Search

Tributary-junction alluvial fans situated at the intersection of confined valleys with 2 tributary catchments are of special interest to evaluate the heterogeneous consequences of extreme rainfall events in arid zones. These fans record the episodic sedimentological behaviour of the hillslope response to rainstorm events within tributary catchments, together with the influence on the main fluvial systems. In this paper, we benefit from the March 2015 event (23–26 March 2015), which produced 75–46 mm of precipitation over four days in the southern portion of the Atacama Desert. This storm event triggered several debris flows in El Huasco River watershed tributaries and, therefore, tributary-junction alluvial fans received a total of ∼106 m3 of sediments across 49 activated catchments. We find that the characteristic storm signature across the catchments can be synthetised in a conceptual fan formation model based on field mapping of facies (F1 to F6) present in the fans. The characteristic signature is a record of initially high sediment-to-water flows restricted to the fan environments (mainly debris flows) followed by later, more dilute (mainly hyper-concentrated to fluvial) flows that incise the tributary-junction alluvial fan deposits and link tributary catchments with the main river. These later-stage flood event deposits, locally, are capable of ponding and compartmentalising the main river where the longitudinal connectivity of the tributary-junction catchment is effective. This situation improves tributary-junction fan slope and main-trunk-channel linkages. This approach provides a reference framework for understanding the distribution and routing of effective runoff from similar rainfall events that control the aggradation and incision of the fluvial system, which is of great value when studying past stratigraphic arrangements in these arid alleys.

In the small fraction of Earth's surface with the highest erosion rates such as the Alps and Himalayas quantifying rates of incision, rock uplift and inferring climatic controls on the landscape can be relatively straightforward once the ages of river terraces cut in bedrock (strath terraces) are constrained. However, in many mid to lower relief settings that are more typical of mountain belts worldwide, periods of net river incision and riverbed lowering are relatively short (punctuated), interrupted by long periods of sediment aggradation or stasis. We define a conceptual model of punctuated river incision and strath terrace formation for the calculation of incision and rock uplift rates, and recommend strategies for geochronological sampling and interpretation. An approach using OSL dating of terrace gravels allows us to constrain a detailed ~150 kyr history of punctuated river incision and strath terrace formation spanning two stratigraphic landform levels in the High Atlas Mountains (NW Africa). Extensive preservation and exposure of strath-top gravels, within a post-orogenic setting unaffected by eustatic influences, enables the derivation of rates of base-level fall, integrated over periods of strath-top aggradation and incision, that are consistent with independently constrained regional rock uplift rates. Combining a punctuated river incision model with our well-constrained terrace formation history allows us to demonstrate how assumptions concerning Quaternary river incision and aggradation can lead to the problematic Sadler Effect, an apparent dependence of incision rates on measured time interval. Subsequently, we demonstrate that an approach to reinterpreting previously published data using the punctuated incision model, even when combined with limited terrace age data, results in more consistent conclusions about rates of river incision, rock uplift and base-level lowering across the mountain belt. Our recommendations for sampling strategies to constrain rock uplift rates require samples to be taken just above the strath surface, and in addition towards the top of the deposit for river incision rates. In a setting with punctuated river incision and strath terrace formation, both rock uplift and incision rates require burial dates, as exclusive use of abandonment ages will not yield constraints on accurate rates of rock uplift or incision. Furthermore, we find that only with multiple along-stream locations and multiple burial dates in each terrace deposit, could a reliable climatic signal be extracted: this signal would not have shown up in terrace abandonment ages such as those derived from cosmogenic exposure dates. The demonstrated effects of assumptions about strath terrace formation, and the recommended approaches for sampling and interpretation, have implications for those attempting to constrain palaeoclimatic, tectonic, and geomorphic histories from strath terrace records in regions exhibiting punctuated river incision.

Structural and lithological controls in mountain valleys have been shown to affect the erosional connectivity of hillslopes, tributaries and alluvial fans, as well as the formation and preservation...

River strath terraces reflect changes in lateral and vertical erosion rates within mountain valleys related to changes in the sediment to water discharge ratio. In contrast to the formation of terraces in high latitude glaciated catchments, little is known about the timing and mechanisms of river valley aggradation and incision in response to climate in low latitude, non-glaciated arid regions. To investigate the timing of river strath terrace formation in North-West Africa, we developed and applied a new approach to OSL dose rate correction of gravels. We sampled terraces in the M’Goun catchment crossing the thrust front and a thrust-sheet-top basin of the south-central High Atlas in Morocco, totalling 23 dated samples. Strath surfaces are elevated 10 to 40 m above the modern river plain, depending on local valley and bedrock configuration, and are overlain by 2 to 10 m of fluvial conglomerates. Burial ages of conglomerates in the first strath terrace level span from 180 to 60 ka, with widespread abandonment and incision post 60 ka throughout the catchment. This timing coincides with an eccentricity-driven decrease in African summer insolation and a decrease in the fluvial signature of Saharan dust recorded in an offshore Atlantic sediment core. We propose enhanced precipitation from the African summer monsoon during high insolation periods led to increased sediment yield and aggradation in the southern High Atlas, whilst low insolation and dry periods led to sediment-starved incision. To our knowledge, the M’Goun river terrace record is the most detailed record of long-term landscape evolution in response to climate fluctuations in northwest Africa to date.

The fluvial archive literature is dominated by research on river terraces with appropriate mention of adjacent environments such as lakes. Despite modern sedimentary basins comprising a significant (>88%) volume of distributive fluvial systems, of which alluvial fans (>1 km

The study of drainage network response to uplift is important not only for understanding river system dynamics and associated channel properties and fluvial landforms, but also for identifying the nature of crustal deformation and its history. In recent decades, geomorphic analysis of rivers has proved powerful in elucidating the tectonic evolution of actively uplifting and eroding orogens. Here, we review the main recent developments that have improved and expanded qualitative and quantitative information about vertical tectonic motions (the effects of horizontal deformation are not addressed). Channel long profiles have received considerable attention in the literature, and we briefly introduce basic aspects of the behaviour of bedrock rivers from field and numerical modelling perspectives, before describing the various metrics that have been proposed to identify the information on crustal deformation contained within their steady-state characteristics. Then, we review the literature dealing with the transient response of rivers to tectonic perturbation, through the production of knickpoints propagating through the drainage network. Inverse modelling of river profiles for uplift in time and space is also shown to be very effective in reconstructing regional tectonic histories. Finally, we present a synthetic morphometric approach for deducing the tectonic record of fluvial landscapes. As well as the erosional imprint of tectonic forcing, sedimentary deposits, such as fluvial terrace staircases, are also considered as a classical component of tectonic geomorphology. We show that these studies have recently benefited from rapid advances in dating techniques, allowing more reliable reconstruction of incision histories and estimation of incision rates. The combination of progress in the understanding of transient river profiles and larger, more rigorous data sets of terrace ages has led to improved understanding of river erosion and the implications for terrace profile correlation, i.e., extrapolation of local data to entire profiles. Finally, planform changes in fluvial systems are considered at the channel scale in alluvial rivers and regional level in terms of drainage reorganisation. Examples are given of how numerical modelling can efficiently combine with topographic data to shed new light on the (dis)equilibrium state of drainage systems across regional drainage divides.

