The Oum Errabia River is a vital aquatic ecosystem that supports both biodiversity and human activities in the region. However, it is increasingly threatened by pollution, habitat degradation, and the effects of climate change. One of the primary contributors to this degradation is the discharge of domestic wastewater, which significantly impacts aquatic fauna. This study focused on benthic communities to assess water quality through faunal analysis at three sampling stations, ranging from upstream to downstream, including the urban area of Khénifra. A total of 52 different taxa were identified, with insects being the most dominant group, followed by crustaceans, mollusks, and annelids. The upstream station (S1) exhibited high species diversity (H' > 2) and a strong biological quality index for macroinvertebrates (IBGN), indicating good water quality. In contrast, station S2—located in the area receiving untreated domestic wastewater—showed low diversity (H' < 2) and a poor IBGN score (< 5). This station was dominated by Chironomidae, a group known for its tolerance to pollution, and showed a marked absence of sensitive taxa such as Ephemeroptera, Plecoptera, and Trichoptera, indicating degraded water quality. Downstream at station S3, water quality was classified as moderate, suggesting some degree of natural self-purification despite the upstream pollution. Although biodiversity at S3 was higher than at S2, it remained lower than that recorded at the upstream station (S1). These results underscore the significant impact of human activities—particularly wastewater discharge—on the ecological integrity of the Oum Errabia River. The findings provide valuable data to support conservation and restoration efforts aimed at preserving this critical ecosystem.