equilibrium disequilibrium and nonequilibrium landforms in the landscape abstract landscapes are composed of a diversity of landforms that may be characterized as equilibrium disequilibrium or nonequilibrium equilibrium is a constant relation between input and output or form toward which a landform tends or around which it fluctuates in time because of recent environmental changes and long relaxation times however many landforms are not adjusted to present inputs disequilibrium landforms are those that tend toward equilibrium but have not had sufficient time to reach such a condition some landforms called nonequilibrium do not tend toward equilibrium even with relatively long periods of environmental stability but rather undergo frequent and large change in form nonequilibrium may be caused by high magnitude thresholds that cause low frequency weather events to dominate form positive feedbacks and or nonlinearities that result in deterministic chaos landscapes contain mixtures of equilibrium disequilibrium and nonequilibrium landforms and these types of landforms interact in geomorphic systems such as sediment cascades the three types of behavior may coexist in the same landscape and the outputs of such mixed geomorphic systems reflect the combined effects of equilibrium disequilibrium and nonequilibrium subsystems this classification provides a framework for analysis and modeling of complex landscapes

geometry mechanisms and significance of extensional folds from examples in the rocky mountain basin and range province u s a abstract geologic mapping and structural analysis in paleogene half graben of idaho and montana have revealed over 70 extensional folds that form orthogonal sets with mostly ne and se trending axes on a regional scale they parallel or lie perpendicular to the strikes of the largest normal faults transverse folds at a high angle to the fault and folds oblique to the fault comprise more than half of the folds and reflect the highly three dimensional nature of the bulk strain detailed geometric analysis of 15 folds in the salmon horse prairie and medicine lodge basins shows that they typically have an upright to steeply inclined bisecting surface an interlimb angle of 141 and a plunge of 19 based on our synthesis of the eight common mechanisms of extensional folding and their distinguishing characteristics we were able to determine how some of the investigated folds formed fault bend folding above both fault parallel and fault perpendicular bends in the underlying normal fault produced most of the folds in the study area but displacement gradients along normal faults fault drag isostatic adjustments and transtension were also factors in the deformation most of the compound folds which result from more than one mechanism of folding are oblique to all adjacent normal faults recognition of such extensional folds is critical because they might be misinterpreted as contractional structures they influence sediment thickness patterns and dispersal in rift basins and may control the migration and trapping of petroleum and groundwater resources

interplay between mass movement and fluvial network organization an example from southern apennines italy abstract relationships between landslides and drainage network arrangement have been investigated by classical and quantitative geomorphological analysis in an intermountain valley i e the upper valley of the sinni river of the southern italian apennines the 120 km 2 wide test area is characterized by very low grade metamorphic rocks mainly shale and associated crystalline metamorphic rocks which underwent high exhumation and uplift rates during quaternary times we recognized and mapped 534 landslides through the stereoscopic interpretation of one set of 20 panchromatic aerial photographs 1 33 000 scale landslides were classified according to both type of movement and relative age we detected and mapped 35 new landslides after a long rainfall period during the winter 2008 2009 the percentage of area affected by landslides d l of the area has a high value about 37 analysis of distribution of landslides areas within different age classes suggests that recent present day landslide distribution is strongly influenced by the legacy of widespread large and ancient landslides probably generated within a different morpho climatic setting a quantitative geomorphological analysis of the drainage network has been performed to evaluate the influence of landslide processes on morphometric properties of the drainage network in particular the degree of hierarchical organization of the drainage network is assessed through the hierarchical anomaly density a measure of the number of anomalous confluences i e channels of u order not flowing in channels of order u 1 in drainage basins on the basis of statistical relationships between morphometric indices of the drainage network and d l we argue that the activity of more recent mass movements rather than very old landslides can act as a disturbing element within the development of the drainage network generating anomalous confluences of the 1st order empirical relationships between suspended sediment yield and some geomorphic parameters of the drainage network allowed the estimation of short term denudation rates in the study area the comparison between uplift and denudation rates suggests that the landscape system has not reached a steady state between uplift and erosion in which tectonic uplift can represent a disturbing element within the recent morphogenesis on the basis of the data presented large mass movements in the study area are mainly linked to the relief growth promoted by quaternary tectonics whereas litho structural and morpho climatic factors assume a minor role

