Understanding how untreated wastewater degrades an intermittently flowing Mediterranean river requires repeated sampling across contrasting hydrological periods and cautious statistical interpretation. Ten stations in the Oued Larbaa catchment, which receives untreated wastewater from Taza City, were sampled during the high-water season in January 2025 (n = 10) and the low-water season in July 2025 (n = 7; the three most upstream stations were dry). At the seven stations sampled in both seasons, pollution-related parameters increased significantly during the low-water period: TSS, TKN, PO₄, BOD₅ and COD all showed significant seasonal differences according to the paired, two-sided Wilcoxon signed-rank test (p = 0.016). Mean BOD₅ increased from 177.9 to 547.6 mg O₂/L, while COD increased from 299.1 to 1,153.7 mg O₂/L, corresponding to increases of 208% and 286%, respectively. Dissolved oxygen decreased significantly, whereas electrical conductivity showed no significant seasonal change (1493.1 versus 1828.1 µS/cm; p = 0.109). A principal component analysis of the nine field and laboratory determinands showed that the first principal component accounted for 79.2% of the total variance, while the first two components together explained 92.6%. The main axis represented an organic-pollution gradient separating naturally mineralised headwaters from sewage-dominated low-water samples. Ward hierarchical clustering recovered the same three descriptive groups. Contrary to a simple upstream-to-downstream deterioration model, TSS and COD decreased significantly with downstream distance from the principal discharge point during low water (Spearman’s rho = −0.79, p = 0.036 for both; n = 7). This pattern indicates downstream attenuation of concentrations, but remains exploratory. A weighted arithmetic pollution index based on Moroccan discharge-limit values classified every summer station and one winter station as poor, whereas all winter headwater stations were classified as excellent. Total Kjeldahl nitrogen and suspended solids contributed most to the index score despite not having the largest nominal weights. Because water discharge, pollutant loads and transformation processes were not measured, the observed concentration decrease cannot be attributed specifically to self-purification. Overall, the results indicate that the Oued Larbaa functions for much of the year as an open wastewater-receiving channel whose quality is strongly controlled by discharge timing, with only modest downstream attenuation of pollutant concentrations.