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Abstract Fluvial cross strata are fundamental sedimentary structures that record past flow and sediment transport conditions. Bedform preservation can be significantly influenced by the presence of larger‐scale topographic features that cause spatial gradients in flow. However, our understanding of the controls on cross strata preservation in the presence of a morphodynamic hierarchy is limited. Here, using high‐resolution bathymetry from a physical experiment, we quantify bedform evolution and cross strata preservation in a zone of flow expansion and deceleration. Results show that the size and celerity of superimposed bedforms decreases along the host‐bedform lee slope, leading to a systematic downstream increase in the sediment accumulation rate relative to bedform celerity. This increase in local bedform climb angle results in the preservation of a larger fraction of formative bedforms. Our results highlight the need to revise current paleohydraulic reconstruction models, and demonstrates that fluvial morphodynamic hierarchy is a fundamental determinant of sedimentary strata. 

Abstract Bedform evolution and preserved cross strata are known to respond to floods. However, it is unclear if autogenic dynamics mask the flood signal in bedform evolution and cross strata. To address this, we characterize the temporal structure of autogenic noise in steady‐state bedform evolution in a physical experiment. Results reveal the existence of bedform groups—quasi‐stable collections of bedforms—that migrate at a similar speed as bedforms. We find that bedform and bedform‐group turnover timescales are the key autogenic timescales of bed evolution that set the transition time‐periods between different noise regimes in bedform evolution. Results suggest that bedform‐group turnover timescale sets the lower limit for detecting flood signals in bedform evolution, and floods with duration shorter than bedform turnover timescale can be severely degraded in bedform evolution and cross strata. Our work provides a new framework for interrogating fluvial cross strata for reconstruction of past floods. 

Abstract River bedforms and their deposits—fluvial cross strata— respond to floods. However, it is unclear if all floods are equally represented in cross strata. Here, using a series of physical experiments in which bedforms were subjected to equivalent flood magnitudes over varying durations, we demonstrate the existence of a lower bound on flood durations that are represented in cross strata. We show that the scour depths and preserved set thickness are indistinguishable from baseflow conditions when the rising‐limb duration of floods is shorter than the baseflow‐equilibrated bedform turnover timescale—time required to displace the volume of a single bedform at baseflow conditions. In contrast, scour depth and preserved set thickness distributions deviate from baseflow conditions when flood rising‐limb duration exceeds the baseflow‐equilibrated bedform turnover timescale, causing preferential preservation of falling‐limb bedform dynamics. Our work provides a previously unrecognized quantitative bound on flood durations that are represented in fluvial cross strata. 

ABSTRACT Braided rivers distribute sediment across landscapes, often forming wide channel belts that are preserved in stratigraphy as coarse-grained deposits. Theoretical work has established quantitative links between the depth distribution of formative channels in a braided river and the geometry of their preserved strata. However, testing these predictive relationships between geomorphic process and stratigraphic product requires examining how braided rivers and their deposits coevolve, with high resolution in both space and time. Here, using a series of four runs of a physical experiment, we examine the controls of water discharge and slope on the resulting geometry of preserved deposits. Specifically, we focus on how a twofold variation in water discharge and initial riverbed slope affects the spatiotemporal distribution of channel depths and the geometry of preserved deposits of a braided river. We find that the channel depths in the laboratory experiment are described by a two-parameter gamma distribution and the deepest scours correspond to zones of erosion at channel-belt margins and channel-thread confluences in the channel belt. We use a reduced-complexity flow model to reconstruct flow depths, which were shallower compared to channel thalweg depths. Synthetic stratigraphy built from timeseries of topographic surfaces shows that the distribution of cut-and-fill-unit thickness is invariant across the experiments and is determined by the variability in scour depths. We show that the distribution of cut-and-fill-unit thickness can be used to reconstruct formative-channel-depth distributions and that the mean thickness of these units is 0.31 to 0.62 times the mean formative flow depth across all experiments. Our results suggest that variations in discharge and slope do not translate to measurable differences in preserved cut-and-fill-unit thickness, suggesting that changes in external forcings are likely to be preserved in braided river deposits only when they exceed a certain threshold of change. 

Abstract A Silurian shift in fluvial stratigraphic architecture, coincident with the appearance of terrestrial vegetation in the fossil record, is traditionally cited as evidence for exclusively shallow, braided planforms in pre‐vegetation rivers. While recent recognition of deep, single‐thread channels in pre‐Silurian strata challenge this paradigm, it is unclear how these rivers maintained stable banks. Here, we reconstruct paleohydraulics and channel planform from fluvial cross‐strata of the 1.2 Ga Stoer Group. These deposits are consistent with deep (4–7 m), low‐sloping rivers (2.7 × 10⁻⁴to 4.5 × 10⁻⁵), similar in morphometry to modern single‐thread rivers. We show that reconstructed bank shear stresses approximate the cohesion provided by sand‐mud mixtures with 30%–45% mud—consistent with Stoer floodplain facies composition. These results indicate that sediment cohesion from mud alone could have fostered deep, single‐thread, pre‐vegetation rivers. We suggest that the Silurian stratigraphic shift could mark a kinematic change in channel migration rate rather than a diversification of planform. 

