Alluvial Bars Research Articles

Evolution of riparian vegetation in response to anthropogenic effects: The Taleqan River, Iran

AbstractDuring the last decades, human interventions, such as dam construction and land‐use change, have caused changes in geomorphic patterns and riparian zones in many rivers in Iran. This study aimed to assess the spatio‐temporal evolution of riparian vegetation and the spatial area of biogeomorphic feedback windows (BFWs) located in the riparian Salicaceae vegetation patches and their traits responding to human interventions in the Taleqan River during 1990–2022. The river was divided into five reaches using the geomorphic unit system (GUS) method, and the river sensitivity index (SI), location of BFWs, vegetation typology and main cover typology were investigated in the reaches using aerial photographs and satellite images from 1990 to 2022. For this purpose, the main cover typology was classified into grassland, woody vegetation, bank‐attached bars, islands, vegetated islands, river channels and urban areas. The stem diameter and density, degradation level and longitudinal/latitudinal alluvial bars in each of the BFWs were investigated using the field survey. Based on the findings, the lowest values of the SI (braiding index [BI], channel activity [CA] and channel width [CW]) and the highest expansion of BFWs were observed in the upstream reaches of the Taleqan River. Human disturbances in these reaches were less than in the reaches downstream, allowing the expansion of vegetation habitats. The vegetation patches were unstable and scattered in the midstream and downstream, and the width decrease of the river channel was created by land‐use change and channelization. A significant direct relationship was observed between BFW establishment, stem diameter and density, and vegetation height. The BFWs create an opportunity for engineer vegetation to prevent erosion and expand new habitats by creating biogeomorphic accumulation landforms and trapping alluvial bars on the lee side of engineer woody vegetation. This study highlights the interactions between riparian areas and hydrogeomorphic processes within riparian corridors exposed to human disturbances to help better define a comprehensive plan for river management.

Dating post-LGM aeolian sedimentation and the Late Palaeolithic in Central Yakutia (northeastern Siberia)

Central Yakutia is a large region in northeastern Siberia characterized by extensive permafrost, large river valleys, mountain glaciers, and large massifs of aeolian sands; the geological history of the region is complex and, at present, poorly constrained. In recent years, it has been shown that aeolian sands cover up to 60% of large parts of Central Yakutia. This paper presents the results of luminescence dating of aeolian sedimentation at the Diring Yuriakh Palaeolithic site located in the middle reaches of the Lena River. Field studies identified several thick units of aeolian sand, which cover an old deflation surface with Late (Duktai culture) and Early Palaeolithic (Diring culture) artefacts. The reliability of the OSL chronology was assessed by comparison of ages based on the optically stimulated luminescence from quartz and the infra-red stimulated luminescence from potassium-rich feldspars; these age pairs are in good agreement, implying that at least the quartz grains were sufficiently bleached before sedimentation. We obtained OSL ages that reflect three periods of accumulation between the LGM and the Holocene: ∼21 ka, 15-14 ka, and 12.5–10 ka. These periods of accumulation broadly coincide with global cooling episodes during the Last Glacial Maximum, the Older Dryas, and the Younger Dryas, with some extension into subsequent warmer intervals, whereas the intervening intervals without preserved sediments are taken to reflect dune stability during warmer periods. The sand on the terraces, sourced from alluvial bars in the river channel, was blown up the valley slope during cold and dry periods when the vegetation cover was sparse. When the climate warmed, the vegetation took some time to spread, and so the accumulation of aeolian sand on the high terraces continued into the warm periods. We also infer periods of deflation (wind erosion) that occurred before 21 ka and between 20 and 15 ka, presumably due to increased aeolian activity and localized remobilization of sediment. The new OSL chronology shows that the younger artefacts located at the cape of Diring Yuriakh, belonging to the Late Palaeolithic Duktai culture, are older than 15 ka. The new ages also show that the post-LGM aeolian sand sequences at Diring Yuriakh are correlated with the regionally developed subaerial Dolkuma Formation.

