For centuries, the water power of rivers and streams has been used in Slovenia. For this purpose, in the past, classical dams represented the most important water facilities. The diploma thesis deals with how the transversal structure in the watercourse influences the use of the waterside space, the course of the surface and other conditions. The goal is to determine the impact of the dam on Dreta and what happens if the existing dam is to be changed. To this end, three situations have been created using the HEC-RAS software tool - the current situation and two situations, when the dam is removed and when it is lowered. The hydraulic conditions of the new situations are then compared with the current situation, which helps us to understand the impact caused by transversal structures in the watercourse.