Geomorphic Dynamics Research Articles

Interplay between soil formation and geomorphic processes along a soil catena in a Mediterranean mountain landscape: an integrated pedological and geophysical approach

Soil spatial distribution often shows high variability because of geo-environmental settings affecting soil formation and geomorphic processes. The paper describes a combined pedological–geophysical approach for investigating the interplay among soil-forming processes, geomorphic dynamics and physico-chemical soil properties in a soil catena in a Mediterranean mountain landscape (southern Italy). Stocks of soil organic carbon (SOC) and total nitrogen (STN) were determined and their interactions with slope position analyzed. Five slope units were defined along the soil catena (summit, shoulder, back, foot, and toe slopes), and in each of them, a soil profile was dug and analyzed. Thickness of soil horizons and contact between soil and parent material were determined by a 2D-electrical resistivity tomography survey. Results showed granitic bedrock is characterized by vary weathering degrees strongly affecting soil type development. Moreover, soil properties variation is related to topographic features and water erosion processes. Soil chemical composition showed high values of SiO2 and Al2O3, probably due to inheritance from underlying granitic parent material. Particularly, Fe2O3 content increased with weathering conditions as shown by chemical index of alteration values. Along foot and toe slopes, reworking and accumulation of colluvial soil lead to the formation of thicker accretionary A horizon. Consequently, summit areas, together with foot and toe slopes had greater SOC and STN stocks than back and shoulder slopes. That because soils are thicker and richer in SOC on summit and valley bottom than in shoulder and back-slope positions. The proposed approach has showed the benefit of an integrated methodology for investigating spatial variations along soil catenas.

Climate and relief‐induced controls on the temporal variability of denudation rates in a granitic upland

AbstractHow soil erosion rates evolved over the last about 100 ka and how they relate to environmental and climate variability is largely unknown. This is due to a lack of suitable archives that help to trace this evolution. We determined in situ cosmogenic beryllium‐10 (10Be) along vertical landforms (tors, boulders and scarps) on the Sila Massif to unravel their local exhumation patterns to develop a surface denudation model over millennia.Due to the physical resistance of tors, their rate of exhumation may be used to derive surface and, thus, soil denudation rates over time. We derived soil denudation rates that varied in the range 0–0.40 mm yr‐1. The investigated boulders, however, appear to have experienced repositioning processes about ~20–25 ka bp and were therefore a less reliable archive. The scarps of the Sila upland showed a rapid bedrock exposure within the last 8–15 ka. Overall, the denudation rates increased steadily after 75 ka bp but remained low until about 17 ka bp. The exhumation rates indicate a denudation pulse that occurred about 17–5 ka bp. Since then the rates have continuously decreased.We identify three key factors for these developments – climate, topography and vegetation. Between 75 and 17 ka bp, climate was colder and drier than today. The rapid changes towards warmer and humid conditions at the Pleistocene–Holocene transition apparently increased denudation rates. A denser vegetation cover with time counteracted denudation.Topography also determined the extent of denudation rates in the upland regime. On slopes, denudation rates were generally higher than on planar surfaces. By determining the exhumation rates of tors and scarps, soil erosion rates could be determined over long timescales and be related to topography and particularly to climate. This is key for understanding geomorphic dynamics under current environmental settings and future climate change. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.

Origin and Holocene geomorphological evolution of the landslide-dammed basin of la Narse de la Sauvetat (Massif Central, France)

