Meandering-anabranching river channel change in response to flow-sediment regulation: Data analysis and model validation

Anabranching river has been recognized as an independent river pattern. A variety of investigations have attempted to disclose the formation of anabranching rivers, however, most previous studies mainly focused on the controls of anabranching channels formation. The mechanism of transition among sub-channels with varying flow-sediment regime in anabranching rivers has been poorly understood. In this study, we used multi-source data sets to investigate the changes in river channels and island (Wugui Island, about 7 km(2)) along the 27-km long Jianli Reach (Tashiyi-Tianziyihao) in the middle Yangtze River. Nine Landsat images (1987, 1990, 1994, 1996, 2000, 2003, 2006, 2010 and 2014) with 30 m spatial resolution were used to map the channel planform dynamics. Obvious changes in channel width and morphology of the island were observed during 1987-2014. The width of the left branch had reduced from 1041 m to 340 m, while the width of the right branch had increased from 632 m to 1531 m. During 1994-1996, the main river channel shifted from the left branch to the right. During the period of 1987-2014, the total width of the compound cross-section increased, and the mean length-width ratio of the island increased from 159 to 4.32. The mean annual flow discharge increased about 9%, and mean annual suspended sediment concentration and sediment transport coefficient (the ratio of mean annual suspended sediment concentration to mean annul flow discharge) decreased about 75% and 76% respectively from 1951 to 2009, in response to the operation of Gezhouba Dam and Three Gorges Dam. The meandering-anabranching channel planform adjusted by narrowing the island width and enlarging compound cross-section in response to upstream water-sediment regime. The shift of the main river channel from left branch to the right may resulted from the increase in flow discharge and decrease in sediment transport coefficient since 1980s. The adjustment has been intensified by the changes in the sediment transport rate after 1995. After the main channel was switched from left branch to right branch, the flow-sediment diversion capacity was kept stable with a diversion percentage of 90% in right branch. Due to the persistent influence of the Three Gorges Dam, the right branch will function as the main channel for a long time before the meandering-anabranching system achieves equilibrium. The shift and stability of main channel was closely related with the relative magnitude of flow-sediment partition in each branch. The significance of this study is that we successfully manifested the main channel shift process of anabranching river with the support of direct hydromorphological observation and theoretical verification.