Abstract The study of meandering patterns created by geophysical flows is important for a number of fundamental and applied research topics, including stream and wetland restoration, land management, infrastructure design, oil exploration and production, carbon sequestration, flood-hazard mitigation and planetary palaeoenvironmental reconstructions. This volume, Meandering Streamflows: Patterns and Processes across Landscapes and Scales , contains 13 papers that present field, laboratory and numerical investigations of meandering channels found in distinct environmental and geological contexts and focus on how the interactions of different autogenic and allogenic processes, both in the horizontal and the vertical dimension, affect meander kinematics and the resulting morphology, sedimentology and stratigraphic architecture. In this introductory chapter, we offer an overview of the evolution of scientific research on meandering streams over time, aiming to review and discuss meandering patterns in both fluvial and non-fluvial settings. Additionally, we present a new compilation of data on meander morphological features, drawn from both existing literature and novel sources, encompassing over 8000 meander bends discovered across a diverse array of environments.

Abstract Despite creating one of Earth's most iconic landforms, and having been studied since antiquity, river meandering is currently explained by several alternative models that remain debated. This is partly because observers have deconstructed meanders to a set of properties thought to be fundamental or significant for meandering, but each of these underpinning properties is itself complex and in turn requires analysis of its causes and mechanisms. This philosophical contribution aims to provide a perspective on the state of the science that makes a rigorous interrogation into what ‘causation’ actually means, and explores what a systems perspective can elucidate about meandering phenomena. We approach the problem through a discussion of the variety of ways in which previous researchers have tackled the problem of meandering, from a channel and floodplain perspective, on Earth and Mars, in the field and laboratory, and at the present day through to the ancient stratigraphic record. It is evident that river meandering has numerous causal pathways rather than a single set of necessary and sufficient conditions. A systemic view on the evolution of plant species shows that their feedbacks on river pattern likewise have several pathways. Employing the conceptual apparatus of the philosophy of science will move us towards a more consensual synthesis of river meandering.

Abstract Fluvial point-bar deposits are described from the c. 1 Ga Diabaig Formation, representing some of the most compelling architectural evidence of planform sinuosity recognized in pre-vegetation sedimentary strata to date. The stratigraphical architecture of the deposits indicates different planform transformations in response to meander-bend expansion and downstream migration. Point-bar architectural motifs are locally abundant in the unit and this is attributed to elevated amounts of mud-calibre cohesive sediment. Mud afforded landscape stability that, together with limited stream power, was sufficient to corral sediment into single-thread, sinuous channels. Significant volumes of fine-grained sediment were encouraged to accumulate by the local basin setting of several topographically confined endorheic depressions. These conditions ceased once the hydrologically closed valleys were filled, with the overlying sandstone-dominated Applecross Formation subsequently deposited by low-sinuosity, braided rivers that bypassed fine-grained sediment oceanward. Other endorheic basins conducive to the formation of depositional meanders should be expected to have been relatively widespread on pre-vegetation Earth, though many individually limited in size, confined by their inherited topographies. Depositional architectures representative of sinuous channel planforms remain scarce within more extensive, externally drained basins.

Abstract Many present-day alluvial floodplains display traces of abandoned meandering channel belts developed during the past millennia (i.e. mid to late Holocene). Deposits of these ancient rivers represent preferential pathways for groundwater flows and related environmental issues, such as contaminant propagation or saltwater intrusion in coastal areas. However, since formative bankfull flows in such old and abandoned routes are hard to estimate, fossil meanders have not been commonly addressed by morphodynamic numerical models, and most of them have been investigated following classical sedimentological approaches based mainly on punctual data derived from sedimentary cores. This study aims at investigating the sediment distribution within different fossil bends on the southern Venetian Plain (NE Italy), and relating such distribution to numerically modelled bed shear stresses used herein as a proxy of sediment sorting patterns. For this purpose, formative flows in the studied palaeomeanders are first inferred from measured sediment grain size and estimates of bend widths. Then, shear stress distributions are computed along the studied palaeobends using a 2D linearized model. Model results are finally compared with conductivity distributions gauged directly in the field through electromagnetic induction investigations in the frequency domain. Our results suggest significant correlations between shear stress distributions and sediment sorting estimated from conductivity data. We deem that the integration between sedimentological reconstructions and state-of-the-art numerical modelling can provide a solid contribution to predicting the spatial distribution of sediment properties within ancient meandering channel belts, with relevant implications for the understanding of shallow aquifer dynamics and soil management.