A high‐resolution inter‐annual framework for exploring hydrological drivers of large wood dynamics

AbstractRivers with alluvial bars store more wood than those without, supplied through channel shifting. However, wood dynamics (arrival or new deposits, departure or entrainment, and stable or immobile pieces) can vary substantially over time in response to critical hydrological drivers that are largely unknown. To evaluate them, we studied the dynamics of large wood pieces and logjams along a 12‐km reach of the lower Allier River using six series of aerial images of variable resolution acquired between 2009 and 2020, during which maximum river discharge fluctuated around the dominant flood discharge (Q1.5) that is potentially the bankfull discharge along this well‐preserved not incised reach. Individual wood departure was best correlated with water levels exceeding dominant flood discharge. The duration of the highest magnitude flood was best correlated with wood depositions, with shorter floods resulting in a higher number of deposits. Finally, most of the wood remained stable when river discharge did not exceed 60% of Q1.5 over a long period of time. Changes in inter‐annual wood budget (reach‐scale) depend on the duration over which discharge exceeded 60% of Q1.5. Hydrological conditions driving jam build‐up and removal were similar to those controlling individual wood piece dynamics. The results suggest that specific hydrological conditions influence the dynamics of large wood and log jams in the Allier River. Understanding the dynamics of large wood and its impact on river morphology is fundamental for successful river management and habitat restoration initiatives.

Assessment of Flood-Induced Geomorphic Changes in Sidere Creek of the Mountainous Basin Using Small UAV-Based Imagery

Floods often cause changes in the hydro-geomorphology of riverbeds and banks. These changes need to be closely monitored to find a balance and exchange between lateral and vertical erosion and deposition, upstream local sediment supply, and a stream’s transport capacity. Low-frequency cross-sectional field surveys cannot map hard-to-reach locations. Innovative techniques, such as small unmanned aerial vehicles (UAVs), must be employed to monitor these processes. This research compared historical data with a UAV survey and the Pix4DMapper structure-from-motion (SfM) program to assess the longitudinal, lateral, and vertical changes of Sidere Creek in the eastern Black Sea, Türkiye. Digitization was undertaken using 2011–2015–2017 Google Earth photographs, 1960s topographic maps, and 2023 orthomosaics. ArcGIS 10.6 was used to delineate the centerlines (thalweg), left/right banks, alluvial bars, active channel widths, and channel confinement layers. Channel Migration Toolbox and CloudCompare were utilized for analyzing lateral and vertical morphological changes, respectively. The active channel migrated 25.57 m during 1960–2011, 15.84 m during 2011–2015, 6.96 m during 2015–2017, and 5.79 m during 2017–2023. Left-bank channel confinement rose from 2.4% to 42% and right-bank channel confinement from 5.9% to 34.8% over 63 years. Neither stream meandering nor sinuosity index changed statistically. Active channel boundary widths varied from 149.79 m to 9.46 m, averaging 37.3 m. It can be concluded that UAV surveys can precisely measure and monitor the stream channel longitudinal, lateral, and vertical morphological changes at a lower cost and in less time than previous methods.

Hydrogeomorphological processes and plant invasion. What interactions in the case of Asian knotweeds along the Herault River (France)?

AbstractConsidered one of the five major threats to biodiversity worldwide, Invasive Alien Species (IAS) particularly threaten riparian ecosystems. Among the IAS found on riverbanks, Asian knotweeds (Reynoutria spp. including R. japonica Houtt.; R. sachalinensis [F.Schmidt] Nakai and the hybrid R. x bohemica Chrtek & Chrtkova) can barely be controlled as, once established, they disperse easily along stream banks via rhizome or stem fragments transported by water. However, the hydrogeomorphological processes underlying the establishment of Asian knotweeds are poorly understood. The objective of this study was to describe and model the hydrogeomorphological preferences of Asian knotweeds along a Mediterranean river. Based on exhaustive presence/absence surveys, we implemented two models related to the presence of Asian knotweeds: (1) at the river reach scale and (2) at the finer scale of the alluvial bar. Areas of low curvature identified as convex banks and the central parts of alluvial bars appear to be more susceptible to knotweed establishment. Highly disturbed areas were less favorable to maintaining plant species, including Asian knotweeds, while less disturbed areas with denser plant cover were more favorable to Asian knotweeds. The results seem to indicate a trade‐off hypothesis in the knotweed establishment strategy between hydrogeomorphological constraints and strong interspecific competition. Analyzed in the light of the current literature, our final models are designed to integrate hydrogeomorphological processes in order to provide an operational tool to help river managers locate the areas most susceptible to knotweed invasion and with important implications for managing these species in riparian ecosystems.

