Avulsion regimes in southeast Texas rivers

Avulsions – relatively sudden changes in course, or establishment of new anabranches – are an important process in alluvial rivers. Their key role in floodplain construction and alluvial architecture, and the general conditions favouring avulsions, are well known. However, avulsion processes and evolution, and the factors controlling avulsion regimes, are poorly understood. In the southeast Texas coastal plain, where avulsions are common features of the river valleys, avulsions were studied on the lower Brazos, Navasota, Trinity, Neches and Sabine rivers using a combination of aerial imagery, digital elevation models and field surveys. Avulsions have important influences on the surface morphology and contemporary processes in all five rivers. Features associated with avulsions are active and distinct throughout the study area, and all the rivers have experienced geologically (if not historically) recent avulsions. However, no two of the study rivers have the same contemporary avulsion regime. First‐order differences in avulsion style are controlled by the stage of valley filling, and within the three rivers characterized by an unfilled incised valley, antecedent morphology associated with late Quaternary and Holocene coastal and fluvial‐deltaic processes accounts for the major differences. In the Navasota (27 avulsions in 185 km) and Neches (21 in 340 km) rivers, subchannels associated with avulsions exist in all stages of development from active to infilled, and some have occurred in recent decades. The other rivers have fewer avulsions, but both the Sabine and Trinity have experienced historic channel shifts. Only the Brazos River has experienced no avulsions within the past c. 300 years. Results show that even within a region of similar environmental controls and geological history local variations in inherited morphology can result in different avulsion regimes. Copyright © 2008 John Wiley & Sons, Ltd.     

Variability in erosion rates related to the state of landscape transience in the semi‐arid Chilean Andes

We quantify erosion rates in the higher sectors of the Huasco Valley, in the Main Cordillera of the semi‐arid Andes of Chile, using elevation differences between three successive geomorphic markers (pediments and paleo‐valleys) and the present day valley. Available Ar‐Ar ages of Neogene pediments are used to estimate mean erosion rates for the three periods (16 to 13 My, 13 to 8 My, and following 8 My). The landscape of the Huasco Valley is in a transient state, as indicated by well‐preserved pediment surfaces in interfluves, valleys deeply incised by fluvial and glacial erosion and scarped head‐valleys that represent the current knickzones. Higher erosion rates (45–75 m/My) are calculated for the more recent period (< 8 My) during which deep incision developed compared to previous periods (6–31 m/My). Quantitative data indicate that glaciers had a much higher erosional capability than fluvial activity in the higher sectors of the Main Cordillera. Comparison with erosion rates calculated in other drainage basins of the Chilean Andes suggests that the variability of erosion rates depends on the landscape's transient erosive state. The landscape's geomorphologic response to the uplift of the Main Cordillera results in the retreat of a knickzone, for which retreat velocity depends on precipitation rate pattern and glacial erosion intensity. Copyright © 2011 John Wiley & Sons, Ltd.     

Mass flows and river response in rapid uplifting regions – A case of lower Yarlung Tsangpo basin,southeast Tibet,China

The fluvial geomorphology in tectonically active (particularly rapid uplift) regions often undergoes continuous change. The rapid uplift is coincident with high erosion rates; consequently, incised valleys are formed. Mass flows (for example, avalanches, landslides, and debris flows) in incised valleys can markedly influence fluvial processes and even reshape valley geomorphology. However, these processes and long-term evolution corresponding to mass flows require further clarification. Field campaigns were carried out in the region near the Yigong Tsangpo and Palong Tsangpo Rivers (hereafter the Yigong and Palong Rivers), the two largest tributaries of the lower Yarlung Tsangpo River, to examine the feedback between fluvial processes and mass flows. Remote sensing images from recent decades were used to compare the channel morphology before and after typical mass flows (particularly catastrophic ones). The morphology of the lower Yigong River has evidently been impacted by landslides, while that of the Palong River has mainly been shaped by glacial processes and debris flows. At present, the morphology of the latter consists of alternating sections of gorges and wide valleys, with a staircase-like longitudinal profile. The gorge sections exhibit single and deeply incised channels with a high-gradient channel bed and terraces. In contrast, the wide valley sections consist of lakes, braided or anabranching channels, gentle bed gradients, and thick alluvial deposits. Debris flows occur more frequently in gullies in the reaches of the gorge sections and rarely in gullies along the wide valley sections. The occurrence of mass flow events has resulted in an imbalance of the previous (quasi-)equilibrium in the river morphology; however, this has triggered negative feedback that is driving the transient river morphology to a new state of (quasi-)equilibrium.     

