The paper concerns the transformation of water resources induced by the construction and functioning of the Brest Fortress defence structure and presents the current water resources resulting from these changes. The study was conducted by analysing historical materials: maps, plans and written documents. Hydrographic changes were analysed for five study periods covering almost 200 years, from 1823, presenting the hydrographic network before the construction of fortifications, up to 2018, when most of these structures ceased or were repurposed. Hydrographic changes were analysed in detail for the area of the Terespol Fortification. The analysis revealed that almost 80% of the wetland area had disappeared after intensive drainage works, and several dozen originally small and isolated areas had been incorporated into a vast drainage network. One of the consequences of these activities was the creation of significantly transformed artificial catchments within the study area.

The paper presents the characteristics of shallow groundwaters of the Polesie National Park – one of five national parks in Poland, where main subjects of protection are water-peatland ecosystems. Its functioning depends on the state of water resources. Data from field mapping at the turn of July and August 2013 used for the analysis show untypically high location of groundwaters in the summer period. Such a situation was caused by abundant atmospheric alimentation that occurred in spring and early summer of the analysed year.

The Włodawka River catchment of an area of 725 km2 covers the central and eastern part of the Łęczna-Włodawa Lake District. Evaluation of the role of hydrogenic areas in runoff creation was based on materials of the Department of Hydrography and the Institute of Meteorology and Water Management data. The analysis was conducted for selected catchments in which additional hydrometric measurements and water quality tests were done. Such parameters as: the share of hydrogenic surfaces in total catchment area, types of wetlands, their hypsometric location and position with reference to drainage streams were taken into consideration for evaluation. The degree of anthropogenic transformation of the marshland was expressed in terms of density and depth of the drainage ditches that dissect it. It was found that the drained gyttja of Krowie Bagno plays a considerable role in increasing the minimum discharge. Wetlands in the Włodawka River catchment influence the conditions of the runoff and water quality, which is noticeable, primarily, in the concentration of organic carbon, and of nitrogen and phosphorus compounds.