Environmental and land use controls of microplastic pollution along the gravel-bed Ain River (France) and its “Plastic Valley”

Understanding microplastic particles (MPs) accumulation and transport along rivers represents a major task due to the complexity and heterogeneity of rivers, and their interactions with their wider corridor. The identification of MPs hotspots and their potential sources is especially challenging in coarse-bed rivers transporting a wide range of particle sizes with a high degree of variability in time and space. This research focuses on the gravel-bed Ain River (Rhône River tributary, France) which is managed by means of various dams and also hosts one of the major plastic production centres in Europe (Oyonnax and Bienne Plastic Valleys). In this research, (i) Geographical Information Systems (GIS) were used to locate plastic factories and to characterise the land use of the Ain River watershed. (ii) On the field, sediment samples were extracted from the hyporheic zone (HZ) of mobile gravel bar heads, while hydro-sedimentary settings were measured in order to describe site conditions. Sampling sites were especially established in downwelling areas (i.e. where the surface water entered the hyporheic zone), upstream and downstream of dams and plastic factories. (iii) After density separation and organic matter digestion of sediment, MPs were characterised with a µFTIR device followed by data processing via the siMPle software. This work highlighted the trapping efficiency of alluvial bars for MPs. The highest MPs concentrations were found along the Plastic Valleys (up to 4400 MPs/kg), while the lower river was less contaminated by MPs. After grain-size correction, a significant breakpoint was identified in the area of the main dams, revealing their major influence on MPs distribution. The variability in MPs concentrations and types suggested a local origin for most of MPs. A particular feature was the dominance of polypropylene (PP) which appears as a critical industrial heritage as the studied region is specialised in the manufacturing of hard plastics. Indeed, multivariate analyses also revealed that MPs concentrations and types were mostly driven by the vicinity of plastic factories and urban areas. This relationship between the land use, the presence of dams and MPs characteristics provides key results for the MPs assessment and the improvement of management issues along coarse-bed rivers.

Black poplar establishment on alluvial bars: seed rain homogeneity over a few kilometres

Black poplar establishment on alluvial bars: seed rain homogeneity over a few kilometres

First fossil record of cave lion (Panthera (Leo) spelaea intermedia) from alluvial deposits of the Po River in northern Italy

A perfectly preserved left felid hemimandible has been found in fluvial deposits of the Po River near Cremona (Northern Italy). The fossil, found in allochthonous position within an alluvial bar, corresponding in morphology and size at the hemimandible of modern and fossil lions. The bone, in excellent conditions, is characterized by the presence of all the teeth and shows a slight erosion of the condyle and angular process indicating a very limited transport (rafting). Based on the comparison with a large dataset of morphometric measurements of cave lion and fossil leopard hemimandibles, the studied fossil is classified as a subadult female of the chronosubspecies Panthera spelaea intermedia.The mammalian fossil record from Po River consists predominantly of large herbivores Palaeoloxodon antiquus, Stephanorhinus kirchbergensis, Bison priscus, Megaloceros giganteus, Mammuthus primigenius, Alces alces, Cervus elaphus, some carnivores like Panthera cf. pardus, Crocuta crocuta, Ursus arctos, Canis lupus, Vulpes vulpes and primates such as Homo neanderthalensis and H. sapiens. The species composition indicates a mixing of different faunal assemblages from interglacial and glacial periods of the Late Pleistocene.

Alluvial landform and the occurrence of paleosols in a humid-subtropical climate have an effect on long-term soil organic carbon storage

