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We present the most comprehensive dataset of bedload transport in ephemeral channels compiled to date. These nine ephemeral channels cover a range of dryland climates and channel types. First, we evaluate these channels and how they compare with each other. Next, we contrast this database with a previously compiled bedload dataset encompassing 92 perennial rivers. While previous studies have identified differences between measured bedload flux in perennial and ephemeral systems, we quantify those differences across a wide range of channel types and shear stress conditions. We find that the ephemeral dataset is statistically distinct, showing greater average transport across flow conditions in normalized shear vs. bedload flux space. Prior researchers have variously attributed these high transport rates to a combination of factors that commonly define ephemeral channels: lack of armoring, mixed sand and gravel, flashy hydrographs, erodible banks, and lack of vegetation. We tested the influence of armoring by comparing transport differences at different transport stages, finding that bed armor contributes to the observed differences, but is not the sole reason. In addition to these previously proposed mechanisms, we add that the abundance of very coarse sand and fine gravels in ephemeral channels provides easily-mobilized but difficult-to-suspend particles. 

Abstract. Despite a rich history of studies investigating fluid dynamics over bedforms and dunes in rivers, the spatiotemporal patterns of sub-bedform bedload transport remain poorly understood. Previous experiments assessing the effects of flow separation on downstream fluid turbulent structures and bedload transport suggest that localized, intermittent, high-magnitude transport events (i.e., permeable splat events) play an important role in both downstream and cross-stream bedload transport near flow reattachment. Here, we report results from flume experiments that assess the combined effects of flow separation–reattachment and flow re-acceleration over fixed two-dimensional bedforms (1.7 cm high; 30 cm long). A high-speed camera observed bedload transport along the entirety of the bedform at 250 frames per second. Grain trajectories, grain velocities, and grain transport directions were acquired from bedload images using semiautomated particle-tracking techniques. Downstream and vertical fluid velocities were measured 3 mm above the bed using laser Doppler velocimetry (LDV) at 15 distances along the bedform profile. Mean downstream fluid velocity increases nonlinearly with increasing distance along the bedform. However, observed bedload transport increases linearly with increasing distance along the bedform, except at the crest of the bedform, where both mean downstream fluid velocity and bedload transport decrease substantially. Bedload transport time series and manual particle-tracking data show a zone of high-magnitude, cross-stream transport near flow reattachment, suggesting that permeable splat events play an essential role in the region downstream of flow reattachment. 

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.