In this study, 25 physicochemical and heavy metal parameters were monitored in a total of 120 water samples collected monthly from 10 different sampling stations in the Büyükdere Basin, located in the Pınarbaşı district of Kastamonu Province, Turkey, between November 2022 and October 2023. The spatial and temporal variability of water quality and potential pollution sources were investigated, and the resulting data were compared with national (YSKY, 2016) and international (WHO, 2022) standards. Factor Analysis/Principal Component Analysis (FA/PCA) identified five main factors accounting for 85.51% of the total variance, revealing that the dominant pollution load in the river originated from agricultural activities and geogenic processes. Seasonal Water Quality Index (WQI) values ranged from 35.42 to 46.03, generally indicating an "excellent" water quality status. However, a gradual increase in WQI values was observed along the downstream flow direction, with Station 10 declining to the "good" quality category with a WQI value of 50.55. Regarding irrigation water suitability, Büyükdere surface waters were classified as "excellent" for Sodium Adsorption Ratio (SAR), "permissible" for Sodium Percentage (%Na), and "suitable" for Kelley's Ratio (KR), whereas they were categorized as "unsuitable" in terms of Magnesium Hazard (MH). Heavy Metal Pollution Index (HPI: 0.01) and Heavy Metal Evaluation Index (HEI: 0.36) values demonstrated that heavy metal toxicity remains extremely low. Although no severe pollution problem currently exists, statistically significant increases in parameter values toward the downstream direction indicate an ascending anthropogenic pressure on the basin. Therefore, implementing long-term monitoring programs and effective water resource management strategies across the river basin is of vital importance