to clearly ascribe the form of rhg to particular physical processes the non linearity of rhg may have multiple origins the presence of lateral gravel bars the hydraulic conditions of riffle cross sections a change in the resistance to flow with water depth vegetation sediment influences further research is needed to focus on the physical processes that govern the rhg non linearity

modelling the flooding capacity of a polish carpathian river a comparison of constrained and free channel conditions abstract the gravel bed biala river polish carpathians was heavily affected by channelization and channel incision in the twentieth century not only were these impacts detrimental to the ecological state of the river but they also adversely modified the conditions of floodwater retention and flood wave passage therefore a few years ago an erodible corridor was delimited in two sections of the biala to enable restoration of the river in these sections short channelized reaches located in the vicinity of bridges alternate with longer unmanaged channel reaches which either avoided channelization or in which the channel has widened after the channelization scheme ceased to be maintained effects of these alternating channel morphologies on the conditions for flood flows were investigated in a study of 10 pairs of neighbouring river cross sections with constrained and freely developed morphology discharges of particular recurrence intervals were determined for each cross section using an empirical formula the morphology of the cross sections together with data about channel slope and roughness of particular parts of the cross sections were used as input data to the hydraulic modelling performed with the one dimensional steady flow hec ras software the results indicated that freely developed cross sections usually with multithread morphology are typified by significantly lower water depth but larger width and cross sectional flow area at particular discharges than single thread channelized cross sections they also exhibit significantly lower average flow velocity unit stream power and bed shear stress the pattern of differences in the hydraulic parameters of flood flows apparent between the two types of river cross sections varies with the discharges of different frequency and the contrasts in hydraulic parameters between unmanaged and channelized cross sections are most pronounced at low frequency high magnitude floods however because of the deep incision of the river both cross section types are typified by a similar low potential for the retention of floodwater in floodplain areas the study indicated that even though river restoration has only begun here it already brings beneficial effects for flood risk management reducing flow energy and shear forces exerted on the bed and banks of the channel in unmanaged river reaches only within wide unmanaged channel reaches can the flows of low frequency high magnitude floods be conveyed with relatively low shear forces exerted on the channel boundary in contrast in channelized reaches flow velocity and shear forces are substantially higher inevitably causing bank erosion and channel incision

archaeological geology of the lubbock lake site southern high plains of texas the lubbock lake site is a well stratified archaeological locality in yellowhouse draw on the southern high plains of texas it has a cultural record that spans the past 11 000 yr that is contained within a thick sequence of sediments and soils the sequence at lubbock lake is one of the most closely integrated records of late quaternary human occupation sedimentation and soil formation documented in north america stratum 1 11000 yr b p and older was deposited by a meandering stream and contains clovis age cultural remains stratum 2 is composed of lacustrine sediments 11000 to 10000 yr b p and overlying marsh deposits 10000 to 8500 yr b p with paleoindian material in both units the firstview soil 8500 to 6300 yr b p formed in the top of stratum 2 stratum 3 6300 to 5000 yr b p contains early archaic material eolian and marsh deposits suggestive of warm arid conditions and the yellowhouse soil indicating a brief period of landscape stability stratum 4 5000 to 4500 yr b p is primarily an eolian unit that marks the second of two periods of severe drought in the middle holocene the lubbock lake soil formed in stratum 4 beginning 4500 yr b p coincides with the late archaic and ceramic periods and documents landscape stability and a return to climatic conditions similar to those of today stratum 5 contains late ceramic protohistoric and historic archaeological material within eolian and slopewash sediments and the apache and singer soils which indicate several shifts toward aridity within the past 1 000 yr

channel pattern and river floodplain dynamics in forested mountain river systems channel pattern effectively stratifies the dynamics of rivers and floodplains in forested mountain river systems of the pacific northwest usa straight channels are least dynamic with relatively slow floodplain turnover and floodplains dominated by old surfaces braided channels are most dynamic with floodplain turnover as low as 25 years and predominantly young floodplain surfaces island braided and meandering channels have intermediate dynamics with moderately frequent disturbances erosion of floodplain patches maintaining a mix of old and young surfaces return intervals for the erosion of floodplains increase in the order of braided island braided meandering and straight 8 33 60 and 89 years respectively a threshold for the lateral migration of a channel occurs at a bankfull width of 15 20 m the most likely mechanism underlying this threshold is that larger channels are deep enough to erode below the rooting zone of bank vegetation above this threshold channels not confined between valley walls exhibit channel patterns distinguishable by slope and discharge and slope discharge domains can be used to predict channel patterns meandering and braided patterns are most consistently identified by the model and prediction errors are largely associated with indistinct transitions among channel patterns that are adjacent in plots of slope against discharge locations of straight channels are difficult to identify accurately with the current model the predicted spatial distribution of channel patterns reflects a downstream decline in channel slope which is likely correlated with a declining ratio of bed load to suspended load ecological theory suggests that biological diversity should be highest where the intermediate disturbance regime of island braided channels sustains high diversity of habitat and successional states through time

