Abstract: In May(the dry season),August(the wet season)and November(the normal season)2009,water samples were collected from a number of rivers,including Ji Canal,Chaobai New River,Yongding New River,Jinzhong River,Beitang Channel,Heizhu River,Haihe River,Dagu Sewage Channel,Duliujian River,Qingjinghuang Drain,Ziya New River and North Canal in Tianjin and analyzed for monitoring of water quality.Principal component analysis was conducted of the monitoring data.On such a basis,a water quality evaluation index system was established,using water quality identification indices to evaluate water quality.Results show that the evaluation combining PCA with the water quality identification indices is objective and reliable.The temporal variation of the water quality on the whole reveals that it is the worst in the dry season,when the water quality of 58.3% of the rivers were worse than Grade V of standard for surface water and some even turned to blackish.However,the water quality turned relatively good in the wet season(August).In light of the comprehensive water quality identification indices,the rivers south to the Haihe River were more seriously polluted,especially with ammonia nitrogen,which was followed by BOD₅,COD_Mn and TP.The pollution was of the organic and eutrophicating type.

Key words: Tianjin, river, water quality assessment, principal component analysis(PCA), water quality identification index