Soil organic carbon (SOC) storage in alluvial depositional zones is important as it serves as a centennial to millennial timescale sink of C. This study examines SOC variability in alluvial bars, floodplains and terraces in a forested, humid-subtropical setting along the Clarks River in the Mississippi River basin (MRB) using replicated soil sampling, characterization and modeling. Soil and depositional profiles range in age from a minimum of 8000 years ago to modern. Normalized (by depth) mean SOC stocks (kg m−2) of the surface soil and buried layers, i.e., buried soils and layers underlying lithologic discontinuities, are highest in the terraces. Normalized mean surface SOC stocks decrease from terraces to floodplains and then to bars; whereas normalized mean buried layer SOC stocks decrease from terraces to bars, and then floodplains. The depth distribution of stocks showed that buried layers accounted for >50% of the total SOC pool, regardless of landform type. A regression tree analysis shows that soil horizon, pH, landform, and magnetic susceptibility were significant predictors of mean SOC content. Notably, the regression tree shows that alkaline pH (>7.9), observed only in buried layers in the terrace, is an important predictor in higher mean SOC. These alkaline soil pH values were associated with paleosol calcite, where the δ13C and 14C from the calcite suggests it formed under C3 vegetation during the drier than modern mid-Holocene hypsithermal, ~8000 years ago. Buried soil and sediment contain the majority of the SOC in this humid-subtropical valley bottom, while landform position and past buried soil-forming environments (paleosols) played an important role in the storage of that SOC. This work reconfirms that there is a long record of a passive, buried SOC pool found deep in our river valley bottoms that holds clues to the past biogeochemical cycling of C on land prior to intensive agriculture and industrialization. The legacy of past Holocene climates and paleosols are directly linked to our current understanding of long-term storage of SOC.

Experiments on patterns of alluvial cover and bedrock erosion in a meandering channel

Abstract. In bedrock rivers, erosion by abrasion is driven by sediment particles that strike bare bedrock while traveling downstream with the flow. If the sediment particles settle and form an alluvial cover, this mode of erosion is impeded by the protection offered by the grains themselves. Channel erosion by abrasion is therefore related to the amount and pattern of alluvial cover; these are functions of sediment load and hydraulic conditions, which in turn are functions of channel geometry, slope, and sinuosity. This study presents the results of alluvial cover experiments conducted in a meandering channel flume of high fixed sinuosity. Maps of quasi-instantaneous alluvial cover were generated from time-lapse imaging of flows under a range of below-capacity bedload conditions. These maps were used to infer patterns of particle impact frequency and likely abrasion rates. Results from eight such experiments suggest the following: (i) abrasion through sediment particle impacts is driven by fluctuations in alluvial cover due to the movement of freely migrating bars; (ii) patterns of potential erosion are functions of sediment load and local curvature; (iii) low sediment supply ratios are associated with regions of potential erosion located closer to the inner bank, but this region moves toward the outer bank as sediment supply increases; and (iv) the threads of high erosion rates are located at the toe of the alluvial bars, just where the alluvial cover reaches an optimum for abrasion.

Influence of hydrology, sediment supply and sediment gradation on river bar morphodynamics: application to the Loire River at Bréhémont (France)

Rivers inherently show heterogeneous sediment sizes and can also show a strong sediment supply variability in time because of natural episodic events or as a consequence of human activities, which alter the characteristics and dynamics of alluvial bars at the macro-scale. The impact of the combination between sediment size heterogeneity and sediment supply variation, or even with other forcings (i.e. hydrology, channel geometry) remains poorly documented. In this work, a physics-based numerical model is applied on a trained reach of a sandy-gravel bed river to investigate the combination of these parameters on bar morphodynamics. The morphodynamic computations are performed with a two-dimensional depth-averaged hydrodynamic solver, internally coupled to a sediment transport and bed evolution module, which estimate the transport of graded sediment and model bed stratigraphy, respectively. A 1 km long reach of the Loire River at Bréhémont (France) is selected to conduct the numerical investigations. The interaction between several forcing mechanisms induces highly complex bar morphodynamic processes in this area.A comprehensive set of high-definition data is available, which allows to study the river morphodynamics for a succession of three flooding events and a period of low flows. Based on this model, a variety of scenarios is presented with the aim of exploring the implications of sediment gradation, geometrical and boundary forcing effects on in situ bars morphodynamics.

