This study investigates the impact of climate changes in natural streamflow in the Melen River Basin. Being the main drivers of variability in streamflow, daily temperature and rainfall data from 1963 to 2014 were analyzed to determine long term trends of each characteristic in the Melen River Basin area. These data also served as a basis for estimating evapotranspiration in the area. With the addition of trend analysis of the streamflow data from 1997 to 2014 the correlation between streamflow data and changes in climate were established. Long-term Melen River Basin temperature and precipitation trends indicate that both the mean annual temperature is increasing steadily every year (y=0.0209x+12.727, R-2= 0.169) and the annual rainfall is also decreasing (y= -2.2014x+877.86, R-2= 0.0602). Thornthwaite's methodology was applied to calculate monthly potential evapotranspiration for last 52 years. A regression analysis for the period yielded y=0.7536x+641.96, and R-2=0.2065, with a steady growth of monthly evapotranspiration values over the last 52 years. The study shows a very high correlation between streamflow of Melen River and the changes in temperature, rainfall and evapotranspiration. This supports the initial hypothesis that streamflow of Melen River is highly sensitive to climate changes and therefore climate changes must be considered in any long term water management project.