The world's population is continuously growing, so the water demand is increased and the water crisis tends to intensify the use of groundwater. With regard to the reduction of groundwater resources, the use of surface water is a priority. Base flow is the minimum flow rate that flows in the river. Base flow is the maximum discharge which is extracted from the river regardless of the environmental flow. In this research, to estimate the main flow and its index at a daily time step for 8 representative gauging stations located at the Gorganroud river basin during a period of 34 years (1981 to 2014) the Hysopp methods (local minimum, fixed intervals and Sliding-interval), B-Flow method, one-parameter digital filter method, Chapman digital filter method, EWMA method and Eckhardt method used to estimate base flow index from the measured streamflow, and their results compared. Using two parameters of α and BFI _max, the base flow was separated by Eckhart's method and the base flow index was calculated. Finally, it can be observed that one-parameter digital filter method (with MAE, RMSE and r values of 0.28, 0.22, and 0.976, respectively), and the Chapman digital filter method (with the MAE, RMSE and r values of 0.29, 0.33 and 0.979, respectively) give the best results. In both methods, the base flow has the least standard deviation (SD≤0.66), which was selected as the best flow separation method in the study area.