Abstract: The water quality in Miyun Reservoir and reaches of its main influents including Chaohe River,Baihe River and Qingshuihe River was monitored and analyzed in 2006.The results showed that the difference of the annual average pH was small,while the pH was lowest in rainy season,relative lower in winter and higher in the other seasons.The concentrations of total nitrogen and nitrate in Miyun Reservoir were lower than that in its influents.Among three rivers,the highest concentrations of total nitrogen and nitrate were found in Chaohe River,while they were lowest in Qingshuihe River.The low concentrations of total nitrogen and nitrate were found during plant growing season,namely from May to September,while they were high in the other months,namely from January to April and from November to December.The concentration of phosphorus in Miyun Reservoir was lower than that in Chaohe River and Baihe River,but it was higher than that in Qingshuihe River.The highest values of phosphorus concentration in Chaohe River and Baihe River were found in July.The COD concentration in Miyun Reservoir was higher than that in other three influents.The COD concentration in Qingshuihe River was the highest,because of the drainage of the waste water rich in organic matters from fishponds along the river.The highest COD values in Miyun Reservoir and its influents were found from July to September,while they were lowest in January and December.

Key words: Miyun Reservoir, Miyun Reservoir, River, Water quality, Seasonal change