Modern sediment yield compared to geologic rates of sediment production in a semi‐arid basin,New Mexico: assessing the human impact

An Erratum has been published for this article in Earth Surface Processes and Landforms 29(13) 2004, 1707. In the semi‐arid Arroyo Chavez basin of New Mexico, a 2·28 km² sub‐basin of the Rio Puerco, we contrasted short‐term rates (3 years) of sediment yield measured with sediment traps and dams with long‐term, geologic rates (～10 000 years) of sediment production measured using ¹⁰Be. Examination of erosion rates at different time‐scales provides the opportunity to contrast the human impact on erosion with background or geologic rates of sediment production. Arroyo Chavez is grazed and we were interested in whether differences in erosion rates observed at the two time‐scales are due to grazing. The geologic rate of sediment production, 0·27 kg m^?2 a^?1 is similar to the modern sediment yields measured for geomorphic surfaces including colluvial slopes, gently sloping hillslopes, and the mesa top which ranged from 0·12 to 1·03 kg m^?2 a^?1. The differences between modern sediment yield and geologic rates of sediment production were most noticeable for the alluvial valley ?oor, which had modern sediment yields as high as 3·35 kg m^?2 a^?1. The hydraulic state of the arroyo determines whether the alluvial valley ?oor is aggrading or degrading. Arroyo Chavez is incised and the alluvial valley ?oor is gullied and piped and is a source of sediment. The alluvial valley ?oor is also the portion of the basin most modi?ed by human disturbance including grazing and gas pipeline activity, both of which serve to increase erosion rates. Copyright © 2004 John Wiley & Sons, Ltd.     

Controls on valley spacing in landscapes subject to rapid base‐level fall

What controls the architecture of drainage networks is a fundamental question in geomorphology. Recent work has elucidated the mechanisms of drainage network development in steadily uplifting landscapes, but the controls on drainage‐network morphology in transient landscapes are relatively unknown. In this paper we exploit natural experiments in drainage network development in incised Plio‐Quaternary alluvial fan surfaces in order to understand and quantify drainage network development in highly transient landscapes, i.e. initially unincised low‐relief surfaces that experience a pulse of rapid base‐level drop followed by relative base‐level stasis. Parallel drainage networks formed on incised alluvial‐fan surfaces tend to have a drainage spacing that is approximately proportional to the magnitude of the base‐level drop. Numerical experiments suggest that this observed relationship between the magnitude of base‐level drop and mean drainage spacing is the result of feedbacks among the depth of valley incision, mass wasting and nonlinear increases in the rate of colluvial sediment transport with slope gradient on steep valley side slopes that lead to increasingly wide valleys in cases of larger base‐level drop. We identify a threshold magnitude of base‐level drop above which side slopes lengthen sufficiently to promote increases in contributing area and fluvial incision rates that lead to branching and encourage drainage networks to transition from systems of first‐order valleys to systems of higher‐order, branching valleys. The headward growth of these branching tributaries prevents the development of adjacent, ephemeral drainages and promotes a higher mean valley spacing relative to cases in which tributaries do not form. Model results offer additional insights into the response of initially unincised landscapes to rapid base‐level drop and provide a preliminary basis for understanding how varying amounts of base‐level change influence valley network morphology. Copyright © 2015 John Wiley & Sons, Ltd.     

