[H-CG22] Deltas and estuaries: multidisciplinary analyses of complex river-mouth systems

[HCG22-P02] Channel deformation of the lower Mississippi and Yangtze Rivers – Implications for future backwater zone studies

*Yi-Jun Xu¹, Maotian Li², Zhongyuan Chen², Bo Wang¹, Shuwei Zheng³, Weiguo Zhang² (1.Louisiana State University, 2.East China Normal University, 3.Shandong Normal University)

Keywords:Channel dynamics, Riverbed deformation, Backwater zone , Alluvial river, Lower Mississippi River, Lower Yangtze (Changjiang) River

The lower reaches of many world’s large rivers serve as transportation hubs, commercial hotspots, and industry and population centers. This is especially the case of the Mississippi River in the United States and the Yangtze River in China. Draining a land surface of 3.2 and 1.8 million square kilometers, respectively, the lower Mississippi and lower Yangtze Rivers have become the world’s two most important shipping channels and their countries’ most vibrant economic corridors. However, as riverine sediment loads in the Mississippi and Yangtze have declined largely, these two alluvial rivers and their deltaic areas face challenges such as, among others, channel deformation, sediment transport, levee stability, and shipping safety. In this study, we analyzed channel changes in the final 500 kilometers of the Mississippi and the final 565 kilometers of the Yangtze in the past approximately 20 years, in order to identify common patterns with respect to where erosion and deposition occurred. For the lower Mississippi River, over 6,000 single-beam cross-sectional measurements surveyed in 1992, 2004, and 2013 were analyzed; For the Yangtze River, navigational charts surveyed in 1972, 2002, and 2013 were digitalized. These data were utilized to create channel bathymetric DEMs, which were used to quantify the changes in the riverbeds from bank to bank. Our study found that the two rivers showed similar fluvial patterns, specifically, 1) the average channel elevation lies below mean sea level; 2) there is a deep channel drawdown with a rapid elevation rise near the river mouth; and 3) widespread riverbed degradation in the recent decade. The lower Yangtze River experienced strong riverbed degradation since the completion of the Three Gorges Dam construction in 2003. Of the entire study river reach, 425 kilometers have undergone substantial erosion (i.e., 2.16 billion m³ with an annual average channel incision rate between 0.01 and 0.19 m), while the other 140 kilometers have experienced marginal deposition (i.e., 0.31 billion m³ with an annual average deposition rate ranging from 0 to 0.17 m). Changes in the riverbed of the lower Mississippi were more variable and can be classified into three geomorphic stages: aggradation, degradation, and dynamic equilibrium. While sediment loads in the lower Yangtze River sharply declined since 2003 (from ~270 MT yr^-1 to ~140 MT yr^-1), sediment loads in the lower Mississippi River showed relatively small fluctuation in the past two decades (averaged 120 MT yr^-1). Large floods seemed to have a strong effect on riverbed erosion in the lower segment of the backwater zone owing to the drawdown effect. These findings suggest that the lower segments of large rivers are likely experiencing substantial channel erosion, and that the backwater zone of these rivers may shift upstream as global sea level rises. The consequence of these changes on sedimentation and geomorphic processes is unknown. Future studies are needed to determine the underlying mechanisms in these two and other rivers and to examine risk implications for channel stability and sediment transport in estuaries.