Bar Accretion Research Articles

A typical point bar with atypical strata in the McMurray Formation, Alberta, Canada: Floods, tides and high suspended sediment concentrations

AbstractStratigraphic positions and the nature of some breccias and mudstones observed in the Lower Cretaceous McMurray Formation type section (Alberta, Canada) challenge the existing fluvial point bar facies models. The currently applied large‐scale, tidally influenced, sandy bedload facies model suggests that point bars fine upward, where breccias and cross‐strata occur at the base of the succession and mud content increases upward. Instead, this study documents that: (i) mudstone clast breccias are not exclusively associated with basal channel‐lag deposits and can occur throughout the point bar; and (ii) stratified mudstones are not limited to the top of the fining‐upward succession and can occur at or near the channel base. This outcrop data, including lithology, sedimentary structures, trace fossils, bed thickness and boundaries, and the nature of stratal packages, further suggests dynamic interplay between fluvial and tidal processes. This article discusses the potential role of high‐magnitude river floods in temporarily elevated suspended sediment concentrations and highlights that fluid mud processes played a crucial role in mud deposition in both tide‐influenced and fluvial parts of the system. Fluid mud processes allowed mud deposition along the channel base and across the point bar accretion surfaces. The breccias predominantly consist of broken pieces of inclined heterolithic strata and mostly occur on accretion sets of point bars, suggesting erosion and collapse of the point bar rather than of the cutbank. This work introduces a novel perspective to the existing models of point bar sedimentation, offering a new end member and concepts for the interpretation of subsurface data. The authors hope that this research encourages further investigations into similar phenomena elsewhere in the world.

Morphodynamics and depositional architecture of mid‐channel bars in large Amazonian rivers

AbstractLarge rivers are characterized by large water discharges, high suspended sediment fluxes and low slope, and typically display multiple channels that are separated by large complex bars. The most common channel style found in these rivers is characterized by the alternating presence of single and multiple channel threads. Mid‐channel compound bars separate a main deeper channel from shallower secondary channels, leading to low‐order braiding and low to moderate sinuosity. Despite the importance of this type of river for global terrestrial sediment transport and the interpretation of ancient fluvial successions, integrated depositional models for large multi‐channel rivers are still not fully developed. This paper interprets the channel morphodynamics and depositional architecture of such large rivers by investigating the distinctive features of their laterally‐accreting mid‐channel bars in the Solimões–Amazonas River. This is achieved by examination of temporal series of satellite images, quantification of bedforms using multibeam echosounding surveys, analysis of shallow seismic and ground penetrating radar surveys of selected areas, and on‐site field observations and sample collections. Such mid‐channel bars produce directional variability in planform scroll bar accretion of up to 180°. As these bars – and their associated channels – evolve, they become progressively shallower, due to the bifurcation of the two channels and the resultant partitioning of discharge that leads to a gradual reduction in transport capacity within the outer channel. The preserved successions of this process in the alluvial plain are characterized by fining‐upward trends, with larger cross‐strata sets at mid‐depths and internal erosional surfaces marking distinct cycles of bar development with potentially differing palaeocurrent trends. The new depositional models proposed for these mid‐channel bars and channels constitute a basis for the recognition of large multi‐channel river deposits in the ancient rock record.

Sediment preservation and accretion rates of fluvial meander-belt deposits: variations with temporal scale and river size

Abstract The preservation and accretion rates of fluvial meander-belt deposits appear to vary with the time over which they are evaluated, but the drivers of this effect are not fully understood. Using channel trajectories tracking the temporal evolution of meandering rivers, constrained with data on past river planforms, a numerical model is used to simulate planform evolutions of meander-belt reaches that underwent different types of meander transformation behaviours and bend cutoffs. Sediment preservation and bar accretion rates are quantified for three hierarchies of depositional products: (i) accretion stages reflecting intervals of bend migration by a certain meander-transformation style, (ii) meander-belt segments encompassing multiple stages but no bend cutoffs and (iii) meander-belt segments that experienced one or multiple cutoffs. Results show that distinct power-law relationships between the accretion rate and the time of sedimentation exist for river systems of different magnitudes in scale. Within each order of magnitude in river-system size, a single power-law relationship fails to effectively capture all depositional hierarchies. Sediment preservation varies with the time over which it is computed, but systematic variations in accretion rates with time primarily portray an apparent dependency of river migration rates on time: these variations largely reflect the temporal resolution and timespan of the modelled examples, highlighting an inherent issue affecting studies of river evolutions.