on evolution of bed material waves in alluvial rivers field and laboratory measurements have shown distinct characteristics of bed sediment waves under differing conditions whilst their theoretical interpretation has emerged to be equivocal this note aims to clarify the interpretation of evolution of bed material waves the complete set of governing equations for the flow sediment morphology system is deduced to demonstrate its universally hyperbolic nature irrespective of the sediment transport functions implemented to close the equations the hyperbolic nature can admit not only attenuating bed material waves but also shock like waves that are not unusual in the real world it is suggested that the theory of dispersion diffusion is not universally appropriate for evolution of bed material waves

the implications of disharmonious assemblages for pleistocene extinctions abstract the terms disharmonious or intermingled are applied to pleistocene faunas that contain associations of species that are now allopatric the apparently conflicting environmental implications of these faunas can be reconciled by postulating that the climate at the time these faunas existed was more equable than the present one associations of species of this kind are a ubiquitous feature of pleistocene faunas in all parts of the world where adequate data are available they imply a higher diversity of habitats which in many instances may be the result of the relatively greater control on the biota by locally more variable factors such as soils and topography these associations have implications for the causes of the late pleistocene extinctions in that their disappearance is coincident in time with the extinction event the implication that these associations indicate a higher habitat diversity suggests that the extinction is linked to a reduction of habitat diversity at the end of the pleistocene

hydraulic geometry of stream reaches abstract variation in river hydraulic geometry throughout stream networks is central to the problems of catchment management because of its influence on flow and sediment routing physical habitats and channel floodplain interactions in its simplest form at a station hydraulic geometry is described by power functions relating the surface width water depth and mean velocity at a channel cross section to discharge this paper extends hydraulic geometry analysis to represent the mean and variation of hydraulic characteristics along a river reach as power functions of discharge using 54 surveys of 17 river reaches mostly in south east australia it is shown that five reach hydraulic variables are sufficient to characterise the gross hydraulic conditions along a river reach the means and coefficients of variation of surface width and hydraulic depth plus the coefficient of variation of cross sectional velocity exponents for the mean width and mean depth hydraulic geometry relations are sensitive to measurement and cross section location and may best be treated as constant for regional studies coefficients for the mean width and mean depth hydraulic geometry relations are correlated positively with mean discharge and negatively with reach gradient respectively further analysis of variations in reach hydraulic geometry parameters particularly the power function coefficients in relation to river type flow parameters sediment load bank material and bank vegetation is warranted reach hydraulic geometry relations show considerable potential for improving analyses of fluvial processes and physical habitats across catchments

channel narrowing by vertical accretion along the green river near green river utah the green river is the longest tributary of the colorado river near the town of green river utah the green river narrowed in two discrete phases during the twentieth century the first phase of narrowing decreased average width by about 5 and occurred between about 1930 and 1940 when the magnitude of 2 yr flood mean annual discharge and effective discharge decreased by about 30 28 and 37 respectively during this first phase of narrowing saltcedar tamarisk spp an invading non native tree began to establish itself in the study area but botanists of that time did not describe the tree as abundant channel width was stable in the 1940s and 1950s even though saltcedar were becoming already abundant on the river9s banks further narrowing of an additional 14 occurred after 1959 this latest period of narrowing began following three successive years when the magnitude of floods was less than the present 1 5 yr recurrence flood and when saltcedar were already abundant along the river the deposits that comprise the banks of the narrowing green river are composed of the suspended load of the river and these alluvial deposits are characterized by horizontal layers which indicate that they formed by vertical accretion a mechanism is proposed to explain the coarsening upward sequence of beds found in these vertically accreted deposits these changes in the channel of the green river are based on analysis of more than 2600 discharge measurements made by the u s geological survey resurvey of an abandoned measurement site matches of historical ground level photography and analysis of historical aerial photography within a geographic information system we have developed analytical techniques that permit analysis of width data from u s geological survey discharge measurements where gaging cross sections have adjustable beds and banks these techniques allow the spatially rich but temporally poor data from aerial photographs to be supplemented with gaging station data which add great detail about the timing and actual processes of channel narrowing that cannot be determined from aerial photographs alone such an analytical strategy provides a more complete record of historical channel adjustment than can be obtained by other means

