In this paper water balance approach has been adopted for estimation the water management rules in the Middle Biebrza Basin. There are few different water users in the area which water demands are in conflict i.e. water use for agriculture and for carp production in the fish ponds vs. renaturization of the previously overdrained area along the Woznawiejski Canal. The main hydraulic structures in the are there are the Rajgrodzki water reservoir and the Kuwaski Canal. Authors propose also use of Drenstwo Lake as a water reservoir. Using optimisation-simulation model of the area the water management rules for the Rajgrodzki reservoir are estimated. The possible use of the Kuwaski Canal for flood protection is proposed. The idea of use of the Drenstwo Lake for water supply for renaturized area is examined. The results of various scenarios of water management in terms of performance criteria are summarized in Tab. 5 a and b.