The debate in historical geomorphological studies about the causes of erosion in regions such as the Mediterranean has been long-standing. The relative roles of climate change and human impacts can be difficult to disentangle in the absence of highly resolved chronologies. Here we reconstruct the erosion history of a small lake catchment in central Anatolia, located on the edge of one of the Mediterranean's most iconic badland landscapes in Cappadocia. Because these lake sediments are annually laminated, precisely dating clastic inwash layers and calculating recurrence intervals and flux rates is possible. Lake cores have been analysed for μXRF elemental geochemistry and via thin sections, along with proxies for hydroclimate (oxygen isotopes) and land cover (pollen). Peaks in titanium, along with other detrital elements, and changes in clastic layers indicate increased sediment influx into Nar Lake between 9300 and 8000 cal BP (ceramic Neolithic, when obsidian mining took place nearby) and again, more importantly, during the last 2500 years (Iron Age to modern), the latter exhibiting three phases of enhanced catchment erosion. Multiproxy comparisons show that these phases were related primarily to periods of increased human impact on vegetation and soils around the lake. Most sediment influx has been in the form of turbidites, linked to the presence of a fan delta at the lake edge, although this store does not appear to have significantly delayed sediment delivery from eroding hillslopes to the lake bed. The marked increase in detrital influx during the late Holocene implies that badland development in the lake catchment is recent and largely anthropogenic, rather than ancient and of climatic causation, and probably involving stream capture. The record also shows that sediment influx diminished markedly at times when human land use disintensified, which in turn indicates that hillslope degradation is reversible with appropriate land management.

Field-based palaeoflood event reconstruction has the potential to contribute to the development of our understanding of long-term landscape evolution. However, the reconstruction of past flow event histories (magnitude and frequency) over long-term (Quaternary) timescales is fraught with difficulties. Here we make a preliminary exploration of some of the practicalities of flood reconstruction from fluvial terrace archives using commonly available sedimentological and geomorphological observations from a field perspective. We utilize Manning and palaeostage indicators to reconstruct historic events that can be used as benchmarks for a lesser used competence based approach, which is applied to coarse-grained strath terrace deposits. We evaluate the results against gauged records for extreme and catastrophic events that affected the same region in 1973 and 2012. The findings suggest that the competence approach is most effectively applied to terrace deposits if the channel geometry is taken into account when sampling both in cross-section and in longitudinal section and calibrated against the sedimentology for palaeo-flow depth. Problems can arise where constrictive channel geometries allow boulder jams to develop, acting as sediment traps for the coarsest material and leading to downstream ‘boulder starvation’. Useful sites to target for palaeoflood reconstruction, therefore, would be upstream of such constrictive reaches where the coarsest transportable bedload has been effectively trapped. Sites to avoid would be downflow, where the deposited material would poorly represent palaeoflood competence. Underestimation from maximum boulder preservation and limited section exposure issues would appear to outweigh possible overestimation concerns related to fluid density and unsteady flow characteristics such as instantaneous acceleration forces. Flood data derived from river terrace deposits suggests that basal terrace geometries and coarse boulder lags common to many terrace sequences are likely the result of extreme flow events which are subsequently filled by lesser magnitude flood events, in this environmental setting. Copyright © 2016 John Wiley & Sons, Ltd.

Modern tributary-junction alluvial fans (cone-shaped depositional landforms formed in confined valley settings) were analysed from a 20-km-long reach of the Dades River in the distal part of the fold-thrust belt region in the south-central High Atlas Mountains of Morocco. Here, a deeply dissected network of ephemeral tributary streams and a perennial trunk drainage characterised by an arid mountain desert climate are configured onto a folded and thrust faulted Mesozoic sedimentary sequence. Out of 186 tributary streams, only 29 (16%) generated alluvial fans at their tributary junctions. The fan-generating catchments possess higher relief, longer lengths, lower gradients, and larger areas than nonfan-generating catchments. Whilst geologically, fan-generating catchments are underlain by folded/steeply dipping weak bedrock conducive to high sediment yield. Tributary-junction fans are built from debris flow or fluvial processes into open or confined canyon trunk valley settings. The proximity of the perennial trunk drainage combined with the valley morphology produces lobate or foreshortened trimmed fan forms. Analysis of fan (area, gradient, process), catchment (area, relief, length, gradient), and tributary valley (width) variables reveals weak morphometric relationships, highlighted by residual plots that show dominance of smaller and lower gradient than expected fan forms. These morphometric relationships can be explained by interplay between the catchment and trunk drainage geology, morphology, climate, and flood regime that are combined into a conceptual ‘build and reset’ model. Ephemeral tributary-junction fans develop progressively during annual localised winter-spring storm events, attempting to build towards a morphological equilibrium. However, the fans never reach an equilibrium morphological form as they are reset by rare (> 10 year) large floods along the River Dades that are linked to regional incursions of Atlantic low pressure troughs. The model highlights the spatial and temporal variability of tributary-junction fan building and illustrates the connectivity/coupling importance of such features in dryland mountainous terrains.

Numerical model simulations and experiments have suggested that when migration of the main drainage divide occurs in a mountain belt, it can lead to the rearrangement of river catchments, rejuvenation of topography, and changes in erosion rates and sediment flux. We assess the progressive mobility of the drainage divide in three lithologically and structurally distinct groups of bedrock in the High Atlas (NW Africa). The geological age of bedrock and its associated tectonic architecture in the mountain belt increases from east to west in the study area, allowing us to track both variations in rock strength and structural configuration which influence drainage mobility during erosion through an exhuming mountain belt. Collection of field derived measurements of rock strength using a Schmidt hammer and computer based extraction of river channel steepness permit estimations of contrasts in fluvial erodibilities of rock types. The resulting difference in fluvial erodibility between the weakest and the strongest lithological unit is up to two orders of magnitude. Published evidence of geomorphic mobility of the drainage divide indicates that such a range in erodibilities in horizontal stratigraphy of the sedimentary cover may lead to changes in erosion rates as rivers erode through strata, leading to drainage divide migration. In contrast, we show that the faulted and folded metamorphic sedimentary rocks in the centre of the mountain belt coincide with a stable drainage divide. Finally, where the strong igneous rocks of the crystalline basement are exposed after erosion of the covering meta-sediments, there is a decrease in fluvial erodibility of up to a factor of three, where the drainage divide is mobile towards the centre of the exposed crystalline basement. The mobility of the drainage divide in response to erosion through rock-types and their structural configuration in a mountain belt has implications for the perception of autogenic dynamism of drainage networks and fluvial erosion in mountain belts, and the interpretation of the geomorphology and downstream stratigraphy.

Geomorphic archives, particularly longitudinal river profiles, are increasingly used as a proxy to reconstruct uplift rates in mountainous regions. Within the Atacama Desert, Northern Chile, slow, long-term erosion creates exceptional preservation of fluvial and alluvial surfaces. This enables river incision patterns to be used on a continental-scale (>250 km) along the western margin of the Andes (18°00'S to 20°15′S) and over a time frame from Miocene to Present day. The data show marked compartmentalisation of fluvial system behaviour with changes in incision rates from south to north creating 3 distinctly different regions. Within these different sectors, incision rates are broadly consistent between rivers suggesting a regional rather than a river specific control on rates. In Sector 1 (18°05′S to 19°20'S) the fluvial systems are exorheic with a terminal base level (the lowest base level to which the river system can erode) in the Pacific Ocean and span the Coastal Cordillera, Longitudinal Valley, Precordillera and western edge of the Western Cordillera. This constrains the total uplift over these regions to a minimum of 1200 m in 11 Myr with incision rates of ~200–120 m/Myr consistent with rapid but sustained uplift of the Andes in the Late Miocene. In Sector 2 (19°20'S to 19°50'S), to the immediate south, the rivers are shorter and terminate in the Longitudinal Valley, spanning only the Longitudinal Valley, Precordillera and the western edge of the Western Cordillera with lower incision rates of 100–50 m/Myr. Comparison of incision rates between Sector 1 and 2 can constrain the uplift of the Coastal Cordillera to 60 m/Myr which is in keeping with previous studies from the region. In southernmost Sector 3 (19°50'S to 20°10'S), the fluvial systems terminate in the Longitudinal Valley and span only the Longitudinal Valley and eastern part of the Precordillera with low incision rates of 50 to 25 m/Myr. Differences between Sectors 2 and 3 are attributable to drainage loss by tectonic beheading of catchments through uplift of the Cordillera de Domeyko fault system, placing a minimum constrain on uplift in this region of 50 to 25 m/Myr. This study demonstrates the applicability of large-scale fluvial archives to access not just the timing of uplift on a continental scale, but also the relative uplift of individual tectonic provinces.