channel adjustments to changing discharges powder river montana sixteen years of annual surveys reveal how powder river responds to varying discharges during 1978 the second largest recorded daily mean discharge occurred cutbank migration bed degradation net bank erosion and overbank deposition all contributed to increase the channel area at 12 cross sections by an average of 62 during the ensuing years the channel area decreased as sediment was stored in low lying benches adjacent to the active bed of the channel the survey data indicate that the balance between bank erosion and deposition varies with discharge in years when the annual maximum daily mean discharge is 3 s a flow of 60 m 3 s has a recurrence interval of 1 1 yr bank erosion and deposition are approximately equal in years when the annual maximum daily mean discharge is between 60 and 150 m 3 s a discharge of 150 m 3 s has a recurrence interval of 2 7 yr bank deposition exceeds bank erosion and the channel contracts often by developing benches in years with higher discharges the channel expands through net bank erosion these results demonstrate that the channel of powder river is influenced by a wide variety of formative discharges powder river9s recent history of expansion and contraction and the development of prominent benches cannot be explained by equilibrium models based on a single channel forming discharge

sediment supply and the development of the coarse surface layer in gravel bedded rivers the bed surface of most gravel rivers is considerably coarser than the sub surface or the gravel load transported over it a phenomenon affecting river dynamics morphology and ecology the coarse surface layer often called an armour or pavement has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1 2 and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3 4 where bedload supply is cut off below dams selective erosion causes coarsening5 6 using a simple quantitative model we propose that surface coarsening develops in gravel bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load we present laboratory results which support this hypothesis and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse less active bed it may therefore be possible to relate the degree of river bed surface coarsening to sediment supply resulting from land use

prediction of discharge from water surface width in a braided river with implications for at a station hydraulic geometry 1 the total discharge of small braided rivers can be monitored nearly continuously using ground based orthorectified oblique images and a locally derived relationship between mean water surface width in a reach and total discharge with accuracy of the order of 10 the width discharge relationship is nearly linear over the normal range of summer flows measured in this proglacial stream the width discharge relation for a given location and channel configuration is transferable to adjacent reaches with different morphology with little loss in accuracy however the prospect of a more universal relation for braided rivers is remote because of differences in channel geometry and hydraulics between rivers the classical at a station hydraulic geometry exponent for width in this case is approximately 1 05 which is unique well above that observed in the few other braided rivers for which this has been derived and implies almost no change in mean velocity and depth with changing stage

the form and function of headwater streams based on field and modeling investigations in the southern appalachian mountains headwater streams drain the majority of most landscapes yet less is known about their morphology and sediment transport processes than for lowland rivers we have studied headwater channel form discharge and erosive power in the humid moderate relief valley and ridge and blue ridge provinces of the appalachian mountains field observations from nine headwater 2k m 2 drainage area mixed bedrock alluvial channels in a variety of boundary conditions demonstrate variation with respect to slope area channel initiation basic morphology slope distribution hydraulic geometry substrate grain size and role of woody debris these channels display only some of the typical downstream trends expected of larger lowland rivers variations are controlled mainly by differences in bedrock resistance from the formation level down to short wavelength outcrop scale variations hydrologic modeling on these ungauged channels estimates the recurrence of channel filling discharge and its ability to erode the channel bed two year recurrence discharge is generally larger and closer to bankfull height in the valley and ridge due to low soil infiltration capacity discharge that fills the channel to its surveyed bankfull form is variable generally exceeding two year flows at small drainage areas 0 5 km 2 and being exceeded by them at greater drainage areas this suggests bankfull is not controlled by the same recurrence storm throughout a channel or physiographic region stream power and relative competence are also variable these heterogeneities contrast relations observed in larger streams and illustrate the sensitivity of headwater channels to local knickpoints of resistant bedrock and armoring of channels by influx of coarse debris from hillslopes the general lack of predictable trends or functional relationships among hydraulic variables and the close coupling of channel form and function with local boundary conditions indicate that headwater streams pose a significant challenge to landscape evolution modeling copyright 2005 john wiley sons ltd