This work is the first geomorphological analysis of La Narse de la Sauvetat, a hydromorphic basin located in the southern Limagne plain (French Massif Central) which had never been studied despite its great palaeoenvironmental interest. We explore the potential of its sedimentary archives to provide valuable data on Holocene geomorphic dynamics and their sensitivity to local and regional hydro-climatic changes. In order to characterize the nature and the morphostratigraphic and pedosedimentary evolution of the basin, we used an integrative approach involving geomorphological mapping, hand auger stratigraphic cross-sections, topographic analysis based on LiDAR data and stereophotogrammetric reconstruction, geophysical prospection and radiocarbon dating. Results revealed a palaeotopography fossilized under 6 m of sedimentary infilling, with a clear shift from fluvial to hydromorphic conditions circa 2800 cal BCE, separated by a level with colluvial and lacustrine features. A detailed analysis of the eastern border of the basin also suggests the existence of a former open valley in place of the current closed depression, now locked in by a topographic threshold forming a dam. The sedimentary aggradation of the basin started with an alluvial phase during the Upper Pleistocene and probably continued into the Middle and Late Holocene, consistently with regional alluvial dynamics. In the early Subboreal, a massive landslide occurred on the western slopes of the Puy-de-Corent volcanic plateau, thereby damming the valley and forming a closed depression. This episode coincided with the mid-Holocene climatic shift and an associated phase of slope instability and increased landslides in Western Europe due to changing climatic conditions. This depression evolved into a hydromorphic basin with several high energy hydro-sedimentary episodes after 2800 cal BCE; however hydro-sedimentary conditions gradually became more stable towards the Late Holocene. Around 500 cal CE, the drainage deteriorated towards palustrine conditions and caused the formation of a marsh and its consequent evolution into a shallow lake throughout the Middle Ages. This lake was drained in the late XVIIIth Century. The long-term sedimentary record of La Narse de la Sauvetat, sensitive to climatic instability periods of the Middle Holocene, makes this landslide-dammed basin a valuable site with an important potential to further development of palaeoenvironmental and geoarchaeological studies.

Morphological characterization of a highly-dynamic fluvial landscape: The River Baker (Chilean Patagonia)

Characterizing river corridors from a hydro-morphological standpoint is a fundamental requisite for the analysis of their past evolution and for the plausible prediction of how rivers may adjust to changing climatic conditions and to increasing human impacts. The River Baker (Chilean Patagonia) is a highly dynamic fluvial system with relatively limited human intervention, which flows throughout a mountainous landscape following a narrow floodplain which widens towards the Pacific Ocean. Here, we characterize its current morphology as a basis for understanding its geomorphic dynamics, using satellite images as data source and following a hierarchical classification procedure. The Baker was divided into 9 segments and 34 reaches based on channel confinement and geometry. The geomorphic classification resulted in several confined (41%) and semiconfined (53%) reaches, whereas only 6% of them were classified as unconfined. Most of the confined reaches were single thread and the majority of the semiconfined ones were sinuous. Braided reaches were identified in all confinement categories and represent 30% of the reaches. We identified and characterized 246 islands and 393 fluvial bars and delimited 170 wetlands. Finally, we could statistically relate the island density to the braiding index, active channel width and sinuosity index, whereas the presence of bars is influenced by the active channel width, and degree of braiding and sinuosity. This characterization represents a starting point in the geomorphological analysis of the river and serves as a basis to plan future investigation efforts aiming at unravelling the morpho-dynamics of this unexplored large austral fluvial system.

The geomorphological role of snow since the Little Ice Age in the Sierra de Ancares (NW Spain)

On the Pico Cuiña cirque, Sierra de Ancares (León, Spain), the seasonal snow cover undergoes both slow and rapid mass displacements. Push associated with moving snow is responsible for an intense geomorphological activity, which is characterised by the plucking and transport of fragments of the bedrock, the abrasion of rock surfaces and the deposition of the mobilized material. Pronival ramparts are the most characteristic accumulation geoform created by pushing snow. Its study enabled us to verify the functionality of the nival processes and to prove the relative antiquity of some of them. The use of lichenometric techniques, based on the prior construction of a growth curve for lichens of the Rhizocarpon subgenus, has made possible to date sectors of the pronival ramparts. Lichenometric dates show a series of events of geomorphic activity of the snow cover fitting chronologically within the so-called Little Ice Age. It can be deduced from the observation of the current geomorphic dynamics of the snow cover that, although Little Ice Age temperature decrease might be important, particularly in the summer, the role of the variations in snow precipitation must be also taken into consideration.

