The impact of high discharge variability on sedimentology and architecture of bar deposits in the meandering Powder River (Montana, USA)

Point bars are emblematic deposits of meandering rivers. Classical facies models that define their architecture and sedimentology are essentially based on rivers with low to moderate peak discharge variability. However, many global rivers experience high peak discharge variability, which may significantly impact point-bar sedimentological features. This study investigates how high peak discharge variability affects the sedimentology and architecture of point-bar deposits along the meandering Powder River in south-eastern Montana, USA. The analysis integrates detailed sedimentological data from trenches and natural exposures at four point bars where century-long discharge records and more than four decades of geomorphic surveys are available. Sedimentological data reveals substantial deviations from classical facies models. Coarsening-upward and blocky vertical grain-size trends are common, in contrast with the classic fining-upward model. Upper-flow regime structures are abundant features that record rapid waning of high-magnitude floods, whereas lateral accretion surfaces matching the original extent of bar slopes are rare due to frequent bar slope reworking. Chute channels are large, occupying significant portions of the bar area, with complex internal architectures of coarse-grained upper-flow regime deposits that further obscure lateral accretion geometries of the hosting bar. Oxidized mud layers suggest prolonged sub-aerial exposure of most of the bar slope between floods. Results from this study challenge assumptions of classical point-bar models and provide new criteria for recognizing ancient deposits of meandering rivers with high peak discharge variability, with implications for palaeohydrological reconstructions, understanding pre-vegetation fluvial dynamics and interpreting deposits on other planetary bodies. This new evidence accounts for hydrological modulation effects on fluvial deposits, enabling more robust facies analysis of alluvial successions across a spectrum of discharge variability regimes.