Abstract: Principal pollutants in the waters were compared between Yili River and Xitiaoxi River located in the west and southwest of Taihu basin, based on field observations at various sampling sites in July and November in 2010. Several factors which might induce the pollutants in these inflows were also analyzed related with the socio-economic, geographical and other aspects in the Taihu basin. Several indicators were selected in this study, including chemical oxygen demand (COD_Mn), total phosphorus (TP), total nitrogen (TN) and ammonia-N (NH₃-N). Field observations were carried out at 22 river sections and all selected indicators were monitored. Results show that COD_Mn, TN, NH₃-N and TP of Yili River in non flood season were significantly higher than that in flood season. Water quality of Xitiaoxi River in flood period was worse than that in non flood season. Water quality of Yili River is worse than that of Xitiaoxi River. Because of the spatial distribution of urban areas, population, industrial pollution and other business factors, the spatial variation of water quality of both rivers is very obvious. Yili River is mainly polluted by point source pollution. While the downstream of the river is also affected by non-point source pollution in the flood period. Water pollution of Xitiaoxi River mainly comes from non-point source pollution, which is caused by rainfall and runoff carrying most of the pollutants into Xitiaoxi River in flood season.The pollution of the downstream of Xitiaoxi River is also affected by emissions from point sources.

Key words: comparative study, river pollution, spatial variation, Taihu Basin