Urbański, JanuszJadczyszyn, JanOleszczuk, RyszardZając, EwelinaBajkowski, SławomirBrandyk, AndrzejPokładek, Ryszard2026-10-022026-10-022026Sustainability 2026, 18, 76952071-105010.3390/su18157695https://bc.iung.pl/handle/123456789/4814https://www.mdpi.com/2071-1050/18/15/7695To protect and sustainably use peat soils, it is important to maintain their high moisture content by keeping proper groundwater levels. This can be achieved by using existing water-retaining structures in their area, such as weirs and sluices. Beaver dams can also serve a similar function, as they block water outflow from peatlands, raising water levels and naturally retaining it. The water conditions created by beaver dams promote secondary environmental succession processes, leading to the sustainable use of environmental resources. The presented research focused on drainage–irrigation perspectives for a lowland fen in the face of hydrologic alterations and a search for proper organic soil and water management. The results of systematic observations of water level variability in the Mała riverbed were analysed, as well as in a selected area of the subsurface irrigation system, influenced by the presence of a beaver dam in the years 2015–2019. The research was conducted on a section of that system, used as permanent grassland in central Poland—the Solec site. Before the construction of the beaver dam, measurement results indicated the draining nature of the Mała River, supported by an extensive network of drainage ditches. After the construction of the beaver dam, water levels in the Mała River bed above the dam increased, as did groundwater levels in the adjacent area, up to 40 m from the riverbank. In the analysed site, the beaver dam caused the groundwater level to increase by 0.25–0.45 m in 2015, compared to the neighbouring quarter, located outside the range of the dam. At observation points A and B, located 40 m and 115 m from the river, respectively, the groundwater table was usually within the acceptable drainage standards for organic soils (complex C—dry), while in the case of point C, located 240 m from the river (medium mineral soil complex), the water table was mostly below the maximum drainage standard. The conducted groundwater table depth estimations were purposely compared to the existing standards to further outline the impact of selected measures, e.g., the bever dams, to maintain hydrologic conditions of a managed fen.enriver valley, mineral and organic soils, drainage system, sustainable environmental developmentArticle