The European beaver (Castor fiber) has long been considered an ecosystem engineer, exerting a profound influence on the habitats it occupies. Its activities significantly shape water conditions, terrain, and biodiversity. Research conducted in the Ciechanów Forest District aimed to assessthe impact of beavers on the local environment. Observations took place from August 2024 to March 2025 and involved documenting beaver activities such as browsing trees, creating burrows, and building lodges. It was found that the tree species most frequently selected by beavers in the study area was gray alder (Alnus incana). Beavers most frequently browsed trees with a diameter of 1–20 cm and a hardness of 2.1 on the Brinell scale. Their most intense impact occurred at the end of the growing season, fromAugust to November. The most preferred species were young trees with soft wood, which may indicate that less effort and energy are required to cut them down, or that they are more palatable, juicy, and tender than older plants.