This study applied a one-dimensional water quality model using HEC-RAS and RAS Mapper to assess the pollution status of the Asa River in Ilorin, Nigeria. Field samples were collected at four stations during both dry and wet seasons and analyzed for physicochemical and nutrient parameters. Hydrodynamic simulation revealed that the river depth ranged from 0.32 m downstream to 9.80 m upstream, with velocities between 0.06 and 2.68 m/s. Water quality modeling indicated persistent low dissolved oxygen (3.9 - 5.2 mg/L) across stations, below the 6.5 mg/L ecological threshold, suggesting oxygen stress. Nutrients showed seasonal variability, with nitrates declining in the wet season due to dilution, while nitrite (0.206 - 0.274 mg/L) and organic phosphorus (>1.0 mg/L) exceeded international guidelines, indicating a risk of eutrophication. Model calibration showed good agreement between observed and simulated values, with minor deviations in organic nitrogen and phosphorus. The results show that the Asa River is unsuitable for domestic use without treatment and emphasize the need for stricter effluent control and sustainable river management.