Morphodynamic Characteristics of a Complex Anabranching System in the Qinghai‐Tibet Plateau and the Implications for Anabranching Stability. Issue 2 (13th February 2023)

• Record Type: Journal Article
• Authors: Guo, Xiwei; Gao, Peng; Li, Zhiwei
• Is Part Of: Journal of geophysical research. Volume 128:Issue 2(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-02-13
• Subjects: anabranching river -- island dynamics -- accretion and erosion -- flow efficiency -- channel stability; Geomorphology -- Periodicals; 551.3
• DOI: 10.1029/2022JF006788
• Languages: English
• ISSNs: 2169-9003

Abstract

Abstract: 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. Plain Language Summary: Anabranching patterns are characterized by multiple channels separated by relatively stable islands, which can be commonly found in fluvial systems, including the world's largest rivers. Anabranching has been considered a stable channel form with the optimal number of channels (i.e., two to four). Nonetheless, anabranching rivers with more complex morphology (i.e., >4 channels) can be observed extensively in nature, but their morphological characteristics, patterns of channel changes, and the linkages with channel stability remain poorly understood. In this study, we characterize the morphology and lateral changes of islands and channels of a complex anabranching system in the Upper Yellow River and assess its ability to transport sediment. We found that although the anabranching system could not transport all supplied sediments out of the system, it remained stable by keeping large channels unchanged while allowing small ones to adjust through interactions between islands and channels. These complex processes are not manifest in the number of channels but instead in island dynamics and the resultant emergence and disappearance of small channels. Key Points: Island formation, elimination, and areal change control the morphology of this complex anabranching system The system is in an accreting disequilibrium state due to the overdevelopment of small, floodplain‐incised anabranches Anabranching stability is manifest in island dynamics and the different impacts on channels with variable widths