Collisional mountain belts commonly develop intramontane basins from mechanical and isostatic subsidence during orogenic development. These frequently display a relict top surface, evidencing a change interval from basin infilling to erosion often via capture or overspill. Such surfaces provide markers that inform on orogenic growth patterns via climate and base level interplay. Here, we describe the top surface from the Sorbas Basin, a key intramontane basin within the Betic Cordillera (SE Spain). The surface is fragmentary comprising high elevation hilltops and discontinuous ridges developed onto the variably deformed final basin infill outcrop (Gochar Formation). We reconstruct surface configuration using DEM interpolation and apply 10Be/26Al cosmonuclides to assess surface formation timing. The surface is a degraded Early Pleistocene erosional pediment developed via autogenic switching of alluvial fan streams under stable dryland climate and base level conditions. Base-level lowering since the Middle Pleistocene focused headwards incision up interfan drainages, culminating in fan head capture and fan morphological preservation within the abandoned surface. Post abandonment erosion has lowered the basin surface by 31 m (average) and removed ~5.95 km3 of fill. Regional basin comparisons reveal a phase of Early Pleistocene surface formation, marking landscape stability following the most recent Pliocene-Early Pleistocene mountain building. Post-surface erosion rate quantification is low and in accordance with 10Be denudation rates typical of the low uplift Betic Cordillera.

Relict landforms provide a wealth of information on the evolution of the modern landscape and climate change in the past. To improve understanding of the origin and development of these landforms we need better spatial measurements across a variety of scales. This can be challenging using conventional surveying techniques due to difficulties in landform recognition on the ground (e.g. weak visual/topographic expression) and spatially variable areas of interest. Here we explore the appropriateness of existing remote sensing datasets (aerial LiDAR and aerial photography) and newly acquired unmanned aerial vehicle (UAV) imagery of a test site on the upland of Dartmoor in SW England (Leeden Tor) for the recognition and automated mapping of relict patterned ground composed of stripes and polygons. We find that the recognition of these landforms is greatly enhanced by automated mapping using spectral two-dimensional imagery. Image resolution is important, with the recognition of elements (boulders) of

Field geologists and geomorphologists are increasingly looking to numerical modelling to understand landscape change over time, particularly in river catchments. The application of landscape evolution models (LEMs) started with abstract research questions in synthetic landscapes. Now, however, studies using LEMs on real-world catchments are becoming increasingly common. This development has philosophical implications for model specification and evaluation using geological and geomorphological data, besides practical implications for fieldwork targets and strategy. The type of data produced to drive and constrain LEM simulations has very little in common with that used to calibrate and validate models operating over shorter timescales, making a new approach necessary. Here we argue that catchment fieldwork and LEM studies are best synchronized by complementing the Pattern Oriented Modelling (POM) approach of most fluvial LEMs with Pattern Oriented Sampling (POS) fieldwork approaches. POS can embrace a wide range of field data types, without overly increasing the burden of data collection. In our approach, both POM output and POS field data for a specific catchment are used to quantify key characteristics of a catchment. These are then compared to provide an evaluation of the performance of the model. Early identification of these key characteristics should be undertaken to drive focused POS data collection and POM model specification. Once models are evaluated using this POM/POS approach, conclusions drawn from LEM studies can be used with greater confidence to improve understanding of landscape change.

River terraces represent important records of landscape response to e.g. base-level change and tectonicmovement. Both these driving forces are important in the southern Iberian Peninsula. In this study,Optically Stimulated Luminescence (OSL) dating was used to date two principal river terraces in theTabernas Basin, SE Spain. A total of 23 samples was collected from the fluvial terraces for dating usingquartz OSL. Sixteen of the samples could not be dated because of low saturation levels (e.g. typical2xD0 < 50 Gy). The remaining seven samples (5 fossil and 2 modern analogues) were investigated usingboth multi-grain and single-grain analysis. Single grain results show that: (i) measurements from multigrainaliquots overestimate ages by up to ~ 4 ka for modern analogues and young samples (

The variability of Quaternary landforms preserved in the Tabernas basin of southeast (SE) Spain raises numerous questions concerning the roles of external forcing mechanisms (e.g. tectonics and/or climate) and internal landscape properties (e.g. lithological controls) in the evolution of the basin-wide fluvial system over Late Quaternary timescales. In this study, we apply the FLUVER2 numerical model to investigate the significance of these landscape controls upon patterns of landscape evolution. We highlight the complications of generating realistic input datasets for use in the modelling of long-term landscape evolution (e.g. discharge and runoff datasets). Model outputs are compared to extensive field mapping of fluvial terraces, their sedimentary architecture and optically stimulated luminescence dating results of the terraces. The results demonstrate the significance of non-linear rates of flexural tectonic uplift towards the west of the Tabernas Basin which have controlled base levels throughout the Quaternary and promoted the formation of a series of diverging fluvial terraces. Our numerical model results further highlight the importance of climate cycles upon river terrace formation. Basin-wide aggradation events were modelled during the transition from Marine Isotope Stage (MIS) 6 to 5 and the Last Glacial Maximum (LGM) as supported by field evidence. This aggradational pattern supports the regional hypothesis of terrace formation during global glacial cycles and cold-to-warm stage transitions and supports the use of sea surface temperature climate proxy data in the modelling exercise. The availability of sediments derived from the surrounding hillslopes and adjacent alluvial fans explains the generation of substantial terrace aggradations. Copyright © 2015 John Wiley & Sons, Ltd.

This study documents river terraces from upstream reaches of the Dad es River, a major fluvial system draining the south-central High Atlas Mountains. Terraces occur as straths with bedrock bases positioned at 10 m altitudinal intervals up to 40 m (T1-T5) above the valley floor, becoming less common between 50 and 140 m. The rock strength, stratigraphy and structure of the mountain belt influences terrace distribution. Terraces are absent in river gorges of structurally thickened limestone; whilst welldeveloped, laterally continuous terraces (T1-T4) form along wide valleys occupying syncline structures dominated by weaker interbedded limestone-mudstone. Terrace staircases develop in confined canyons associated with weaker lithologies and influence from structural dip and stratigraphic configuration. Terraces comprise a bedrock erosion surface overlain by fluvial conglomerates, rare overbank sands and colluvium. This sequence with some OSL/IRSL age control, suggests terrace formation over a 100 ka climate cycle with valley floor aggradation during full glacials and incision during glacial-interglacial transitions. This integrates with other archives (e.g. lakes, glaciers, dunes), appearing typical of landscape development along the NW Saharan margin south of the High Atlas, and similar to patterns in the western-southern Mediterranean. The 100 ka climate cycle relationship suggests that the terrace sequence documents Late-Middle Pleistocene landscape development. Consistent altitudinal spacing of terraces and their distribution throughout the orogen suggests sustained base-level lowering linked to uplift-exhumation of the High Atlas. Low incision rates (