Morphological characterization of calanchi (badland) hillslope connectivity

AbstractCalanchi, a type of Italian badlands created by a combination of water erosion processes and local geomorphological and tectonic controls, is a striking example of long‐term landscape evolution. In small temporal/spatial scales, the calanchi exhibit many of the geomorphic processes and landforms that may be observed in fluvial landscapes; hence, they may be considered as microbasins where geomorphic dynamics and landscape features can be related. The goal of this research is testing the use of simple morphometric variables for assessing sediment connectivity of calanchi landforms. In order to detect the morphological characteristics controlling the landscape connectivity of calanchi basins, 2 areas located in Sicily (Italy) were examined. The investigated features were identified on 119 calanchi basins and mapped using orthophotographs and 2‐m‐resolution digital elevation models. Application of Hack's law to the calanchi basins showed that the width/length ratio increases with the drainage area, suggesting that calanchi may have a limited connectivity of their hillslopes to the main channel. Moreover, the analysis of the drainage network composition suggested that calanchi are sediment removal systems more efficient than river basins. The empirical frequency distribution of the travel time, which is the ratio between the length and the square root of the hillslope steepness, of each cell of the calanchi digital elevation model was established. Finally, for each calanchi basin, an index of hillslope connectivity was devised. This was explored as a function of the sediment transport efficiency itself estimated by the travel time and the corresponding sediment delivery ratio of each calanco hillslope cell.

Coastal dune dynamics in response to excavated foredune notches

Abstract Dune management along developed coasts has traditionally focussed on the suppression of the geomorphic dynamics of the foredune to improve its role in sea defence. Because a stabilized foredune acts as an almost total barrier to aeolian transport from the beach, the habitat diversity in the more landward dunes has degraded. With the overarching objective to mitigate this undesirable loss in biodiversity, dune management projects nowadays increasingly intend to restore aeolian dynamics by reconnecting the beach-dune system with notches excavated through the foredune. Here, we use repeat topographic survey data to examine the geomorphic response of a coastal dune system in the Dutch National Park Zuid-Kennemerland to five notches excavated in 2012–2013 within an 850-m stretch of the 20-m high established foredune. The notches were dug in a V-shape (viewed onshore), with a width between approximately 50 and 100 m at the top, a (cross-dune) length between 100 and 200 m, and excavation depths between 9 and 12.5 m. The 1 × 1 m digital terrain models, acquired with airborne Lidar and UAV photogrammetry, illustrate that during the 3-year survey period the notches developed into a U-shape because of wall deflation, and that up to 8-m thick and 150-m long depositional lobes formed landward of the notches. Sand budget computations showed that the sand volume of the entire study area increased by about 22,750 m 3 /year, which, given the 850-m width of the study area, corresponds to an aeolian input from the beach of approximately 26.5 m 3 /m/year. Between 2006 and 2012 all wind-blown beach sand deposited on the seaward side of the foredune; since 2013, the notches have caused 75% of the sand to be deposited landward of the foredune. This highlights that the notches are highly effective conduits for aeolian transport into the back dunes. Future monitoring is required to determine for how long the notches will stimulate aeolian dynamics and if (and when) vegetation eventually starts to regrow and enforces the degeneration of the notches.

100 Years of Competition between Reduction in Channel Capacity and Streamflow during Floods in the Guadalquivir River (Southern Spain)

Reduction in channel capacity can trigger an increase in flood hazard over time. It represents a geomorphic driver that competes against its hydrologic counterpart where streamflow decreases. We show that this situation arose in the Guadalquivir River (Southern Spain) after impoundment. We identify the physical parameters that raised flood hazard in the period 1997–2013 with respect to past years 1910–1996 and quantify their effects by accounting for temporal trends in both streamflow and channel capacity. First, we collect historical hydrological data to lengthen records of extreme flooding events since 1910. Next, inundated areas and grade lines across a 70 km stretch of up to 2 km wide floodplain are delimited from Landsat and TerraSAR-X satellite images of the most recent floods (2009–2013). Flooded areas are also computed using standard two-dimensional Saint-Venant equations. Simulated stages are verified locally and across the whole domain with collected hydrological data and satellite images, respectively. The thoughtful analysis of flooding and geomorphic dynamics over multi-decadal timescales illustrates that non-stationary channel adaptation to river impoundment decreased channel capacity and increased flood hazard. Previous to channel squeezing and pre-vegetation encroachment, river discharges as high as 1450 m3·s−1 (the year 1924) were required to inundate the same areas as the 790 m3·s−1 streamflow for recent floods (the year 2010). We conclude that future projections of one-in-a-century river floods need to include geomorphic drivers as they compete with the reduction of peak discharges under the current climate change scenario.

