<p>One of the most important surface water sources in the districts of Gebze and Dilovası (Kocaeli, Turkey) is the Dil Stream and its tributaries. Today, while part of the Dil Stream has high-quality water, the southern section, which is open to settlement, has become low-quality water due to the controlled/uncontrolled discharge of agricultural, domestic, and industrial wastewater. Based on its characteristics and the QUAL2K model structure, the Dil Stream has been divided into a total of 11 reaches, including its tributaries. Samples were collected over 12 months from 28 sampling points in these 11 reaches. Model calibration was performed using April data and model validation was performed using November data, for the parameters pH, carbonaceous biological oxygen demand (CBOD), dissolved oxygen (DO) and total kjeldahl nitrogen (TKN). During the calibration, the R² values were calculated as 0.72 for DO, 0.98 for CBOD, 0.96 for TKN and 0.92 for pH, while during the validation, they were calculated as 0.81 for DO, 0.92 for CBOD, 0.97 for TKN and 0.91 for pH. The RMSE and MAE values applied to evaluate model performance, in the calibration values were 0.99 and 0.80 for DO, 0.91 and 0.88 for CBOD, TKN 1.20 and 1.03, and pH 0.73 and 0.66. In the validation, the values were calculated as 1.22 and 1.09 for DO, 1.82 and 1.51 for CBOD, 1.80 and 1.63 for TKN and 0.40 and 0.39 for pH. The QUAL2K model, applied for the first time in Dil Stream, will provide a basis for water and environmental management in future decision-making processes for different scenarios.</p>