The critical flow is a concept used in water management: it means the amount of water to be left in the water reservoir without disturbing the environmental equilibrium. It is the minimum amount of water to be maintained in a given cross-section of the river due to biological and social considerations. The critical flow is determined on the basis of hydrological and fishing modalities and criteria related to nature conservation and water tourism. There is no alternative for critical flow. Polish legislation has sanctioned the critical flow for many years, in various legal acts, including the Water Act of July 18, 2001 and the ordinance of the Minister of Environment issued in April 2004.