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.

Abstract Floodplains are important organic carbon (OC) storage sites, and the evolution of watercourses modulates the transport, burial and reworking of organics with cascading effects on watershed CO 2 budgets. Meandering rivers have broadly predictable patterns of channel-planform evolution, providing an opportunity to assess floodplain OC budgets of relation to characteristic migration paces and floodplain ages. However, due to geomorphic complexities of individual meandering rivers, assessments of relationships between channel and OC dynamics have thus far remained limited. We illustrate evolving OC budgets in relation to channel migration in the Vermilion River (Ontario) located in the boreal forest of eastern North America. We combine photogrammetric analyses and dynamic time warping of channel centrelines with analysis of top-soil bulk density and OC. We found that variations of OC stock per unit surface area and soil development are modulated by meander migration through the development of typical boreal-forest vegetation successions. Our results support the hypothesis that meander migration controls soil development, forest age and floodplain OC budgets. We anticipate our study to inform wider applications to rivers in different bioclimates – an approach that may in turn help carbon assessment in the context of changing climate or land use.

Abstract Cutoffs, which include neck and chute cutoffs, are the results of the fluvial processes that fundamentally influence evolution of meandering rivers. A neck cutoff happens when the two limbs of a highly sinuous bend touch, whereas a chute cutoff refers to the formation of a shortcut channel passing through a meander bend. In this review, we begin by distinguishing the morphological and hydrological conditions of the two cutoff types. Mechanisms driving the development of a neck cutoff are embodied in a variety of kinematical and hydrodynamic models simulating processes governing the long-term evolution of meandering rivers. These models adopt a morphological threshold for judging the occurrence of a neck cutoff, b = αw , where α is a constant ranging between 0 and 1, b is the bend neck width and w is the mean channel width. The potential underestimation of the evolutionary period during the late stage of bend evolution toward a neck cutoff when using this morphological threshold and the uncertainties in quantifying the migration–curvature relationship limit the abilities of existing models to predict the occurrence of neck cutoffs. We then suggest three possible directions for future research on meander neck cutoffs. Mechanisms controlling chute cutoffs are relevant to six key factors representing the meander hydrological regime, planform morphology, bed topography and floodplain characteristics. The combination of these factors gives rise to four distinguishable triggering mechanisms: headward-erosion, embayment, mid-channel bar and scroll–slough chute cutoffs, though the initiation of the chute cutoff may be caused by some combination of these. However, the hydraulic and morphological characteristics of meander bends under these triggering mechanisms are so complex that they are often site-specific, making it extremely challenging to generalize the known morphodynamic and hydrodynamic processes driving the formation of chute cutoffs in individual meander bends. We close the review by recommending three possible research directions on chute cutoffs for tackling the existing challenges in the future.

Abstract Neck and chute cutoffs along meandering rivers are near-instantaneous morphodynamic perturbations that drive modification of planform channel geometry and kinematics at the cutoff site and in adjacent bends. Despite their ubiquity, quantitative and predictive linkages between post-cutoff planform bend geometry and subsequent bend kinematics remain limited. Here, using a 37-year record of Landsat-derived river channel centrelines from a meandering reach of the Trinity River in Texas (USA) and a newly developed directed graph-based approach for high-resolution channel tracking, we identify a linkage between post-cutoff bend curvature and the magnitude, spatial extent, and temporal duration of cutoff influence on local and non-local channel bend kinematics. We further explore this relationship using a kinematic model of meandering applied to synthetically generated river reaches and the Landsat-derived Trinity River centrelines. While the model captures the observed linkages between post-cutoff bend curvature and resultant bend kinematics, it underpredicts the magnitude of cutoff influence observed in the real centreline data – particularly at the cutoff site – by up to one order of magnitude. Our results provide a promising advance in the characterization and prediction of the spatiotemporal impact of cutoffs on local and non-local post-cutoff channel bend kinematics.