The evolution of meandering and anabranching rivers in postglacial and loess landscapes of Europe

This study aims to explain the influence of sediments and landforms of postglacial and loess landscapes of Europe on processes forming meandering and anabranching rivers. Based on a literature review, sedimentary architecture, types and grain-size of deposits, and channel planform changes since the Late Pleniglacial were collected for extensive set of European rivers. These data were used to create a conceptual model explaining the evolution of meandering and anabranching rivers in both types of landscape. The present study shows that the inheritance of glacial and fluvioglacial landforms and sediments, and loess formation, influence types of processes forming anabranching and meandering rivers. Point bar accretion formed meandering rivers in postglacial zone whereas oblique accretion dominated in loess areas. Anabranching rivers of postglacial zone evolved through the formation of crevasse channels and meandering anabranches. Anabranching rivers of loess zone formed sustained bifurcations. Postglacial and loess rivers reacted differently to increased deposition and humidity in the Late-Holocene. Meanders of postglacial zone formed channel bars owing to increase in stream power and sediment load. In cases of flow energy decrease, and sustained upstream sediment delivery, they turned into anastomosing rivers. Anabranching rivers in ice-marginal valleys maintained its courses until hydro technical works in the 19th century. Low-energy meandering rivers of loess areas became wetlands owing to deposition of silts. High-energy meandering and anabranching rivers maintained the planforms throughout the Holocene. These findings need to be refined by detailed studies on the evolution of rivers in subarctic zone, and large rivers of Europe.

Biogeomorphic recovery of a river reach affected by mining

AbstractEnvironmental changes are impacting river systems worldwide. These arise from factors such as flood magnitude–frequency changes, direct human management interventions, inadvertent human impacts on sediment supply and fluvial regimes and landscape‐scale changes in climate. Historical and active metal mining is significant in this regard. Here, we investigate morphodynamic changes within a reach of the River Ystwyth, Wales, since 1845. We analyse historical and contemporary information derived from maps, river flow records (1962–2021), metal analyses of sediment samples (1970s and 2021), ground geomorphological surveys (1970s and 1986–1987) and remotely sensed imagery (2001–2021) to investigate changes during a period of active metal mining followed by a century of post‐mining recovery. During the studied period, an initially meandering river was transformed into a braided one, subsequently reverting to a single sinuous channel. Sinuosity reduced from 1.31 in 1845 to 1.09 in 1982 before recovering to 1.39 in 2019. Inversely, the braiding index reduced from a maximum of 2.0 in 1987 to 1.5 in 2021. Evolution in planform was associated with a change from expansive bar formation and avulsion under braided conditions to lateral bar accretion and associated bank erosion along a sinuous single channel. The initial 19th‐century channel pattern and floodplain instability seems to have been related to mining sediment toxicity effects rather than a response to high sediment volumes, with recent recovery and channel style reversion being attributable to vegetation encroachment and biomass stabilization of the floodplain. Causal factors of recent recovery appear to be colonization by gorse (Ulex europeaus) in the absence of physical control measures and a reduction in grazing by the native rabbit population because of a disease‐induced decline in their numbers. These results highlight the importance of riparian vegetation in addition to sediment balance and hydrological processes in controlling fluvial responses to environmental changes.

Ontogeny of a subtidal point bar in the microtidal Venice Lagoon (Italy) revealed by three‐dimensional architectural analyses

