Dams are known as the one of the most important water supplier and it is essential to maintain its water quality. Erosional instability of the slopes is a gradual phenomenon and threatens the water quality of the dam reservoirs. In physical type, erosion occurs due to poor adhesion between particles and grain separated and in chemical erosion, the chemical composition is the main cause of erosion which is activated in contact with water and particles undergo chemical dispersion. Darbandikhan dam is located in southeast of Kurdistan region of Iraq and its reservoir with length of more than 20 km is located on various geological formations. West and northwest slope of reservoir is composed of cyclic deposit of sandstone, siltstone and claystone which has erosional instability and has led to form badlands. In this study, by sampling different units, the cause of erodibility of units was investigated. Petrographic studies and SEM and EDX showed that sandstone has an undeveloped matrix of clay and carbonate and in many cases the grains are in contact with each other so they do not have the necessary adhesion. chemical tests showed that most of samples havedispersion potential and Crumb and hydrometric experiments showed that most of the samples have dispersion potential.
