The article considers the problem of designing a robust controller for an open-channel hydraulic system with multiple dams for controlling discharge in rivers. The open-channel system considered is made of a network of river reaches controlled by two dams located upstream, one large dam far upstream, and one closer but smaller. As shown by the authors previously, the performance of such systems is structurally limited. The present paper shows how to recover the performance of the system controlled by the small dam, without emptying it too soon in the season. The problem is casted into the H_∞ framework, using frequency weighting functions to specify design requirements. Note that a strong interest of H_∞ controllers is to ensure robustness margin and performance.