The objective of this study is to assess the diversity of benthic macro invertebrates. And the Physic-chemical and biological quality of the waters of OuedDrader. the main watercourse of the Drader-Souierbasin. It is located in northwest Morocco. The results show the physicochemical parameters reveal that the upper course is not polluted. While the middle and lower parts show a high concentration of nitrates during certain periods of the year. This temporary pollution is explained by the contamination of the water of the river by the residues of the chemical fertilizers spread in the catchment area. The values of the organic pollution parameters also show a gradual deterioration from upstream to downstream of water quality. The calculated values of the Chemical Water Quality Index show that, along the river. The water is of good quality during the winter period. Which corresponds to the period of heavy rainfall. For other times of the year. The water becomes polluted. The decrease in flow and the concentration of certain pollutants under the effect of water evaporation are the main causes. The Shannon and Weaver index (H') shows that during the two sampling periods. The diversity of benthic macro-invertebrates is high during spring and early summer, and low during the winter period. And the highest values of the BWI show that in spring and at the beginning of summer upstream and downstream. The waters are of good quality compared to the average river. This difference could be a consequence of the clayey nature of the substrate of the middle part of the river. Note also that the comparative study of the results of the two study campaigns showed results showed only a slight difference that can be attributed to annual variations in climatic conditions.