Overpopulation has some administrative, social, economic and environmental problems in the world. Istanbul is the city with the largest population in Turkey causing pollution problems on the surface water reserves. Buyukcekmece Reservoir is one of the 8 major drinking water reservoirs of Greater Metropolis Istanbul. Buyukcekmece Watershed covers the third largest drinking water source of Istanbul, Buyukcekmece Lake. The purpose of this study is to assess the water quality changes of Buyukcekmece Reservoir using Remote Sensing and Geographic Information System technologies in support of operational management. The results are obtained by image processing of different spectral and spatial resolution data belonging to dates of 1992, 1996, 2000, 2001, 2004, 2005 and 2006. Rapid population growth, industrial development, lack of sufficient infrastructure, and also illegal settlements highly affect the watersheds and water quality as a typical reflectance of urban sprawl in mega cities of developing countries. The pollution load from settlements and industrial areas to the watersheds has a negative effect on drinking water quality, and also many species in the examined lake.