Chemical composition of water and dissolved organic carbon (DOC) level were investigated over three years, along two large rivers in NE Poland, with different hydrological regimes. One river from lakeland and second typical lowland river with forests and high contribution of peatlands in the catchment. The study involved hydrological (specific runoff, river discharge) effects on the variability of water chemistry and DOC concentrations. Specific runoff and discharge influence on DOC level and water quality. DOC concentrations and chemical water parameters were highest in the upper part of lowland river and lowest in the upper course of lakeland river. The two times lower DOC concentration was observed in the river draining lakeland with high values of specific runoff than in lowland river. In typical lowland rivers organic compounds (mainly natural humic substances) intensified water eutrophication along the river course.