Abstract. Hydrotechnical activities on the Słupia river above the town of Słupsk (northern Poland) were performed against floods in the valley. The results based on the data on fauna and flora gathered before (in 2003, T+0) and after (in T+2, T+6 and T+11) the reclamation showed that the river canalization and oxbow lake dredging can significantly disturb the ecology of water bodies adjacent to the river. Observations of the oxbow lake (OLS5), showed the decrease in the number of macrophyte phytocenoses from 11 before the dredging to 6 very valuable Potametea phytocenoses, after wards. The development of common pleustonic phytocenoses of Lemnetea minoris and the massive development of green algae resulted from the bottom sediment removal. In the case of population, Stratiotes aloides was rebuilt after elimination during dredging already in T+2. Before hydrotechnical works, the oxbow lake was characterized by 12 invertebrate taxa inhabiting S.aloides, whereas only 9 were found first period after the dredging, 15 taxa in second period and 23 in a recent study. During both observation periods, dominated by Chironomidae larvae (T+0), Trichoptera larvae (T+2 and T+6) and again Chironomidae larvae (T+11). The hydrotechnical activities significantly altered in the first period the structure of fauna and flora inhabiting the water body. Such radical interference in natural values of oxbow lakes is un-suitable for the preservation of ecosystems due to the significant decrease in their biodiversity. Achieved on a similar or higher level of macrophytes diversity and epiphytic invertebrates was noticeable ten years after the dredging of a water body.