<p>The chemistry of the Stara River changes along its course due to both natural processes and human activities. Its chemical composition is influenced by tributary inflows, groundwater infiltration, and anthropogenic pollution. Particularly noticeable are sudden changes in the concentrations of nitrogen and phosphorus compounds, as well as Na⁺ and Cl⁻ ions, indicating a strong impact of human activities, especially in agricultural and urbanized areas. Although no significant trends in total precipitation have been recorded, higher temperatures lead to increased evaporation, which results in higher ion concentrations. Climate change accelerates rock weathering, leading to increased ion release, particularly HCO₃⁻ and Ca²⁺. Land use changes significantly impact water quality. The expansion of urbanized areas leads to water quality deterioration, especially through increased concentrations of nitrogen, phosphorus, and chlorides. In contrast, an increase in forested areas contributes to water quality improvement by reducing erosion and retaining pollutants. In agricultural regions, a decrease in SO₄²⁻ concentrations is observed due to reduced fertilizer use. PCA analysis indicates that hydrometeorological conditions are the most important factor shaping water chemistry. Land use also plays a significant role – urban areas increase the biogenic load, while agriculture influences NH₄⁺, NO₂⁻, and PO₄³⁻ concentrations.</p>