Beavers (Castor spp.) shape and change their habitat more than almost any other animal species. Beaver dams, beaver ponds and their marginal bogs, stream splits, side channels, beaver meadows and large amounts of deadwood used to be common structures in small streams. The reintroduction and distribution of beavers in the northern hemisphere is accompanied by a continuous “restoration” of the small watercourses and their floodplains, the consequences of which has scarcely been explored to date. To investigate the influence of beaver activities, I studied aquatic habitats created by beavers and compared them with aquatic habitats in comparable stream sections without beavers. I used the abundance, species richness and functional groups of the macrozoobenthos community, as well as the size, diversity and complexity of the habitats as indicators. In the first chapter, I explored the impact of beaver activities on stream and floodplain morphology and habitat complexity, using aerial photography, transect mapping and a geographic information system. I focused on changes in wetted surface, macro- and microhabitats, as well as the connectivity of the stream-floodplain complex. The results revealed that beaver activities drastically increased the wetted surface area and created a diverse complex of lentic, lotic and semi-aquatic habitats. Furthermore, beaver activities improved the connectivity of the stream-floodplain complex by lengthening the shoreline, reducing stream incision and thus enhancing the hydrological connection between the aquatic and the riparian area. Additionally, the number, diversity and heterogeneity of micro-habitats increased due to beaver activities. Especially, the amount of deadwood increased extremely, and - together with the presence of emergent macrophytes – contributed to a higher habitat complexity and diversity in comparison with stream sections without beavers. In the second chapter, I studied the macrozoobenthos community in the beaver created habitats, such as ponds and side channels and compared them to habitats stream sections without beavers, e.g. riffles and pools. For this, I collected 188 habitat-specific macrozoobenthos samples that yielded more than 82,000 individuals. The differences between stream section types were analysed in terms of the abundance and species richness, as well as the flow preferences of the macrozoobenthos community. Beaver activities had a profound positive impact on macrozoobenthos diversity, significantly enhancing both species richness and abundance. In beaver territories, the flow preferences are more heterogeneous than in stream sections without beavers which are dominated by lotic taxa. Community composition was most similar between habitat types with comparable flow patterns, such as beaver ponds and pool-habitats in non beaver territories. In both stream sections types, rheophilous taxa accounted for the largest percentage (> 50 %) of the community. Furthermore, the results show that beaver activity increased the functional and taxonomical diversity of macrozoobenthos. In the third and final chapter, the macrozoobenthos fauna of beaver dams was investigated in detail. Beaver dams are special habitats in the aquatic-terrestrial interface, but their macrozoobenthos community is hardly known. This study aimed to quantify and characterize this community, taking into account the maintenance state of the dams. Nine different areas of a dam were systematically sampled, from the top to the middle and bottom areas using a suction device specially developed for this purpose. The macrozoobenthos community of beaver dams proved to be diverse and predominantly rheophile. Differences in the colonizing structure were directly dependent on the degree of maintenance and the area of a dam. The species distribution indicated an increase of flow velocity from the top to the bottom of the dams. Thereby, the flow gradient was higher in maintained dams than in abandoned ones. In terms of feeding types, shredders were most strongly represented. However, these were less common in middle and bottom areas, especially in maintained dams, where passive filter feeders predominated. In addition, next to the typical running water fauna, semi-aquatic taxa also colonized beaver dams, preferably in middle and bottom areas of abandoned dams. The results show that beaver dams offer an impressively wide range of environmental conditions and habitat types that promote a high biodiversity in streams and floodplains. Due to the sample size and the methods used, this study is one of the most detailed on this subject conducted to date. Therefore, this thesis provides new insights into the complexity of beaver engineering of aquatic ecosystems. In the context of the ongoing devastation of aquatic environments, the expansion of the beaver represents an outstanding potential for species and habitat conservation, restoration, maintenance and protection. In addition, the results may update the classic concept of the hydromorphology and invertebrate colonization of mountain streams, in which beaver activities have not yet been taken into account.