A powerful tool for assessing distribution and fate of potentially toxic metals (PTMs) in soils: integration of laser ablation spectrometry (LA-ICP-MS) on thin sections with soil micromorphology and geochemistry.

The dynamic behavior and inherent spatial heterogeneity, at different hierarchic levels, of the soil system often make the spatial distribution of potentially toxic metals (PTMs) quite complex and difficult to assess correctly. This work demonstrates that the application of laser ablation spectrometry (LA-ICP-MS) to soil thin sections constitutes an ancillary powerful tool to well-established analytical methods for tracing the behavior and fate of potential soil contaminants at the microsite level. It allowed to discriminate the contribution of PTMs in distinct soil sub-components, such as parent rock fragments, neoformed, clay-enriched or humified matrix, and specific pedogenetic features of illuvial origin (unstained or iron-stained clay coatings) even at very low contents. PTMs were analyzed in three soil profiles located in the Muravera area (Sardinia, Italy), where several, now abandoned mines were exploited. Recurrent trends of increase of many PTMs from rock to pedogenic matrix and to illuvial clay coatings, traced by LA-ICP-MS compositional data, revealed a pedogenetic control on metal fractionation and distribution, based on adsorption properties of clay minerals, iron oxyhydroxides or organic matter, and downprofile illuviation processes. The main PTMs patterns coupled with SEM-EDS analyses suggest that heavy metal-bearing mineral grains were sourced from the mine plants, in addition to the natural sedimentary input. The interplay between soil-forming processes and geomorphic dynamics significantly contributed to the PTMs spatial distribution detected in the different pedogenetic horizons and soil features.

Stormy geomorphology: geomorphic contributions in an age of climate extremes

AbstractThe increasing frequency and/or severity of extreme climate events are becoming increasingly apparent over multi‐decadal timescales at the global scale, albeit with relatively low scientific confidence. At the regional scale, scientific confidence in the future trends of extreme event likelihood is stronger, although the trends are spatially variable. Confidence in these extreme climate risks is muddied by the confounding effects of internal landscape system dynamics and external forcing factors such as changes in land use and river and coastal engineering. Geomorphology is a critical discipline in disentangling climate change impacts from other controlling factors, thereby contributing to debates over societal adaptation to extreme events. We review four main geomorphic contributions to flood and storm science. First, we show how palaeogeomorphological and current process studies can extend the historical flood record while also unraveling the complex interactions between internal geomorphic dynamics, human impacts and changes in climate regimes. A key outcome will be improved quantification of flood probabilities and the hazard dimension of flood risk. Second, we present evidence showing how antecedent geomorphological and climate parameters can alter the risk and magnitude of landscape change caused by extreme events. Third, we show that geomorphic processes can both mediate and increase the geomorphological impacts of extreme events, influencing societal risk. Fourthly, we show the potential of managing flood and storm risk through the geomorphic system, both near‐term (next 50 years) and longer‐term. We recommend that key methods of managing flooding and erosion will be more effective if risk assessments include palaeodata, if geomorphological science is used to underpin nature‐based management approaches, and if land‐use management addresses changes in geomorphic process regimes that extreme events can trigger. We argue that adopting geomorphologically‐grounded adaptation strategies will enable society to develop more resilient, less vulnerable socio‐geomorphological systems fit for an age of climate extremes. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

Flood‐driven topographic changes in a gravel‐cobble river over segment, reach, and morphological unit scales

