—This article presents two methods for open-loop river flow control, based on a classical hydraulic model (the Hayami model, a partial differential equation resulting from a simplification of the Saint-Venant equations). The proposed method shows the equivalence of differential flatness and a frequency approach to design a controller, by expressing the upstream flows as a function of the downstream flow (model inversion). The controller is represented as an infinite series and a statement about the convergence of this series is made. A comparison is made with controllers for thermal systems from a similar problem available in the literature. The numerical simulations show the application of the open-loop controller for irrigation canals modeled by the full Saint-Venant equations.