AbstractSedimentological and architectural features of meandering subtidal channels are relatively unexplored, and their deposits are commonly investigated based on facies models set up for intertidal meandering channels. The Venice Lagoon (northern Adriatic Sea, Italy) is affected by a micro‐tidal regime and hosts a dense network of active and buried tidal channels. It represents an excellent natural laboratory to improve the current knowledge on subtidal meander morphodynamics and related deposits. In this study, the integration of high‐resolution geophysical images and core data allows reconstruction of the architectural three‐dimensional model of a meandering subtidal palaeochannel, which is buried below a modern subtidal flat. The study palaeochannel was 35 m wide and 3 m deep, and formed three adjacent meander bends and related point bars. A detailed three‐dimensional architectural reconstruction was carried out for deposits associated with one of these meander bends, that was crossed by a minor, low‐sinuosity channel with two minor bank‐attached bars. This reconstruction highlights that the study point bar has a horseshoe shape, which arose from the onset of bar accretion from an already‐sinuous channel. Reconstructed growth stages of the studied bends show that point‐bar accretion can follow different styles of planform transformation, also experiencing simultaneously landward (or seaward) deposition according to the dominant flow direction (i.e. local tidal asymmetry). The analyses show that planform transformations occurred in parallel with elevation changes of the related channel thalweg, which shaped pools with geometry varying with the radius of curvature of the bend. The present study highlights the relevance of high‐resolution three‐dimensional reconstructions to link palaeomorphodynamic processes with related sedimentary products.

A stratigraphic example of the architecture and evolution of shallow water mouth bars

AbstractImproved understanding of mouth bar morphodynamics, and the resulting stratigraphic architectures, is important for predicting the loci of deposition of different sediment fractions, coastal geomorphic change and heterogeneity in mouth bar reservoirs. Facies and architectural analysis of exceptionally well‐exposed shallow water (ca 5 m depth) mouth bars and associated distributaries, from the Xert Formation (Lower Cretaceous), of the Maestrat Basin (east‐central Spain), reveal that they grew via a succession of repeated autogenic cycles. An initial mouth bar accretion element forms after avulsion of a distributary into shallow standing water. Turbulent expansion of the fluvial jet and high bed friction results in rapid flow deceleration, and deposition of sediment in an aggradational to expansional bar‐form. Vertical bar growth causes flattening and acceleration of the jet. The accelerated flow scours channels on the bar top, which focuses further expansion of the mouth bar at individual loci where the channels break through the front of the mouth bar. Here, new mouth bar accretion elements form, downlapping and onlapping against a readily recognizable surface of mouth bar reorganization. Vertical growth of the new mouth bar accretion elements causes flattening and re‐acceleration of the jet, leading to channelization, and initiation of the next generation of mouth bar accretion elements. Thus the mouth bar grows, until bed‐friction effects cause backwater deceleration and super‐elevation of flow in the feeding distributary. Within‐channel sedimentation, choking and upstream avulsion of the feeding channel, results in mouth bar abandonment. In this study, mouth bars are formed of at least two to three accretion elements, before abandonment happened. The results of this study contrast with the notion that mouth bars form by simple vertical aggradation and radial expansion. However the architecture and facies distributions of shallow water mouth bars are a predictable product of intrinsic processes that operate to deposit them.

Bed Particle Displacements and Morphological Development in a Wandering Gravel‐Bed River

AbstractBed particles were tracked using passive integrated transponder tags in a wandering reach of the San Juan River, British Columbia, Canada, to assess particle movement around three major bars in the river. In‐channel topographic changes were monitored through repeat LiDAR surveys during this period and used in concert with the tracer data set to assess the relationship between particle displacements and changes in channel morphology, specifically, the development and re‐working of bars. This has direct implications for virtual velocity and morphologic based estimates of bedload flux, which rely on accurate estimates of the variability and magnitude of particle path lengths over time. Tracers were deployed in the river at three separate locations in the Fall of 2015, 2016, 2017, and 2018, with recovery surveys conducted during the summer low‐flow season the year after tracer deployment and multiple mobilizing events. Tracers exhibited path length distributions reflective of both morphologic controls and year to year differences related to the annual flow regime. Annual tracer transport was restricted primarily to less than one riffle‐pool‐bar unit, even during years with a greater number of peak floods and duration of competent flow. Tracer deposition and burial was focused along bar margins, particularly at or downstream of the bar apex, reflecting the downstream migration and lateral bar accretion observed on Digital Elevation Models of difference. This highlights the fundamental importance of bar development and re‐working underpinning bedload transport processes in bar‐dominated channels.

