This paper presents an evaluate the benefit of streamflow forecasting in the deterministic optimization of the long term hydroelectric sheduling problem. In the approach considered, at each stage of optimization a forecast of the inflows during the period of planning is made and an operational decision for the following stage is obtained by a deterministic optimization model, in a partial open-loop feedback control framework. The influence of the forecasting model in the performance of this control policy is analised by simulation using historical inflows record. The effectiveness of the approach was measured using the mean and standard deviation values for hydro generation and operational costs during the planning period, using the hydroelectric plants of the Southeast Brazilian as a case study.