Geomorphological response of fluvial and coastal terraces to Quaternary tectonics and climate as revealed by geostatistical topographic analysis

Geostatistical topographic analysis is widely recognized as a useful tool for the statistical reconstruction of planar geomorphic markers from relict surfaces. This work is aimed at improving the geostatistical approach used in previous works and developing a method for evaluating the incision rates of rivers in their lower catchments during the Late Quaternary. We chose the major valleys of the Adriatic foothills (central Italy), affected since Late Miocene by a differential tectonic uplift which is still active. In particular, (i) we applied the geostatistical analysis to reconstruct the original top‐surfaces of fluvial‐to‐coastal terrace bodies at the Metauro River and Cesano River mouths; (ii) we performed correlations between the height distribution of the alluvial terrace sequences and the Quaternary climatic curve to estimate the average long‐term fluvial incision rates in the lowermost reaches of the Metauro, Cesano, Misa and Esino Rivers. The obtained averaged incision rates have been interpreted also in the light of the Stream‐Length Gradient Index (SL Index), Steepness Index (K_s), and Concavity Index (θ) as proxies of the stream‐power per unit length. Results confirm that geostatistical and terrain analysis of topographic and geometric arrangements of fluvial and coastal terraces is an effective tool in detecting geomorphic and tectonic factors inducing perturbations on planar geomorphic markers. In particular, we better delineated the surface geometry and boundaries of well‐developed coastal fans at the mouths of the Metauro and Cesano Rivers, already recognized in previous works through sedimentological, morphostratigraphic, and chronological data. Moreover, we found evidence for cut‐and‐fill phases that took place during and immediately after the river aggradation of the late Quaternary glacial periods. Despite the Slope–Area analysis evidenced a widespread influence of the regional differential uplift on single river basin configuration, we observed some space and time variability of averaged incision rates for adjacent valleys, mainly explained by physiographic configuration and dynamics of drainage network. Copyright © 2011 John Wiley & Sons, Ltd.     

Log‐jams and avulsions in the San Antonio River Delta,Texas

The San Antonio River Delta (SARD), Texas, has experienced two major avulsions in the past 80 years, and a number of other historical and Holocene channel shifts. The causes and consequences of these avulsions – one of which is ongoing – were examined using a combination of fieldwork, geographic information system (GIS) analysis, and historical information to identify active, semi‐active, and paleochannels and the sequence of shifting flow paths through the delta. The role of deposition patterns and antecedent morphology, large woody debris jams, and tectonic influences were given special attention. Sedimentation in the SARD is exacerbated by tectonic effects. Channel aggradation is ubiquitous, and superelevation of the channel bed above the level of backswamp areas on the floodplain is common. This creates ideal setup conditions for avulsions, and stable, cohesive fine‐grained banks favor avulsions rather than lateral migration. Flood basins between the alluvial ridges associated with the aggraded channels exist, but avulsions occur by re‐occupation of former channels found within or connected to the flood basins. Large woody debris and channel‐blocking log‐jams are common, and sometimes displace flow from the channel, triggering crevasses. However, a large, recurring log‐jam at the site of the ongoing avulsion from the San Antonio River into Elm Bayou is not responsible for the channel shift. Rather, narrow, laterally stable channels resulting from flow splits lead to accumulation of wood. Some aspects of the SARD avulsion regime are typical of other deltas, while others are more novel. These includes avulsions involving tributaries and subchannels within the delta as well as from the dominant channel; tectonic influences on delta backstepping and on channel changes within the delta; avulsions as an indirect trigger for log‐jam formation (as well as vice‐versa); and maintenance of a multi‐channel flow pattern distinct from classic anastamosing or distributary systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Oblique aggradation: a novel explanation for sinuosity of low‐energy streams in peat‐filled valley systems