The case of the braided river that meandered: Bar assemblages as a mechanism for meandering along the pervasively braided Missouri River, USA

Abstract This paper offers a mechanism for meandering in an otherwise braided river and then discusses its general implications for river processes and fluvial deposits. Braided rivers manage to meander without the paired point bars and single-thread channels that are instrumental in developing bends in other meandering rivers. The driving processes for meandering in these braided systems remain enigmatic. The unchannelized and prechannelized Missouri River is an example of a braided meandering river, and it provides an opportunity to gain insight into these processes. This study utilized historical maps, sequential air photos, and surficial geologic maps both to define the processes by which this braided river meanders, and to characterize the deposits produced by these processes. These data show that the Missouri River meanders by building point assemblages instead of point bars. Repeated accretion of midchannel and lateral bars to a common point on the bank forces development of a meander bend around a point assemblage comprising multiple amalgamated compound bars. This differs from single-thread systems, which expand and translate bends around a single compound point bar. Alternating development of point assemblages forces meandering over successions of meander bends. Braided meander loops grow by expansion and translation like single-thread rivers, but they also may contract to produce counterpoint assemblages. Contraction appears to be the more common means of loop abandonment compared to loop cutoff for the braided Missouri River. This differs from single-thread meandering rivers, where contraction is limited, and loop cutoff is consistently the dominant abandonment process. Deposits of the braided meandering Missouri River differ from deposits of single-thread rivers in the rarity of both meander scrolls and single-thread channel fills. Instead, point and counterpoint assemblages comprise fusiform bar elements bound by small filled remnants of anabranch channels. These assemblages are commonly bound by meander cutbank scars. Cutbank scars associated with contraction, however, tend to be composite rather than discrete erosional surfaces, and they do not tend to bind river-scale abandoned channel fills. The braided meandering Missouri River also differs from wandering rivers because wandering rivers meander by building compound bars instead of assemblages, are more gravelly, have less pervasive and much less mobile midchannel bars, and appear to reflect a transitional intermediate pattern instead of a stable hybrid pattern. Braiding and meandering both expend stream power, and both are mechanisms for achieving channel equilibrium. The Missouri River exhibits both of these processes in tandem; thus, meandering and braiding are not mutually exclusive processes. Braided meandering rivers like the Missouri River are less common than either straight-braided or single-thread-meandering rivers, but they are not unique. The long-held distinction of braided versus meandering patterns for rivers thus may be practical but is not definitive.

Spatiotemporal variability of channel and bar morphodynamics in the Gorai-Madhumati River, Bangladesh using remote sensing and GIS techniques

Bangladesh is a riparian country that is criss-crossed by the many tributaries and distributaries of the mighty Ganges, Brahmaputra, and Meghna river systems. Gorai-Madhumati, a distributary of the Ganges River is an example where morphological development of the river is associated with frequent channel shifting within the catchment area. The main objective of this research is to quantify the extent of channel migration, erosion-accretion, river width, sinuosity, and charland morpho-dynamics from 1972 to 2018 using geospatial techniques combined with satellite images and hydrological data. The study also addressed the impacts of Farakka Barrage construction in India on the shifting, flow behavior, and siltation of Gorai-Madhumati River. The study shows that bar surface areas have abnormally increased in both segments after 1975 due to the construction of Farakka Barrage. Water flow in the Gorai-Madhumati has dropped remarkably in the downstream and instigated huge sedimentation in this region. Analysis of the time series satellite images revealed that the morphology of the river channel experienced huge changes simultaneously with the changes in the seasonal flow and sedimentation all over the study period. Migration trend has frequently shifted and taken place in the NW and NE direction in the observed sections of the river. Throughout the study period, total amount of accretion was greater than the net percentage of erosion on both banks of the river. River discharge, bar accretion, and erosion history show that the Gorai-Madhumati River will no longer exist with the present flowing condition without attention and proper river management.