One of the challenges facing Earth Scientists is to determine the extent to which geomorphic features can be used to extract tectonic signals from landscapes. Here, we quantitatively analyse the long profiles of rivers that drain southwards across the South Atlas Fault (SAF), a thrust fault that forms the southern margin of the High Atlas Mountains in Morocco, to derive new data on the Late Cenozoic activity of this fault system. River long profiles were extracted for 32 major rivers flowing southwards into the Ouarzazate Basin. Of these, eleven exhibit concave-up river profiles with a mean concavity of 0.64 and normalized steepness indices in the range 47.5 – 219.0 m 0.9 . By contrast, 21 rivers exhibit at least one knickpoint upstream of the thrust front. Knickpoint height varies from 100 – 1300 m, with calculated incision at the range bounding fault ranging from 80-900 m, despite the drainage areas upstream of the knickpoint ranging over several orders of magnitude. In map view, knickpoint locations generally plot along sub-parallel lines and there are no obvious relationships with lithological units for knickpoints exhibiting slope-break morphology. Channel reaches below slope-break knickpoints have higher mean concavities (0.76) than above the knickpoint indicative. This observation combined with a lithological or river-capture origin for the knickpoints having been ruled out suggests that an increase in uplift rate along a planar fault zone during the Plio-Quaternary caused the initiation of the transient response (i.e., knickpoint formation) to a change in base-level observed in the river profiles. This uplift event can be correlated to the convective removal of the lithospheric root to the Atlas Mountains resulting in the anomalously high topography at the present day.

Deposits of aeolian sand are known to have accumulated in periglacial environments during the cold phases of the late Quaternary. In many instances, however, they form low-relief topographic units which may not be readily identified without detailed field survey. This study aims to use a multidisciplinary approach, combining remotely sensed data analysis and field survey, to investigate the extent and palaeoenvironmental significance of sand ramps in the Drakensberg/Rooiberge foothills of South Africa. Analysis of Google Earth™ imagery has demonstrated that gully systems are a common component of the landscape, and heterogeneously distributed across the landscape. Field investigation confirmed the hypothesis that the gullies are mainly eroding into sand ramps of fine sands and very coarse silts which mantle many of the lower hillslopes of the region. These sand units include palaeosols and occasional gravel lags, but are otherwise remarkable for their homogenous composition, cross-bedding and the complete absence of clasts. Much of the sediment is thus interpreted as aeolian in origin. The deposits are sufficiently similar in many respects to the Masotcheni Formation, a late Quaternary colluvium which outcrops abundantly in the Drakensberg, to propose an assignation to this unit. However, an aeolian component in the Masotcheni has not previously been described. The distribution of aeolian accumulation in the region is consistent with southward transport during late Quaternary cold phases from a source on the Highveld to the north of the study area. The low relief and complex fluvial network in this region would concentrate sediment eroded from the Drakensberg/Rooiberge, which would subsequently be available for deflation when the balance between fluvial flow regime, seasonally frozen ground and north-westerly trade winds were optimal for aeolian entrainment. Deposition is primarily topographically controlled, and is in places sufficiently extensive that it may be better described as a discontinuous coversand. This study suggests that aeolian deposits may be overlooked in other environments subject to past periglacial landscape development, and develops a potential methodology by which this problem may be overcome.

Erosional landform features and their associated sedimentary assemblages (river terraces) often provide important records of long-term landscape evolution. However, the methods available for spatial representations of such records are typically limited to the generation of two-dimensional transects (valley long profiles and cross sections). Such transects limit the full quantification of system responses in a three-dimensional landscape (e.g., the identification of spatial changes in net sediment flux within a hydrological basin). The purpose of this paper is to explore the use of geospatial interpolation methods in the reconstruction of Quaternary landform records. This approach enables more precise quantifications of terrace landform records at a range of spatial scales (from a single river reach to geological basin scales). Here we use a case study from the Tabernas basin in SE Spain to test the applicability of multiple methods of geospatial interpolation in the reconstruction of Quaternary landforms (river terrace and alluvial fan remnants). We take steps in (1) refining the terrace data sets and the methods of technique application in order to reduce modelling errors, and (2) in highlighting the requirements for an assessment of interpolation method suitability when modelling highly fragmented landform records. The results from our study show that the performance of interpolation methods varies considerably and is dependent upon the data modelled. Method performance is primarily controlled by the inherent geomorphological characteristics (surface morphology and elevation) of the data; however, the attributes of data structure are significant. We further identify the importance of predefined model parameters (e.g., search radius) upon technique performance, increasing the appreciation of these commonly neglected variables in such studies. Ultimately, the overall applicability of the interpolation process is evidenced by the close correlation of surface volume data generated by all interpolation methods. These data would suggest that the interpolation technique can be applied in many forms as a useful tool in the reconstruction of Quaternary landform features.

Documented for the first time are an extensive suite of late Neogene giant terrestrial coastal landslides along the classic convergent margin of western South America (18° to 24° south). These are remarkable in terms of their unusual abundance and atypical setting, such failures previously being linked with oceanic volcanic edifices or over-steepened glaciated coastlines. Located within the hyper-arid Coastal Cordillera of the Atacama Desert of Northern Chile we report the presence of more than 60 individual large-scale landslides with individual volumes up to 9 km 3 developed over a horizontal coastline distance of some 650 km. These landslides were emplaced as a combination of rock avalanches and multiple rotational failures. The majority terminated directly into the Pacific – likely generating significant tsunami hazard to the Chilean and south Peruvian coastline in a region which is today considered to be part of a notorious seismic gap. The proliferation and scale of these Late Neogene giant landslides in this actively uplifting, hyperarid terrain suggests they are the main geomorphic agent for relief reduction, probably triggered by megathrust earthquakes and potentially providing a unique palaeoseismic archive. The temporal and spatial distribution of these giant landslides corresponds with a period of surface steepening of the forearc wedge in the Central Andes and south to north differential uplift associated with factors such as aseismic ridge subduction. The resulting surface gradient increases, combined with the persistent climatic aridity of the region, have served to limit effective relief-reducing geomorphic processes in this oversteepened terrain to large-scale landsliding. The phenomena documented here geospatially link previously recognised large-scale slope failures from the off-shore environment and higher altitude areas of the Andean forearc, suggesting that large-scale landsliding is capable of transferring sediment on a regional scale to the off-shore Peru–Chile trench. This has implications for the friction of the subducting Nazca plate and associated seismicity and uplift.

