Abstract: Flow velocity, water temperature, nutrients and chlorophyll-a were monitored in the three tributaries (Xiangxi Bay, Daning Bay and Xiaojiang Bay) of the Three Gorges reservoir (TGR) in July 2017 to study the characteristics of density flow and its influence on thermal stratification, nutrients distribution and algal blooms in this reservoir. The results show that bidirectional density currents could be a common flow pattern in these tributaries, and they are primarily caused by the difference in water temperature between the mainstream and tributaries in summer. And the water from the TGR mainstream intrudes into the three tributaries from middle layers, and the backward density currents feature with a depth and thickness mainly related to the water temperature distribution in the tributary estuaries. Under the influence of the mainstream backward flows, nutrients in the middle and lower streams of the three tributaries generally exhibit a vertical distribution structure of “middle layer > bottom layer > surface layer”. Since phosphate rock miners are located upstream of the Xiangxi Bay, its TP concentration presents a tendency of gradual decreasing from upstream to downstream, quite different from the other tributaries. Temperature difference between the mainstream and tributaries varies across the three tributaries, and it results in differences in the intensities of backward flows into the tributaries. The backward flow will play a crucial role in the thermal stratification and the species and intensity of algal blooms in the tributary bays, and then dominate the algal blooms.

Key words: Three Gorges reservoir, stratified density currents, thermal stratification, nutrients, algal blooms