The objective of the paper was the hydrological analysis, in terms of categorizing main watercourses (based on coupled catchments) and marking areas covered by potential impact of the occurrence and activities of the European beaver Castor fi ber . At the analysed area – the Forest District Głogów Małopolski there is a population of about 200 beavers in that Forest District. Damage inflicted by beavers was detected on 33.0 ha of the Forest District, while in the area of 13.9 ha the damage was small (below 10%). The monitoring of the beavers’ behaviour and the analysis of their influence on hydrology of the area became an important element of using geoinformationtools in the management of forest areas. ArcHydro ArcGIS Esri module was applied, as an integrated set of tools for hydrographical analysis and modelling. Further steps of the procedure are hydrologic analyses such as: marking river networks on the DTM, filling holes, making maps of the flow direction, making the map of the accumulation flow, defining and segmentation of streams, marking elementary basins, marking coupled basins, making dams in the places, where beavers occur and localization of the area with a visible impact of damming. The result of the study includes maps prepared for the Forest District: the map of main rivers and their basins, categories of watercourses and compartments particularly threatened by beaver’s foraging.

The research was carried out on a small (52.8 ha) and shallow (4 m) Lake Track in Olsztyn, exposed to a considerably intense anthropogenic pressure. In the middle 1800s this reservoir was dried out and most of the contiguous land was designated for agricultural purposes. The lake was restored in the mid 1900s. At present, the drainage basin of Lake Track is 216 ha. Urban land comprises the largest portion of this area, i.e. 49.3%, barren land 41.4% and forests 6. I%. The lake receives storm waters but for years it had also received sanitary sewage. The results of this research allowed classifying the lake as nutrient-rich, with fairly advanced eutrophication processes. The waters were characteristic of very high nutrients content, up to 0.75 mg P/dm3 and 3.87 mg N/dm3. The high fertility of Lake Track was additionally confirmed by high BOD, values, i.e. up to 9.5 mg 0/dm3, high chlorophyll content, usually from 30 to 40 mg/m3 but reaching 123 mg/m3, and low water transparency, oscillating between 0.6 and 0.9 m. The reason for the lake's high trophic level was no doubt the excessive loading from the drainage basin. The actual nutrient runoff from the drainage basin to the lake exceeded the critical loads, as defined by Vollenweider. Low quality of the lake's waters and the parallel high external loading indicate that preventive measures should be taken, aimed at reducing the external loading.

Flooding in the northern part of The Netherlands has caused serious economic threats to densely populated areas. Therefore a project has been carried out in a pilot area to assess the retention of water in two river basins as a way to reduce flooding. The physically-based groundwater and sur-face water model SIMGRO was used to model the hydrology of the basins. The model was calibrated using discharges and groundwater levels. Scenarios of measures to assess the possibility of retaining water in the basin were then defined and tested. The first measure was the retention of higher dis-charges using culverts or gates in the upstream part of the basin. The second measure was to make the streams shallower and thereby, increase flood plain storage. The last measure was flood water storage in a designated area in the downstream part of one basin. The analysis indicates that holding water in the upstream parts of the basins proved to be feasible and can result in significant reductions of peak flows.