Need for water increases from day to day due to enhancing life standards and developing industry. It is to our knowledge that millions of people in different regions of the world are living deprived of sufficient water in quality and quantity. Due to lessening and pollution of water in the third millenium together with population increase, sufficient and clean water supply to large cities has gained more importance. The spatial and temporal distribution of water in nature does not generally coincide with that of the demand. To overcome this problem a system of reservoirs has to be established for water supply to large settlements. From the perspective of escaping floods and dry periods with minimum loss, management of water supply systems is especially important. Under such conditions management of water supply system of any metropolis becomes a very complex hydrological, sociological and economical problem. Sufficient water supply in quality and quantity is a significant issue also for Istanbul which has received a large migration throughout the years and where industry is rapidly growing. Since the establishment of the city, during the Roman, Ottoman and the Republican periods many structures have been constructed with water supply objective. Within the framework of the subject, alternative operations of Istanbul Water Supply System during 1989-1994 drought have been given.