Criticality in the evolution of river meanders in the Lower Yellow River in response to anthropogenic-induced hydrological regime shift

To interpret the planform transition from meandering to straight in depositional river channels, a new approach has recently been proposed, incorporating the channel mobility number $M$ (quantifying the relative activity of lateral channel migration versus vertical channel-bed variation) alongside channel sinuosity $S$. To evaluate the applicability of this approach in determining cutoff conditions in the wandering Lower Yellow River (LYR), we analyzed the evolution of six actively migrating meanders using remote sensing images and river cross-sectional profile measurements. These meanders progressively developed cutoffs between 1990 and 2021 under conditions where $M\approx 1$ and $S\approx 1.6$, with a notable increase in cutoffs following the Xiaolangdi Reservoir's operation in 2000. The increase in cutoffs reflects the increased lateral migration rates of these meanders, primarily driven by a drastic hydrological regime shift in the LYR. Before 2000, the river was dominated by hyper-concentrated flows and frequent overbank flooding. However, after the reservoir's operation, a sharp decline in sediment concentration left the river with excess energy. This energy was expended through both vertical incision and accelerated lateral migration, resulting in highly sinuous meanders that eventually underwent cutoffs, reverting to low-sinuosity channels. This study demonstrates that the channel mobility number $M$, together with channel sinuosity $S$, can be used to track the critical planform transition points of sinuous channels experiencing aggrading or incising processes. Although meander cutoffs arise from the self-organizing process of river evolution, their critical sinuosities are modulated by allogenic factors, particularly bank erodibility and hydrological regime shift.