controls on the strength of coupling among climate erosion and deformation in two sided frictional orogenic wedges at steady state 1 many important insights regarding the coupling among climate erosion and tectonics have come from numerical simulations using coupled tectonic and surface process models however analyses to date have left the strength of the coupling between climate and tectonics uncertain and many questions unanswered we present an approximate analytical solution for two sided orogenic wedges obeying a frictional rheology and in a condition of flux steady state that makes explicit the nature and sensitivity of the coupling between climate and deformation we make the simplifying assumption that the wedge grows in a self similar fashion consistent with airy isostasy such that topographic taper is invariant with orogen width tectonic influx rate and climate we illustrate first how and why the form of the erosion rule matters to orogen evolution and then derive a physically based orogen scale erosion rule we show that steady state orogen width crest elevation and crustal thickness are controlled by the ratio of accretionary flux to erosional efficiency to a power dictated by the erosion process remarkably we show that for most combinations of parameters in the erosion law rock uplift rate is more strongly controlled by erosional efficiency than it is by the accretionary flux further assuming frontal accretion with no underplating the spatial distribution of erosional efficiency dictates the relative rock uplift rates on the pro wedge and retro wedge and the time averaged trajectories of rocks through the orogen the restriction to invariant frictional properties is conservative in these respects systems subject to positive feedback between erosion and rheology will exhibit even stronger coupling among climate erosion and deformation than shown here

the control mechanisms of erosion and weathering at basin scale from cosmogenic nuclides in river sediment the study of a sample of river sediment enables the determination of spatially averaged denudation rates that provide an exceptional perspective on erosion and weathering processes that have taken place within a landscape these measurements are done with in situ produced cosmogenic nuclides e g 10be 26al mostly in quartz from alluvial sediment cosmogenic nuclides are produced when secondary cosmic rays interact with the very uppermost layer of the earth s surface they are produced within a characteristic depth scale of about 1 m which means that the measured concentrations record an integrated denudation history while material passed through this depth interval depending on the denudation rate the resulting integration time scales are 103 to 105 years and one obtains a robust long term estimate of natural denudation that is relatively insensitive to short term changes the last 10 years have seen significant research activity using these methods and an array of fascinating tectono geomorphologic and geochemical insights are emerging amongst these is the ability to identify the physical and chemical processes with which a landscape responds to tectonic activity or climate change a compilation of world wide denudation rates in non glaciated areas that however does not yet include some of the world s most active mountain belts has resulted in the following findings some of which have been unexpected 1 no obvious relationship between precipitation or mean annual temperature and total denudation is apparent 2 topographic relief alone does not result in high rates of denudation 3 denudation rates are high in areas of landscape rejuvenation that is triggered and controlled by tectonic activity faulting escarpment formation and retreat rifting surface uplift 4 rates of weathering using a combination of cosmogenic nuclides and zirconium normalised cation loss balances co vary primarily with physical erosion rates and much less with temperature or precipitation 5 in some areas of high land use short term rates from river load gauging exceed those from cosmogenic nuclides by several orders of magnitude which serves to highlight the severity of geomorphic change caused by human action in the future the control mechanisms over denudation will be determined on all spatial scales ranging from the single soil section to entire river basins the same analysis can be done back through time on well dated terraces lake records and marine sediment cores which is possible with 10be for the past 1 2 my the rates obtained will be used to develop a quantitative understanding of tectonic geomorphologic and geochemical landscape processes which in turn is a prerequisite to design and calibrate models of the response of landscapes to tectonic climate and anthropogenic forcing

mechanical stratigraphy and the transition from trishear to kink band fault propagation fold forms above blind basement thrust faults a discrete element study abstract we employ a discrete element technique to investigate the influence of mechanical stratigraphy on the transition from trishear to kink band fault propagation fold forms above a blind basement thrust fault we find that a classic trishear fold consisting of a broad upwards widening monocline with limb dips increasing towards the fault tip occurs where the cover stratigraphy is weak and has no strong mechanical layering in contrast we find that marked bedding parallel mechanical layering in the cover promotes much narrower kink like fold geometries with more constant limb widths and dips we discuss the modelling results in terms of their implications for structural interpretation and the application of kinematic models of fault related folding in contractional settings

the morphology of graded gravel rivers a network perspective analytical hydraulic geometry equations are combined with a watershed model to quantify downstrem trends in equilibrium fluvial morphology in gravel streams the hydraulic geometry equations relaet fluvial morphology to discharge bedload transport rate and grain size these independent variables are in turn determined by routing gravel supplied at zero order basins through a channel network as gravel is routed downstream it decreases in size according to an empirical power law the resulting model specifies the discharge bedload transport rate grain size width depth slope and river bed elevation at every point in the drainage basin when the downstream fining law is calibrated using data from central pennsylvania dowstream hydraulic geometry exponents for the width and depth are accurately reproduced longitudinal profiles calculated from the model fit observed profiles in largerwatersheds but are inaccurate in smaller watersheds the model also reproduces hack s 1957 empirical result relating slope to the 0 6 power of a ds where a is the drainage basin area and ds is the mean grain size unfortunately the model requires an equilibrium time scale which is unreasonably long suggesting that the channels of the study area are best described as systems with multiple time scales and multiple rates of response the model probably is successful because the width and depth have relatively short relaxation times and also because downstream trends in discharge and sediment transport rate are controlled by network topology thus downstream hydraulic geometry equations do not necessarily provide strong evidence for the equilibrium of fluvial landscapes rather they reflect the constancy of network topology and the rapid response times of width and depth

