Discharge variability drives point-bar macroform degradation in the meandering Powder River (Montana, USA)

Abstract Most extant facies models depict the internal architecture of fluvial point bars as large-scale, inclined beds formed by the lateral migration of channel bends. However, recent studies have revealed that discharge variability can significantly influence these architectures, highlighting that the genetic processes governing their formation remain poorly understood. This study examines point-bar deposits along the meandering Powder River in Montana, USA, to understand how a highly variable hydrological regime impacts bar depositional architecture. Sedimentological data reveal that the bars consist of amalgamated beds and lack well-preserved, large-scale, inclined (macroform) bedding. By integrating architectural data with 27 years of hydrological and geomorphological records, we linked distinct beds to specific water discharge conditions, highlighting that major floods play a crucial role in constructing and reshaping point-bar architecture, whereas minor floods have marginal impacts. Bar-scale sedimentation occurs briefly during early flood-waning stages, forming limited bar-scale, inclined beds, which are later disrupted by localized scouring from subsidiary flood pulses. The preservation of these deposits is primarily controlled by the frequency of subsidiary peaks during the waning phase, rather than by intermittency or total duration of the waning stage. Consequently, the complex bedding in the Powder River point bars reflects autogenic intraflood reworking rather than interflood reworking. Major floods produce both slope-scale and localized deposits, with subsidiary peaks during waning stages driving macroform-bedding disruption. These results provide valuable insights for interpreting hydroclimatic environments and refining interpretations of past depositional processes from the rock record, and they emphasize the need to explicitly account for discharge variability in fluvial facies models.