Above- and belowground responses of Populus nigra L. to mechanical stress observed on the Allier River, France

Pioneer riparian trees such as Populus nigra L. which establish on alluvial bars within dynamic riparian corridors strongly influence fluvial geomorphology by trapping sediments and constructing landforms during floods. The engineering effects (changes in the physical state of the habitat by organisms) of P. nigra on alluvial bars depend on its biomass and its exposure to mechanical stress. P. nigra has a strong phenotypic plasticity that enables individuals to adapt their morphological and biomechanical traits, according to the local hydrogeomorphic conditions. The comprehension and quantification of the variation of morphological and biomechanical response trait attributes of P. nigra populations according to their exposure to mechanical stress is fundamental to better understand why riparian plants are capable to impact fluvial geomorphology. In an empirical in situ study, we quantified the relation between response trait attributes of P. nigra and its exposure to three different levels of mechanical stress. At a highly exposed bar-head, plants clearly developed response traits such as small flexible stems and a strong root system which favour higher mechanical resistance, while at the less exposed bar-tail plants developed taller, less flexible stems and finer root systems. Plants that established in the lower reach of the chute channel developed some common trait attributes in comparison to the bar-tail population and some other trait attributes which were common to the bar-head population. Poplar plants which established on bar-tails favoured bioconstruction, and thus are potentially faster disconnected from hydrogeomorphic disturbances. These results further suggest that fine scale biogeomorphic feedbacks have an influence on larger scale processes within the fluvial corridor requiring hierarchical biogeomorphic bottom-up and top-down cross scale studies for a better understanding of complex biogeomorphic fluvial ecosystems.

Niche construction within riparian corridors. Part I: Exploring biogeomorphic feedback windows of three pioneer riparian species (Allier River, France)

Within riparian corridors, biotic-abiotic feedback mechanisms occur between woody vegetation strongly influenced by hydrogeomorphic constraints (e.g., sediment transport and deposition, shear stress, hydrological variability), fluvial landforms, and morphodynamics, which in turn are modulated by the established vegetation. During field investigations in spring 2015, we studied 16 alluvial bars (e.g., point and lateral bars) within the dynamic riparian corridor of the Allier River (France) to assess the aptitude of three pioneer riparian Salicaceae species (Populus nigra L., Salix purpurea L., and Salix alba L.) to establish and act as ecosystem engineers by trapping sediment and constructing fluvial landforms. Our aim is to empirically identify the preferential establishment area (EA; i.e., the local areas where species become established) and the preferential biogeomorphic feedback window (BFW; i.e., where and to what extent the species and geomorphology interact) of these three species on alluvial bars within a 20-km-long river reach. Our results show that the EA and BFW of all three species vary significantly along the longitudinal profile, i.e., upstream-downstream exposure on the alluvial bars, as well as transversally, i.e., the main hydrological connectivity gradient from the river channel toward the floodplain. In the present-day context of the Allier River, P. nigra is the most abundant species, appearing to act as the main engineer species affecting landform dynamics at the bar scale; S. purpurea is established and acts as an ecosystem engineer at locations on alluvial bars that are most exposed to hydrosedimentary flow dynamics, while S. alba is established on the bar tail close to secondary channels and affects the geomorphology in mixed patches along with P. nigra. Our study highlights the role of functional trait diversity of riparian engineer species in controlling the extent of fluvial landform construction along geomorphic gradients within riparian corridors exposed to frequent hydrogeomorphic disturbances.

Video‐Based Electroshocking Platform to Identify Lamprey Ammocoete Habitats: Field Validation and New Discoveries in the Columbia River Basin

AbstractA deepwater electroshocking platform (DEP), developed to sample larval lampreys (ammocoetes) and associated habitat in depths up to 15 m, was recently tested in the field. Searches were conducted at a known rearing location (mouth of the Wind River, Washington) and at locations on the Cowlitz River, Washington, where ammocoetes had not previously been found. At the Wind River, video‐imaged ammocoetes ranged from 50 to 150 mm in water depths between 1.5 and 4.5 m, and were more common in sediments containing organic silt. Ammocoetes (n = 137) were detected at 61% (summer) and 50% (winter). Following the field verification, the DEP was used on the lower 11.7 km of the Cowlitz River. Ammocoetes (n = 41) were found at 26% specific search locations. Cowlitz River sediment containing ammocoetes was also dominated by silt with organic material, often downstream of alluvial bars in water depths from 0.8 to 1.7 m. Test results indicated the DEP was successful at detecting ammocoetes with little to no impact in relation to other sampling methods. The DEP can also be used to characterize lamprey ammocoete habitats.Received October 21, 2016; accepted March 17, 2017 Published online May 12, 2017

