The reservoir storage-yield-reliability (S-Y-R) curve defines the required volume to meet a specific yield. It is typically obtained through the historical streamflow time series; however, as an effect of nonstationarity, the statistical properties of a streamflow series may vary, which might lead to a change in the reservoir’s operational risk. In this study we explore this issue by analyzing two sets of annual data: (i) natural energy flows to aggregated reservoirs, and (ii) streamflow time series of four hydropower plants currently in operation in Brazil. The study is supported by Monte Carlo simulations to account for the reliability of the S-Y-R curves. Results suggest that the time series from the Southern and Northeast regions exhibit upward and downward trends, respectively. Consequently, the regularization capacity of the Southern reservoir decreased, however only in relative terms. On the other hand, the Northeastern reservoir had an actual loss of its regularization capacity as an effect of lower average streamflow.