The reduction of flood flow is not only dependent on proper capacity of retention reservoir but also possibilities of control of hydrotechnic devices. The analysis of work of retention reservoir Wisła-Czarne was based on a hydrograph of water flow to the reservoir whose culmination is equal to the maximal annual flow with probability of superiority 1%. This flow was determined on the basis of precipitation formula. In order to make hydrograph of flow to reservoir Snyder unit hydrograph was applied and model SCS (Soil Conservation Service) for determination of effective precipitation. The daily precipitation with 1% probability was showed in such a way (gamma distribution) to gain the expected value of culmination flow. In the analysis several simulations of reservoir work dependently on the initial state of filling the tank and work of overflow-drain devices was carried out. The final effect was to depict hydrographs of outflows from the reservoir in relation to particular variants.