Urbanization is a pervasive global trend. The development of residential areas and road network in Hajighoshan watershed (northern Iran) has been observed in the recent several decades. The objective of this study is the quantitative investigation of the effects of impervious surfaces development and urban development on runoff generation and flood hazard. The study of urban area development was carried out using aerial photos, topographic maps and satellite images. Also a rainfall-runoff model was presented using GIS (HEC-GeoHMS extension) and HEC-HMS model. Then, the model was optimized with initial loss and SCS-Lag parameters. The optimized model was evaluated using the other 4 events of flood .In the next stage, the development of impervious surfaces was included in the validated hydrologic model and their effects on intensifying runoff generation and flood hazard was investigated quantitatively during the recent forty years. The results showed that the runoff generation potential has increased in Hajighoshan watershed because of urban development during forty years ago
