How do braided rivers evolve: Insights from 2D shallow water hydro-sediment-morphodynamic modeling

Braided rivers commonly exhibit frequent channel shifting and bar migration, which remains a major challenge to predict. Numerical modeling has evolved into a proactive approach for resolving fluvial processes for over half a century. To date, however, it has remained poorly understood how flow–sediment–morphology interactions dictate braided river evolution. Here, a fully coupled two-dimensional shallow-water hydrosediment-morphodynamic model is developed, and a timescale-based dynamic braiding index is proposed to characterize channel evolution. To unravel the roles of flow–sediment–morphology interactions, secondary flow effects, slope collapse, and sediment heterogeneity in braided river evolution, the model is applied to flume experiments and a typical braided reach of the Tuotuo River in the source region of the Yangtze River. Braided channel development follows a three-stage pathway, i.e., bar initiation with flow division, bifurcation with active channel formation, and channel reworking toward dynamic equilibrium. Fully coupled flow–sediment–morphology modeling produces higher braiding intensity during multithread network development. Secondary flow enhances lateral sediment transport and channel asymmetry, whereas slope collapse limits excessive local incision and regulates steep bar margins. Critically, sediment heterogeneity and mobility modify braided channel evolution through bed surface sorting, leading to different bar-channel sorting patterns under partial and full mobility conditions. In the Tuotuo River flood case, nonuniform sediment amplifies sediment transport peaks and directly affects channel evolution. Overall, this work clarifies the distinct roles of key physical mechanisms in braided river evolution and provides a basis for informing the effective management of sediment and cross-river infrastructure safety.