The present study aimed to assess the water quality of the Khosr and Tigris rivers for aquatic life by applying two mathematical models: the model proposed by Rodriguez de Bassoon and the Canadian model (CCMEWQI). Physical, chemical, and biological properties were estimated to calculate the values for these models. The results indicate that the Khosr River has poor water quality for aquatic life, with Canadian model values ranging between 30.9 and 41.5. In contrast, the Tigris River water at the comparison site was classified under the Marginal water category, with quality further deteriorating after the Khosr River merges with it, showing values between 52.7 and 40.2. Similarly, using Rodriguez de Bassoon's model, the Khosr River was classified as having Bad water quality with values ranging between 26.35 and 35.96, while the Tigris River exhibited Medium water quality with values between 54.81 and 64.33. The deterioration in water quality was mainly attributed to a significant reduction in oxygen levels, reaching as low as 1.2mg/ L, coupled with an increase in organic load to 56mg/ L. This reflects a considerable degradation of the Khosr River, which negatively impacts the water quality of the Tigris River within the Mosul City area.