Characterization of Mixing Processes in the Confluence Zone between the Three Gorges Reservoir Mainstream and the Daning River Using Stable Isotope Analysis

Understanding the interaction processes between the mainstream and its tributaries and detailing the rates of contribution of water and nutrients from two different waterbodies in the confluence zone are essential for water management in the Three Gorges Reservoir (TGR). The stable isotope ratios of hydrogen (SD) and oxygen (5180) were applied to explore the interactions between the TGR mainstream and a typical tributary, the Daning River. The results of the model calculations showed that approximately 78.9% of the water and 88% of the nitrate in the confluence zone were from the TGR mainstream. The dynamic vertical distributions of the mixing ratios, major ion contributions, and flow velocities indicated that the water mass from the Yangtze River mainstream flowed backward from the confluence zone up to the tributary along the surface and upper-middle layers, whereas water from the tributary flowed into the mainstream through the lower-middle and bottom layers. This study demonstrates the value of hydrogen and oxygen isotope tracers in accurately describing water mass mixing processes and estimating the rates of contribution of different nutrient sources in the confluence zone, which will provide valuable information for controlling algal blooms in the future.