AbstractRegulated rivers generally incise below dams that cut off sediment supply, but how that happens and what the consequences are at different spatial scales is poorly understood. Modern topographic mapping at meter‐scale resolution now enables investigation of the details of spatial processes. In this study, spatial segregation was applied to a meter‐scale raster map of topographic change from 1999 to 2008 on the gravel‐cobble, regulated lower Yuba River in California to answer specific scientific questions about how a decadal hydrograph that included a flood peak of 22 times bankfull discharge affected the river at segment, reach, and morphological unit scales. The results show that the river preferentially eroded sediment from floodplains compared to the channel, and this not only promoted valley‐wide sediment evacuation, but also facilitated the renewal and differentiation of morphological units, especially in the channel. At the reach scale, area of fill and mean net rate of elevational change were directly correlated with better connectivity between the channel and floodplain, while the mean rate of scour in scour areas was influenced by the ratio of slope to bankfull Froude number, a ratio indicative of lateral migration versus vertical downcutting. Hierarchical segregation of topographic change rasters proved useful for understanding multi‐scalar geomorphic dynamics. Copyright © 2016 John Wiley & Sons, Ltd.

Land Cover Data for the Mississippi–Alabama Barrier Islands, 2010–2011

Land cover on the Mississippi–Alabama barrier islands was surveyed in 2010–2011 as part of continuing research on island geomorphic and vegetation dynamics following the 2005 impact of Hurricane Katrina. Results of the survey include sub-meter GPS location, a listing of dominant vegetation species and field photographs recorded at 375 sampling locations distributed among Cat, West Ship, East Ship, Horn, Sand, Petit Bois and Dauphin Islands. The survey was conducted in a period of intensive remote sensing data acquisition over the northern Gulf of Mexico by federal, state and commercial organizations in response to the 2010 Macondo Well (Deepwater Horizon) oil spill. The data are useful in providing ground reference information for thematic classification of remotely-sensed imagery, and a record of land cover which may be used in future research.

Gully catchments as a sediment sink, not just a source: Results from a long‐term (~12 500 year) sediment budget

AbstractSediment delivery from hillslopes to trunk streams represents a significant pathway of mass transfer in the landscape, with a large fraction facilitated by gully systems.The internal gully geomorphic dynamics represent a considerable gap in many landscape and empirical erosion models, therefore a better understanding of these processes over longer timescales (10–104 years) is needed. This study analyses the sediment mass balance and storage dynamics within a headwater gully catchment in central Europe over the last ~12 500 years. Human induced erosion resulted in hillslope erosion rates ~2.3 times higher than under naturally de‐vegetated conditions (during the Younger Dryas), however the total sediment inputs to the gully system (and therefore gully aggradation), were similar. Net gully storage has consistently increased to become the second largest term in the sediment budget after hillslope erosion (storage is ~45% and ~73% of inputs during two separate erosion and aggradation cycles). In terms of the depletion of gully sediment storage, the sediment mass balance shows that export beyond the gully fan was not significant until the last ~500 years, due to reduced gully fan accommodation space. The significance of storage effects on the gully sediment mass balance, particularly the export terms, means that it would be difficult to determine the influences of human impact and/or climatic changes from floodplain or lake sedimentary archives alone and that the sediment budgets of the headwater catchments from which they drain are more likely to provide these mechanistic links. Copyright © 2015 John Wiley & Sons, Ltd.

Los efectos geoecológicos del cambio global en el Pirineo Central español: una revisión a distintas escalas espaciales y temporales