Geophysical and Sedimentological Investigations Integrate Remote-Sensing Data to Depict Geometry of Fluvial Sedimentary Bodies: An Example from Holocene Point-Bar Deposits of the Venetian Plain (Italy)

Over the past few millennia, meandering fluvial channels drained coastal landscapes accumulating sedimentary successions that today are permeable pathways. Propagation of pollutants, agricultural exploitation and sand liquefaction are the main processes of environmental interest affecting these sedimentary bodies. The characterization of these bodies is thus of utmost general interest. In this study, we particularly highlight the contribution of noninvasive (remote and ground-based) investigation techniques, and the case study focuses on a late Holocene meander bend of the southern Venetian Plain (Northeast Italy). Electromagnetic induction (EMI) investigations, conducted with great care in terms of sonde stability and positioning, allowed the reconstruction of the electrical conductivity 3D structure of the shallow subsurface, revealing that the paleochannel ranges in depth between 0.8 and 5.4 m, and defines an almost 260 m-wide point bar. The electrical conductivity maps derived from EMI at different depths define an arcuate morphology indicating that bar accretion started from an already sinuous channel. Sedimentary cores ensure local ground-truth and help define the evolution of the channel bend. This paper shows that the combination of well-conceived and carefully performed inverted geophysical surveys, remote sensing and direct investigations provides evidence of the evolution of recent shallow sedimentary structures with unprecedented detail.

Anastomosing-to-meandering transitional river in sedimentary record: A case study from the Neogene of central Poland

We present a sedimentological analysis of the Wielkopolska Member, the upper unit of the Poznań Formation which formed in a fluvial environment within a tectonic graben during the middle Miocene to early Pliocene in Poland. Large excavations in a lignite opencast mine have enabled our study of palaeochannel infills, in which sandy to silty-clayey ribbons are scattered within overbank clays. Channel sedimentation was controlled mainly by silty-clayey suspension settling and counter point bar accretion, with sinuous dunes and cut-and-fill of scours as secondary depositional agents. Lithofacies frequency indicates that the channel facies derived from both fine-grained suspension and bedload. The river was low energy, with the power of flood flows calculated in the range of 2.5 to 21 W m−2. Washload concentration was high, as demonstrated by thick silty-clayey packages within palaeochannels, the presence of climbing dunes and the relatively low number of ripple-derived beds. Alluvial channels were deep and narrow (3–4 m and 70–90 m for main channels; 2–3 m and <30 m for secondary channels, respectively). The average width to depth (w/d) ratio approaches 13. This value indicates that lateral channel migration was limited. Some palaeochannel successions consist of thin basal sandy bed overlain by thick silty-clayey interval. They prove an abrupt channel abandonment, that is, avulsion. Avulsion often occurred as channel reoccupation, as is evidenced by the frequent superposition of two or three ribbons. The interpreted environmental features suggest that the fluvial system was transitional between an anastomosing and a meandering river. This transitional nature was most likely due to climatic and tectonic conditions. During the deposition of the sediments studied, slow regional subsidence occurred together with the general trend for cooling and drying climate with its clear seasonality. At that time, the deposition in the anastomosing river prevailed, while in periods of limited subsidence and increased rainfall the river transformed into a meandering type. This study provides new facts about sedimentary successions of low-energy rivers, and broadens the discussion on the evolution of ancient fluvial systems.

Planform and stratigraphic signature of proximal braided streams: remote-sensing and ground-penetrating-radar analysis of the Kicking Horse River, Canadian Rocky Mountains

ABSTRACTBraided rivers have accumulated a dominant fraction of the terrestrial sedimentary record, and yet their morphodynamics in proximal intermountain reaches are still not fully documented—a shortcoming that hampers a full understanding of sediment fluxes and stratigraphic preservation in proximal-basin tracts. Located in the eastern Canadian Cordillera near the continental divide, the Kicking Horse River is an iconic stream that has served as a model for proximal-braided rivers since the 1970s. Legacy work on the river was based solely on ground observations of small, in-channel bars; here we integrate field data at the scale of individual bars to the entire channel belt with time-lapse remote sensing and ground-penetrating-radar (GPR) imaging, in order to produce a more sophisticated morphodynamic model for the river.Cyclical discharge fluctuations related to both diurnal and seasonal variations in melt-water influx control the planform evolution and corresponding stratigraphic signature of trunk channels, intermittently active anabranch channels, and both bank-attached and mid-channel bars. Three-dimensional GPR fence diagrams of compound-bar complexes are built based on the identification of distinct radar facies related to: i) accretion and migration of unit bars, ii) both downstream and lateral outbuilding of bar-slip foresets; iii) buildup of bedload sheets, iv) channel avulsion, and v) accretion of mounded bars around logs or outsized clasts. Trends observed downstream-ward include decreases in gradient and grain size decreases, trunk-channel shrinkage, intensified avulsion (with increase in abundance for anabranch channels), and a shift from high-relief to low-relief bar topography. The integration of ground sedimentology, time-lapse remote sensing, and GPR imaging demonstrates that proximal-braided streams such as the Kicking Horse River can be critically compared to larger systems located farther away from their source uplands despite obvious scale differences.

