Several water quality and hydrologic parameters were measured to determine the physical, chemical and biological characters of six streams which flow into the Ömerli Reservoir (İstanbul). It was aimed to investigate the effects of the streams on drinkable water reservoir. For this purpose water samples were collected from twelve sites in six streams at monthly intervals between June 2005 and July 2006. Fifteen physical and chemical variables were measured to determine the water quality. Principal components analysis (PCA) results indicated that stream width, flow rate and Chlorophyll-a were the most important variables in the streams. Total phosphate, ortho-phosphate, nitrite and dissolved oxygen were also found important. It is determined that human impacts, land use and geology of streams were the most important factors influencing chemical features of the stream water. Ömerli Reservoir was affected negatively by mainly the Paşaköy Stream. The Riva Stream which is discharge water of the reservoir had also a serious pollution problem. The other streams are relatively clear in terms of nutrient enrichment. Zooplankton species (25) were identified and canonical correspondence analysis (CCA) results indicated that nitrate, nitrite, orthophosphate, Chl-a, pH, water temperature, suspended solid material, oxygen and stream width were the most important variables influencing zooplankton diversity. Zooplankton species were quite poor in all of the streams with the exception of Paşaköy and Riva streams. Indicator zooplankton species of eutrophication were found as dominant species in Paşaköy and Riva streams.