Abstract: The threeroute (i.e. East, Middle, and West) SouthtoNorth Water Transfer Project which began in 2002 will divert a total of 44.8 billion m3 of water annually from the Yangtze River to the drought North and Northwest China by 2050. It will somewhat alleviate water shortage in those areas and help a balanced development across the nation. However, such a large interbasin water transfer project will change the aquatic environment of the upper Yellow River by increasing its runoff and also the environment of the lower reach of the Han River due to the decrease in its runoff. The resultant decrease of discharge into the mainstream of the Yangtze River will cause water contamination and seawater intrusion into the Yangtze Delta. Meanwhile, water quality in the upper Han River basin is of great concern, and secondary salinization seems unavoidable in the water receiving areas. This article briefly discusses the necessity and its environmental implications in particular species migration, ecological security of water environment, watershed management for water resource conservation, and rehabitation of degraded ecosystems in the west. A longterm monitoring programme in combination with the Chinese Ecosystem Research Network and National Observatory System for the Environment will be necessary for evaluating its environmental implication.

Key words: interbasin water transfer, species migration, water ecosystem security, ecosystem management, degraded ecosystem