The influence of a vegetated bar on channel-bend flow dynamics

Abstract. Point bars influence hydraulics, morphodynamics, and channel geometry in alluvial rivers. Woody riparian vegetation often establishes on point bars and may cause changes in channel-bend hydraulics as a function of vegetation density, morphology, and flow conditions. We used a two-dimensional hydraulic model that accounts for vegetation drag to predict how channel-bend hydraulics are affected by vegetation recruitment on a point bar in a gravel-bed river (Bitterroot River, Montana, United States). The calibrated model shows steep changes in flow hydraulics with vegetation compared to bare-bar conditions for flows greater than bankfull up to a 10-year flow (Q10), with limited additional changes thereafter. Vegetation morphology effects on hydraulics were more pronounced for sparse vegetation compared to dense vegetation. The main effects were (1) reduced flow velocities upstream of the bar, (2) flow steered away from the vegetation patch with up to a 30 % increase in thalweg velocity, and (3) a shift of the high-velocity core of flow toward the cut bank, creating a large cross-stream gradient in streamwise velocity. These modeled results are consistent with a feedback in channels whereby vegetation on point bars steers flow towards the opposite bank, potentially increasing bank erosion at the mid- and downstream ends of the bend while simultaneously increasing rates of bar accretion.

Sedimentary Facies and Architecture of a Gigantic Gravelly Submarine Channel System in a Cretaceous Foredeep Trough (the Magallanes Basin, Southern Chile)

The Lago Sofia conglomerate in southern Chile is a deep-marine gravelly deposit, which is hundreds of meters thick and kilometers wide and extends laterally for more than 100 km, filling the foredeep trough of the Cretaceous Magallanes Basin. For understanding the depositional processes and environments of this gigantic deep-sea conglomerate, detailed analyses on sedimentary facies, architecture and paleoflow patterns were carried out, highlighting the differences between the northern (Lago Pehoe and Lago Goic areas) and southern (Lago Sofia area) parts of the study area. The conglomerate bodies in the northern part occur as relatively thin (< 100 m thick), multiple units intervened by thick mudstone-dominated sequences. They show paleoflows toward ENE and S to SW, displaying a converging drainage pattern. In the southern part, the conglomerate bodies are vertically interconnected and form a thick (> 400 m thick) conglomerate sequence with rare intervening fine-grained deposits. Paleoflows are toward SW. The north-to-south variations are also distinct in sedimentary facies. The conglomerate bodies in the southern part are mainly composed of clast-supported conglomerate with sandy matrix, which is interpreted to be deposited from highly concentrated bedload layers under turbidity currents. Those in the northern part are dominated by matrix- to clast-supported conglomerate with muddy matrix, which is interpreted as the products of composite mass flows comprising a turbidity current, a gravelly hyperconcentrated flow and a mud-rich debris flow. All these characteristics suggest that the Lago Sofia conglomerate was formed in centripetally converging submarine channels, not in centrifugally diverging channels of submarine fans. The tributaries in the north were dominated by mass flows, probably affected by channel-bank failures or basin-marginal slope instability processes. In contrast, the trunk channel in the south was mostly filled by tractive processes, which resulted in the vertical and lateral accretion of gravel bars, deposition of gravel dunes and filling of scours and channels, similar to deposits of terrestrial gravel-bed rivers. The trunk channel developed along the axis of foredeep trough and its confinement within the trough is probably responsible for the thick, interconnected channel fills. The large-scale architecture of the trunk-channel fills shows an eastward offset stacking pattern, suggesting that the channel migrated eastwards most likely due to the uplift of the Andean Cordillera.

Palaeovalleys in foreland ramp settings: what happens as accommodation decreases down dip?

