The presented work aims to evaluate the changes in hydraulic conductivity and the planning possibilities for the naturalization of regulated streams in the Southeast Lithuania. The investigations have been carried out in sections of the regulated streams Nemėža, Ž–2, Žalesa and Girija. The hydraulic roughness coefficient, varying within 0,030 and 0,094, has been determined on the basis of morphometric parameters of the regulated streams, and on distribution and density of woody vegetation in the cross profile of the channel. Sections of the flood hydraulic conductivity reserve decreased to the water overflow to the valley limit already exist in the regulated streams of southeast Lithuania. Having calculated the hydraulic roughness coefficients, HEC–RAS (River Analysis System) is used to estimate the influence of woody vegetation on the hydraulic conductivity of the stream. According to simulation results in the investigated sections of the regulated streams the hydraulic conductivity reserve is lost when the hydraulic roughness coefficient reaches the limit of 0.060–0.080. Thus, if the channel slopes were overgrown with woody and grass vegetation and the roughness coefficient exceeded 0.060–0.080, the hydraulic conductivity of the channels could be used to reduce the channel roughness to the admissible limit (0.060)
