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Alluvial fans create a special type of flash flood threat, especially in and and semi-arid areas where the fans support urban development. Most of the cities in Iran have developed in such environments, and are facing flash and urban floods because of the rapid growth of the cities, and the deficiency of urban planning in the last four decades. The present study aims at analyzing the urbanization effect on natural drainage patterns and their immediate result, namely the increase of vulnerability of the metropolitan city of Mashhad. Mashhad, as the second religious metropolis of the world, and second metropolis of Iran has an annual 12 million pilgrims, and 3 million population. The results of this study indicate that the rapid growth of the city from 1966 to 2002 has had excessive destructive effects on the catchments area, and has caused the decrease of impervious surfaces, and the increase peak discharge of the urban floods.