We review the effects of climate variability and land use / land cover changes in the Central Spanish Pyrenees at different spatial and temporal scales. Paleoclimatic studies based upon multi-proxy analyses of lacustrine, glacial and speleothematic deposits, among others, have demonstrated the occurrence of intense climatic fluctuations not only during the Late Upper Pleistocene associated to deglaciation, but also during the Holocene, affecting plant cover distribution, runoff generation, flood frequency, and the spatial organization of human activities, particularly during the 8.2 event, the Bronze Age, the Medieval Climatic Anomaly and the Little Ice Age. The study of the impact of human activity on landscape dynamics during the last 150 years has revealed dramatic changes in plant cover structure and distribution, in both the montane and the sub-alpine belts and, consequently, changes in runoff generation, soil erosion intensity, and sediment sources, whatever the spatial scale used. In general, the maximum human pressure on the territory was reached during the middle of the 19th century, with the cultivation of most of the south-facing slopes up to approximately 1650 m a.s.l., and the frequent use of fire to control shrub colonization, resulting in intense soil erosion and degradation processes, as well as the development of braided rivers with a high torrentiality. Farmland abandonment since the beginning of the 20th century (and, particularly, since the 1960s) and the declining livestock pressure have favoured plant recolonization, with the expansion of shrubs and forests in the old cultivated and grazing areas, the human-induced reforestation of large hillslopes, and the treeline upward in the subalpine belt, in this latter case with the contribution of climate warming. The most outstanding consequences of such an evolution have been: (i) the sustained decline of discharge in the Pyrenean rivers since the 1970s because of increasing actual evapotranspiration; (ii) the spatial shrinkage of sediment source areas; (iii) the increasing uncoupled geomorphic dynamics between hillslopes and channels; (iv) the decreasing importance of snow accumulation and snowmelt processes in the fluvial regime of the Pyrenean rivers, with earlier onset of the high spring discharges; (v) the changes in reservoir hydrological dynamics and the need for new management policies; and (vi) the increasing scouring processes that affect most of the Pyrenean rivers. The studies carried out on Global Change and its consequences in the Central Pyrenees confirm the extreme complexity of interactions occurring in the natural systems, and the quick response of plant cover, runoff generation and soil erosion as a consequence of climatic and land use changes.

Investigating decadal‐scale geomorphic dynamics in an alpine mountain setting

AbstractWe know little about the effect of recent climate variability upon landscapes at the timescale of decades because of (1) the complex, nonlinear, and path‐dependent nature of the response of a landscape to climate forcing and (2) the difficulty of quantifying spatially distributed impacts at the timescale of decades to centuries, despite this being the timescale over which significant hypotheses have been raised over human impacts upon climate change and hence geomorphic systems. A unique resource to investigate the linkages between climatic variability and geomorphic response is provided by the extensive coverage of aerial imagery commonly available since the 1950s. Here we use archival digital photogrammetry to produce high‐precision digital elevation models over large spatial scales, and so to reconstruct the quantitative history of surface downwasting and sediment flux in a high mountain alpine system, over the timescales of decades. Propagation of error methods is used to identify locations of significant landscape response and to compute volumes of significant surface change. Orthorectified aerial images are used in an image correlation framework to detect horizontal and vertical displacements of components of the landscape. Results are coupled to extant climate data and modeled snow cover to show how the landscape responds to climate forcing and to geomorphological maps to understand how this response varies between landscape elements. The results show distinct landscape response to both warming and cooling periods and a tendency for the acceleration of surface displacement under warming conditions. Precipitation and snow cover are critical in controlling glacier dynamics and rock glacier displacement velocities. However, while some landforms might lead to locally high sediment flux, landscape heritage can disconnect zones of high change rates from the valley bottom. Hence, the landscape response to climate forcing is not necessarily reflected in valley system processes or sediment deposits.

Hydrologic and geomorphic flow thresholds in the Lower Brazos River, Texas, USA

River science and management often require a design or reference discharge. The common (and sometimes unavoidable) use of such discharges may, however, obscure the fact that the magnitude and frequency of critical flows can differ due to various hydrological, geomorphological, and ecological criteria. Threshold stages and discharges were identified for six lower Brazos River, Texas gaging stations corresponding to thalweg connectivity, bed inundation, high sub-banktop flows, channel–floodplain connectivity (CFC), and overbank flooding. Critical flows were also identified for estimated thresholds for sandy bedform and medium gravel mobility, critical specific stream power for potential channel modifications, and cohesive-bank channel erosion. These thresholds have variable relationships to mean, median, and maximum flows. For four of the six stations, daily recurrence probabilities for all but flood flows are at least 1%, and as high as 11%. All stations achieve channel–floodplain connectivity at stages less than banktop. Estimated threshold flows for sediment mobility and channel erosion occur relatively frequently, with daily probabilities of 2–77%. Critical flows for bank erosion occur least often, and for sandy bedform and gravel mobility most often. Thalweg connectivity is always maintained at all sites, while bed inundation flows have a daily probability of about 80% or more. Overall, results suggest that no single flow level is dominant in hydrological or geomorphic dynamics, and that the frequency of a given threshold varies considerably even along a single river. The results support the idea that multiple flow levels and ranges are necessary to create and maintain the hydrological, geomorphological, and ecological characteristics of rivers, and that no single flow level is a reliable determinant of fluvial state.Editor Z.W. Kundzewicz; Associate editor Q. Zhang

