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International audience; The eastern part of Morocco, surrounded by the High Atlas chain close to Errachidia, represents a very sensitive area in a semi-arid context. Average annual rainfall does not exceed 300 mm/year. Most of the regional water resources are linked to the aquifer systems in the Atlas area. The study site coincides with the Eastern Upper Guir Basin, one of the main rivers in Eastern Morocco. This basin is located close to the eastern border of the Moroccan High Atlas. The geology of the basin consists mainly of Jurassic and Cretaceous formations. The structure of the basin is strongly affected by the Atlasian tectonics. This results in successive large synclines separated by major WE faults. The hydrogeological system of the basin is accordingly constituted by sub-basins associated with each syncline and separated by faulted anticlines, whose overlapping faults act as semi-impermeable barriers. This compartmentalization causes the very complex functioning of the whole system. The study undertaken, based on structural, meteorological, hydrogeological, hydrochemical and isotopic data, allowed us to advance significantly in the understanding of this system. The compartments behave as hydrogeological sub-units but remain interdependent on each other. The recharge of the aquifer system is essentially associated with the rains and snowfalls at high altitudes. This is consistent with the stable isotope results. Important development projects are planned in this region, associated with a drastic increase in water demands. Groundwater suitability was assessed using the WHO standards for drinking purposes and the SAR (sodium adsorption ratio) for irrigation. Complementary investigations should be considered to further the results presented here and move towards a quantitative assessment and management of the water resources.