Biomonitoring is a key solution for assessing the effects and risks of pollutants to preserve the most vulnerable ecosystems, including aquatic ecosystems. This study aims in establishing the gastropod Lymnaea stagnalis, as a sentinel species to assess the water quality of the El-Malah river in the Algerian North-West. Three sites were chosen along the river: upstream (US), midstream (MS), and downstream (DS). The responsiveness of the aquatic snails has been compared using physiological and biological markers: condition index (CI), volumetric condition index (VCI), acetylcholinesterase (AChE), glutathione s-transferase (GST), and catalase (CAT). Additionally, the occurrence of changes in biometric parameters of the specimens has also been treated: shell height (SH), shell thickness (ST) total weight (TW), and the ratio ST/SH. Snails from the site MS reacted in front to the water deterioration with low biometric values (ST 1.28 ± 0.17 cm; SH 1.83 ± 0.20 cm; TW 2.95 ± 0.69 cm), and condition indices values (CI 31.19 ± 3.58; VCI 2.09 ± 0.53 g.cm-3), thereby signaling smaller individuals compared to the population of site US. Whereas, no specimen was recorded in the site submitted to wastewaters discharge (DS). This indicates that the degradation of the water quality affected the growth and the viability of the snails. Furthermore, a significant induction in the GST activity (88.98 ± 10.72 nmol min-1mg-1), and a significant inhibition in the CAT activity (82.85 ± 9.49 μmol min-1mg-1) were recorded in the site MS, whereas no statistically significant variation was observed in AChE activity. L. stagnalis demonstrated biological and physiological variability between the studied contrasting sites of the El-Malah River. These results allow us to propose this species as a model in the ecotoxicology of western Algerian freshwaters.