on steady states in mountain belts the dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state this concept is often invoked but the nature of the steady state is commonly not specified we identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical model results and natural examples these types are 1 flux steady state 2 topographic steady state 3 thermal steady state and 4 exhumational steady state they refer to the erosional flux the topography the subsurface temperature field and the spatial pattern of cooling ages respectively models suggest that the topography will reach a steady mean form at the scale of an orogenic belt but perfect topographic steady state is unlikely to be achieved at shorter length scales thermal steady state is a precondition for exhumational steady state and in the case of temperature dependent deformation topographic steady state exhumational steady state is characterized by reset age zones spatially nested according to closure temperature as illustrated in natural systems from new zealand the cascadia accretionary margin and taiwan

ecogeomorphology in the australian drylands and the role of biota in mediating the effects of climate change on landscape processes and evolution

soil and water components of banded vegetation patterns abstract banded landscapes are comprised of alternating bands of vegetation and bare ground aligned along the contours in arid and semi arid regions 50 750 mm rainfall on very gentle and uniform slopes 0 2 2 vegetated bands can be perpendicular to the direction of the dominant wind or more frequently of the slope under given climatic conditions slope gradient is the controlling factor of the type of pattern spotted broadly or typically banded for a given slope gradient mean annual rainfall determines the contrast between the vegetated and bare phase as well as the band length and the interband width band width ratio a typical transect through such two phase mosaic includes a dynamic succession of surface crusts which generates a run off run on system this favours the capture of limited water resources and thus a biomass production greater for banded patterns than spotted patterns or uniformly scattered vegetation moreover vegetated bands act as natural bench structures that greatly limit soil erosion as suggested by modelling banded vegetation patterns can equally be derived from nearly bare areas or from dense vegetation patterns because banded vegetation patterns have often been considered as a form of degradation of previous continuous vegetation cover many attempts have been made by foresters to restore this initial cover by reforestation of the bare interbands however the numerous failures of reforestation of the bare interbands illustrate the key role of these mosaic components in the maintenance of the tiger bush ecosystem banded vegetation patterns are more resilient to climate change than to human disturbances the loss of landscape patchiness due to the clearing of the bands either for fuelwood or for ephemeral cropping induces a rapid decline in soil fertility and water infiltration potential long term monitoring shows that the interband width band width ratio adapts to rainfall variations that are intrinsic of semi arid zones moreover climatic changes might also induce a change in the type of patterning suggesting that banded vegetation patterns are highly self sustainable the ecological differences between the pioneer and the decaying edges of the vegetated bands the temporal spatial succession of soil crusts as well as other indices such as the spatial distribution of soil organic matter and termite nests suggest the upslope migration of the bands nevertheless field evidence of such a shift remains scarce profound lessons can be learnt from the study of banded landscapes in terms of ecosystems functions when designing water harvesting systems or restoring degraded arid and semi arid land banded vegetation patterns should be imitated to optimize biomass production and limit land degradation

controls on flash flood magnitude and hydrograph shape upper blue hills badlands utah although many arid badland channels erode rapidly channel flows appear to be rare raising the issue of the character and frequency of flows responsible for channel erosion using an acoustic stream gauge we recorded nine flash flood hydrographs over a 3 yr period from the upper blue hills badlands utah with maximum discharges up to 9 m3 s flow hydrographs reveal bores and rapid depth changes that are similar to flash floods observed elsewhere bore and hydrograph peak translation velocities are greatest in narrow channel segments rapid runoff generation during short duration thunderstorms produced complex hydrographs whose shapes appear to reflect channel network geometry storm runoff response is highly sensitive to antecedent moisture which greatly reduces the regolith infiltration capacity high antecedent moisture coupled with a relatively low intensity long duration rainstorm produced the largest flow event estimating flow frequency in this landscape therefore requires knowledge of the distributions of both storm sizes and temporal spacing relative to the short time required for the regolith infiltration capacity to recover following wetting here roughly 24 hr landscape changes can be produced not only by rare large rainfall events but by a broad range of storm size and frequency under optimal antecedent moisture conditions

