Impacts of water release operations on algal blooms in a tributary bay of Three Gorges Reservoir

Water release operation is crucial for water quality in large reservoirs such as Three Gorges Reservoir (TGR), because it determines the hydrodynamics and hence the self-purification capability. As algal blooms were often observed in some tributary bays of TGR during the release periods, high frequency field observations were carried out in a typical eutrophic tributary bay (Xiangxi River) from February 9 to May 10, 2009. In this paper we assess the hydrodynamic behaviour, density stratification, and trophic status in this bay, respectively using a series of observations for flow, physical, chemical and biological parameters. Then, we analyze the effects of reservoir release operations on algal blooms using correlation analysis method. An empirical prognosis is concluded for the likelihood of algal bloom occurrence as a function of daily fluctuation of water level (DFWL) and water temperature. Our results indicate that during the release period, the algal bloom occurrences are closely tied to the DFWL in that if the ratio of DFWL to total water depth ranges from -10x10(-4) to 0, the possibility of algal blooms may reach up to 70%, and if the ratio is less than -10x10(-4), then that risk can be significantly reduced to less than 10%. This paper finally suggests that a wave-type water release operation should be beneficial in reducing bloom frequencies in the tributary water bodies, which is helpful for TGR water quality management, especially for the water release operation optimization.