Abstract The morphology of Earth’s surface reflects the interaction of climate, tectonics and denudational processes operating over a wide range of spatial and temporal scales. These processes can be considered catastrophic or continuous; depending on the timescale of observation or interest. Recent research had required integration of historically distinct subjects such as geomorphology, sedimentology, climatology and tectonics. Together, these have provided new insights into absolute and relative rates of denudation, and the factors that control the many dynamic processes involved. Specific subject areas covered are sediment transport processes and the timescales of competing processes, the role of the geological record and landscapes in constraining different processes, the nature of landscape evolution at different spatial scales and in contrasting geological environments.

Abstract Flood events within rivers are responsible for much erosion and deposition. Thus, deposits laid down during floods could potentially comprise the bulk of the Quaternary fluvial record. However, it is difficult to detect individual flood events, as effectively illustrated by the Middle Devensian (Weichselian) to Holocene fluvial sequence from the Nene Valley, Northamptonshire, described in this paper. This is due to limits in the resolution of sedimentological, palaeontological and geochronological techniques. Geochronological techniques have the highest resolution, but error bars of c . 50 years (radiocarbon) and up to 2 ka (optically stimulated luminescence) in the Late-glacial do not allow detection of floods lasting only a few weeks or less. Geochronology is, however, essential for linking periods of fluvial deposition to climatic phases at the marine isotope substage scale. Thus, multiple age determinations show remnant Middle Devensian deposits within a facies association mainly of Younger Dryas age, showing similar fluvial response to climate during both time periods. Palaeontological assemblages suggest that climate was also similar, although with some subtle differences. Determining ‘average’ fluvial activity in response to broad climate phases improves understanding of how rivers behave over long time periods, even though determination of the role of flood events in the Quaternary fluvial record remains elusive.

Abstract The fluvial history of the Mediterranean basin and the Near East includes depositional evidence for a latitudinally diachronous, locally bipartite, episode of aggradation by equable streams during the period AD 500–1900. According to the Leopold gullying model, the key requirement would have been an increase in the proportion of small, non-erosive rains. Theoretical considerations supported by general climate models suggest that a decrease in solar radiation at the UV wavelengths would lead to equatorward displacement of the subtropical jet streams and the associated mid-latitude depressions. Atmospheric Δ 14 C values show a gradual decline from c . 7000 bp followed by a temporary resurgence after AD 500 which includes peaks corresponding to the Oort, Wolf, Spörer, Maunder, Dalton and other solar minima. Reduced irradiance could account for channel aggradation by favouring cyclonic at the expense of convectional precipitation. Confirmation of a solar–fluvial link would benefit both solar history and flood forecasting.

Abstract Rates of landscape evolution and landform development depend on the capacity of the main transporting medium, predominantly water and the river system, to move material away from the site of production in the upland slopes. In upland areas the main river bed material is coarse gravels, with various admixtures of finer sand and silt. This paper reports a series of flume experiments to investigate the impact of admixtures of finer material on the entrainment of coarse particles. In all experiments the main framework of the bed was made up of quartz-density gravels with a mean particle size of 8 mm. In some of the experiments unimodal, 0.09 mm mean particle size, quartz sands were introduced upstream of the experimental section and transported into place in order to simulate a common condition in natural river beds, of sand migrating over a static gravel bed. New image analysis techniques were developed to extract data from video recordings of the experimental runs. These revealed important differences in entrainment processes among the experiments with a distinctive contrast between the clean gravel and sand–gravel runs. Observations suggest that the presence of sand increases the rates of gravel entrainment and leads to a distinctive patchiness in bed break-up which will encourage bed form development. In the mixtures, sand removal prior to gravel entrainment destabilizes the bed and allows large areas to become entrained. This contrasts with clean gravels where grains tend to entrain individually. These observations show the importance of bed material character in controlling river form and process and point to its role in controlling sediment flux through the landscape.

Abstract Geologists and geomorphologists have long been concerned with rates of sediment transfer as bedload in gravel-bed rivers, especially as rates of sediment transfer are important factors controlling river aggradation and incision. Bedload transport equations, originally derived for Holocene streams, have been used widely in modern gravel-bed river systems. However, palaeohydraulic reconstructions have received less attention and are generally dismissed as inaccurate since most are estimated to be at least an order of magnitude out. This study focuses on deriving stream power, bedload transport rates and efficiency estimates for Oligo–Miocene and Plio–Pleistocene gravel-bed river deposits from the south central Pyrenees, Spain. The basic data used in the palaeohydraulic calculations are estimates of palaeoslope, palaeovelocity, palaeodepth and the volume of sediment accreted in yearly flood events on gravel bars. Analyses of data from these ancient river systems yield more accurate estimates of relative stream power, bedload transport rates and efficiency parameters. This study illustrates the need for understanding the palaeohydraulics of river systems in order to characterize ancient rivers. Gravel-bed rivers with low sediment supply and high bedload transport rates incise. Conversely, when sediment supply is abundant, bedload transport rates and efficiency are low and the river system aggrades.

