Abstract: Eutrophication is caused by hydrological,physical or chemical factors such as temperature,illumination,or shapes of waters.It is very important to identify the main factors to understand and control the process of eutrophication.In this paper,Beijing north circle water system was divided into river subsystem and lake subsystem by cluster analysis.Factor analysis indicated that the main factors of river subsystem eutrophication were TP,TN and NH₄-N,while those for lake subsystem were TN and NH₄-N,which meant that in the study area,the main cause for eutrophication was the overload of nutrients.Stepwise regression was introduced to build forecasting model,and the choice results of independent variables in the regression model inferred that the type of eutrophication in river subsystem was P limited,while that of lake was N limited.Two main causes for nutrient load were found from water quantity and quality analysis of the study area,one was the ecological water demands be not able to satisfy from 1990 to 1998 except 1998,and the other was the increase of nutrient load coming from domestic waste and non-point pollutant,resulting from the rapid population increase and city expansion.The measures to control the eutrophication process were put forward.

Key words: Urban water system, Eutrophication, Characteristics, Factor analysis, Stepwise regression