Morphodynamic Characteristics of a Complex Anabranching System in the Qinghai-Tibet Plateau and the Implications for Anabranching Stability

The understanding of fluvial processes controlling morphological adjustments and stability of anabranching rivers remains incomplete. Focusing on a complex anabranching system in the Upper Yellow River, this study quantifies the morphological characteristics and lateral dynamics of islands and channels in four reaches over a 56-km river course from 1986 to 2017. Using five heuristic anabranching structures derived from the studied anabranching system, we estimate sediment transport capacities and assess their implications for the morphodynamics of anabranching rivers. These results show that the system exhibited a highly complex pattern featuring high channel multiplicity and dominance of islands. Over the study periods, the system exhibited an accreting disequilibrium state, which was highlighted by spatiotemporally diverse and dynamic patterns of islands. Morphological changes in islands were dominated by expansion and shrinkage, which caused more areal changes than other modes of change, including coalescence, cleavage, new island formation, and elimination. These processes controlled the creation and elimination of small anabranches, whereas the size and morphology of large channels remained comparatively stable, which maintained the stability of this anabranching pattern. The stability of the anabranching pattern is further supported by estimations of transport capacity in that the main channels transport most sediments and in that excess development of small anabranches promotes sediment deposition within the system. Overall, this study provides new insight into the morphodynamic properties of anabranching rivers, demonstrating that, instead of channel numbers, island dynamics and interactions with channels are controls of the evolutionary processes and stability of anabranching rivers.