Low‐energy streams in peatlands often have a high sinuosity. However, it is unknown how this sinuous planform formed, since lateral migration of the channel is hindered by relatively erosion‐resistant banks. We present a conceptual model of Holocene morphodynamic evolution of a stream in a peat‐filled valley, based on a palaeohydrological reconstruction. Coring, ground‐penetrating radar (GPR) data, and ¹⁴C and OSL dating were used for the reconstruction. We found that the stream planform is partly inherited from the Late‐Glacial topography, reflecting stream morphology prior to peat growth in the valley. Most importantly, we show that aggrading streams in a peat‐filled valley combine vertical aggradation with lateral displacement caused by attraction to the sandy valley sides, which are more erodible than the co‐evally aggrading valley‐fill. Owing to this oblique aggradation in combination with floodplain widening, the stream becomes stretched out as channel reaches may alternately aggrade along opposed valley sides, resulting in increased sinuosity over time. Hence, highly sinuous planforms can form in peat‐filled valleys without the traditional morphodynamics of alluvial bed lateral migration. Improved understanding of the evolution of streams provides inspiration for stream restoration. Copyright © 2016 John Wiley & Sons, Ltd.     

贵州龙宫水系坡立谷湖群水文地貌结构与功能特征

龙宫是贵州著名的喀斯特网景点,该区在第四纪构造运动中由于地貌回春,形态逆向发育,水系频繁变迁,喀斯特地块渗漏导致地表流集中转入地下,从而使流水谷地首先盲谷化并逐步形成洼地,在这个过程中,流域地貌复杂响应基面变化,发育了龙宫坡立谷,这些坡立谷的形成与流域形态演化相联系而不同于I.Gams对划分的类型,它们在水系结构中与地下管道相患联,每逢雨季,将大量上游汇水滞蓄盆内,形成季节性喀斯特湖,对下游起到良好的天然调节作用。     

The Role of Fluvial and Glacial Erosion in Landscape Evolution: The Ben Ohau Range,New Zealand

A morphometric comparison of valleys has been made for the Ben Ohau Range in the central Southern Alps of New Zealand. The range is undergoing rapid tectonic transport and uplift. The humid north of the range is a glacial trough-and-arête landscape, with a temperate glacial climate. The dry south has rounded divides and plateau remnants dissected by fluvial valleys. Assuming that space–time substitution allows today's spatial valley-form transition to represent evolutionary stages in valley development, the tectonic history allows time constraints to be placed on the rate of transition to an alpine glacial landscape. Morphometric change has been quantified using hypsometric curves, and distance–elevation plots of cirque and valley-floor altitudes. Ancestral fluvial valleys have less concave long profiles but are stepped at altitude owing to the presence of high-level cirques and remnant plateau surfaces, and possess a low proportion of land area at low elevation. Increasing glacial influence is manifest as smoother, more deeply concave long profiles and U-shaped cross-profiles associated with a higher proportion of the land area at lower elevation. The full morphological transition has involved up to 2.4 km of vertical denudation over the 4 Ma lifetime of the mountain range, of which 80 per cent would have occurred by preglacial fluvial erosion. Combining the trajectory of tectonic transport with reconstructed glaciation limits and climatic history, it is indicated that about 200 ka of temperate glacial erosion produces recognizable trough-and areête topography. Mean and modal relief increase where glacial activity is confined to cirques, but decrease when trough incision by ice becomes established as a dominant process in the landscape. © 1997 by John Wiley & Sons, Ltd.     

