Time series techniques are applied to Ghara-Aghaj flow records, in order to generate forecast values of the mean monthly river flows. The study of data and its correlogram shows the effect of seasonality and provide no evidence of trend. The autoregressive models of order one and two (AR1, AR2), moving average model of order one and ARMA (1,1) model are fitted to the stationary series, where the AR2 model yields results which are statistically compatible with the past records. For a better understanding of the behavior the spectral analyses of the data and exhaustive statistical tests are also provided. Synthetic data generated by AR2 model together with historical data are then used to obtain reservoir storage for varying design periods, and draft rates. For this, the Stall's method which is a more sophisticated form of mass curve is employed. Results are given as two design curves.