In Poland in recent years, the number of engineered and technical structures in forest areas has increased, and these have in turn affected water conditions. The purposes of these structures include protecting habitats, enabling rehabilitation of wetland areas, and protection of biodiversity. In addition to the State Forests National Forests Holding, numerous other organisations and associations have taken part in work to improve water conditions. One of these organisations is the Polish Society for the Protection of Birds, which, at the beginning of the 21st century, worked to improve the habitat conditions of birds. This effort was made by constructing dams on watercourses in the Białowieża Forest. As part of this work, several structures were built on the Braszcza River. One of these dams has been monitored since 2004 to assess the impact of the structure on water level. This dam has become an ideal location for habitation of the beaver, which was first noticed in the spring of 2006. Beaver activity has increased the height of the dam by approximately 50 cm. In this study, we analyzed changes in water level prior to the arrival of beavers, during their stay, and after they left their lodges.