Abstract Scroll bars are curvilinear ridges of sediment deposited as meandering channels migrate laterally. It is unclear exactly which variables control the spacing, height, and curvature of scroll ridges, and in turn how these properties record channel migration histories. This study tests the relationship between channel and scroll bar geometries in river systems with laterally migrating chute channels that deposit their own scroll packages. LiDAR- and Shuttle Radar Topography Mission-derived digital elevation models (DEMs) are used to calculate the spacing, relief, wavelength, and curvature of scroll bars in eight different chute-channel-dominated river systems with a range of channel dimensions, slopes, and water and sediment discharge regimes. Seven of the eight rivers have statistically indistinguishable chute channel (CC) and main channel (MC) scroll spacings, even though the chute channels have smaller channel dimensions. Additionally, there is no correlation between scroll spacing and relief for main or chute channels. For LiDAR-derived DEMs, the ratio between median scroll spacing and median channel width is low and falls within a narrow range for MC scrolls (from 0.13 to 0.22; mean 0.16), but is higher with a wider range for CC scrolls (from 0.48 to 1.33; mean 0.8). CC scrolls are straighter overall than MC scrolls, but there is no correlation between curvature and scroll spacing. Overall, these results show that (1) chute channel scrolls are often indistinguishable from main channel scrolls based on spacing and relief, and (2) MC and CC scrolls have different relationships with formative channel widths, which suggests that other variables, such as lateral migration rate and flood inundation depth, are key controls on scroll bar geometries.

Abstract Bank erosion commonly occurs in alluvial rivers, shaping landscapes and riparian habitats and impacting water quality and infrastructure. Several models have been proposed that link shear stresses to bank erosion. However, data to test these hypotheses for characteristic geometries of meandering channels are sparse and technically challenging to acquire. Here we present results from a controlled experiment in a naturalistic channel to isolate the relationships between turbulent flow and nascent bank erosion. We ran the experiments at the Outdoor StreamLab (St Anthony Falls Laboratory, University of Minnesota) and gathered high-precision, contemporaneous measurements of the turbulent flow field and topography near a standardized, erodible bank at five locations along a single meander. The measurements show that the rate of bank erosion varied both along the channel and vertically and, local bank erosion was not correlated with any single hydrodynamic parameter. Upstream of the meander apex, erosion correlated with the near-bank time-averaged streamwise velocity magnitude while downstream of the apex, bank erosion correlated more strongly with near-bank turbulence parameters and depth. These results support field measurements that suggest that fluid shear contributions to outer bank erosion reflect multiple components of turbulent flow structure in river meanders.

Abstract There has been debate over the processes acting on deep-water channels, with comparisons made to the evolution of meandering fluvial systems. We characterized a three-dimensional seismic-reflection dataset of the Joshua deep-water channel–levee system located in the eastern Gulf of Mexico and interpreted 13 horizons showing its kinematic evolution over a 25 km reach. Over this reach, we documented channel migration through systematic bend expansion and downstream translation, which was sustained through channel aggradation as sinuosity increased from 1.25 to 2.3 at abandonment. An abrupt decrease in sinuosity was associated with a neck cutoff, which changed the subsequent migration direction of the channel in that locality. These processes are analogous to the evolution of meandering fluvial systems. We show that increasing channel sinuosity correlates to a reduction in channel slope and hypothesize that this promoted increasingly depositional turbidity currents that led to channel aggradation. Using a simple forward stratigraphic model in which vertical movements of the channel are governed by a stream power law, we show how aggradation can be driven autogenically. Trends in sinuosity, aggradation and slope are in broad agreement between the Joshua Channel and the model. This highlights the potential importance of intrinsic channel processes as a control on system evolution.

Abstract Channel kinematics play a pivotal but underappreciated role in determining submarine meandering channel stacking patterns. A stratigraphic forward model is here applied to turbidite systems to generate synthetic stratigraphic architectures and to compare channel trajectories recorded on cross-sections with the kinematics of meander bends. Various channel stacking patterns, which may be classified into four types, are obtained from a single set of constant parameters. This variety of stacking patterns observed from one 2D section to another is thus autogenic. It is the consequence of the 3D migration of meander bends. It reflects the number of consecutive bends that have intercepted the cross-section, which depends on the channel downstream migration rate. These 3D effects alone are sufficient to explain variations in the apparent migration rate and the reversal of migration direction. As a result, sigmoidal geometries can be preserved in aggrading systems without a significant change in channel geometry or varying aggradation and migration rates. Finally, despite these 3D effects, stratigraphic mobility numbers still provide good constraints on bend kinematics. These results should apply to meandering rivers and other meandering systems.