A series of palaeoflood estimation techniques are applied to an inset sequence of Middle to Late Pleistocene river terrace landforms associated with the Rio Almanzora, SE Spain. The study area is a 7 km-long transverse reach, where 4 terrace levels (Level 1 = highest and oldest, Level 4 = lowest and youngest) document some 200 m of incision across an uplifted basement block during the Pleistocene. For the broader region, river terrace aggradation is attributed to increased sediment supply during glacial to interglacial transitions (Level 1 = Marine oxygen isotope stages [MOIS] 12/11, L2 = 10/9, L3 = 8/7 and L4 = 6/5). Within the transverse reach, terrace Levels 2, 3 and 4 are characterised by coarse boulder-rich gravels (D max = 2.5 m) organised into 3–5 m-high cross-beds. Level 4 is characterised by a series of km-scale abandoned meander loops, with Levels 2 and 3 showing evidence for similar degraded high sinuosity forms. The fluvial setting is interpreted as a braided river environment, with lateral and longitudinal gravel barforms. Using field-derived sedimentary (boulder size etc) and morphological (width, slope etc) features, flow competence- and regime-based methods were applied. Palaeoflood estimates varied from 40 to 2859 m 3 /s, with mean maximum discharges showing an increase through time (L2 = 278, L3 = 413, L4 = 1361 m 3 /s). Palaeoflood estimates were compared to flood events associated with the ephemeral modern Rio Almanzora (catchment = 2500 km 2 ) to establish whether palaeoflood values were reasonable estimations. A 46-year (1963–2009) modern flood gauge record shows infrequent flood events with discharges typically 3 /s. A rare flood event in 1973 (5680 m 3 /s) provides an upper value for comparison with the palaeoflood estimates. Although estimates appear reasonable they should be considered as minimum values due to 1) some disparities between flow depths derived from the palaeoflood equations (0.7–4 m) and field-based evidence (3–5 m-high cross-beds); 2) issues related to field sampling of boulders from sections, and 3) uncertainties as to the relationship between flood hydrology and transportation of boulders, formation of 3–5 m-high cross-beds and the position of the terrace remnants in the river valley. The palaeoflood estimates suggest that relatively large discharges occurred during valley floor aggradation in the glacial/interglacial transitions, periods characterised by low vegetation cover and high sediment yield. Discharge increases through time can be explained by the long-term drainage evolution of the Rio Almanzora, characterised by an expanding catchment area linked to river capture, driven by regional uplift patterns. The 3.5-times larger discharge estimates recorded in terrace Level 4 are coeval with large and rapid changes in precipitation or elevated seasonal discharge linked to the MOIS 6/5 glacial interglacial climate transition. Overall, within tectonically active regions affected by river capture, fluvial systems could be responding to loss or expansion of catchment areas. Long-term palaeoflood records from inset flights of river terraces could store such signals and may mask/complicate any climate–palaeoflood relationships.

Vertical stereographic aerial photographs obtained as photographic plates or, more recently, as digital images represent a primary source of data for desk studies on all engineering projects. New methods of data acquisition from both airborne and spacecraft-based scanners provide unparalleled opportunities to obtain high-resolution data. However, interpretation of the images remains as much an art as a science and is critically dependent on the skill and experience of the interpreter. Relevant experience can be obtained only by the practice of ground-truthing an interpretation that the interpreter has carried out. Although it has rarely been quantified there are important lessons for all engineering projects on the divergence between the interpreted image and the actual ground situation. Some of these are exemplified in a case study undertaken by the authors using large-scale aerial photographs for a landslide hazard investigation in Spain. Based on this investigation it can be assumed that up to 50% of landslide features of engineering geological significance might not have been identified by the aerial photograph interpretation. The alternative is to use the images to divide the landscape into terrain units that allow the landslide potential to be evaluated. It can be demonstrated that this approach is more effective than trying to create a landslide inventory based on aerial photographs alone. Given the potential expansion in the use of large-scale image analysis associated with the recent improvement in data acquisition systems this has implications for the use of images and the training of interpreters.

The Atacama Planation Surface is an extensive west-dipping surface developed between 16°S and 27°S along the Pacific margin of South America. It is considered to have formed between 16 and 7.5 Ma and to have important chronostratigraphic significance. Here we present new cosmogenic 3He exposure dates of boulders on the planation surface using pyroxene and amphibole. Exposure ages display good within- and between-site consistency and range from 22 to 1.2 Ma, with ages of ca. 14.6, 7, and 3 Ma recurring at more than one site on the planation surface. The 14.6 Ma peak records the cessation of the main period of planation surface development, but, contrary to popular opinion, the younger ages reflect subsequent modification of the planation surface by alluvial activity. Comparison with other climate proxies for western South America suggests that since 14.6 Ma, a predominantly hyperarid climate, interspersed with short-lived phases of more intense runoff driven by global climate change, has prevailed. The longevity and composite nature of the Atacama Planation Surface suggest that regionally extensive planation surfaces may have a multiphase history, are unlikely to have any chronostratigraphic significance, and cannot be used to reconstruct uplift histories.

Transverse drainages are discordant river patterns whose flow directions cut across geological structures such as faults, folds and the regional tectonic fabric (strike) of mountain belts. Such drainages can form spectacular geomorphological features, with reaches often occupying narrow and deeply dissected canyons/gorges that cut through prominent topographic barriers. Within collisional plate margins (e.g. foreland basin systems), studies have frequently examined transverse drainage development in response to active tectonics (e.g. thrusting). Here, transverse drainage will develop via incision or become deflected and fixed in response to oblique fault/fold growth structures, particularly within wedge-top, thrust front and foredeep basin settings. Within this paper the development of transverse drainage within the little studied fold–thrust component of an orogen is examined using a spectacular series of river gorges along the River Dades from the High Atlas Mountains of Morocco. Integrated field-derived geological and geomorphological data are used to explore the interplay between active tectonics, lithology and structural configuration (passive tectonics) of bedrock for the creation of transverse drainage and associated river gorge reaches. At the southern edge of the High Atlas fold–thrust belt, the River Dades passes through the deeply dissected Tarhia n' Dades and Main Dades Gorges before emerging into the Ait Seddratt wedge-top basin. Fluvial incision is principally controlled by Plio-Quaternary uplift of the High Atlas Mountains. The larger Main Dades Gorge has formed a deeply dissected antecedent course via drainage inheritance into structurally thickened Jurassic bedrock via uplift between two key structural components of the foreland basin system (the fold–thrust belt and the wedge-top basin). The smaller Tarhia n' Dades Gorge involves a more complex interplay between active tectonics and the passive structural arrangement of an asymmetric plunging syncline combined with the stratigraphic arrangement and strength of the Jurassic bedrock. The drainage network is configured to the fold and associated fault-joint structures to form respective NW–SE and NE–SW orientated dip slope and strike drainage. Progressive incision by tributary dip slope streams, cutting down through resistant limestone bedrock into underlying weaker mudstone bedrock, has resulted in accelerated headward erosion and drainage network expansion by river captures. This has resulted in a minor re-routing of the River Dades as a transverse drainage through a former dip slope tributary stream to form the Tarhia n' Dades Gorge.

Armoured mudballs have been reported from modern fluvial catchments and preserved in ancient (Quaternary and older) sedimentary sequences. Despite many descriptions of mudballs in the literature there is little systematic analysis of their physical properties, genesis, transport and deposition within alluvial sequences. This study uses two adjacent catchments developed on identical geology in southeast Spain that offer a unique opportunity to examine the key controls on armoured mudball development as one catchment produces armoured mudball whilst the second does not. Field survey using a Total Station together with laboratory tests for material composition and aggregate stability were used to examine the catchment and the armoured mudballs. The results show that the generation of the armoured mudballs is heavily dependent on the correct sediment supply (suitable aggregates to act as mudball ‘seeds’, and suitable fragments to source the armour) and good slope-channel coupling within the catchment. Coupling is facilitated by steep gully sides which deliver the weathered blocks of marl (mudball ‘seeds’) to the gully floor. The ‘seeds’ form the core of the mudballs in the ’mudball factory’ zone. If the ‘seeds’ are subjected to sealing by moisture from rainfall the mudball will hold together through negative pore pressures and can be transported by the ensuing gully flow. As the mudball rolls it picks up a surface armour which becomes embedded into the mudball. With longer transport paths the turbulent nature of the flow creates spherical mudballs which are size sorted downstream by the hydraulics of the flow rather than attrition. Armoured mudball deposition occurs within unit braid bars in areas of waning flow (due to transmission losses), within vegetated, point or mid-channel bars. The armoured mudballs are preferentially deposited within the coarser bar head of these depositional units, with smaller marl fragments forming the finer bar tail. Trenching of the bars reveals that the armoured mudballs are preserved in the subsurface although they may become increasingly flattened upon burial at depth, and may be difficult to recognise due to disaggregation by roots. This study suggests that armoured mudballs recognised in ancient alluvial sequences imply seasonality of climate and good slope-channel coupling within the catchment area, and that they are capable of transporting delicate flora undamaged, making them good targets for pollen recovery in ancient alluvial sequences. Where armoured mudballs were present but go unrecognised in the ancient record the mode and energy of transportation and deposition of the fine sediment may be misinterpreted as the fines are in fact being transported as bedload rather than suspended load. The armoured mudballs have also been noted to transport pristine microfauna such as ostracod shells from the source area material. This may provide challenges to palaeoenvironmental interpretations of the mud-dominated unit as environmentally significant fauna may not be contemporaneous with the unit.

