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The impact of a hydropower plant (HPP) on the environment, first at all in downstream reaches is analyzed in the paper. The impact on habitat quality changes can be estimated by the ecohydraulic parameters such as velocity, water depth, Froude Number (Fr), turbulence and vorticity regimes fluctuation Analysis of literature and experiment results from Virvyte and Susve HPP turbines operation regimes suggest, that dams and reservoirs may impart change in the environment though facility operations over a sustained period. In pounded reaches nearby above HPP dams of Virvyte River low Fr number habitats (0.04>Fr> 0.0007) were created and negative dam impact was observed. A large disturbance experiment when a positive wave front resulting from an increase of initial sanitary flow rate Qs =0.4m3/s till total flow rate Qtot=10.4 m3/s moving downstream after the switch on turbines was made The maximal water level increase speed Δh/Δt= 10.18 m/min=10.8 m/h was fixed on the second minute from the beginning. As a positive surge can travel over very long distance without losing much energy because it is self-perpetuating damaging of downstream biota, it is evident that in practice rapid operation of HPP turbines switch must be avoided. [ABSTRACT FROM AUTHOR]