First fossil record of leopard-like felid (Panthera cf. pardus) from alluvial deposits of the Po River in northern Italy

A slender right felid tibia has been found in river deposits of the Po River in the territory of Cremona (Northern Italy). The fossil, found in allochthonous position within an alluvial bar, shows an overall slenderness and subtlety, different shape of the shaft from lateral view, or less pronounced medial malleolus, corresponding in morphology and dimensions rather with tibiae of modern leopards than with shinbones of larger pantherines or feline cats. The bone is, however, faintly rounded (surface erosion of the edges in particular on the head of the tibia), indicating a limited transport (rafting). Based on that, it is determined as Panthera cf. pardus only. The whole mammalian fossil record from the site consists predominantly of large herbivores (Elephas (Palaeoloxodon) antiquus, “Dihoplus” (Stephanorhinus) kirchbergensis, Bison priscus, Megaloceros giganteus, Mammuthus primigenius, Alces alces, Cervus elaphus), whereas carnivores (Ursus arctos, Crocuta crocuta ssp., Canis lupus, Vulpes vulpes) are very rare and composed only by a few specimens. The species composition indicates probably a mixing of different faunal assemblages from warm (interglacial) and cold (glacial) periods of the Late Pleistocene.

The role of floodplain width and alluvial bar growth as a precursor for the formation of anabranching rivers

Anabranching rivers are defined as a system of multiple channels separated by stable alluvial islands. While substantial progress has been made in developing a physics-based understanding of what drives the differences between meandering and braided river channels, anabranching rivers have not been well-integrated into these models. Here, we propose that alluvial bar growth on the floodplain may be a precursor for the formation of anabranching rivers. Alluvial bar growth strongly depends on the aspect ratio of the flow (width divided by depth) and rivers with wide floodplains have flows with a large aspect ratio. Consistent with this idea, remotely sensed measurements of floodplain width of four rivers from different climatic and tectonic settings show that anabranching river patterns are often associated with relatively wide floodplains. To explore the physics behind that empirical relationship we carried out two sets of morphodynamic numerical simulations using boundary conditions from field-scale modern anabranching rivers spanning different climatic and geologic settings as well as hypothetical floodplain widths. Results from the simulations show that, for a given flood discharge, widening the floodplain changes the river pattern from single channel to multi-threaded with mobile bars to multi-channeled with immobile islands. Multi-channeled patterns arise because the emergence of bars causes flow bifurcation. Subsequent bifurcation instability leads to the emergence of multiple stable channels. As the channels increase their cross-sectional area, shields stresses on intervening bars are reduced until the bars stabilize into islands. We suggest that the presence of stable islands allows vegetation to grow or cohesive sediment to accumulate leading to enhanced channel bank strength and a stable anabranching pattern. Our results suggest that alluvial bar growth can be a precursor to formation of anabranching rivers. Compared with field measurements our simulations accurately predict the number of active channels to within ~20% which seems to support the idea that some anabranching rivers may originate from alluvial bar growth.

A multi-scale approach of fluvial biogeomorphic dynamics using photogrammetry

Over the last twenty years, significant technical advances turned photogrammetry into a relevant tool for the integrated analysis of biogeomorphic cross-scale interactions within vegetated fluvial corridors, which will largely contribute to the development and improvement of self-sustainable river restoration efforts. Here, we propose a cost-effective, easily reproducible approach based on stereophotogrammetry and Structure from Motion (SfM) technique to study feedbacks between fluvial geomorphology and riparian vegetation at different nested spatiotemporal scales. We combined different photogrammetric methods and thus were able to investigate biogeomorphic feedbacks at all three spatial scales (i.e., corridor, alluvial bar and micro-site) and at three different temporal scales, i.e., present, recent past and long term evolution on a diversified riparian landscape mosaic. We evaluate the performance and the limits of photogrammetric methods by targeting a set of fundamental parameters necessary to study biogeomorphic feedbacks at each of the three nested spatial scales and, when possible, propose appropriate solutions. The RMSE varies between 0.01 and 2 m depending on spatial scale and photogrammetric methods. Despite some remaining difficulties to properly apply them with current technologies under all circumstances in fluvial biogeomorphic studies, e.g. the detection of vegetation density or landform topography under a dense vegetation canopy, we suggest that photogrammetry is a promising instrument for the quantification of biogeomorphic feedbacks at nested spatial scales within river systems and for developing appropriate river management tools and strategies.