Vegetation–Hydrogeomorphology Interactions in a Low‐Energy, Human‐Impacted River

AbstractThe geomorphological dynamics of rivers have been traditionally explained largely by the physical processes of water flow, and sediment erosion and deposition, but the significant role of a third element, vegetation, in driving geomorphological changes has been increasingly highlighted recently. However, few studies have documented how both aquatic and woody riparian plants interact with fluvial processes to induce landform development and initiate channel adjustment. This paper presents analyses of historical maps, recent aerial images and field observations from the River Frome (Dorset, UK), which, as a result of human pressures, has been subject to an increased supply of sand and finer sediment, particularly over the last 50–60 years. Analysis of these information sources indicates that this low‐energy river has adjusted to this delivery of finer sediment by narrowing and increasing its sinuosity. The analysis also indicates that this has been achieved through interactions amongst vegetation, water flow and sediment. Emergent aquatic macrophytes were observed to retain sediment, which leads to the development of submerged shelves that aggrade and become colonized by other plant species to form bars, berms and benches, eventually leading to the extension of river banks into the channel and also the formation of islands. Where woody riparian vegetation is well developed, complex geomorphic changes were observed, with fine sediment being absorbed into a diverse mosaic of geomorphic features initiated by living trees and large wood. These observations underline the importance of vegetation for the geomorphic dynamics and adjustment of lowland, low‐energy rivers and its potential for inclusion in the development of sustainable, process‐based river management and restoration strategies. Copyright © 2015 John Wiley & Sons, Ltd.

Analysis of geomorphic dynamics in ancient quartzite landscape using phytolith and carbon isotopes, Espinhaço Mountain Range, Minas Gerais, Brazil

Although there are important landscapes developed on underlying quartzite in Brazil, there is still very little known about the processes and mechanisms involved in their formation. Some recent research has drawn attention to the presence of karstic features associated with silica dissolution in this lithology. However, there are other relief forms and materials commonly encountered in the domain of quartzite in the Espinhaco Mountain Range in the southeast region of Brazil. The main objective of this research is to analyze the geomorphic processes associated with the material overlying relatively flat surfaces of low inclination that always occur near quartzite outcrops in the study area. This is relevant in order to attempt to understand the development of landforms in quartzite domains in tropical humid areas. A Geomorphic approach for the analysis of the material was used, in which geomorphic sites were identified and two soil profiles were described and sampled for laboratory analysis. This was complemented with laboratory analysis of phytoliths found in the soil, 13C analysis and 14C-AMS dating. The relatively shallow profiles show a sequence of similar horizons, in which the material varies from sandy organic material at the top to stony immediately below. The superficial horizons have predominantly finer sand and also clay, which is expected in humid tropical environments. Both profiles present a stony layer formed of angular to subangular quartz pebbles lying just on the top of the weathered quartzite in the C horizons where the oldest SOM 14C dating values were found. Considering the mean residence time (MRT), it is possible to assume that the first depositions in P2 are more than 10.000 yrs old (10,506-10,230 yr cal BP) and that the first profile in P1, located on the upper slope, presents younger SOM (5,919-5,664 yr cal BP). This suggests that the erosive processes were either more intense or more recent on this portion of the slope and that these deeper horizons may have previously been on the surface, when the material was deposited. Also, the high amount of phytoliths which are unclassified due to their destruction, along with the presence of the most robust types, can be interpreted as evidence of intense erosion and transportation of the material by surface runoff, enabling the assumption that the material is colluvium and that its source is a nearby quartzite outcrop. After deposition along the gently inclined slope, identified as a “colluvial ramp”, the material was relatively undisturbed, which allowed pedogenic processes to take place, forming Quartz-sand Neosols. The ramp itself seems to be the result of very slow rates of weathering and removal of the material, together with strong control from relatively flat Mesoproterozoic depositional macro-structures of the underlying rock. This study is one of the first attempts to use phytoliths combined with carbon isotope analysis for geomorphologic purposes, with very interesting results. Their relevance points to their potential for geomorphic studies.