Abstract For over half a century the Pyrenees were considered to be a mountain range in which compressional structures were ancient (pre-Oligocene) but topography was young due to late Neogene tectonic uplift. Sufficient time had been afforded for a ‘peneplain’ to form at low elevations, undergo vertical uplift and remain partially preserved at high elevations until present times. This model of topographic growth has since been challenged by alternative theories. One of these postulates that topography in active orogens is in a steady-state, hence mountain ranges must be monocyclic and their ‘peneplains’ must have formed at high altitudes during continental convergence as a result of raised foreland base levels. Here we investigate Pyrenean denudation chronology using a range of evidence including provenance stratigraphy, the cross-cutting relations between topographic and tectonic features, and the age of regolith based on fossil faunas and floras. We find that the Eastern Pyrenees underwent a punctuated topographic evolution until recent times driven primarily by tectonic forcing, including kilometre-scale rock and surface uplift after 12 Ma. Climatic and eustatic inputs were subsidiary driving mechanisms.

Abstract From fifty 1:50,000 scale topographic maps of South Africa, the following attributes were recorded at every intersection of one minute longitude and latitude: basic land facet description, annual rainfall, rainfall concentration, rainfall seasonality, monthly rainfall, local relief, contour interval, rock type, vegetation, pediment length or interfluve width, stream order at valley bottom, local drainage pattern, altitude, terrain morphology, physiographic region and postulated age of planation surface as defined by King ( The Morphology of the Earth: a Study and Synthesis of World Scenery , Oliver & Boyd, 1962) and Partridge & Maud ( South African Journal of Geology , 90 , 179–208, 1987). A strong relationship was found between the propensity for concave slopes (or escarpments) where there is low annual rainfall and high local relief. The relationship with rock type was not so strong, but concave slopes are shown to be more likely on fine-textured rather than coarser-textured rock types. The relationship between drainage density and annual rainfall decreases in value from less than 200 mm annual rainfall up to 400–600 mm annual rainfall, and then increases above this value. Concave slopes are particularly prone in lightly vegetated areas, require some local relief, and are more common on easily eroded rocks and older land surfaces protected from recent drainage dissection. Sheetwash seems the most likely agent of erosion.

Abstract Far less is known of the processes involved in erosion of submarine channels compared with channels eroded subaerially by water runoff, but geometrical properties derived for canyons of the USA Atlantic continental slope reveal some intriguing similarities. Slope-confined canyons are concave-upwards, displaying decreasing channel gradient with increasing contributing area, as observed in many bedrock-eroding rivers. Tributaries join principal channels at the same elevation (without intervening waterfalls), in effect obeying Playfair's law, as do many river networks. Gradient and contributing area data for channels at confluences also reveal a tendency for tributaries to have steeper gradients than their associated principal channels, reflecting their smaller drainage areas. The concavities of bedrock-eroding rivers are often explained by a balance between river discharge, which increases with increasing rainfall catchment area, and gradient, which declines to offset the erosive effect of the discharge. It is unclear, however, if such a balance can be invoked for submarine canyons because erosion is probably caused when sedimentary flows are active only in individual canyon branches, originating from isolated slope failures. Instead, the frequency of sedimentary flows experienced by canyon floors may increase downstream simply because the area of unstable canyon walls available to source sedimentary flows increases, and this effect becomes compensated by declining gradient. Knickpoints created by faults in tectonically active slopes provide a further way to infer the form of erosion by sedimentary flows. Such knickpoints typically lie upstream of the faults that probably generate them, implying that detachment-limited erosion is enhanced where sedimentary flows become more vigorous on steep gradients, leading to knickpoint migration.

Abstract Debate about the relative roles of catastrophic v. continuous processes of landform evolution is as old as the discipline of Earth Science itself. Over the last 10 years or so, research in the Earth Sciences has focussed strongly on the Earth's surface and particularly in terms of quantifying rates of processes. This research parallels developments in geomorphology and sedimentology in the quantification of surface processes since the 1950s and 1960s. These surface processes are the manifestation of the large-scale interaction of climate and tectonics operating over a wide range of spatial and temporal scales. Thus, recent research had required integration of the historically distinct subjects of geomorphology, sedimentology, climatology and tectonics. Partly as a cause and partly as a consequence of this integration, there have been many recent developments in quantitative modelling and both laboratory and field-based analytical tools. Together, these have provided new insights into absolute and relative rates of denudation, and the factors that control the many dynamic processes involved. One of the outstanding issues concerns the balance between tectonics, climate and denudation, and in particular the limiting effects of one on the others and the nature of dynamic feedback mechanisms. The fact that processes can be considered catastrophic or continuous, depending on the timescale of observation or interest, can hinder the predictability of models, depending on how they are formulated. Certain conditions may lead to a steady-state situation in which denudation balances tectonic uplift, leading to a more or less constant topography. Steady-state topography means that detailed study