Populus nigra L. establishment and fluvial landform construction: biogeomorphic dynamics within a channelized river

AbstractPopulations of the riparian pioneer species Populus nigra L. which establish on alluvial bars within river channels modulate sediment dynamics and fluvial landforms. Dense cohorts of P. nigra have colonized gravel point bars along the channelized River Garonne, France, during the last 20 years and have enhanced the vertical, lateral and longitudinal development of the bars. For this period, the geomorphic characteristics of two wooded point bars on this laterally stable river are closely linked to the spatial distribution and intensity of establishment and resistance of different cohorts of P. nigra. Furthermore, P. nigra colonization dynamics were controlled by engineer effects of this same species. This relationship is illustrated by a significant correlation between key geomorphic and biological variables measured in situ and characterized with a set of four aerial photographs taken between 2000 and 2010. The development of wooded point bars, which are discrete biogeomorphic units, over the studied period, appear to result from a specific biogeomorphic positive feedback of matter aggregation and vegetation establishment related to sediment trapping and stabilization by pioneer engineer plants. We propose a conceptual model of biogeomorphic unit construction for channelized, lateral stable rivers. We consider the resultant biogeomorphic units as functional from an ecological point of view because P. nigra enhances at the cohort scale (i) its own inherent capacity to resist hydrogeomorphic disturbances, and (ii) its resilience capacity as a result of successful colonization, especially downstream of mature poplar stands. Copyright © 2016 John Wiley & Sons, Ltd.

Seed retention by pioneer trees enhances plant diversity resilience on gravel bars: Observations from the river Allier, France

Pioneer riparian trees which establish in river active tracts on gravel bars enhance fine sediment retention during high flows within their stands and in their lee side, forming obstacle marks. Fine sediment retention can be accompanied by deposition of seeds transported by water dispersal, i.e. by hydrochory. We tested the hypothesis that pioneer riparian trees significantly control seed deposition on gravel bars by forming sediment obstacle marks. We described the seed bank structure and compared samples collected from obstacle marks and bare coarse-grained bar surfaces. At the surface (at 2cm depth), seed abundance (N) and richness (S) (expressed as mean ± sd) were significantly higher in areas directly affected by riparian trees, i.e. obstacle marks, (N: 693±391; S: 17±3) than in bare surfaces (N: 334±371; S: 13±5). Surface and sub-surface (at 20cm depth) samples were also significantly different, with the sub-surface samples almost devoid of seeds (respectively N: 514±413; S: 15±5 and N: 3±6; S: 1±2). These results suggest a biogeomorphic feedback between sediment and associated seed retention mediated by hydrochory, vegetation growth and local seed dispersal mediated by barochory. Such feedback may improve plant diversity resilience on gravel alluvial bars of high-energy rivers.

Evolutionary trajectory of channel planforms in the middle Garonne River (Toulouse, SW France) over a 130-year period: Contribution of mixed multiple factor analysis (MFAmix)

The purpose of this study is to propose a methodological essay for defining evolutionary trajectories of channel planforms and to examine the channel change in the middle Garonne River (southwest France) over a 130-year period. The study focuses on a reach of ~90km situated downstream from the city of Toulouse. A set of four historical maps (1868, 1940s, 1970s, and 2000s) is used to build a geomorphometric diachronic database. Data processing through mixed multiple factor analysis (MFAmix) and hierarchical cluster analysis (HCA) allows distinction between four homogeneous zones within the study reach, depending on their evolutionary trajectories. Channel behavior in the upstream and median zones evolved as of the beginning of the study period (narrowing of the fluvial area, colonization by vegetation, and removal of alluvial bars), likely owing to punctual anthropogenic actions. The downstream zone is characterized by stabilization of the channel and alluvial bar removal over the second half of the twentieth century, coinciding with the campaign undertaken by French local authorities between 1960 and 1984 to protect river banks. The role of climate transition between the Little Ice Age (LIA) and the onset of the Global Warming period (GW) is also discussed. Results generally are consistent with the chronology established for most European rivers.