In this study, a 1D model of reservoir hydrodynamics DYRESM has been applied to Sau Reservoir, a river valley reservoir in the North-Eastern Spain. Simulation is undertaken for 3 years (1995-1997). Meteorological input data measured at the dam are only available from May of 1997. In this case the simulation results fit measured temperatures very well. In the remaining periods, some meteorological data (radiation, wind and rainfall) were obtained from two nearby stations. Simulated temperature distribution in 1996 is close to the observed one. In 1995, however, the simulated result is far from the observed data. Inflows, outflow and local meteorological events such as storms and gusts of wind seem to be responsible for the differences. By changing some parameters, the effects of flow, light extinction coefficient and outlet elevation on thermal stratification are investigated. Simulations demonstrate that the inflow with high temperature is the main factor controlling the thermal structure in Sau Reservoir and demonstrate that the effect of residence time on thermal stratification is manifested mainly by the changes in the depth of thermocline.