energy time and channel evolution in catastrophically disturbed fluvial systems abstract two diverse fluvial systems show that with time channels adjust such that the rate of energy dissipation is minimized one fluvial system characterized by high relief and coarse grained sediment was subjected to an explosive volcanic eruption the other system characterized by low relief and fine grained sediment was subjected to dredging and straightening study of the expenditure of kinetic and potential energy components of total mechanical energy provide an energy based rationale of the interdependency between processes and forms during channel evolution spatial and temporal trends of aggradation and degradation are similar although relative amounts of aggradation in the high energy system are greatly enhanced by the deposition of large amounts of eroded bank material from upstream reaches degradation accompanied by widening is the most efficient means of energy dissipation because all components of total mechanical energy decrease with time widening dominates energy dissipation in the coarse grained system to offset increases in hydraulic depth caused by incision in the low energy fine grained system channel adjustment and energy dissipation are dominated by vertical processes because of low relative values of kinetic energy and because eroded bank sediment is transported out of the drainage basin and does not aid in downstream aggradation energy dissipation or channel recovery specific energy is shown to decrease nonlinearly with time during channel evolution and provides a measure of reductions in available energy at the channel bed data from two sites show convergence towards a minimum specific energy with time time dependent reductions in specific energy at a point act in concert with minimization of the rate of energy dissipation over a reach during channel evolution as the fluvial systems adjust to a new equilibrium

equilibrium and time scales in geomorphology application to sand bed alluvial streams equilibrium is defined as a single valued temporally invariant functional relationship between the values of an output variable and the values of the input variable s in a geomorphic system disequilibrium occurs if the output deviates from the functional relationship by more than a consensual degree natural geomorphic variables are characterized by a relaxation time output variables are insensitive to cyclical inputs with frequencies much greater than the relaxation time but can respond completely for sufficiently low frequencies rapid trends recent step changes or pulse inputs and intermediate frequency inputs can cause disequilibrium the gradient of sand bed alluvial channels the output variable is determined by sediment and water delivery from slopes the input variables and changes in this hydraulic regime require regrading by erosion and deposition initial stages of adjustment to changed regime in a long unbranched channel with sediment and water delivery only at the upstream end propagate downstream but later stages of adjustment occur simultaneously throughout the reach in a dendritic channel network the gradient responds rather uniformly throughout the network to changes in regime during all stages of adjustment the time scale of adjustment to changed regime depends upon the size of the channel network or stream length the sediment and water discharges and to a lesser degree upon the magnitude of the change grade as defined by mackin 1948 is synonymous with equilibrium as used in this paper if a period of years is replaced by a time period commensurate with the relaxation time of the gradient the use of the term grade is best restricted to a single valued relationship between channel gradient and the hydraulic regime

decoupling of erosion and precipitation in the himalayas the hypothesis that abrupt spatial gradients in erosion can cause high strain rates in active orogens has been supported by numerical models that couple erosional processes with lithospheric deformation via gravitational feedbacks1 2 3 most such models invoke a stream power rule in which either increased discharge or steeper channel slopes cause higher erosion rates spatial variations in precipitation and slopes are therefore predicted to correlate with gradients in both erosion rates and crustal strain here we combine observations from a meteorological network across the greater himalaya nepal along with estimates of erosion rates at geologic timescales greater than 100 000 yr from low temperature thermochronometry across a zone of about 20 km length spanning the himalayan crest and encompassing a more than fivefold difference in monsoon precipitation significant spatial variations in geologic erosion rates are not detectable decreased rainfall is not balanced by steeper channels instead additional factors that influence river incision rates such as channel width and sediment concentrations must compensate for decreasing precipitation overall spatially constant erosion is a response to uniform upward tectonic transport of greater himalayan rock above a crustal ramp

geomorphology as science the role of theory abstract because geomorphology is a science it is permeated by theory overt recognition of this actuality is frequently resisted by geomorphologists earth history does not represent an alternative to earth science it is an essential component of earth science in its broadest sense science seeks to discover new knowledge through a two stage activity involving the creation and justification of ideas theory deduction is generally regarded as the only logically consistent method of justifying ideas the creation of ideas is a much more controversial topic some methodologists deem it beyond logic certainly deductive arguments which are nothing more than formal logical expressions of theory play no role in the conception of new ideas many earth scientists generate possible explanations of observed phenomena based on abductive reasoning others advocate reliance on purported forms of pure induction such as serendipity and intuition in which observations assume primacy over theory besides lacking consistency and educability the latter posture is flawed because it mistakenly implies that becoming well versed in theory is irrelevant to or impedes scientific discovery irrational or subjective factors play a role in the creation of ideas but it is erroneous to claim that these factors are divorced from theory science is first and foremost a cognitive activity thus the primacy of observations in science is a myth all observations are theory laden in the sense that the act of observation inherently involves interpretation and classification both of which can only occur within the context of theoretical preconceptions even discoveries based on unexpected observations require the fortunate investigator to recognize the theoretical importance of what is seen or measured the most useful view of geomorphology as a science is one in which theory is seen as central but fragile and in which theory and observation are viewed symbiotically with theory providing the generative force and observation providing a vital policing role much of the current debate in geomorphology centers around differences in characteristics of theory type of scientific arguments and metaphysical perspectives among investigators working at different temporal scales full recognition and understanding of these differences are essential for developing a unified approach to the science of geomorphology