The complex multiple-age badlands at El Cautivo are cut in upper Miocene marls of the uplifted and dissected Tabernas Basin, Almeria, in semi-arid SE Spain. Previous work identified six episodes of erosion and subsequent stabilisation, with ages ranging from the late Pleistocene to the present day. This paper uses newly-acquired digital elevation models, orthorectified aerial photographs, and field survey data to illustrate the development sequence of a series of gullies which drain into the Rambla de Tabernas. Changing drainage directions and phases of incision and stabilisation are related to the filling and subsequent dissection of the Tabernas lower lake sediments, differential material strengths, changing process mixes on hillslopes and, in more recent times, land-use changes. We propose that the erosional phases differed considerably in both time-span and depth of incision. A long period of stability during the existence of the lake led to extensive pediment development in the area. Subsequent incision into the lake sediments by the Rambla de Tabernas produced a limited amount of localised pediment incision in the tributary catchment. Subsequent aggradation occurred in the lower reaches of gullies when incision by the Rambla de Tabernas reached the underlying bedrock. Following incision through the lip of the lake the Rambla de Tabernas cut rapidly into the bedrock leading to an altered drainage direction in the tributary catchments and the major phase of badland development visible today. Stabilisation of the pediments and some of the north-facing slopes occurred subsequently. Increased sediment loads, caused by climatic shift, a change in land use, or both, gave rise to a later phase of valley fill in the gullies. The modern channels have reduced this fill to isolated terrace benches and there are also localised remnants of a lower, more recent fill. Whilst supporting the general interpretation of the site's development by Alexander et al. [Alexander, R.W., Harvey, A.M., Calvo, A., James, P.A., Cerda, A., 1994. Natural stabilisation mechanisms on badlands slopes: Tabernas, Almeria, Spain. In: Millington, A.C., Pye, K. (eds.), Environmental Change in Drylands: Biogeographical and Geomorphological Perspectives. Wiley, Chichester, pp 85–111.], this re-analysis highlights the importance of changing drainage direction to the pattern of landform development and attempts to assess the magnitude and overall rate of incision.

A research programme underway in south–east Spain has the overall aim of developing a long-term landscape evolution model for the Tertiary depositional basins that lie within the eastern part of the Betic cordillera. As part of the work it has become apparent that there are multiple natural hazards to development in the region, and the nature and distribution of these is presently under investigation. For one hazard, namely landsliding, a database of over 300 cases has been compiled within one defined 425 km2 river catchment, namely the Rio Aguas. Evaluation of the database has demonstrated that the contemporary distribution of landslides correlates with areas of steepest slopes across a range of the different lithologies. However, the “slope” component of the landscape is controlled by a wave of incision associated with a river capture event c. 100000 years ago which locally increased erosion by between 5 and 10 times. This event was a function of differential uplift between the depositional basins and resulted in over-steepened slopes within parts of the catchment which have yet to reach equilibrium in this evolving landscape.

The sedimentary succession in the Atacama Desert records deposition under an arid to semiarid climate from the late Jurassic (150 Ma) to the present day. Palaeomagnetic data indicate no significant latitudinal movement of this area since the late Jurassic. The present-day location of the Atacama within the dry subtropical climate belt is the principal cause of aridity. This situation is likely to have prevailed since the late Jurassic, supplemented by (1) the continentality effect (enhanced by the Gondwanan landmass), and (2) the presence offshore of a cold, upwelling current (from at least the early Cenozoic onwards and possibly earlier), resulting in conditions promoting climatic stability and desert development. Rapid and extreme climatic fluctuations during the Plio-Pleistocene were not sufficient to destabilize the climate within the Atacama. Comparison with other long-lived deserts (e.g. SW USA, Namib, Sahara and Australia) suggests that the Atacama is the oldest extant desert on Earth.

Pleistocene landslide failures are rarely recorded in the literature due to difficulties in recognition in the field. An adaptation of standard landslide classification based on activity is used to identify and examine the significance of a ‘fossil’ Pleistocene landslide from the Mocatan catchment of the Sorbas Basin, SE Spain. Recognition of the landslide is based on identifying key, but subtle anomalies in the geology and drainage. The fossil landslide at Mocatan developed principally in response to a rapid increase in valley incision rates associated with capture-induced base-level changes in the master drainage. This was facilitated by the weak sand and silt lithology of the study area. The landslide was a significant morphological modifier of the environment. Holocene drainage follows the original landslide morphology, whilst more recent gullying and piping exploits planes of weakness developed within the internal body of the landslide feature. In addition, the landslide removed much of the Pleistocene divide, facilitating future river capture.

The Rio Almanzora forms one of the larger drainage systems within the Betic Cordillera, Southeast Spain. In its distal reaches, prior to joining the Mediterranean Sea, the Rio Almanzora cuts across a major topographic barrier, the Sierra Almagro, forming drainage that is transverse to the regional geological structure. The long-term drainage evolution of the Rio Almanzora and its creation as a transverse drainage across the Sierra Almagro has been examined by reconstruction of an evolving basin-scale drainage network using a combination of the geological and geomorphological record. The early stages of drainage evolution (Late Miocene–Pliocene and Plio-Pleistocene) reveals the emergence of marine basins to the north and south of the Sierra Almagro and the development of unconnected drainage systems. This emergence records the development of the proto Rio Almanzora on the south side of the Sierra Almagro as a prograding fan-delta and alluvial fan system. Expansion of this drainage network via incision and headward erosion across the Sierra Almagro took place during the Pleistocene. During the Early–?Mid-Pleistocene, the proto Rio Almanzora became fully transverse across the Sierra Almagro and linked the endorheic Almanzora/Huercal-Overa basin with the Vera basin. A tectonically induced lowering of regional base level is proposed for the incision, headward erosion and drainage network expansion that has resulted in the creation of the Rio Almanzora as a transverse drainage. Differential uplift between the Huercal-Overa/Almanzora and Vera basins has resulted in southwards tilting and the creation of a regional gradient. The fluvial response to tilting was for incision into the steepened regional surface gradient via an increase in stream power. The incision, headward erosion and drainage network expansion by the proto Rio Almanzora and its creation as a transverse drainage appears to be part of a regional response by fluvial systems to differential uplift recorded throughout the Betic Cordillera of Southeast Spain during the Plio-Pleistocene.

