Karaj Dam reservoir is one of the main suppliers of Tehran's drinking water. The rate of sedimentation on the dam's reservoir is influenced by many factors including lithology, geological structures, drainage system, and morphology and landuse of the watershed area. Different map was drawn referring to the above named factors using the GIS system. The square mesh method was used to evaluate the weight of each mesh. Finally an erosion hazard map was drawn for the whole the watershed area. According to the zonation map, more than 33% of the area is susceptible to high erosion and about 28% is least erosion capability.