The Euphrates River Basin system is of vital importance for water, food, energy security in Turkey, Syria, Iraq and under adverse climate change impact. It is therefore this paper mainly focuses on the naturally changing trend in long term streamflow with temperature and precipitation on the Upper Euphrates Basin that lies in Turkey. Because of the complexity of the setting up a basin-wide comprehensive hydrological model in the basin which have several large dams under operation ,we mainly used “run off data” in the statistical regression and time series analyses to predict the long term streamflow trends. We conclude that the annual natural flow has decreased mainly due to natural factors not because of construction dams. Obtained long term precipitation and air temperature trends in study area support the obtained streamflow trends. Results obtained from this study indicate that collaborative approach on transboundary water management between riparian’s is an urgent need.