Deteriorating water quality has become a global concern. Chungju Reservoir is the largest reservoir in South Korea. Here we describe the long-term spatiotemporal variabilities and prevailing trends in physicochemical and nutrient contributing factors using datasets of 1992-2016. Seasonal, annual, multivariate analytical techniques, and water pollution index (WPI) investigations revealed monsoon rainfall events as major influential element in fluctuating water quality. Total nitrogen (TN) and total phosphorus (TP) concentration was largely affected by the intensity of monsoon rainfall along the longitudinal gradient. Seasonal and inter-annual analyses indicated the reservoir system as being N-rich and strongly P-limited. They also indicated progressive escalation of pH, chemical oxygen demand (COD), TN, and TP – clarifying greater industrial and agricultural runoff. The transition zone (Tz) was crucial for leaching of suspended solids pointing toward an imminent siltation problem. Mann Kendal test confirmed a significantly increasing trend in pH, COD, TN, TP, and ammonium. Principal component analysis disclosed that water quality deterioration cannot be ascribed to a single point source but instead diffused sources. The trophic state index exhibited deviation from larger particles to severe zooplankton grazing as an explicit sign of oligo-mesotrophic state, leading to increased Secchi disk depth. WPI categorize the chemical health status as ‘good.’ Overall, it could be inferred that water quality was good but may face an impending degradation plus increasing nutrients and chemical loads.