response of bankfull discharge to discharge and sediment load in the lower yellow river this paper presents a study on the response of bankfull discharge to incoming discharge and sediment load based on long term hydrological and bankfull discharge data measured on the lower reaches of the yellow river the analysis indicated that the bankfull discharge was a function of the preceding 5 to 6 years flow regimes revealing that bankfull channel dimensions were a result of the accumulative effect of several consecutive years flow discharge and sediment load conditions in other words the mean relaxation time was about 5 to 6 years for channel adjustment theoretical methods for the prediction of bankfull discharge were developed in which the response time or the relaxation time was fully considered testing of the methods using data observed at five hydrologic stations in the lower reaches of the yellow river from 1960 to 2003 showed that the proposed methods can predict the variation of bankfull discharge in response to changes in the incoming discharge and sediment load the proposed methods hold promise for predicting the magnitude and trend of channel response to other rivers undergoing aggradation or degradation from changes in hydrologic or sediment regime

bedload transport of fine gravel observed by motion picture photography motion pictures taken at duck creek a clear stream 6 5 m wide and 35 cm deep near pinedale wyoming provide detailed quantitative information on both the modes of motion of individual bedload particles and the collective motions of large numbers of them bed shear stress was approximately 6 pa 60 dynes cm 2 which was about twice the threshold for movement of the 4 mm median diameter fine gravel bed material and transport was almost entirely as bedload the displacements of individual particles occurred mainly by rolling of the majority of the particles and saltation of the smallest ones and rarely by brief sliding of large angular ones entrainment was principally by rollover of the larger particles and liftoff of the smaller ones and infrequently by ejection caused by impacts whereas distrainment was primarily by diminution of fluid forces in the case of rolling particles and by collisions with larger bed particles in the case of saltating ones the displacement times averaged about 0 2 0 4 s and generally were much shorter than the intervening repose times the collective motions of the particles were characterized by frequent brief localized random sweep transport events of very high rates of entrainment and transport which in the aggregate transported approximately 70 of the total load moved these events occurred 9 of the time at any particular point of the bed lasted 1 2 s affected areas typically 20 50 cm long by 10 20 cm wide and involved bedload concentrations approximately 10 times greater than background the distances travelled during displacements averaged about 15 times the particle diameter despite the differences in their dominant modes of movement the 8 16 mm particles typically travelled only about 30 slower during displacement than the 2 4 mm ones whose speeds averaged 21 cm s 1 particles starting from the same point not only moved intermittently downstream but also dispersed both longitudinally and transversely with diffusivities of 4 6 and 0 26 cm 2 s 1 respectively the bedload transport rates measured from the films were consistent with those determined conventionally with a bedload sampler the 2 4 mm particles were entrained 6 times faster on finer areas of the bed where 8 16 mm particles covered 6 of the surface area than on coarser ones where they covered 12 even though 2 4 and 4 8 mm particles covered practically the same percentage areas in both cases the 4 8 and 8 16 mm particles in contrast were entrained at the same rates in both cases to within the statistical uncertainty the rates of distrainment balanced the rates of entrainment for all three sizes and were approximately proportional to the corresponding concentrations of bedload

variation in the reference shields stress for bed load transport in gravel bed streams and rivers 1 the present study examines variations in the reference shear stress for bed load transport r using coupled measurements of flow and bed load transport in 45 gravel bed streams and rivers the study streams encompass a wide range in bank full discharge 1 2600 m3 s average channel gradient 0 0003 0 05 and median surface grain size 0 027 0 21 m a bed load transport relation was formed for each site by plotting individual values of the dimensionless transport rate w versus the reach average dimensionless shear stress the reference dimensionless shear stress r was then estimated by selecting the value of corresponding to a reference transport rate of w 0 002 the results indicate that the discharge corresponding to r averages 67 of the bank full discharge with the variation independent of reach scale morphologic and sediment properties however values of r increase systematically with average channel gradient ranging from 0 025 0 035 at sites with slopes of 0 001 0 006 to values greater than 0 10 at sites with slopes greater than 0 02 a corresponding relation for the bank full dimensionless shear stress bf formulated with data from 159 sites in north america and england mirrors the relation between r and channel gradient suggesting that the bank full channel geometry of gravel and cobble bedded streams is adjusted to a relatively constant excess shear stress bf r across a wide range of slopes

particle size variations between bed load and bed material in natural gravel bed channels particle sizes of bed load and bed material that represent materials transported and stored over a period of years are used to investigate selective transport in 13 previously sampled natural gravel bed channels the ratio d of median particle size of bed material to the transport and frequency weighted mean of median bed load size decreases to unity with increasing drainage area bank full discharge dimensionless stream power and bed material sorting in channels with high values of d significant volumes of fine bed load are transported during discharges that are less than bank full which is commonly associated with general entrainment of the coarse pavement in many gravel bed channels this indicates transport of fine bed load over a more stable substrate of coarser bed material the apparent breakdown in equal mobility of the bed as a whole may be caused by areal segregation of poorly sorted bed material into superiorly sorted patches of varying mean size likely sources of selectively transported fine bed load include fine patches that have low entrainment thresholds and high virtual particle velocities a simple sediment budget applied to measurements from three channels indicates that velocities of material from fine patches in pools relative to velocities of average bed material are high in low order channels and decrease distally as more bed material that represents the bed as a whole is accessed for bed load by deeper annual scour