The influence of ground ice distribution on geomorphic dynamics since the Little Ice Age in proglacial areas of two cirque glacier systems

AbstractHolocene glaciers have contributed to an abundance of unstable sediments in mountainous environments. In permafrost environments, these sediments can contain ground ice and are subject to rapid geomorphic activity and evolution under condition of a warming climate. To understand the influence of ground ice distribution on this activity since the Little Ice Age (LIA), we have investigated the Pierre Ronde and Rognes proglacial areas, two cirque glacier systems located in the periglacial belt of the Mont Blanc massif. For the first time, electrical resistivity tomography, temperature data loggers and differential global positioning systems (dGPS) are combined with historical documents and glaciological data analysis to produce a complete study of evolution in time and space of these small landsystems since the LIA. This approach allows to explain spatial heterogeneity of current internal structure and dynamics. The studied sites are a complex assemblage of debris‐covered glacier, ice‐rich frozen debris and unfrozen debris. Ground ice distribution is related to former glacier thermal regime, isolating effect of debris cover, water supply to specific zones, and topography. In relation with this internal structure, present dynamics are dominated by rapid ice melt in the debris‐covered upper slopes, slow creep processes in marginal glacigenic rock glaciers, and weak, superficial reworking in deglaciated moraines. Since the LIA, geomorphic activity is mainly spatially restricted within the proglacial areas. Sediment exportation has occurred in a limited part of the former Rognes Glacier and through water pocket outburst flood and debris flows in Pierre Ronde. Both sites contributed little sediment supply to the downslope geomorphic system, rather by episodic events than by constant supply. In that way, during Holocene and even in a paraglacial context as the recent deglaciation, proglacial areas of cirque glaciers act mostly as sediment sinks, when active geomorphic processes are unable to evacuate sediment downslope, especially because of the slope angle weakness. Copyright © 2014 John Wiley & Sons, Ltd.

Benchmark soils on alluvial, fluvial and fluvio-glacial formations of the upper-Segre valley .

The upper reaches of the Segre river, flowing through the Pyrenees, offers a variety of geomorphic surfaces that allow us to study soil chronosequences. The objective of this work is to widen the knowledge about the main characteristics and formation processes of some benchmark soils developed on fluvio-glacial, alluvial-fan and terrace materials of Pleistocene and Holocene age related to the Segre river, either siliceous or carbonatic. This knowledge will allow us to identify soil forming processes, commonly found in Mediterranean environments such as carbonate redistribution, clay formation and mobilization and rubefaction, all as functions of parent material and age. Five profiles, ranging from the Lower Pleistocene to the Holocene were classified according to Soil Taxonomy/WRB. The Montferrer profile (Calcic Palexeralf /Calcic Cutanic Luvisol (Chromic) is a deep, partly decarbonated soil, with calcium carbonate accumulation in depth covering glacial features. The Torre del Remei profile (Typic Paleustalf /Cutanic Luvisol) developed on silicic moraines and shows an extreme clay formation and illuviation. The Alp (Typic Haplustalf /Cutanic Luvisol) and Tartera (Petric Calciustept /Petric Calcisol) soils are developed on alluvial fans with calcium carbonate sources. The former is partly decarbonated, whilst the latter is rubefacted on top and shows speleothem-like carbonate pendants with superposition of clay illuviation. The youngest profile, Abellerols, (Typic Calciustept /Typic Calcisol) shows only a partial decarbonation and calcite accumulation at depth. The results show that soil development is determined by the age of the surface and the source of calcite, either in the parent rock or brought by subsurface flow: clay illuviation is extreme in absence of it. Special morphologies of carbonate pendants are indicators of environmental conditions. The coexistence of clay coatings and secondary calcite can be explained by recarbonatation or by spatial differentiation of soil environments in the profile. One of the implications of this research is the inconsistence of using soil development indices based on morphological indicators when soils are formed on different parent materials and are subjected to different geomorphic dynamics.