Echo state networks (ESNs) are promising options for performing time series prediction, as they establish an attractive tradeoff between dynamical processing capability and simplicity in the training process. The recurrent character of these structures is essentially concentrated on a nonlinear processing stage known as dynamic reservoir, whereas the output layer–also termed readout–is allowed to assume the form of a combiner that is, as a rule, linear with respect to its free parameters. Recently, Butcher et al. and Boccato et al. proposed readout design paradigms–based, respectively, on Volterra filtering and extreme learning machines–that were shown to enhance the information extraction potential of ESNs without compromising the intrinsic simplicity of their training process. In this work, these approaches are comparatively analyzed, using different reservoir configurations, in the context of monthly streamflow series forecasting. The analysis is based on data from the hydroelectric plant of Furnas, covering three different periods with distinct hydrologic profiles and including the canonical linear approach for readout design as a benchmark. The results reveal that the aforementioned proposals may lead to a relevant performance improvement, thus indicating that the reservoir is not per se capable of fully exploring the nonlinear relationships underlying the focused data.