Confluence Effects of Dongting Lake in the Middle Yangtze River: Discontinuous Fluvial Processes and Their Driving Mechanisms

Abstract Confluence‐induced river discontinuities pose several challenges for flood control, navigation, and maintenance of ecological integrity. River‐connected lakes in lowland regions represent a unique type of confluence, fulfilling significant floodplain functions in fluvial processes and forms of main streams, yet they remain insufficiently studied. This study utilized Dongting Lake in the Middle Yangtze River as a representative case to examine the confluence effects of lake‐type tributary on the fluvial processes of the main stream and to identify key driving factors. The findings revealed that during the equilibrium period, both the flow frequency curves and sediment‐transport rating curves exhibited segmentation characteristics due to the floodplain functions of river‐lake system. The confluence effects of the lake amplified the segmentation behaviors across spatial scales, which inherently depended on the river‐lake flow combination. Following the Three Gorges Reservoir impoundment, alterations in the confluence effects of the lake led to recent abnormal local erosion‐deposition adjustments under sediment subsaturation conditions, whereas upstream reservoir regulation dominated a long‐term reduction in channel‐forming discharge with a channel shrinkage trend downstream of the confluence. Despite the altered water‐sediment regimes, tributary inflow qualitatively remains a critical factor influencing the discontinuity of fluvial processes in the main stream. Our research demonstrated that the hydrological characteristics of confluence, rather than sediment transport, significantly influence the fluvial processes of lowland alluvial river. This insight enhances our understanding of the evolutionary mechanisms of large alluvial rivers with complex river‐lake relationships and offers important implications for the management of water resource systems.