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River meandering dynamics are here explored in light of unsteady water flows. While mathematical models have usually focused on constant discharges—a reasonable and widely adopted approach for long‐term considerations—we show that varying flows strongly affect the short‐term planimetric evolution of meanders, before the cutoff occurrence. In particular, flow variability slows down the meanders' dynamics while does not significantly influence the wavelength selection. We support our arguments with numerical simulations and theoretical (linear and nonlinear) analyses, showing that an interplay between out‐of‐phase river geometry and flow is responsible for the meander‐dynamics slowdown. Our results suggest that accounting for flow variability is critical in assessing yearly to decadal meander dynamics with important implications for river engineering and management strategies. Plain Language Summary Meandering rivers are natural sinuous channels shaping the Earth's surface with their lateral migration motion. Beyond theoretical interest, the investigation of such dynamics is fundamental for river management and restoration. A common way to study river meander dynamics is through numerical simulations that simulate the planform evolution of the river geometry in space and time. A classic assumption in these simulations is a constant water flow. In this study, we show that considering more realistic temporally varying flow discharges slows down the meander growth compared to the case with a constant flow discharge. Key Points Numerical simulations and linear and nonlinear theoretical analyses feature how the river planimetry evolves under variable flows Unsteady water flows slow down the planimetric evolution of river meanders compared to constant discharges Flow variability is crucial for meander dynamics on yearly to decadal timescales, with implications for river engineering and restoration

A recent discovery of two unique meandering streams near the Yarlung Tsangpo Grand Canyon facilitates the present study. Given the contrasting channel patterns compared with the surrounding bedrock and braided reaches, as well as their recent formation due to dam-induced topographic changes within the valley, this study offers critical insights into the formation and evolution processes of meandering channels. It is found that, first, the prolonged sedimentation process due to the backwater of the mainstream of the floodplain proves a material base for the formation of the meandering river. Proper bank strength provided by the floodplain (stratified layer of root-soil composite and silty clay) contrasts the stream from a braided pattern into a single-threaded pattern, then the alternate bar in the upstream preludes the meandering channel formation. The annual migration rate of the stream is consistent with other large-scale natural meandering rivers. Congruences and disparities with the analytical meandering migration model of the present stream (that the meandering path follows the Kinoshita curve with noticeable flatness but no skewness) highlight the complex interplay of local factors in shaping meandering processes, offering valuable insights into both the unique characteristics of the Cuoka streams and the broader principles governing meander formation.

Sinuous channels wandering through coastal wetlands have been thought to lack lateral‐migration features like meander cutoffs and oxbows, spurring the broad interpretation that tidal and fluvial meanders differ morphodynamically. Motivated by recent work showing similarities in planform dynamics between tidal and fluvial meandering channels, we analyzed meander neck cutoffs from diverse tidal and fluvial environments worldwide, and show that tidal cutoffs are widespread. Their perceived paucity stems from pronounced channel density and hydrological connectivity in coastal wetlands, comparatively small size of most tidal channels, and typically dense vegetation cover. Although these factors do not efface tidal meander cutoffs, they collectively inhibit oxbow formation and make tidal cutoffs ephemeral features that can escape detection. We argue that similar morphodynamic processes drive cutoff formation in tidal and fluvial landscapes, with differences arising only during post‐cutoff evolution. Such process similarity has important implications for understanding coastal wetland ecomorphodynamics and predicting their long‐term evolution. Plain Language Summary The sinuous channels that wander through tidal coastal wetlands look like meandering rivers. However, features of alluvial floodplains that indicate active river meandering over time, such as oxbow lakes and meander cutoffs, are difficult to find in tidal settings. Their apparent absence has led researchers to infer that tidal and fluvial meanders evolve differently. We re‐examined this inference by identifying, measuring, and compiling examples of meander cutoffs from a variety of tidal coastal wetlands and fluvial floodplains worldwide. Our analysis suggests that the shapes and geometric properties of tidal and fluvial cutoffs are indeed remarkably similar. This indicates that while tidal and fluvial environments differ in many ways, they nevertheless share the same physical mechanism affecting meander morphodynamical evolution. Differences between tidal and fluvial meanders do arise after a meander is cut off. We observe that tidal meanders remain preferentially connected to the parent channel, preventing the formation of crescent‐shaped oxbow lakes and thus making tidal cutoffs more difficult to detect. Our results indicate a close similarity in meandering channel behavior across tidal and fluvial systems, which opens new opportunities for how researchers model tidal wetlands, with important implications for the effective conservation and restoration of these critical ecosystems. Key Points Tidal meander cutoffs are far more common than typically thought and share remarkable morphometric similarities with fluvial counterparts Similar mechanisms trigger cutoffs in both tidal and fluvial landscapes, with differences arising only during post‐cutoff evolution Tidal cutoffs seldom disconnect from parent channels and rarely form oxbows due to the high hydrological connectivity of tidal wetlands