AbstractRecent advances in our understanding of palaeovalleys are largely guided by examples from passive margins, in which accommodation increases down depositional dip. This study tests these models against a dataset from the Pennsylvanian Breathitt Group of the central Appalachian foreland basin, USA. This fluvio‐deltaic succession contains extensive erosionally based fluvio‐estuarine sand bodies that can be tracked over 80 km down depositional dip from a proximal zone of high accommodation close to the orogenic margin to a distal, lower accommodation zone close to the cratonic margin of the basin. The sand bodies are up to 25 m thick, multi‐storey and characterized in their lower parts by strongly amalgamated storeys containing sandy fluvial to estuarine bar accretion elements, and in their middle to upper parts by more fully preserved storeys up to 10 m thick and laterally extensive over 100s of metres. The upper storeys include abandonment channel‐fills of heterolithic marine or marginal marine deposits or muddy to sandy point‐bar elements. Three major regional‐scale architectures include: (i) Tabular sand bodies that everywhere incise open marine prodelta and mouth bar facies and are interpreted as palaeovalleys formed during falling stage and lowstand systems tracts, when eustatic sea‐level fall outpaced tectonic subsidence across the entire study area. (ii) Sand bodies that incise genetically related floodplain lake and/or bay‐fill minor mouth bar deposits up depositional dip and open marine prodelta and mouth bar facies down dip. These stacked distributary channel deposits map down dip into palaeovalleys and formed when up dip subsidence rate resulted in positive, but reduced rate of accommodation creation, while lower tectonic subsidence rate down‐dip resulted in incision. (iii) Sand bodies that incise genetically related floodplain, lake and/or bay‐fill minor mouth bars up dip and pass down‐dip into genetically related unconfined floodplain, prodelta and mouth bar deposits. These sand bodies represent stacked distributary channel fills and channel amalgamation was the product of high rates of lateral migration, typical of the behaviour of channels above their backwater reach. Case (2) sand bodies demonstrate that in rapidly subsiding foreland basins, cross‐shelf palaeovalleys may form down depositional dip from aggradational, distributive fluival strata. Additionally, the genetic relationship between stacked distributary channels and palaeovalleys supports recent models for palaeovalley formation that emphasize diachronous, cut‐and‐fill during falling stage and lowstands of relative sea level.

Highly Variable Precambrian Fluvial Style Recorded In the Nelson Head Formation of Brock Inlier (Northwest Territories, Canada)

Abstract Spectacular canyon exposures of the ∼ 1 Ga Nelson Head Formation along the modern Brock River, Northwest Territories, provide a rare opportunity to assess the deposits of pre-vegetation, braided to sinuous-channelized fluvial systems. We analyze the sedimentology, architecture, and depositional evolution of 16 stacked fluvial-channel belts at this site, demonstrating greater variability in sedimentary style and morphodynamics than is typically interpreted for Precambrian rivers. Inferences on fluvial planview based on integrated analysis of depositional architecture and paleoflow reveal complex patterns of channel-planform evolution, and four depositional stages are recognized: (1) floodbasin-splay progradation with bounding episodes of eolian reworking; (2) deposition in wandering-channel belts characterized by deep anabranches and prominent lateral accretion; (3) progressive shift towards shallow, braided-channel belts confined within alluvial valleys and dominated by downstream accretion; and (4) deposition by marine-influenced braided-channel belts with mixed downstream, lateral, and upstream accretion. Overall, the studied fluvial-channel belts point to significant morphodynamic complexity and geomorphic variability, challenging the assumption that all pre-Silurian rivers shared poor channelization and low sinuosity. Observed channel bodies also have width:thickness ratios strongly overlapping with those of rivers postdating the rise of vegetation. In both wandering- and braided-channel belts, complex patterns of channel migration, bar accretion, and dissection were the result of co-acting processes such as development of transverse-velocity zones within the channels, local paleoflow disturbance induced by depositional topography, and avulsion. In concert with the regional extent and paleoflow, and increasingly distal character of correlative stratigraphic units towards the northwest, the magnitude of the observed trunk-fluvial channels is consistent with a mature drainage capable of transecting the Laurentian craton over thousands of kilometers.

