Abstract: In this study, we have conducted field measurements in Xiangxi River, a typical tributary of the Three Gorges reservoir, to collect the meteorological and hydrological data of daily variations for the period of June 5 to 30 August, 2019, including water temperature and water quality measurements. This paper focuses on an analysis of the effects of water level fluctuation in Xiangxi Bay on its thermal stratification and water quality in flood season. The results show the daily cycle of an upper warm layer that led to sub-daily variations in the mixed layer depth in a range of 0.0 - 19.4 m, but in long term this depth was significantly negatively correlated with water level fluctuations (P = - 0.47). The bottom water temperature increased slowly and this increase was impacted by the cold inflows at the bay inlets significantly. No stable thermocline was observed due to strong mixing driven by the intrusion of density currents and vertical convection. Significant correlations of dissolved oxygen content and saturation degree with Chl.a concentration indicate the variations in dissolved oxygen can be caused by phytoplankton distribution. Further analysis reveals that large water level fluctuation could vary the plunge depth of density currents, and a shift from the overflow to interflow mode of the currents will result in altered mixing regime in the tributary bay, i.e., reducing the surface mixed layer and increasing vertical stratification, both significantly different from those of the reservoir filling period. This study sheds light on new understanding for effective utilization of water level fluctuation (through reservoir regulation) to improve water quality in the tributaries of Three Gorges Reservoir.

Key words: reservoir regulation, water level fluctuation, water temperature stratification, density current, mixed layer