The paper refers to the determination of land-use changes within a 14-years period in one of the rapidly urbanizing districts of the mega city, Istanbul. The significance of the district, Pendik, is that almost half of its land lies within the boundaries of the Omerli reservoir watershed, that supplies 36% of drinking water needs to the mega city with a population over 10 million. The main objectives of the study were to discuss and evaluate the land-use changes within 1987-2001, by utilizing the satellite images of LANDSAT with a spatial resolution of 30 m, and to anticipate the associated impacts of land-use changes on the water quality of the reservoir. The changes that have occurred within the inspection period indicated the expected outcomes of high population increase. It is clearly seen that there is a significant increase in the urban elements, even at the absolute and short-range protection zones. This occurrence is an expected outcome of population increase, despite the current regional strict regulation regarding the protection zones established with the purpose of protecting the surface water reservoirs of the mega city from further pollution. It is observed during the analyses that the situation since the last decade has not been so satisfactory, and that the urban elements have caused the reservoir water to be highly deteriorated.