AbstractUnits in a hydroplant are usually committed after the operational results are derived for multiple cascaded reservoirs. Such a procedure may make hydro units difficult to harmonize their constraints (e.g., allowable operating zones, start-up frequency, minimum up/down duration, and so on) with the boundary conditions imposed by the operation of cascaded reservoirs. This paper investigated how and how much the cooperation between reservoir operation and unit commitment can contribute to the reduction in water and energy spillages when constraints on units are enforced. With multiple iterations involved, this work employed a two-level synergic mechanism, where the unit commitment provides feedback on the boundary conditions expecting from the higher-level reservoir operation. The procedure was applied to a hydropower system that consists of 45 reservoirs. Even starting with a very good initial solution to the operational problem of the reservoir system, the results strongly suggest the necessity to employ the two-level synergic mechanism, which contributes to the reduction in scheduled water and energy spillages by 34% and 41%, respectively.