Spatial and temporal assessment of water quality in the Gorganrud River (Golestan Province), one of the most critical hydrological systems in northern Iran, was comprehensively conducted across 14 sampling stations over four distinct seasons. To precisely evaluate water quality, 12 physical, chemical, and microbiological parameters—including temperature, dissolved oxygen (DO), Biochemical Oxygen Demand (BOD5), Chemical Oxygen Demand (COD), nitrate, phosphate, electrical conductivity (EC), Total Dissolved Solid (TDS), turbidity, pH, ammonium, and fecal coliform (FC)—were measured. Laboratory data were analyzed using three standard indices, namely Iranian Water Quality Index for surface water (IRWQIsc), National Sanitation Foundation Water Quality Index (NSFWQI), and Canadian Council of Ministers of the Environment Water Quality Index (CCMEWQI). Additionally, advanced multivariate statistical methods, including Pearson correlation matrix, Principal Component Analysis (PCA), and Hierarchical Cluster Analysis (HCA), were applied for pollutant source identification and data dimensionality reduction.

Results indicated that river water quality is strongly governed by seasonal variations and anthropogenic activities, such as municipal wastewater discharge and agricultural runoff. Warm seasons experience severe dissolved oxygen depletion and the dominance of point-source pollution, whereas during cold and rainy seasons, surface runoff plays a dominant role in increasing turbidity and suspended solids. Evaluation of quality indices showed that the overall water quality falls within moderate to poor classes, facing serious environmental challenges at certain stations due to high nutrient concentrations and eutrophication, highlighting the urgent necessity for dynamic management and pollution control.