The paleoecological research of biogenie sediments sampled in the different lake-mire ecosystems of the ŁęcznaWłodawa Lake District were the basis of studies on hydrological changes during the Late Glacial and Holocene. The lithological differentiation (spatial and temporal) of lacustrine and mire sediments in the studied sites indicates that the hydrological changes were of local nature. The investigations also evidence a specific functioning of the lakes in this region. Lake basins were formed as a result of the ground ice degradation and the transformation of groundwater circulation in the Late Glacial. The total area of lakes in the Lake District was the largest from the Younger Dryas to the Subboreal chronozone. The gradual lowering ofwater level during the early Holocene resulted in the transformation of sedimentation process, and in the development of typologically differentiated mires and lacustrinc-mire complexes