some comments on equilibrium concepts and geomorphic systems abstract equilibrium is commonly used to describe geomorphic systems that can adjust to changes by reaching a steady state as a conceptual framework equilibrium emphasizes the relation between present form and process systems analysis is a conceptual framework that can be used to quantify and relate material or energy flows that comprise a geomorphic system despite the apparent similarity between the language of systems analysis and the descriptive examples of geomorphic equilibrium these two conceptual frameworks are divergent in many respects from the vantage point of a systems approach in geomorphology many types of equilibrium purported to describe geomorphic systems such as dynamic equilibrium and graded equilibrium are seen to be very different from one another and in turn very different from the kinds of equilibrium resulting from systems analysis there are several differences between concepts of equilibrium that serve to illuminate the workings of natural systems in a way that were undefined within each framework additionally several important characteristics of geomorphic systems are analytically invisible using equilibrium concepts such as spatial self organization scale invariance and inherent instability time used quite intuitively in all non systems definitions of equilibrium is an important source of insight to the behavior of geomorphic systems in systems analysis time is defined by the process under consideration and is not defined a priori as it is with equilibrium concepts whereas time consists of a single duration for equilibrium concepts it must consist of a duration and a smaller interval over which the process is realized for the systems approach iterative processes whose affects can be different depending on their sequence introduce a degree of complexity into the analysis of geomorphic systems that although long recognized has been unmanageable because of a predisposition to the use of average rates

probabilistic landslide hazard assessment at the basin scale abstract we propose a probabilistic model to determine landslide hazard at the basin scale the model predicts where landslides will occur how frequently they will occur and how large they will be we test the model in the staffora river basin in the northern apennines italy for the study area we prepare a multi temporal inventory map through the interpretation of multiple sets of aerial photographs taken between 1955 and 1999 we partition the basin into 2243 geo morpho hydrological units and obtain the probability of spatial occurrence of landslides by discriminant analysis of thematic variables including morphological lithological structural and land use for each mapping unit we obtain the landslide recurrence by dividing the total number of landslide events inventoried in the unit by the time span of the investigated period assuming that landslide recurrence will remain the same in the future and adopting a poisson probability model we determine the exceedance probability of having one or more landslides in each mapping unit for different periods we obtain the probability of landslide size by analysing the frequency area statistics of landslides obtained from the multi temporal inventory map assuming independence we obtain a quantitative estimate of landslide hazard for each mapping unit as the joint probability of landslide size of landslide temporal occurrence and of landslide spatial occurrence

emergence and pseudo equilibrium in geomorphology abstract many geomorphic system states and behaviors often interpreted as tendencies toward establishment and maintenance of steady state equilibrium are actually emergent outcomes of two simple principles gradient selection and threshold mediated modulation the principle of gradient selection is simply that geomorphic features associated with gradient driven flows persist and grow relative to other features and pathways the principle of threshold mediated modulation reflects the inherent limits on system development along any particular pathway thresholds not only define a restricted state space for any given geomorphic system but may also result in oscillatory behavior around an intermediate condition that resembles fluctuations around a steady state equilibrium together these principles often produce outcomes that mimic steady state equilibria examples are given involving several aspects of fluvial systems channel profiles alluvial channel changes and drainage density hillslope gradients and barchan dunes steady states indeed occur in earth surface systems and are a useful simplification in some models however the assumption that such states are somehow normative or the only natural condition is incorrect and can lead to problems in geomorphic interpretations environmental restoration and management and conceptions of how earth systems work

