This paper presents results of three years of geochemical research carried out in the upper Łyna catchment in NE Poland. Water in the system is of Ca²⁺ - HCO₃^- - SO₄^2- type and reflects mainly the impact of chemical weathering of bedrock. The results indicate that natural components such as Ca²⁺ and HCO₃^-, derived from mineral weathering, dominate chemical composition of the Łyna River system, reaching 90%. However, agricultural input is clearly visible for ions such as Na and K. Under natural conditions their content usually reaches 4.3 and 1.2, respectively, while in the agriculture-dominated subcatchments in the study area the proportion of Na⁺ and K⁺ reached 8.3 and 4.1 mg L^-1.
Besides land use pattern, the main factors modifying riverine transport of the dissolved substances along watercourses are open lakes. They play an important role as sinks for the ions in agricultural areas. The highest decrease in concentration was stated for K⁺ by 50%, Na⁺ by 36%, SO₄^2- by 25%, and Ca²⁺ by 20%. Retention of solutes in lakes is highly dependent on the location within the river catchment and water table area. The largest reservoirs in the region are capable for the retention of 8-12% of the river input. Based on changes in water quality parameters, the Łyna River profile was divided into three distinct zones: headwater zone, middle zone with lakes and lower zone with anthropogenic influences.