This study monitored the physicochemical and microbiological quality of six rivers in the coastal Oueds Basin between Casablanca and Rabat, Morocco, to assess seasonal variations in water quality, identify pollution sources, and evaluate the efficacy of water quality indices (CCME-WQI and WGQI) in a Moroccan context. Water quality was assessed during the dry and wet seasons (March and September 2024) using fifteen parameters: Temperature, pH, EC, Turbidity, DO, COD, BOD5, TP, NH4+, TKN, TSS, NO3-, Cl-, SO42- and FC. Water quality was classified as "poor" to "very poor" across all stations. The dry season showed particularly severe organic pollution (BOD₅ reaching 58.04 mg/L; COD up to 222.58 mg/L) and fecal contamination, primarily from untreated domestic wastewater and industrial discharges. Elevated ionic concentrations (Cl⁻ 4133 mg/L; SO₄²⁻ 255.3 mg/L) indicated significant agricultural and industrial runoff impacts. Intermittent streams exhibited extreme sediment loads (TSS >600 mg/L) following seasonal rewetting. This study presents the first comparative analysis of water quality index (WQI) performance in one of Morocco’s most heavily polluted downstream reaches. The findings contribute to this growing body of research using WQI, while awaiting the development of a Morocco-specific WQI framework that considers hydro-climatic variability and anthropogenic pressures.