dynamic single thread channels maintained by the interaction of flow and vegetation most rivers on earth today flow as a single channel in some cases with occasional islands and follow a more or less sinuous course however single thread channels have proven difficult to reproduce and study experimentally experimental self formed channels tend to widen and subdivide leading to a braided pattern cohesive sediment has been the main mechanism studied for stabilizing banks and producing a single thread channel we show how laboratory experiments using vegetation to stabilize banks can organize the flow and convert the planform morphology from braided to single thread our experimental strategy a repeated cycle of short periods of high water discharge alternating with longer periods of low discharge accompanied by plant seeding and growth leads to the evolution of a dynamic self maintaining single thread channel with well defined banks and floodplain by eliminating weak flow paths the vegetation corrals the water into a single dominant channel until the reduction in total wetted width leads to a new self organized state in which the flow removes vegetated area as fast as it is produced the new channel is deeper and has a broader distribution of depths than the braided one with channel size adjusted to carry almost all the flood flow the resulting system maintains a dynamic steady state via similar mechanisms to those that operate in meandering channels in the field specifically erosion at the outside of bends bend growth and bar development our methodology provides a basis for experimental development of self sustaining high amplitude meanders and has applications for river management and basic research purposes

investigation of spatially distributed braided river flows using a two dimensional hydraulic model this paper reports the application of a two dimensional hydraulic model to a braided reach of the avoca river new zealand field measurements of water surface elevation depth and velocity obtained at low flow were used to validate the model and to optimize the parameterization of bed friction the main systematic trends in the measured flow variables are reproduced by the model however field data are characterized by greater spatial variability than model output reflecting differences in the scale of processes measured in the field and represented by the model additional model runs were conducted to simulate flow patterns within the study reach at five higher discharges the purpose of these simulations was to evaluate the potential for using two dimensional hydraulic models to quantify the reach scale hydraulic characteristics of braided rivers and their dependence on discharge changes in flow depth and velocity with increasing discharge exhibit trends that are consistent with the results of previous field investigations although the tendency for the wetted area of the braidplain within particular depth and velocity categories to remain fixed as discharge rises as has been noted for several braided rivers in new zealand was not observed modelled shear stress frequency distributions fit gamma functions that incorporate a distribution shape parameter the value of which follows clear systematic trends with rising discharge these results illustrate both the problems of and potential for using two dimensional hydraulic models in braided river applications this leads to something of a paradox in that while such models provide a means of generating hydraulic information that would be difficult to obtain in the field at an equivalent spatial resolution they are due to the problems inherent to data collection difficult to validate conclusively despite this limitation the application of spatially distributed models to investigate relationships between discharge and reach scale form and process variables appears to have considerable potential copyright 2003 john wiley sons ltd

reply to comment by cao and hu on long waves in erodible channels and morphodynamic influence 1 we thank cao and hu 2008 very much for their comment we must honestly say that at first sight we felt that their point reproducing that raised by cao and carling 2003 to challenge the formulation of the 1 d governing equations proposed by lisle et al 2001 had already been proven to be erroneous by cui et al 2005 however after more careful examination of the matter we have concluded that both the final forms of the continuity equation derived by cao and hu 2008 hereinafter referred to as ch and lanzoni et al 2006 hereinafter denoted as lsfs are not wholly correct indeed while the main point raised by ch is definitely correct they reach slightly incorrect conclusions employing a framework which ignores the distinction between bed load and suspended load on the other hand the lsfs derivation is also incorrect in that it will be seen to ignore the defect of water flux due to the presence of particles transported as bed load an effect which turns out to be of the same order of magnitude as that of the retained contributions this effect modifies the final form of the continuity equation for the fluid phase which does indeed include a sediment transport correction whose structure will be seen to be intermediate between the one derived by lsfs and that suggested by ch while as shown in section 4 this modification does not affect the treatment of one dimensional long sediment waves presented by lsfs the issue is of conceptual importance we are therefore grateful to ch for motivating us to provide a hopefully more conclusive clarification of the matter we are confident that the analysis presented below will be instructive for the reader as it was for us