Geomorphic evolution of small river–lake‐systems in northeast Germany during the Late Quaternary

This study investigates the post‐glacial development of four small river–lake systems in the Weichselian belt of northern central Europe. The valleys investigated are part of an immature drainage system characterized by frequent and abrupt changes in flow direction and the presence of numerous stagnant‐ice depressions in the valley course. The depressions contain thick sedimentary sequences which provide excellent archives for the reconstruction of the post‐glacial valley development. Study results indicate that the valleys reuse segments of former subglacial meltwater channels. During the Late Pleniglacial these channels carried meltwater streams. Stagnant‐ice melting occurred in stages from the Oldest Dryas to the early Holocene and was often followed by the formation of lakes in the valley course. Flow reversals occurred during the Late‐glacial–Holocene transition and were in response to general base‐level lowering caused by stagnant‐ice melting, headwater erosion and lake overspills. Lacustrine deposition typically started during the early Late‐glacial comprising mainly silicate gyttjas, whereas organic gyttjas and peats accumulated during the Allerød. The Younger Dryas is associated with a marked increase in fluvial and aeolian sedimentation, and lake‐level high stands. This was followed by early Holocene lake‐level low stands and a subsequent stabilization phase with decreasing silicate input and increasing organic lacustrine deposition. In general, dramatic changes in Late Pleniglacial to early Holocene sedimentation suggest that small‐scale catastrophic events played a more important role in triggering geomorphic changes then previously recognized. Infilling continued until peat accumulation and terrestrialization of lake basins became widespread during the mid‐ to late Holocene. Beginning in the late Holocene anthropogenic influences become important mainly involving an increase in sediment supply due to forest clearing and land use, followed by mill stowage, river course correction and anthropogenic lake‐level manipulations. Copyright © 2007 John Wiley & Sons, Ltd.     

Tributary valley impoundment by trunk river floodplain development: a case study from the KwaZulu‐Natal Drakensberg foothills,eastern South Africa

This paper investigates the origin and geomorphic evolution of Stillerust Vlei, a 189 ha wetland located approximately 150 km northwest of Durban in the temperate submontane foothills of the KwaZulu‐Natal Drakensberg Mountains. The investigation confirms the findings of previous research on the arid to semi‐arid South African interior, which established that many floodplain wetlands in eastern South Africa are located upstream of resistant rock barriers (dolerite intrusions) that cross river courses and form stable local base levels. Upstream of these barriers, rivers laterally plane less resistant Karoo sedimentary rocks (sandstones, mudstones), creating broad, low gradient valleys conducive to the formation of floodplain wetlands. In addition, the study examines how local levee and alluvial ridge accretion on the floodplain of Stillerust Vlei has impounded a small tributary valley, and drawing on observations of similar wetlands in the region, the paper explains the origin and geomorphic evolution of wetlands in floodplain‐abutting valleys, and associated streams that commonly become discontinuous toward their confluence with the trunk (floodplain) river. Controls on the origin and geomorphic evolution of Stillerust Vlei are placed within the context of slope‐channel decoupling and (dis)connectivity in sediment delivery, illustrating that wetlands are environments of deposition. As a result of dynamic trunk‐tributary relations, Stillerust Vlei holds a diversity of geomorphic features, and thus provides potential habitat for a diversity of biota. Copyright © 2008 John Wiley & Sons, Ltd.     

Lateral bedrock erosion and valley formation in a heterogeneously layered landscape,Northeast Kansas

In this study, we present direct field measurements of modern lateral and vertical bedrock erosion during a 2-year study period, and optically stimulated luminescence (OSL) ages of fluvial material capping a flat bedrock surface at Kings Creek located in northeast Kansas, USA. These data provide insight into rates and mechanisms of bedrock erosion and valley-widening in a heterogeneously layered limestone-shale landscape. Lateral bedrock erosion outpaced vertical incision during our 2-year study period. Modern erosion rates, measured at erosion pins in limestone and shale bedrock reveal that shale erosion rate is a function of wetting and drying cycles, while limestone erosion rate is controlled by discharge and fracture spacing. Variability in fracture spacing amongst field sites controls the size of limestone block collapse into the stream, which either allowed continued lateral erosion following rapid detachment and transport of limestone blocks, or inhibited lateral erosion due to limestone blocks that protected the valley wall from further erosion. The OSL ages of fluvial material sourced from the strath terrace were older than any material previously dated at our study site and indicate that Kings Creek was actively aggrading and incising throughout the late Pleistocene. Coupling field measurements and observations with ages of fluvial terraces can be useful to investigate the timing and processes linked to how bedrock rivers erode laterally over time to form wide bedrock valleys.     

