The present study focuses on the assessment of the biological quality of Drader river waters, the main watercourse of the Drader-Souier basin located in northwestern Morocco. The study focused on carrying out the benthic macro invertebrate inventory from three stations and the calculation of biotic indices: Shannon and Weaver diversity index and the Belgian Biotic Index. The analysis of the results makes it possible to distinguish:

• An upper (temporary) watercourse, characterized by a rich and diversified fauna attesting to the excellent quality of the water of this section.
• Permanent middle and lower watercourses with a significant decrease in biodiversity that can be attributed to intensive agricultural activity in this area and the nature of the substrate and habitat.