Changes of Divergence and Confluence Relationship Between Dongting Lake and the Yangtze River After the Operation of the Three Gorges Project and Its Impact on the Waterway Depth

Abstract

Human activities and climate change have profoundly changed the hydrological and geomorphological evolutionary processes and trends of rivers and lakes, which, in turn, has affected the utilization of their waterway depth. This study chose the Dongting Lake–Jingjiang confluence of the Changjiang River (Yangtze Rvier), since the operation of the Three Gorges Project, the split ratio of water and the sediment distribution of the three outlets of Dongting Lake—that is, Songzikou, Taipigkou, and Ouchikou—have continued to decrease. Along with the decreasing flow of Dongting Lake, the relative increase in the runoff of the Jingjiang reach and the increase in riverbed erosion intensity have increased the relative height difference between Jingjiang and Dongting Lake, intensifying the reduction of the split ratio of water and the sediment distribution of the three outlets. The riverbed erosion of the Jingjiang reach has created a promising foundation for an increase in the waterway dimensions. However, the length of the reach not meeting the requirements of 4.5× 200 m (water depth × width) is 18.4 km, of which the length in the river-lake confluence is 12.6 km, accounting for 68.35% of the total length of obstructed navigation. Furthermore, at the reach (Zhicheng–Dabujie reach) affected by the first outlet (Songzikou), a 4.5-m flume has been formed, but its width is less than 200 m. Moreover, the “steep slope and rapid current” phenomenon is clear, and it affects the safety of ships. At the reach (Taipingkou reach) affected by the second outlet, the shrinkage of the bottomland, and the intersecting braided river channels make the low-flow routes unstable, the water depth being less than 4.5 m. At the reach (Tianxingzhou–Ouchikou reach) affected by the third outlet, the shoreline of the reach has collapsed and retreated significantly. Additionally, beach erosion and the downstream movement of cut banks have caused the downstream waterway width at the reach to be reduced to less than 200 m or formed shoals with a water depth of less than 4.5 m. Owing to the jacking effect of the lake’s outflow backwater, the flow rate of the reach (Xiongjiazhou–Chenglingji reach) affected by the confluence of Dongting Lake decreased, creating a shoal with a water depth of less than 4.5 m in the navigation waterway. The results of this study have guiding significance for understanding the changes of channel conditions and the improvement of channel scale in the intersection area of rivers and lakes.

Effect of water depth and waterway obstructions on the divergence and confluence areas of Dongting Lake and the Yangtze River after the operation of the Three Gorges Project

“…A mathematical model has predicted that the decrease in the middle‐ and high‐water levels of the Jingjiang reach would be limited to 20 years after the TGP (Y. T. Li et al, 2009); however, the riverbed elevation of the flood channels at the three distributaries would decrease significantly, although the diversion ratio of the three distributaries would not decrease, and the ratio of the current distribution of the three distributaries would not be reduced. The prediction was quite similar to what has ensued (Y. P. Yang, Liu, et al, 2022; Y. P. Yang, Zheng, Zhang, et al, 2022). Under a scenario of the lake volume remaining constant, the reduced outflow from Dongting Lake would be approximately 23% of the water storage capacity of the Three Gorges Reservoir (TGR) (D. D. Zhang et al, 2019).…”

Mechanisms Controlling Water‐Level Variations in the Middle Yangtze River Following the Operation of the Three Gorges Dam

“…For example, in the United States, striking increases in vegetation have occurred in the Arkansas River downstream of John Martin Dam (+90%), the Republican River downstream of Trenton Dam (+55%), the Salt Fork River downstream of Arkansas Dam (+33%), the North Canadian River downstream of Canton Dam (+40%), the Platte River downstream of Kingsley Dam, the Canadian River downstream of Sanford Dam, and the Missouri River downstream of Garrison Dam (Johnson et al., 2012; Williams & Wolman, 1984). Nevertheless, other factors not discussed here but which affect water levels require further investigation, including waterway regulation projects (Yan et al., 2019; Yang et al., 2022).…”

Critical threshold of periodic point bar scour and sediment body transport path in tidal reaches: A case study of Fujiangsha reach, Yangtze River

“…During field observations of the middle reaches of the Yangtze River, the bank in the straight sections exhibited periodic upstream and downstream movements, causing adjustments in the diversion of downstream branches or the alternation of branches, such as the Jiepai reach (Liu et al, 2014). Owing to adjustments in the flow processes during the year, after the Three Gorges Project became operational, the side banks on the convex side of curved reaches eroded and showed a trend of under scouring, such as the Xiongjiazhou-Chenglingji reach, with some sharp bends straightened through scouring to form oxbow lakes (Yang et al, 2022a(Yang et al, , 2022b(Yang et al, , 2023b. In complex branched reaches, frequent scouring of the side banks and the Jiangxin island have deteriorated the river and waterway conditions, such as in the Shashi reach (Zhao et al, 2020;Yang et al, 2021a).…”