The watershed of Wadi Larbaa, in the Prerif area at the north of Taza, is a fragile and vulnerable milieu, where water erosion is causing annually important loss of land (soils) and excessive production of sediments. The structural setting is characterized by the predominance of soft materials, mainly marl, in a turbulent structure. The fragility of materials and their susceptibility to erosion are all the more important as most land is bare and exposed to direct morphogenic effects of rainfall events. The human pressure is spectacular in the area. The ancient settlement and early development of the land, based mainly on cereals and livestock have destroyed natural vegetation. This inherent vulnerability of the environment is exacerbated by a marked concentration of extreme weather events causing in time various erosion processes. Gully erosion is very active and it is responsible for most of the sediment transfer in the region. Landslides are very diverse. Their later evolution by gulling processes, evacuates large quantities of sediments from moving masses. Soil loss is consequently important; quantification tests show that the average loss in soil clearly exceeds the tolerance thresholds known in the Rif Mountains. In the case of the Oued Tleta sub-basin (situated in the upper part of Wadi Larbâa), the weighted average loss of the surface is of 61.42 t / ha / year. Maximum losses according to the land use are estimated at 129.12 t / ha / year as recorded level on wastelands and grazing lands. This study presents the main results obtained by means of field measurements and the quantification of erosion in this catchment area. It includes the factors involved in hydrous erosion in the studied area, in order to identify the main processes acting on slopes.