the initiation of northern hemisphere glaciation for well over a century scientists have speculated that variations in the atmospheric concentration of radiatively important trace gases such as carbon dioxide could control the earth s climate e g arrhenius 1 896 recent observations of significant glacial interglacial variation in atmo spheric co2 in ice cores put this idea on a firm observational as well as theoretical footing e g barnola et ai1 987 in this century as atmospheric co2 levels creep steadily upwards in response to the burning of fossil fuels and forests the specter of manmade global climate change looms before us ipcc 1990 a possibility that has heightened interest in the natural variability of climate in the past in particular the 3 c warming pre dicted for the next century has stimulated interest in past warm climates such as the middle pliocene around 3 0 million years ma ago an interval that has often been invoked as a greenhouse world budyko et al 1985 in this paper the climate transition from the warm mid pliocene around 3 2 ma to the onset of northern hemisphere ice ages around 2 4 ma is examined evidence for the initiation of significant northern hemisphere glaciation is examined as well as how this event affected climate around the globe while the cause of individual glacial interglacial oscillations is tied to milankovitch variations in the earth s orbit around the sun e g imbrie et ai 1 992 these insolation changes cannot account for the long term cooling trend which culminated in northern hemisphere glaciation in the final section of this paper mechanisms of long term climate change

coupling between hillslopes and channels in upland fluvial systems implications for landscape sensitivity illustrated from the howgill fells northwest england abstract the sensitivity of upland fluvial systems depends on the magnitude and frequency of sediment and flood producing events modified by the internal coupling characteristics of the system this paper assesses the role of hillslope channel coupling for the sensitivity of upland geomorphic systems using evidence from a 30 year monitoring programme of geomorphic change in the carlingill valley howgill fells northwest england in the hillslope zone there is little sediment supply to the stream system locally slope failures occur in response to extreme events six such events in 30 years within the ca 6 km 2 carlingill valley in the footslope coupling zone basally induced gullies are major sediment sources to the stream sediment production events occur ca 30 times per year feeding sediment to basal debris cones stream floods which can entrain these sediments occur once every ca 2 5 years stream channel morphology is adjusted to this regime downstream of gullies channels are wide and braided elsewhere they are narrow and single thread as the gullies develop however this coupling weakens and the eroding slopes eventually stabilise by revegetation over the 30 year monitoring period there has been a progressive trend towards gully stabilisation and an associated reduction in channel instability massive destabilisation may occur in response to rare extreme flood events such an event return period 100 years occurred in neighbouring langdale and bowderdale valleys in june 1982 that event destabilised the system causing slope failures fan deposition and in some places a switch in channel style to wide braided channels since 1982 there has been a progressive recovery by slope stabilisation and single thread sinuous channels have become reestablished a different style of extreme event occurred in carlingill in october 1998 in response to the wettest week in the 30 year period a slope failure fed debris flows 400 m downslope almost coupling with the channel system future climatic change could render the system prone to destabilisation through either extreme flood events or through major slope failures

three dimensional measurement of river channel flow processes using acoustic doppler velocimetry this paper describes and assesses i the use of a new instrument for the determination of three dimensional flow velocities in natural rivers the acoustic doppler velocimeter adv and ii a method for positioning and orienting such measurements relative to a single local coordinate system to relate flow velocity vectors with the bed and water surface the adv uses the doppler shift principle to measure the velocity of small particles assuming to be moving at velocities similar to the fluid velocity is resolved into three orthogonal components and measured in a volume 5 cm below the sensor head minimizing interference of the flow field and allowing measurements to be made close to the bed a simple method for positioning and orienting the instrument using digital tacheometry is described and is used to obtain velocity measurements concurrently with measurements of both bed and water surface topography the paper includes a preliminary field assessment of the adv by comparing velocity profiles with those generated from marsh mcbirney electromagnetic current meters and a full field assessment of the position and orientation methodology these results suggest that the recommended methods in combination with an adv are able to provide reliable mean three dimensional velocity field information and accurate bed and surface topography copyright 1998 john wiley sons ltd

simulation of the frictional stick slip instability a lattice solid model capable of simulating rock friction fracture and the associated seismic wave radiation is developed in order to study the origin of the stick slip instability that is responsible for earthquakes the model consists of a lattice of interacting particles in order to study the effect of surface roughness on the frictional behavior of elastic blocks being rubbed past one another the simplest possible particle interactions were specified corresponding to radially dependent elastic brittle bonds the model material can therefore be considered as round elastic grains with negligible friction between their surfaces although breaking of the bonds can occur fracturing energy is not considered stick slip behavior is observed in a numerical experiment involving 2d blocks with rough surfaces being rubbed past one another at a constant rate slip is initiated when two interlocking asperities push past one another exciting a slip pulse the pulse fronts propagate with speeds ranging from the rayleigh wave speed up to a value between the shear and compressional wave speeds in agreement with field observations and theoretical analyses of mode ii rupture slip rates are comparable to seismic rates in the initial part of one slip pulse whose front propagates at the rayleigh wave speed however the slip rate is an order of magnitude higher in the main part of pulses possibly because of the simplified model description that neglected intrinsic friction and the high rates at which the blocks were driven or alternatively uncertainty in slip rates obtained through the inversion of seismograms particle trajectories during slip have motions normal to the fault indicating that the fault surfaces jump apart during the passage of the slip pulse normal motion is expected as the asperities on the two surfaces ride over one another the form of the particle trajectories is similar to those observed in stick slip experiments involving foam rubber blocks bruneet al 1993 additional work is required to determine whether the slip pulses relate to the interface waves proposed by brune and co workers to explain the heat flow paradox and whether they are capable of inducing a significant local reduction in the normal stress it is hoped that the progressive development of the lattice solid model will lead to realistic simulations of earthquake dynamics and ultimately provide clues as to whether or not earthquakes are predictable

