The effect of 11% thinning on selected stream water properties in an oak-beech forest ecosystem in the Belgrad Forest of Istanbul, Turkey was studied with a paired catchment experiment. Regression equations for some physical properties of the stream water were developed between control (W-I) and treatment (W-II) watersheds for calibration (with their 95% confidence limits) and treatment periods. The study discovered significant linear regressions between control and treatment watersheds for color, turbidity, air and water temperatures, suspended sediment concentration, pH and electrical conductivity in the calibration period but not turbidity and suspended sediment concentration in the treatment period. The impact of the timber removal was calculated as the difference between measured and predicted values derived from calibration equations. Results showed that timber harvest caused significant decreases in the color, turbidity, temperature, pH, and electrical conductivity values of the stream water and air temperature under the canopy but did not affect the suspended sediment concentration in the stream water.