Abstract Mobile river channels endanger human life and property and over centuries shape ecosystems, landscapes, and stratigraphy. Quantifying channel movements from remote sensing is difficult, in part due to the diversity of river mobility processes (e.g., channel migration, cutoffs, avulsion) and planform morphologies (e.g., meandering, braided). Here, we present a framework for quantifying riverbank migration from remote sensing that upscales recent methodological advances from laboratory flume studies utilizing particle image velocimetry (PIV). We apply PIV to image time series of 21 rivers worldwide, showing PIV ignores cutoff and avulsion processes by design and is well suited for tracking riverbank migration regardless of planform morphology. We show that PIV‐derived results for riverbank migration are consistent with published results from centerline‐ and bank‐based Lagrangian methods. Unlike existing methods, PIV offers a grid‐based Eulerian framework where defining channel centerlines is unnecessary and quantified uncertainty in riverbank positions is propagated into uncertainty in migration rates. PIV offers means to efficiently extract global patterns in riverbank migration from decades of satellite data, as well as investigate river response to climate change and human activities in our rapidly changing world. 

Abstract Rivers are the primary conduits of water and sediment across Earth's surface. In recent decades, rivers have been increasingly impacted by climate change and human activities. The availability of global‐coverage satellite imagery provides a powerful avenue to study river mobility and quantify the impacts of these perturbations on global river behavior. However, we lack remote sensing methods for quantifying river mobility that can be generally applied across the diversity of river planforms (e.g., meandering, braided) and fluvial processes (e.g., channel migration, avulsion). Here, we upscale area‐based methods from laboratory flume experiments to build a generalized remote sensing framework for quantifying river mobility. The framework utilizes binary channel‐mask time series to determine time‐ and area‐integrated rates and scales of river floodplain reworking and channel‐thread reorganization. We apply the framework to numerical models to demonstrate that these rates and scales are sensitive to specific river processes (channel migration, channel‐bend cut‐off, and avulsion). We then apply the framework to natural migrating and avulsing rivers with meandering and braided planforms. Results show that our area‐based framework provides an objective and accurate means to quantify river mobility at reach‐ to floodplain‐scales, which is largely insensitive to spatial and temporal biases that can arise in traditional mobility metrics. Our work provides a framework for investigating global controls on river mobility, testing hypotheses about river response to environmental gradients, and quantifying the timescales of terrestrial organic carbon cycling. 

Abstract Fluvial cross strata are depositional products of bedform migration that record formative flow and sediment transport conditions on planetary bodies. Bedform evolution varies with transport stage even under constant flow depths, but our understanding of how prevailing sediment transport conditions affect preserved cross strata is limited. Here, we analyzed experimental bedform evolution and preserved set thickness spanning threshold‐of‐motion to suspension‐dominated transport conditions at multiple equilibrium flow depths. Results show that bedform trough depth and mean preserved set thickness have a parabolic dependence on transport stage, with maximum values observed at intermediate transport stages. Our results indicate that transport stage is a key control on the flow‐depth‐normalized set thickness but set thickness is a poor indicator of flow depth. Thus, the dependence of bedform dimensions on transport stage should be considered in paleohydraulic reconstruction, and the analysis of set thickness may aid in the estimation of ancient fluvial sediment flux. 

Abstract River deltas grow through repeated lobe‐scale avulsions, which often occur at a location that correlates with the backwater lengthscale. Competing hypotheses attribute the avulsion node origin to either the morphodynamic feedbacks caused by natural flood discharge variability (backwater hypothesis) or to the prograding delta lobe geometry (geometric hypothesis). Here, using theory, historical flood records, and remotely sensed elevation data, we analyzed five lobe‐scale delta avulsions in Madagascar, captured by Landsat imagery. Avulsion lengths were 5–55 km, distances significantly longer than the backwater lengthscale and inconsistent with the geometric hypothesis. We show that the steep, silt‐bedded rivers of Madagascar have flood‐induced bed scour, driven by backwater hydrodynamics, that propagates farther upstream than the backwater lengthscale. The avulsion lengths are 3.1 ± 1.5 times the predicted flood scour lengths, similar to low‐gradient deltas, and consistent with backwater hypothesis. Results demonstrate that erosion initiated by nonuniform flows in the backwater zone is a primary control on delta avulsion locations. 

Abstract The low temporal completeness of fluvial strata could indicate that recorded events represent unusual and extreme conditions. However, field observations suggest that preserved strata predominantly record relatively common transport conditions—a paradox termed thestrange ordinarinessof fluvial strata. We theorize that the self‐organization of fluvial systems into a morphodynamic hierarchy that spans bed to basin scales facilitates the preservation of ordinary events in fluvial strata. Using a new probabilistic model and existing field and experimental data sets across these scales, we show that fluvial morphodynamic hierarchy enhances the stratigraphic preservation of medial topography—ordinary events. We show that lower‐order landforms have a higher likelihood of complete preservation when the kinematic rates of evolution of successive levels in the morphodynamic hierarchy are comparable. We highlight how relative changes in kinematic rates of evolution of successive levels in the morphodynamic hierarchy can manifest as major shifts in stratigraphic architecture through Earth history.