Reconstructing fluvial bar surfaces from compound cross‐strata and the interpretation of bar accretion direction in large river deposits

AbstractThe interpretation of fluvial styles from the rock record is based for a significant part on the identification of different types of fluvial bars, characterized by the geometric relationship between structures indicative of palaeocurrent and surfaces interpreted as indicative of bar form and bar accretion direction. These surfaces of bar accretion are the boundaries of flood‐related bar increment elements, which are typically less abundant in outcrops than what would be desirable, particularly in large river deposits in which each flood mobilizes large volumes of sediment, causing flood‐increment boundary surfaces to be widely spaced. Cross‐strata set boundaries, on the other hand, are abundant and indirectly reflect the process of unit bar accretion, inclined due to the combined effect of the unit bar surface inclination and the individual bedform climbing angle, in turn controlled by changes in flow structure caused by local bar‐scale morphology. This work presents a new method to deduce the geometry of unit bar surfaces from measured pairs of cross‐strata and cross‐strata set boundaries. The method can be used in the absence of abundant flood‐increment bounding surfaces; the study of real cases shows that, for both downstream and laterally accreting bars, the reconstructed planes are very similar to measured bar increment surfaces.

Fluvial islands: First stage of development from nonmigrating (forced) bars and woody-vegetation interactions

Fluvial islands can develop from the channel bed by interactions between pioneer trees and bars. Although vegetation recruitment and survival is possible on all bar types, it is easier for trees to survive on nonmigrating bars developed from a change in channel geometry or to the presence of a steady perturbation. This field study details the first stages of development of a vegetated mid-channel, nonmigrating (or forced) bar and its evolution toward an island form. Over six years, analysis of bed topographical changes, vegetation density and roughness, scour and fill depths, sediment grain size and architecture, and excess bed shear stress highlighted a specific signature of trees on topography and grain size segregation. Two depositional processes combining the formation of obstacle marks and upstream-shifting deposition of sediments led to the vertical accretion of the vegetated bar. During the first stage of the bar accretion, bedload sediment supply coming from surrounding channels during floods was identified as a key process modulated by the presence of woody vegetation and a deflection effect induced by the preexisting topography. Grain size segregation between vegetated and bare areas was also highlighted and interpreted as an important process affecting the development of surrounding channels and the degree of disconnection (and hence the speed of development) of a growing island. The heterogeneity of bedload supply can explain why sediment deposition and density of trees are not strictly related. A general conceptual model detailing the first stages of evolution from a bar to an established island is proposed for relatively large lowland rivers.

Intra‐event scale bar–bank interactions and their role in channel widening

AbstractChannel bars and banks strongly affect the morphology of both braided and meandering rivers. Accordingly, bar formation and bank erosion processes have been greatly explored. There is, however, a lack of investigations addressing the interactions between bed and bank morphodynamics, especially over short timescales. One major implication of this gap is that the processes leading to the repeated accretion of mid‐channel bars and associated widenings remain unsolved. In a restored section of the Drau River, a gravel‐bed river in Austria, mid‐channel bars have developed in a widening channel. During mean flow conditions, the bars divert the flow towards the banks. One channel section exhibited both an actively retreating bank and an expanding mid‐channel bar, and was selected to investigate the morphodynamic processes involved in bar accretion and channel widening at the intra‐event timescale. We repeatedly surveyed riverbed and riverbank topography, monitored riverbank hydrology and mounted a time‐lapse camera for continuous observation of riverbank erosion processes during four flow events. The mid‐channel bar was shown to accrete when it was submerged during flood events, which at the subsequent flow diversion during lower discharges narrowed the branch along the bank and increased the water surface elevation upstream from the riffle, which constituted the inlet into the branch. These changes of bed topography accelerated the flow along the bank and triggered bank failures up to 20 days after the flood events. Four analysed flow events exhibited a total bar expansion from initially 126 m2 to 295 m2, while bank retreat was 6 m at the apex of the branch. The results revealed the forcing role of bar accretion in channel widening and highlighted the importance of intra‐event scale bed morphodynamics for bank erosion, which were summarized in a conceptual model of the observed bar–bank interactions. Copyright © 2015 John Wiley &amp; Sons, Ltd.