River meandering controls the age of floodplains through its characteristic paces of growth and eventual cutoff of channel bends, forming oxbows. Hence, floodplain‐age distributions should reflect a river's characteristic size and migration rate. This hypothesis has been previously tested in numerical simulations, yet without systematic comparisons with natural systems. Here we analyze oxbow spacing and timescales of bend evolution and abandonment in natural and numerically simulated meander belts. In both cases, a saturated state is achieved whereby oxbows are spaced ∼1 meander radius apart. At saturation, the distribution of floodplain ages and probability of sediment‐storage time can be constrained from characteristic timescales of bend evolution and abandonment. Owing to the similar relationships between floodplain width and characteristic timescales in natural and simulated rivers, we postulate that this approach should apply to unconfined meandering rivers elsewhere—a hypothesis to be tested with independent geo‐ or dendrochronological data. Plain Language Summary Meandering rivers have curvy channels characterized by erosion and deposition along their inner and outer banks, respectively. Over time, continued erosion and deposition shuffle sediment along the river plain, and lead to channel bends joining each other, through a process—called neck cutoff—that isolates a channel segment in between. These processes control the age of sediment and soil in a river plain over timespans much longer than human life, such that evolution models of meandering rivers often rely on numerical simulations. Here, data from both natural and simulated rivers show that, over time, neck cutoffs find themselves in closely spaced arrangements, and that their position can inform typical sediment ages once the river's characteristic pace of erosion and deposition are accounted for. These results may be tested in the future with direct age determination and, if corroborated, could further inform future studies on river organic‐carbon budgets. Key Points Natural and simulated river meander belts reach a saturated state whereby oxbows are arranged ∼1 meander radius away from each other Distribution of floodplain sediment age is controlled by meander location, sizes, and channel migration rate Natural and numerically simulated floodplains display similar relationships between their width and sediment‐age distribution

Meandering rivers are diagnostic landforms of hydrologically active planets, and their migration regulates the continental component of biogeochemical cycles that stabilize climate and allow for life on Earth. The rise of river meanders on Earth has been linked to riverbank stabilization driven by the Palaeozoic evolution of plant life about 440 million years ago. Here we provide a fundamental test for this hypothesis using a global analysis of active meander migrations that includes previously ignored unvegetated rivers from the arid interiors of modern continents. When normalized by channel size, unvegetated meanders universally migrate an order of magnitude faster than vegetated ones. While providing irrefutable evidence that vegetation is not required for meander formation, we demonstrate how profoundly vegetation transformed the pace of change for Earth’s landscapes, and we at last offer a mechanistic explanation for the radically distinct stratigraphic records of barren and vegetated rivers. We posit that the migration slowdown driven by the rise of land plants dramatically impacted biogeochemical fluxes and rendered Earth’s landscapes even more hospitable to life. Therefore, the tenfold faster migration of unvegetated rivers may be key to deciphering the environments of barren worlds such as early Earth and Mars.

Most morphodynamic models of river meandering assume spatially constant width; depending on the intensity of spatial width variations, different meandering styles actually exist, often associated with midchannel bars and islands. When intense enough, width oscillations characterize transitional planforms between meandering and braiding. We investigate, on a modeling basis, morphodynamic feedbacks between spatial curvature and width oscillations in river meanders and related bedform patterns. Our review of existing mathematical models suggests that width-curvature interactions can be comprehensively analyzed by a hierarchy of models that descend from a two-parameter perturbation solution of the governing depth-averaged morphodynamic model. The focus is on in-stream, autogenic hydromorphodynamic processes, and not explicitly on bank processes. Curvature-width interactions are fundamentally nonlinear: the perturbation approach allows us to investigate the key effects at the first nonlinear interaction. In meanders with initially constant width, curvature nonlinearly forces midchannel bar growth, promoting symmetrical bank erosion further downstream, possibly triggering width oscillations. These in turn can significantly affect the process of bend stability and therefore condition the curvature dynamics. Wider-at-bends meanders develop shorter bends and are morphologically more active compared to equiwidth meanders, coherently with the few available field observations. River evolution models aiming to separately simulate bank erosion and accretion processes should incorporate these autogenic flow-bed nonlinearities. Because of its focus on meandering morphologies close to the transition with braiding, the proposed approach can be taken as a novel, physically based viewpoint to the long-debated subject of channel pattern selection.

We utilized the random forest (RF) machine learning algorithm, along with nine topographical/morphological factors, namely aspect, slope, geomorphons, plan curvature, profile curvature, terrain roughness index, surface texture, topographic wetness index (TWI), and elevation. Our objective was to identify flood-prone areas along the meandering Kashkan River and investigate the role of topography in riverbank inundation. To validate the flood susceptibility map generated by the random forest algorithm, we employed Sentinel-1 GRDH SAR imagery from the March 2019 flooding event in the Kashkan river. The SNAP software and the OTSU thresholding method were utilized to extract the flooded/inundated areas from the SAR imagery. The results showed that the random forest model accurately pinpointed areas with a “very high” and “high” risk of flooding. Through analysis of the cross-sections and SAR-based flood maps, we discovered that the topographical confinement of the meander played a crucial role in the extent of inundation along the meandering path. Moreover, the findings indicated that the inner banks along the Kashkan river were more prone to flooding compared to the outer banks.