ABSTRACT: The Sarma Stream is located in southwest of Akcakoca town in Duzce City, Turkey. Its drainage basin is approximately 121.2 km2 . The Sarma stream flows muddy in to the Black Sea with an output of 85 l/s during rainy season . It was decided that the Sariyayla reservoir should be built on the Sarma Stream for the drinking water needs of Akcakoca. Chemical compounds in natural water are altered by interactions with geologic units, by physical and chemical weathering and environmental factors. Generally, research on acid rain and its effect has been done in Istanbul. In this study, effects of acid rainfall on the hydrogeochemistry of the Sarma stream water were investigated. Therefore, samples of rock, soil, rainfall, stream water, suspended and bed sediments were taken in the Sarma Stream basin. The pH of rainfall and snowmelt in the research area are below 5.6. Acid rain affects the dissolution of the geological units and the abundance of principal ions in stream water. The Chemical Index of Alteration (CIA) values of the bed sediment sample and suspended sediment sample are 76 and 77, respectively. Sandstone, which erodes easily and is rich in clay minerals, causes the Sarma Stream to become muddy during the rainy season. The suspended sediment amount of the Sarma Stream in the rainy season is 70 mg/l. The water of the Sarma Stream is rich in calcium and bicarbonate. Rain water is richer in NH4, NO3 and SO4 ions than snowmelt. Snowmelt is richer than rain water in heavy metals such as Al, Ba, Cu, Pb, Mn, Ni, Si, U and Zn. The concentration of heavy metals and some elements such as Al, Ba, B, Fe, Mn and Zn are 10 ppb and more in rain, snowmelt and stream water. Some ions in the Sarma Stream basin waters exceed the drinking indicator water limit values (e.g. Al, Fe and Mn ions). Hence, water in the Sariyayla Reservoir might need to be treated