Hydrogeomorphological differentiation between floodplains and terraces

Floodplains and terraces in river valleys play important roles in the transport dynamics of water and sediment. While flat areas in river valleys can be identified from LiDAR data, directly characterizing them as either floodplain or terraces is not yet possible. To address this challenge, we hypothesize that, since geomorphic features are strongly coupled to hydrological and hydraulic dynamics and their associated variability, there exists a return frequency, or possibly a narrow band of return frequencies, of flow that is associated with floodplain formation; and this association can provide a distinctive signature for distinguishing them from terraces. Based on this hypothesis we develop a novel approach for distinguishing between floodplains and terraces that involves transforming the transverse cross‐sectional geometry of a river valley into a curve, named a river valley hypsometric (RVH) curve, and linking hydraulic inundation frequency with the features of this curve. Our approach establishes that the demarcation between floodplains and terraces can be established from the structure of steps and risers in the RVH curves which can be obtained from the DEM data. Further, it shows that these transitions may themselves be shaped by floods with 10‐ to 100‐year recurrence. We additionally show that, when floodplain width and height (above channel bottom) are normalized by bankfull width and depth, the ratio lies in a narrow range independent of the scale of the river valley. Copyright © 2017 John Wiley & Sons, Ltd.     

Geomorphic process from topographic form: automating the interpretation of repeat survey data in river valleys

The ability to quantify the processes driving geomorphic change in river valley margins is vital to geomorphologists seeking to understand the relative role of transport mechanisms (e.g. fluvial, aeolian, and hillslope processes) in landscape dynamics. High‐resolution, repeat topographic data are becoming readily available to geomorphologists. By contrasting digital elevation models derived from repeat surveys, the transport processes driving topographic changes can be inferred, a method termed ‘mechanistic segregation.’ Unfortunately, mechanistic segregation largely relies on subjective and time consuming manual classification, which has implications both for its reproducibility and the practical scale of its application. Here we present a novel computational workflow for the mechanistic segregation of geomorphic transport processes in geospatial datasets. We apply the workflow to seven sites along the Colorado River in the Grand Canyon, where geomorphic transport is driven by a diverse suite of mechanisms. The workflow performs well when compared to field observations, with an overall predictive accuracy of 84% across 113 validation points. The approach most accurately predicts changes due to fluvial processes (100% accuracy) and aeolian processes (96%), with reduced accuracy in predictions of alluvial and colluvial processes (64% and 73%, respectively). Our workflow is designed to be applicable to a diversity of river systems and will likely provide a rapid and objective understanding of the processes driving geomorphic change at the reach and network scales. We anticipate that such an understanding will allow insight into the response of geomorphic transport processes to external forcings, such as shifts in climate, land use, or river regulation, with implications for process‐based river management and restoration. Copyright © 2017 John Wiley & Sons, Ltd.     

Epigenetic gorges in fluvial landscapes

Epigenetic gorges form when channels that have been laterally displaced during episodes of river blockage or aggradation incise down into bedrock spurs or side‐walls of the former valley rather than excavating unconsolidated fills and reinhabiting the buried paleovalley. Valley‐filling events that promote epigenetic gorges can be localized, such as a landslide dam or an alluvial/debris flow fan deposit at a tributary junction, or widespread, such as fluvial aggradation in response to climate change or fluctuating base‐level. The formation of epigenetic gorges depends upon the competition between the resistance to transport, strength and roughness of valley‐filling sediments and a river's ability to sculpt and incise bedrock. The former affects the location and lateral mobility of a channel incising into valley‐filling deposits; the latter determines rates of bedrock incision should the path of the incising channel intersect with bedrock that is not the paleovalley bottom. Epigenetic gorge incision, by definition, post‐dates the incision that originally cut the valley. Strath terraces and sculpted bedrock walls that form in relation to epigenetic gorges should not be used to directly infer river incision induced by tectonic activity or climate variability. Rather, they are indicative of the variability of short‐term bedrock river incision and autogenic dynamics of actively incising fluvial landscapes. The rate of bedrock incision associated with an epigenetic gorge can be very high (>1 cm/yr), typically orders of magnitude higher than both short‐ and long‐term landscape denudation rates. In the context of bedrock river incision and landscape evolution, epigenetic gorges force rivers to incise more bedrock, slowing long‐term incision and delaying the adjustment of rivers to regional tectonic and climatic forcing. Copyright © 2008 John Wiley & Sons, Ltd.     

