Abstract: To study the influence of regulating small and medium floods into the Three Gorges reservoir in flood season on the algal blooms in its typical tributaries, we focus on a case study of Xiangxi Bay and examine the environmental factor parameters and their variations during its regulation from 2010 to 2019. The results show that in flood season, an outbreak of algal blooms will occur in the bay because it is featured with favorable conditions－suitable water temperature, sufficient light, and adequate nutrient content; and to suppress and prevent the blooms, flood regulation is still the most direct, effective method at present. In 2010-2019, flood regulation by this reservoir modified the hydrodynamic and water environmental conditions in this bay. The depth ratio of its mixing layer over photic zone was correlated positively with its river stage fluctuation (P = 0.32*) and negatively with Chlorophyll a concentration (P = -0.239*). Flood regulation was a frequent practice in the reservoir operation, often with a long regulation duration and a relatively large overall flow. During flood regulation, the depth of density currents in the bay was increased, and its form was switched to plunging in middle and bottom layers. Reservoir dispatching promoted disturbance to the tributary water bodies, weakening their stratification and suppressing algal blooms. This study offers new insights for effective use of flood regulation to improve water environment in the tributaries of the Three Gorges reservoir and is useful for further studies on the methods of flood regulation and its optimization.

Key words: Three Gorges Reservoir, Xiangxi Bay, flood regulation, algal blooms