Multiscale Bedform Reorganization at the Onset of Substantial Suspended Sediment Transport

Sand‐bedded rivers self‐organize into bedforms regulating flow resistance and sediment transport. Emerging research focuses on characterizing multiscale bedform configurations, but their stability remains unknown. Here, we analyzed twelve flume experiments of varying sizes to uncover a sharp transition between two independent multiscale bedform configurations. Bedform groups—quasi‐stable collections of bedforms—that have similar heights and celerities as constituent bedforms are found in flows with suspension number (ratio of shear velocity to sediment settling velocity; ) less than 1. In contrast, large dunes with smaller, faster‐moving superimposed bedforms characterize flows with > 1. The transition between these two states abruptly occurs at in low Froude‐number flows, and is further supported by distinct autogenic noise characteristics in bed evolution. Our work provides novel insight into factors controlling multiscale bedform configurations, and highlights the critical influence of suspension number on river morphology.