Water is considered one of the most important natural resources on Earth. Rivers are the main source of freshwater in many countries around the world. However, these rivers are exposed to pollution due to several factors, the most significant of which is the direct discharge of partially treated or untreated municipal wastewater into receiving rivers. This leads to an increase in the river Carbonaceous Biochemical Oxygen Demand (CBOD), which in turn causes a decrease in Dissolved Oxygen (DO) levels. In this study, the Streeter-Phelps model was used to evaluate the impact of Al-Muamirah Wastewater Treatment Plant on the water quality of the Hilla River in central region of Iraq and to monitor the variation in CBOD and DO values over a certain distance from the wastewater discharge point. The results indicated that CBOD values was the highest at the point where the wastewater was released, leading to a direct decrease in DO levels. Subsequently, the river experienced a self-purification process that restored oxygen levels to its background value. However, the high CBOD values caused that oxygen concentrations in some distances have fallen below the permissible limit, with the lowest dissolved oxygen concentration of 4.7 mg/L recorded at 20 km from the wastewater discharge point. The results also showed that the highest acceptable 5-day CBOD value in wastewater that can be discharged into the river, ensuring that dissolved oxygen is greater than its minimum permissible value at a distance of about 15 Km, is 8.4 mg/L. Accordingly, the study recommends continuous monitoring of CBOD levels in treated wastewater, and implementing additional treatments, when necessary, to ensure the preservation of river quality and prevent any adverse impacts on its environmental condition.