Abstract: Based on field observations in November 1998 and October 1999,comparisons were made between them in quality and spatial distribution of principal pollutants(such as DO,COD Mn ,TP,TN and Chla)in the waters at various sampling sites. It is revealed that there is no significant improvement in water quality after the emission control. Non?point source pollution and release of nutrients from the sediment are probably the two major causes of the hysteresis.