equilibrium scale and inheritance in geomorphology abstract of the many equilibrium terms that have been proposed only two are needed dynamic equilibrium in the original sense of g k gilbert 1877 and steady state the former refers to the relationship between the process components of a system the latter to the system as a whole the tendency towards establishing a dynamic equilibrium is inherent in all natural systems that contain negative feedbacks between processes capable of compensating each other in geomorphology equilibrium tendencies of the mass budget are more relevant than those of the energy budget the validity of the equilibrium concept is discussed at several scales from a slope point to entire slopes drainage basins and mountain ranges the effects of eksystemic changes vary depending upon the length of the intervals between such changes in comparison to the length of the required relaxation times this relationship also applies to the influence of inherited forms patterns and materials that have become components of present day systems

free air gravity over the hoggar massif northwest africa evidence for alteration of the lithosphere abstract analysis of the free air gravity field over the broadly elevated hoggar region in northwest africa suggests that the uplift is similar in origin to the midplate hotspot swells observed on the seafloor the hoggar massif is a dome of precambrian basement approximately 1 km high 1000 km wide and capped by late tertiary alkali basalt volcanos gravity measurements reveal a broad free air high with maximum amplitude of approximately 40 mgal which is coincident with the areal extent of the hoggar using the reasonable assumption that the hoggar is in local isostatic balance calculations indicate that the amplitude of the gravity anomaly is best explained if the isostatic root of the elevated area is 60 km below ground surface this root depth is similar to that observed for both the hawaiian swell in the pacific and the bermuda rise in the atlantic and is shallower than the expected base of the lithosphere the gravity data suggest that the lithosphere beneath the hoggar has been reduced in density perhaps by reheating as inferred for oceanic swells

nonlinear dynamical systems in geomorphology revolution or evolution abstract geomorphic systems typically exhibit complex apparently random behaviors and patterns in both spatial and temporal domains this complexity can arise from the cumulative impacts of individual process response mechanisms which are far too numerous to be accounted for in individual detail or due to multiple controls over process response relationships which operate over a range of spatial and temporal scales stochastic complexity nonlinear dynamical systems nds theory which includes specific techniques and concepts such as chaos dissipative structures bifurcation and catastrophe theory and fractals shows that extreme complexity can also arise due to the nonlinear dynamics and couplings of relatively simple systems represented by relatively small equation systems deterministic complexity while emphasis in earth science has been largely on stochastic complexity geomorphologists have long recognized the presence of deterministic complexity as well recent developments in the study of nonlinear dynamical systems have implications for the ability to make reliable long term predictions and therefore applications of nds in geomorphology may be revolutionary to some extent however virtually all the basic conceptual underpinnings of nds theory can be mapped onto existing and even traditional theoretical concepts in geomorphology in this sense the use of nds concepts in understanding earth surface processes and landforms is evolutionary the link between traditional and nds based concepts is illustrated by analyzing a generalized geomorphic mass flux system it is shown that the system is in most cases unstable consistent with chaos or deterministic complexity however the three attractor states of the system correspond exactly to the states of aggradation degradation and steady state

differences in hillslope runoff and sediment transport rates within two semi arid catchments in southeast spain this paper focuses on hillslope runoff and sediment transport within two catchments in southeast spain five monitoring sites were established on hillslope concavities throughout the two catchments the techniques used were mini crest stage recorders spray painted lines sediment traps and tipping bucket rain gauges established during previous research results show that a storm event in the rambla nogalte on 30 june 2002 of 83 0 mm was responsible for a maximum runoff depth of 12 cm and a maximum hillslope sediment transport of 1886 cm3 m 1 the same storm in the rambla de torrealvilla produced 53 4 mm of rainfall on the 1 july 2002 had a maximum runoff depth of 26 cm and resulted in 2311 cm3 m 1 of sediment transport there is evidence to suggest that measured sediment transport is related to runoff and a qualitative estimate of morphological runoff zones mrz sediment transport and depth of runoff varied dramatically with lithology marl sites produced most runoff and sediment transport followed by the sites of mixed red and blue schist then blue schist these results are important for understanding the behaviour of slopes and show that for short lived storms with high but not rare rainfall intensities and total rainfall amounts runoff can cause significant hillslope sediment transport in semi arid areas