Mineral magnetic and soil iron oxide data are applied to questions of relative age correlation of alluvial fans and lake sediments in the Tabernas basin, southeast Spain, within a context of interaction between tectonics and climatic change. Within the Tabernas basin, the sediment sequences and morphological evolution of late Quaternary alluvial fans suggest climatic change as the primary control. The fans toe out at the upper margins of a former lake, created in response to tectonic uplift. Magnetic and iron oxide data from soils, particularly dithionite-extractable iron (Fe d ), and frequency-dependent magnetic susceptibility ( χ FD %) accord with the relative age relationships suggested by more conventional field-based geomorphic observations. Magnetic data from the lake sediments suggest the main provenance characteristics of the sediments, but also reveal a shift in sediment sources towards the end of the lake period (probably during the late Pleistocene) to sediment supplied from a more active fluvial system from soil erosion within the Sierra de los Filabres part of the catchment. Hence, although the locations of the fans and the existence of the lake relate primarily to tectonics, the fan sequences themselves appear to be primarily climatically controlled, and there is evidence of a climatic influence over the source of sediment input into the lake during the late Pleistocene.

The definitions of hazard and risk in natural hazard studies are well established in the scientific literature. However, many examples of ‘landslide hazard assessment’ only identify the susceptibility of slopes to failure and make no statement on the frequency of occurrence that would be necessary for a complete hazard evaluation. In a research programme undertaken in SE Spain the issue of landslide susceptibility in a semi-arid, neotectonic environment was examined, with some attempt to evaluate the hazard. This work involved establishing the occurrence of landsliding within the 550 km 2 Rio Aguas catchment through remote sensing interpretation and field mapping. These data were compiled in an inventory containing the records of nearly 250 landslides that was analysed to establish the nature and extent of landslide susceptible situations. Within the catchment anticipated combinations of geological materials proved to be susceptible to failure, and relationships between landslide volume and travel angle were examined in relation to standard models. The highest incidence of contemporary landsliding appeared to be related to the proximity of a major river capture site, a geomorphological event that had been dated at 100 000 BP. This produced localized rapid incision, a ten fold increase in sediment removal and the creation of oversteepened slopes that were only recently degrading to their long-term angle of stability. In addition to contemporary landslides, field mapping identified anomalous geological structures that proved to be degraded erosional remnants of ancient landslides. Relating these remnants to the river terrace sequence in the region provided some control on the relative ages of these ancient or ‘fossil’ landslides. It was concluded that any assessment of landslide risk in the study area would need to take into account not only geological materials and the contemporary geomorphological environment but also the geomorphological history of the region.

Recent research into the long-term landscape development of a tectonically active terrain in arid SE Spain has revealed the significance of river capture in understanding current landscape instabilities (badlands and landslides). The river capture was initiated at c.100 ka BP and effected a 90 m base-level change at the point of capture. This stimulated a wave of incision to propagate through the landscape to 20 km upstream of the capture site. The net effect of the associated increase in erosion has been to change valley shapes from broad and shallow to narrow and deep. The associated unloading and steepening of valley sides has led to a focus of landslide activity in lithologies with more unconfined compressive strength (limestones) and a dominance of gullying, piping and badland development in the lithologies with lower unconfined compressive strengths (marls and sands). Post-capture rapid valley widening was initially achieved through landslide development. This form of slope degradation was sustained in the more resistant, joint-controlled lithologies. In weaker lithologies it was superseded by badland development. The elevated sediment fluxes associated with the c.100 ka BP base-level perturbation will continue into the near future, but are expected to decay, assuming that no additional environmental disturbances occur. The patterns of landscape instability witnessed today are controlled by (1) proximity to the areas affected by the base-level change and (2) the robustness of the local geology. Understanding of this long-term temporal context of the landscape provides a valuable spatial and temporal framework for land system management, facilitating the prediction of future natural trends in landscape stability. Copyright © 2002 John Wiley & Sons, Ltd.

An understanding of rates and mechanisms of incision and knickpoint retreat in bedrock rivers is fundamental to perceptions of landscape response to external drivers, yet only sparse field data are available. Here we present eye witness accounts and quantitative surveys of rapid, amphitheatre-headed gorge formation in unweathered granite from the overtopping of a rock-cut dam spillway by small-moderate floods (~100–1,500 m3 s−1). The amount of erosion demonstrates no relationship with flood magnitude or bedload availability. Instead, structural pattern of the bedrock through faults and joints appears to be the primary control on landscape change. These discontinuities facilitate rapid erosion (>270 m headward retreat; ~100 m incision; and ~160 m widening over 6 years) principally through fluvial plucking and block topple. The example demonstrates the potential for extremely rapid transient bedrock erosion even when rocks are mechanically strong and flood discharges are moderate. These observations are relevant to perceived models of gorge formation and knickpoint retreat., Consolider Ingenio 2006: ‘Topoiberia’ (CSD2006- 00041)

The evolution of an incising drainage network controls regional geomorphic development, but is in turn controlled by four sets of dynamic factors. These are as follows: tectonics, including both regional epeirogenic uplift and more local tectonic deformation; climatic change, affecting variations in flood power and sediment supply; base level; and local factors such as river capture, related to the development of the drainage network itself. The geomorphology of four uplifted Neogene sedimentary basins in the eastern Betic Cordillera of Almeria, Spain, demonstrates how these factors interact and operate over a range of temporal and spatial scales. The basins were marine basins until the early Pliocene, when differential epeirogenic uplift caused emergence and the initiation of the drainage networks; first in the Tabernas, then in the Sorbas, and finally in the Vera and Almeria basins. The last two became terrestrial in the early Pleistocene. The modern landscape reflects the influence of differential regional uplift rates on the long-term dissectional history, operating regionally over the whole period of landform development. The extremes are represented on the one hand by the deeply dissected Tabernas basin and on the other hand by the centre of the Almeria basin, which is dominated by coalescent aggrading alluvial fans. The Quaternary climatic signal is another regional signal, expressed by the sediment-led terrace sequence, with aggradation occurring primarily during Pleistocene global glacials and incision during the interglacials. These regional signals are modified locally by the other factors. Local neotectonic deformation is particularly important in the Tabernas and Almeria basins. Base-level change induced by tectonic activity and by river capture is important locally throughout the area, but the effects of base-level change induced by Quaternary sea-level change are restricted to the coastal zone. River capture has had profound effects, modifying the drainage areas. The Vera basin has gained drainage area substantially, whereas that of the Almeria basin has decreased. The most important effects have been on base level and incision rates, especially in the Sorbas basin. The overriding long-term control on drainage development and therefore on landform dynamics has been the pattern of regional epeirogenic uplift, onto which the Quaternary climatically controlled aggradation/dissection sequence has been imposed. These are regional signals that have been modified locally by the more spatially and temporally restricted signals generated by base-level change and river capture.

In mountain areas affected by uplift, significant reorganization of drainage networks can occur through river capture. This modification can dramatically affect sediment flux and routing into adjacent sedimentary basins. It is thus important to obtain information on rates and direction of changes in catchment areas in such environments. This can be achieved where the original, precapture catchments can be compared with the postcapture scenario. This paper examines three modern mountain catchments from southeast Spain that were affected by river capture, and that fed relict Pliocene–Pleistocene alluvial fan systems. Morphometric data have been collated for complete Quaternary mountain catchment piedmont fan systems. These data are used to establish regressions to determine catchment area and basin relief from a combination of fan characteristics (area and gradient). These relationships are then used to predict the original catchment characteristics for the three Pliocene–Pleistocene relict fans and compared with the modern catchments to quantify the magnitude and direction of changes, which have affected the drainage basins since fan abandonment in the early Pleistocene. The figures suggest that the catchment areas are responding to regional uplift, most significantly by drainage net expansion facilitated by river capture rather than lowering of surface relief. As a result, sediment routing into the sedimentary basins has been radically altered.