Geomorphic imprint of landslides on alpine river systems,southwest New Zealand

An inventory of 846 mass movements, mainly landslides, in two alpine regions of southwest New Zealand was created to explore the geomorphic impacts of slope‐failure processes on river channels and valley floors. In total, 213 (i.e. 27 per cent) of the slope failures descended to valley floors, affecting the geomorphology of trunk channels (catchment area A_C > 10 km²) and valley floors in recurring patterns. A nominal classification system is introduced for characterizing (a) the physical contact nature between landslides and river channels, and (b) the resulting geomorphic consequences for drainage. Although landslide area A is useful for estimating the length of channel directly impacted by debris, it does not necessarily predict the direction of fluvial response or type of impact. Dominant persistent geomorphic imprints of bedrock landslides include channel occlusions and landslide dams in South Westland and Fiordland, respectively. Differences in size distribution and geomorphic effects on river systems between the two study regions are attributed to bedrock geology, tectonics and sediment flux. Although South Westland rivers are more frequently affected by landslides, disrupting long‐term effects such as blockage are more persistent in Fiordland. Copyright © 2005 John Wiley & Sons, Ltd.     

Reconstructing fluvial incision rates based on palaeo-water tables in chalk karst networks along the Seine valley (Normandy,France)

Quantifying rates of river incision and continental uplift over Quaternary timescales offer the potential for modelling landscape change due to tectonic and climatic forcing. In many areas, river terraces form datable archives that help constrain the timing and rate of valley incision. However, old river terraces, with high-level deposits, are prone to weathering and often lack datable material. Where valleys are incised through karst areas, caves and sediments can be used to reconstruct the landscape evolution because they can record the elevation of palaeo-water tables and contain preserved datable material. In Normandy (N. France), the Seine River is entrenched into an extensive karstic chalk plateau. Previous estimates of valley incision were hampered by the lack of preserved datable fluvial terraces. A stack of abandoned phreatic cave passages preserved in the sides of the Seine valley can be used to reconstruct the landscape evolution of the region. Combining geomorphological observations, palaeomagnetic and U/Th dating of speleothem and sediments in eight caves along the Lower Seine valley, we have constructed a new age model for cave development and valley incision. Six identified cave levels up to ∼100 m a.s.l. were formed during the last ~1 Ma, coeval with the incision of the Seine River. Passage morphologies indicate that the caves formed in a shallow phreatic/epiphreatic setting, modified by sediment influxes. The valley's maximum age is constrained by the occurrence of late Pliocene marine sand. Palaeomagnetic dating of cave infills indicates that the highest-level caves were being infilled prior to 1.1 Ma. The evidence from the studied caves, complemented by fluvial terrace sequences, indicates that rapid river incision occurred during marine isotope stage (MIS) 28 to 20 (0.8–1 Ma), with maximal rates of ~0.30 m ka⁻¹, dropping to ~0.08 m ka⁻¹ between MIS 20–11 (0.8–0.4 Ma) and 0.05 m ka⁻¹ from MIS 5 to the present time. © 2020 John Wiley & Sons, Ltd.     

