Changes in the climate system and the hydrologic regime strongly affect all water uses and human activity. The main goal of this study is to evaluate the impact of rainfall pattern change on streamflow for 26 Brazilian basins with hydropower plants. More precisely, the goal is to estimate the trends on average streamflow for the 2011–2100 period. The estimated trends result from the analysis of rainfall obtained from a possible climate scenario, among others. The annual average streamflow for this 90-year period is simulated and compared with records from 83 years of observations (1931–2013). Simulations were carried out using two rainfall-to-streamflow hydrological models: Soil Moisture Accounting Procedure (SMAP) and Stochastic Linear Model (SLM). Results from simulations show important impacts, namely, an increase of streamflow in the southern basins and a decrease in the northern basins. Such changes can lead to disastrous consequences, considering the historical exposure to floods and droughts in the southern and northeast regions, respectively. These findings, in addition to the recent severe drought events that have occurred in such regions, provide awareness of a new cycle of reform to the existing water policies and Brazilian institutional framework, which saw the completion of its first 20 years in 2017.