QI Dexuan, MA Wei, DANG Chenghua, SONG Tingting, SUN Lei

Eutrophication of the tributary bay is one of the main environmental problems brought in by the Three Gorges Reservoir. Clarifying the driving forces of the eutrophication in the tributary reservoir bay is the key to solving the problem. This paper takes the Meixi River in the Fengjie County as the study object, which is a typical tributary. The driving mechanism of hydrodynamics and the nutrient sources and composition of the tributary bay are investigated through the 3-D simulation technology for hydrodynamics and water environment. Investigations indicate that the density flow caused by the temperature difference between the mainstem and the tributary water is the main driving force in the formation of the Meixi River tributary reservoir bay. From the estuary to the upstream, the hydrodynamics influence of density flow on the tributary bays gradually weakens. The input of nitrogen and phosphorus nutrients from the mainstream backflow accounts for less than 20% of the nutrients in the tail area of Meixi River where the algal bloom is prone to occur. However, terrestrial input accounts for 70% and thus is the main source of nutrients for the eutrophication of the reservoir bay of the Meixi River. As a matter of fact, the water quality of the mainstem of the Yangtze River and the water temperature of the main and tributary streams are difficult to control. It is therefore suggested that such treatment ideas be followed as “source control-process interruption - terminal interception - in-situ reduction of water body - water ecosystem restoration”. On this basis, regulation measures for the eutrophic water body of the tributary reservoir bay have been developed, which are expected to provide support for water pollution control and water quality target adaptive management in tributary bays.