Abstract: With the development of hydroelectric power source in Lancangjiang River valley, the water quality change and the crossboundary effect are concerned more and more by the downstream countries and many international organizations. This paper takes Manwan hydroelectric station as an example, using water quality monitoring materials of 6 sections and 21 items in the downstream of Lancangjiang River from 1988 to 2002, adopting contrast analysis method, evaluates the water quality status and trend of main pollutants of the whole mainstream of Lancangjiang River downstream since the construction and running of Manwan hydroelectric station for 15 years. The results indicate that the construction and running of Manwan hydroelectric station has not produced obvious impacts on water quality of mainstream of Lancangjiang River downstream currently. But Manwan reservoir was polluted by heavy metal and other pollutants, and its water quality tends to go down. Pollution of the main pollutants such as MnO\+-\-4 index, NH\+-\-4-N, Hg, Pb and Cd tend to decrease, and pollution of TP was heavier. Pollution of Pb and Cd heavy metal was mainly concentrated at Gajiu (Manwan) section and Jinglinqiao bridge section, and organic pollution produced by MnO\+-\-4 index, TP and NH\+-\-4-N mainly concentrated at Xishuangbanna hydrology station section and Menghan ferry section. Water quality of Lancangjiang River at leaveboundary section was good and basically meet the requirement of water environmental function.

Key words: Manwan hydropower station, downstream of Lancangjiang River, changes of water quality