Abstract In regions undergoing active tectonics, the coupling between the tectonic displacement field, the overlying landscape and the redistribution of mass at the Earth's surface in the form of sediment routing systems, is particularly marked and variable. Coupling between deformation and surface processes takes place at a range of scales, from the whole orogen to individual extensional fault blocks or contractional anticlines. At the large scale, the attainment of a steady-state between the overlying topography and the prevailing tectonic conditions in active contractional orogens requires an efficient erosional system, with a time scale dependent on the vigour of the erosional system, generally in the range 10 6 –10 7 years. The catchment–fan systems associated with extensional fault blocks and basins of the western USA are valuable natural examples to study the coupling between tectonic deformation, landscape and sediment routing systems. Even relatively simple coupled systems such as an extensional fault block and its associated basin margin fans have a range of time scales in response to a tectonic perturbation. These response times originate from the development of uniform (steady-state) relief during the accumulation of displacement on a normal fault ( c . 10 6 years), the upstream propagation of a bedrock knickpoint in transverse catchments following a change in tectonic uplift rate ( c . 10 6 years), or the relaxation times of the integrated catchment–fan system in response to changes in climatic and tectonic boundary conditions (10 5 –10 6 years). The presence of extensive bedrock or alluvial piedmonts increases response times significantly. The sediment efflux of a mountain catchment is a boundary condition for far-field fluvial transport, but the fluvial system is much more than a simple transmitter of the sediment supply signal to a neighbouring depocentre. Fluvial systems appear to act as buffers to incoming sediment supply signals, with a diffusive time scale ( c . 10 5 –10 6 years) dependent on the length of the system and the extent of its floodplains, stream channels and proximal gravel fans. The vocabulary for explaining landscapes would benefit from a greater recognition of the importance of the repeat time and magnitude of perturbations in relation to the response and relaxation times of the landscape and its sediment routing systems. Landscapes are best differentiated as ‘buffered’ or ‘reactive’ depending on the ratio of the response time to the repeat time of the perturbation. Furthermore, landscapes may be regarded as ‘steady’ or ‘transient’ depending on the ratio of the response time to the time elapsed since the most recent change in boundary conditions. The response of tectonically and climatically perturbed landscapes has profound implications for the interpretation of stratigraphic architecture.

Abstract Equilibrium is a central concept in geomorphology. Despite the widespread use of the term, there is a great deal of variability in the ways equilibrium is portrayed and informs practice. Thus, there is confusion concerning the precise meanings and usage of the concept. In this chapter we draw on examples from dryland environments to investigate the practical implications of applying and testing the concept of equilibrium. Issues that we cover include the importance of scale and spatial variability, time, the assumption of constant environmental feedbacks and nonlinearities. The evaluation demonstrates that there are a range of problems inherent with using ideas of geomorphological equilibrium explicitly or implicitly to structure research in drylands. Many of these problems also apply to other environments.

Abstract The purpose of this article is to present a model of the formation processes of cockpit karst landscapes. The CHILD software was used to simulate landscape evolution including dissolution processes of carbonate rocks. After examining briefly how the CHILD model operates, two applications of this model involving dissolution of carbonate rocks are presented. The simulated landscapes are compared with real landscapes of the Cockpit Country, Jamaica, using morphometric criteria. The first application is based on the hypothesis that dissolution of carbonate rocks is isotropic over time and space. In this case, dissolution is constant throughout the whole area studied and for each time step. The simulated landscapes based on this hypothesis have morphometric features which are quite different from those of real landscapes. The second application considers that dissolution of carbonate rocks is anisotropic over time and space. In this case, it is necessary to take into account subsurface and underground processes, by coupling surface runoff and water infiltration into the fractured carbonates.

Abstract Flood effectiveness observations imply that two families of processes describe the formation of debris flow volume. One is related to the rainfall–erosion relationship, and can be seen as a gradual process, and one is related to additional geological/geotechnical events, those named hereafter extraordinary events. In order to discuss the hypothesis of coexistence of two modes of volume formation, some methodologies are applied. Firstly, classical approaches consisting in relating volume to catchments characteristics are considered. These approaches raise questions about the quality of the data rather than providing answers concerning the controlling processes. Secondly, we consider statistical approaches (cumulative number of events distribution and cluster analysis) and these suggest the possibility of having two distinct families of processes. However the quantitative evaluation of the threshold differs from the one that could be obtained from the first approach, but they all agree in the sense of the coexistence of two families of events. Thirdly, a conceptual model is built exploring how and why debris flow volume in alpine catchments changes with time. Depending on the initial condition (sediment production), the model shows that large debris flows (i.e. with important volume) are observed in the beginning period, before a steady-state is reached. During this second period debris flow volume such as is observed in the beginning period is not observed again. Integrating the results of the three approaches, two case studies are presented showing: (1) the possibility to observe in a catchment large volumes that will never happen again due to a drastic decrease in the sediment availability, supporting its difference from gradual erosion processes; (2) that following a rejuvenation of the sediment storage (by a rock avalanche) the magnitude–frequency relationship of a torrent can be differentiated into two phases, the beginning one with large and frequent debris flow and a later one with debris flow less intense and frequent, supporting the results of the conceptual model. Although the results obtained cannot identify a clear threshold between the two families of processes, they show that some debris flows can be seen as pulse of sediment differing from that expected from gradual erosion.