Quaternary catchments in the south of the Sorbas Basin, SE Spain have been affected by two regionally significant river captures. The river captures were triggered by changes in regional gradients associated with sustained Quaternary uplift in the region of 160 m Ma−1. The first capture occurred in the early Pleistocene and re-routed 15% of the original Sorbas Basin drainage into the Carboneras Basin to the south. The second occurred in the late Pleistocene and re-routed 73% of the original Sorbas Basin drainage to the east. This latter capture had dramatic consequences for base-level in the Sorbas Basin master drainage. Local base-level was lowered by 90 m at the capture site, 50 m at 7 km upstream and 25 m at 13 km upstream of the site. The base-level change instigated a complex re-organisation of the drainage networks in systems tributary to the master drainage over the ensuing period (some 100 ka). After the capture, drainage systems closer to the capture site experienced a tenfold increase in incision rates over most of their network. Those located some 13 km upstream of the capture site experienced a fivefold increase in incision, although in this instance, the changes do not appear to have propagated to the headwater regions of the drainage nets. The sensitivity of individual catchments was largely governed by geological controls (structure and lithology). The detailed network evolution in the most sensitive areas can be traced by reconstructing former drainage pathways using abandoned drainage cols and the alignment and degree of incision of the drainage networks. Three main stages of evolution can be identified which record the progressive spread of base-level changes from the master drainage. These are Stage 1 (pre-capture): original south-to-north consequent drainage; Stage 2 (early stage, post capture): aggressive subsequent southwest-to-northeast and east–west drainage developed along structural lineaments first in the east of the area (Stage 2a), and later in the west of the area (Stage 2b); and Stage 3 (late stage, post capture): obsequent drainage developing on the topography of the Stage 2 drainage. All stages of the network evolution are associated with drainage re-routing as a function of river capture at a variety of scales. The results highlight the complex response of the fluvial system, and the very different geomorphological histories of adjacent catchments, emphasising the need for regional approaches for examining long-term changes in fluvial systems.

The Plio-Pleistocene sedimentary rocks of the Sorbas Basin, SE Spain are used to reconstruct the palaeogeography, palaeoclimate and active tectonics operative in the Late Pliocene–Early Pleistocene. The deposits are generated from two distinct source areas. These are (1) relatively larger catchments developed across a structural lineament situated in the foothills of the Sierra Alhamilla and (2) a more basinal, smaller catchment developed to the north of the structural lineament. The alluvial systems show evidence for sheet flooding, channelized flow, and overbank sedimentation in distal and marginal areas and are interpreted as fluvial distributary systems. The deposits were subjected to syn-sedimentary folding which exerted a strong control on the general topography at the time of alluvial system development. The balance between the accommodation space created by the tectonics operative over the Plio-Pleistocene, coupled with the sediment discharge, determined the alluvial system morphology and sedimentological architecture. The eventual cessation of deposition from the larger catchments is explained by their capture by an aggressive external drainage developing south of the structural lineament. The switching off of sediment supply from the larger catchments, coupled with high subsidence rates in the sediment dispersal area enabled the smaller alluvial system, still connected to its source area north of the lineament, to expand. Eventually the remaining catchment areas were also reduced by continued river capture. The study emphasizes the significance of river capture in re-routing both sediment and water discharge between sedimentary basins (the Sorbas Basin lost 15% of its original sediment and water budget to the Lucainena fan delta and associated coastal system of the Carboneras Basin to the south) and its subsequent effect on the pirated area (reduction in rates and changes in style of deposition). These latter factors outweighed the direct impact of tectonics and climate on the later development of the studied alluvial systems. The study emphasizes that to maximize the effects of sediment re-routing on actively aggrading alluvial systems (in the sedimentary basins) the positioning of the capture point is crucial and is most effective where (1) the sediment supply areas (mountain catchments) are pirated closest to their outlet (mountain front) into the receiving sedimentary basin and (2) the pirating drainages are external to the sedimentary basin.

Tectonics are perceived to be a major control on the positioning and long‐term evolution of alluvial systems. By increasing slope gradients through uplift and tilting, or by changing local base‐level, incision can be stimulated leading to a switch in the active area of sedimentation. An example of such a sedimentary response to tectonic activity is provided by well‐exposed late Pliocene/early Pleistocene alluvial sediments of the Salmerón Formation from the western margins of the Vera Basin, SE Spain. Early stage palaeogeographic reconstructions demonstrate the occurrence of two alluvial fan bodies with distinct palaeocurrent and provenance signatures that suggest sediment source areas from the north (Sierra Lisbona) and south (Sierra de Bédar) of the study area. Late stage reconstructions suggest fan abandonment and indicate the occurrence of a braided river system sourced from the Sierra de Bédar in the south. Proximal parts of this braided river are incised by up to 100 m into underlying fan sediments sourced from the Sierra de Bédar. In distal areas, incision is negligible and the braided system forms a conformable sedimentary succession with underlying fan sediments sourced from the Sierra Lisbona. The switch from alluvial fan to braided river sedimentation and the spatially variable patterns of incision into the alluvial fan bodies can be accounted for by a phase of deformation which affected the Vera Basin during the early Pleistocene. Extensional faulting resulted in uplift and subsidence, leading to localized tilting of depositional surfaces in distal areas of the southern fan. Increased stream power resulted in headward incision through mid and proximal fan areas. Once the system became fully trenched distal base‐level controls became effective in generating an enlarged catchment area and continuing incision. The resultant changes dramatically increased both sediment and water discharge to the alluvial system and a switch to braided river sedimentation.

Conventionally, a fall in base level is seen as stimulating incision into the distal zones of alluvial fans. In the Cabo de Gata ranges of southeast Spain evidence exists to the contrary. Two sets of Quaternary coastal alluvial fans demonstrate the interaction between climatically-driven variations in the supply of sediment and eustatically-driven changes in base level. The fans are supplied from Miocene volcanic terrain within which no evidence can be found for major tectonic deformation during the period of fan development. The evolution of the east-coast fans has been affected by variations in sediment supply and changes in sea level. The west-coast fans were buffered from the effects of changes in sea level by coastal barriers. Three phases of past sedimentation can be identified on the fans. These can be differentiated on the basis of field observations of soil profiles (particularly colour of the B horizons and accumulation of CaCO3), and laboratory analyses of sequential iron oxide extractions and magnetic mineral properties. The two earlier (major) sedimentation phases were coincident with global glacials (>ca. 135 ka and ca.85-10 ka, based on the stratigraphy and uranium/thorium dating of the coastal sediments). High sea levels during the intervening interglacial and during the Holocene caused erosion of the distal zones of the east-coast fans which led to channel incision into the fan surfaces. On the west-coast fans no such incision occurred, simply proximal incision by small fanhead trenches. The youngest (relatively minor) phase of fan sedimentation has occurred during the Holocene. These contrasting contexts have produced differing styles of fans, with telescopic fan morphology on the east-coast and stacked morphology on the west-coast fans. The differences are reflected in the fan profiles, with steeper gradients dominating the east-coast fans, and extensive lower gradient distal surfaces on the west-coast fans. Fan morphometry, based on analysis of the residuals from drainage area to fan area and gradient regressions, also differentiates between the fan contexts. The fan building phases appear to be controlled proximally by climatically-driven pulses of sediment supplied to the fans. These occurred during global glacials coincident with low sea levels, and caused fan progradation onto the exposed foreshore. The intervening global interglacials were times of little fan sedimentation, and on the east coast, where high sea levels were able to erode the fan toes, deep through-fan dissection ensued., This research has been partially supported by the British Council/Spanish Government—Acciones Integradas Programme. Diane Spivey helped with some of the field surveying. John Dearing gave help in the interpretation of the mineral magnetic data. We also thank the staff of the Graphics section of the Department of Geography, University of Liverpool for producing most of the illustrations.