Abstract: Extensive research has been carried out to evaluate the possible environmental effect of the south to north water diversion project in Hanjiang River, and algal blooms have been the focus of these studies,with most of them being related to the river reaches between Xiantao and its joint to the Yangtze River. However,little has been carried out in the reaches upper to Shayang. In consideration of two algal blooming events in 1998 and 2003 in reaches around Zhongxiang City of Hubei Province,the reduction of water flow and the increasing content of nutrients after the implementation of south to north water diversion project will cause the water velocity and the nutrient contents suitable for the occurrence of algal blooms in the backwater area in Xinglong hinge in the period of low water 505.7 m/s)as indicated by the weighed grey associate analysis and fuzzy mathematical analysis. In the year 2020,a similar result may be expected even in the Xiangfan reach of the Hanjiang River in lower water period.

Key words: algal bloom, water resources, water carrying capacity, sustainable development