Fluvial terraces and their implications for Weihe River valley evolution in the Sanyangchuan Basin

Fluvial terraces are important geomorphic markers for modern valley development.When coupled with numeric ages,terraces can provide abundant information about tectonic,climatic,paleohydrological and the paleoenvironmental changes.On the basis of the paleomagnetic,electron spin resonance(ESR) and optically stimulated luminescence(OSL) dating,in addition to an investigation of local loess-paleosol sequences,we confirmed that 13 fluvial terraces were formed,and then preserved,along the course of the Upper Weihe River in the Sanyangchuan Basin over the past 1.2 Ma.Analyses of the characteristics and genesis of these terraces indicate that they resulted from the response of this particular river system to climate change over an orbital scale.These changes can further be placed within the context of local and regional tectonic uplift,and represent an alternation between lateral migration and vertical incision,dependent upon the predominance of climatic and tectonic controls during different periods.Most of the terraces are strikingly similar in that they have several meters of paleosols which have developed directly on top of fluvial deposits located on the terrace treads,suggesting that the abandonment of terraces due to river incision occurred during the transitions from glacial to interglacial climates.The temporal and spatial differences in the distribution patterns of terraces located on either side of the river valley indicate that a tectonic inversion occurred in Sanyangchuan Basin at-0.62 Ma,and that this was characterized by a transition from overall uplift to depression induced by fault activity.Synthesized studies of the Basin's terraces indicate that formation of the modern valley of the Upper Weihe River may have begun in the late Early Pleistocene between1.4-1.2 Ma.     

Late Quaternary tectono-geomorphological investigations of Zanskar Basin,NW Himalaya,India, using geospatial techniques

Geospatial techniques play a crucial role in geomorphic studies, particularly in the challenging terrains like mountainous regions, inaccessible areas and densely vegetated landscapes, where geomorphic features cannot be recorded easily. Tectono-geomorphologic observations provide important clues regarding the landscape evolution, morpho-dynamics and ongoing tectonism of the region. The present study has been carried out in the Zanskar Basin (ZB), located to the south of the Indus Tsangpo Suture Zone (ITSZ), in the hinterland of the NW Himalaya. This study has been carried out to assess and evaluate active tectonics by employing tectono-geomorphic analysis, dynamics in drainage networks, geomorphological field observations and the Geographic Information System (GIS) environment. High-resolution satellite images, topographic maps and the Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) were used to generate primary data sets, which were corroborated with field investigations for valid inferences. The geometry of the ZB suggests that continuous tectonic activity exerts first-order control on the overall shape, size and structure of the ZB. This first-order response is clearly reflected in the landforms modified by tectonic processes, namely, linear mountain fronts, elongated shape and tilting of the basin, braided and meandering river courses and lower stream length gradient index values in hard rock terrain. The ZB exhibits several eye-catching geomorphic features, such as well-defined triangular facets with wide base lengths and wine-glass valleys with small outlets along the footwall block of the Zanskar Shear Zone/South Tibetan Detachment System (ZSZ/STDS), as well as the presence of wind gaps, water gaps, bedrock incision, incised and entrenched valleys, narrow gorges and a high incision rate inferring active tectonics and recent uplift in the region. In addition, the existence of uplifted river terraces, as well as the stepped morphology of fans and strath terraces, suggests that the region is experiencing recent activity and ongoing tectonic uplift. These modified geomorphic characteristics suggest that the hinterland, which is part of the NW Himalaya, is tectonically quite active and has experienced a differential rate of tectonics during its evolution. The quantified geomorphic indices and their relations with the tectonics, climate and erosion activity infer that the basin geometry is mostly controlled by the ZSZ/STDS that dips 20°–70° NE, the south-dipping Zanskar Counter Thrust (ZCT) and other local tectonic elements like the Choksti Thrust (CT), Stondgey Thrust, Zangla Thrust and tectonic structures. The synergised results of quantified geomorphic indices and tectono-geomorphic evidence in the ZB strongly indicate that both the past and ongoing tectonism have significantly shaped and modified geomorphology of the ZB.