holocene paleofloods of the ross river central australia abstract the rivers of central australia rise in the macdonnell ranges and flow out across broad low relief plains into the surrounding desert the stratigraphy of the ross river plain records the areal extent and frequency of holocene floods this floodout plain is underlain by deeply weathered alluvial deposits characterized by red earth soils dated by thermoluminesence at 59 000 yr this old alluvium is covered by a sheet like deposit of very silty sand of probable eolian origin dated by thermoluminesence at 9200 900 yr the oldest holocene alluvium occurs as broad low relief bars and levee deposits flanking the modem channel and as low relief long wavelength bedforms that fan out across the plain this deposit resulted from a flood flow up to 10 km wide that covered the entire plain evidence for several large floods between 1500 and 700 yr b p is also preserved in a 500 to 1500 m wide paleochannel thus the surface features on the floodout plains are the product of a few rare large flood events this paleohydrologic record is additional evidence of the dynamic nature of the hydrometerological regime of central australia

experimental models of extensional forced folds we have used single layer and multilayer clay models to study the development of forced folds above normal faults our modeling results show that the deformation patterns associated with extensional forced folding depend on the dip of the underlying normal fault and the presence of layer parallel detachments in single layer clay models extensional forced folds are upward widening monoclines anticlinal axial surfaces dip in the same direction as underlying master normal faults and synclinal axial surfaces dip in the opposite direction of master normal faults most secondary faults are upward steepening normal faults if master normal faults are steeply dipping however many secondary normal faults become high angle reverse faults at shallow depths the propagation and linkage of secondary faults into through going normal faults terminates the development of extensional forced folds more folding occurs prior to fault linkage if the master normal fault is steeply dipping rather than gently dipping most dipping beds and secondary faults are preserved in the hanging walls of the through oing normal faults in multilayer clay models with layer parallel detachments extensional forced folds are also upward widening monoclines slip on the lowest detachment laterally transfers extension induced by normal faulting and forced folding from the master normal fault to the detachment edge slip on overlying detachments accommodates minor thickness changes associated with upward widening of the fold secondary faults include low angle normal faults near the anticlinal axial surface minor thrust faults near the synclinal axial surface and high angle normal faults above the detachment edge the model predicted deformation patterns are similar to those of extensional forced folds from the gulf of suez and offshore norway this similarity suggests that our modeling results apply to extensional forced folds and can provide guidelines for interpreting field well and seismic data

process interactions temporal scales and the development of hillslope gully systems howgill fells northwest england abstract hillslope gully development is often controlled by interactions between multiple processes rather than by individual processes such interactions may have both spatial and temporal characteristics a twenty year monitoring program of hillslope gullying in the howgill fells northwest england has identified the roles of both on slope interactions and slope stream coupling interactions which can be related to several temporal scales on slope interactions are primarily seasonal whereas those related to coupling produce a cyclicity over timescales of up to several years over the longer term decades or more progressive changes in gully morphology modify the nature of both sets of interactions and through negative feedback mechanisms may ultimately lead to gully stabilisation gully development is controlled by the rates of headwall recession and the basal incision stabilisation behaviour after basal stabilisation vegetation encroachment onto the eroding slopes takes place faster than headwall retreat resulting in a finite age and limiting size for gully development a simple model based on the current rates of development adequately describes the morphology of modern gullies but is inappropriate when applied to an older generation of stabilised gullies this suggests that there have been changes in sediment generation and removal rates during the late holocene

flood related channel change in an arid region river a review of 112 years of change in the channel of the salt river central arizona u s a shows that this arid region river has a main flow channel that has migrated laterally up to 1 6 km 1 mi in response to floor events maps showing locational probabilities indicate that along the channel zones of relative locational stability alternate with zones of relative instability at a 3 2 km 2 mi interval construction of upstream reservoirs has reduced sediment input into the main river but has not controlled floods the channel width has not changed except for moderate fluctuations around mean values the main flow channel has incised approximately 6 m 20 ft over most of the 48 km 30 mi study reach during six recent floods gradient has remained unchanged during floods bed material was mobilized to a depth below the original bed level that was greater than the height of the water surface above the original bed calculations based on tractive force indicate a threshold discharge of instability that is equal to the flow with a five year return interval the river exhibits remarkable stability with respect to gradient and sinuosity irrespective of water and sediment discharges but horizontal channel location exhibited selective instability over the record period of more than a century the channel appears not to have been in equilibrium considering geometry discharge and sediment