To evaluate water quality conditions, Water Quality Indices (WQIs) are considered efficient and appropriate tools. The present study aims to assess the water quality of the Shahrchai and Nazluchai rivers, located west of Lake Urmia, Iran, using the Iranian Surface Water Quality Index (IRWQI_sc). In this study, samples were collected from 6 stations along the Shahrchay River and 5 stations along the Nazluchai River during the years 2017–2022 .Water quality parameters were measured and evaluated for the calculation of the IRWQI index. Subsequently, to investigate the changes in water quality along the rivers, the IRWQI values were compared over the six-year period. Based on the results, in the Shahrchai River, the average IRWQI values in Keshtiban village and behind the University of Technology were lower than those at other stations, indicating lower water quality. In contrast, the highest water quality was recorded at the Barde Sur village station. In the Nazluchai River, the average water quality was lower at the river’s inlet to the lake and in Tabuk village. Notably, in 2022, the station located downstream of the Nazlu Dam experienced a sudden decline in water quality, reaching a relatively poor category. The results of Pearson correlation analysis between the IRWQI and water quality parameters in the Shahrchai River show a positive and significant correlation with Dissolved Oxygen at the 95% confidence level (P ≤ 0.05), and a negative and significant correlation with Biochemical Oxygen Demand, Chemical Oxygen Demand, and pH at the 99% level. Additionally, the correlations with Total Coliform and Electrical Conductivity were negative and significant at the 95% level. In the Nazluchai River, the correlation between the quality index and BOD and COD was significantly negative at the 99% level, while the correlation with Total Coliform and Phosphate (PO₄³⁻) was significantly negative at the 95% level.