Abstarct: Gotvande Dam, Khuzestan, Iran, is the last dam on the Karoon River prior to the plain of Khuzestan. The dam currently is not filled and water filled the coffer dam. The formations in the reservoir are Aghajari, Mishan, Gachsaran, and Bakhtyari and the present research aimed at the investigation on the effects of these formations in the dam reservoir on the water quality after the filling. To achieve this goal, water samples were taken from the surface as well as 1 and 2 m depth in upstream coffer dam in order to assess the physico-chemical properties of water prior to the construction of the dam. The evaluation was baxsed on the measurement of cations and anions and the ICP-MS tests on the water samples. Samples were also taken from the formations in the reservoir and their anions and cations were measured through two methods: saturated mud and 1:20 (soil:water) solution while the measurement of the elements and heavy mextals was also performed. The surface area of the formations at the elevations of 150, 200, and 250 m and the solubility of each were calculated to predict the most effective formation on the water quality after filling the reservoir. Among the existing formations, Gachsaran and Mishan showed the highest effect due to high solubility rate of lime, salt, and gypsum structures exist in the formations. The final ECs were predicted at 9115.6, 8658.4, and 1237.5 S/cm for the elevations of 150, 200, and 250 m respectively. By filling the dam and submerging the two formations, in the first year the water will be classified as high saline. Since the water in coffer dam is also chlororo-sodic, the effect of the formations on the water quality is confirmed. In elevations of 150 and 200 m, the water is predicted to be in C4S4 class which an indication of very high salinity but by reaching the elevation of 250 m, the class of water will be C3S2 or moderate to high salinity. Among the elements and heavy mextals measured in the reservoir formations, strontium had the highest concentration which is justifiable according to the high correlation of this element with carbonate and sulfate environments which are widely available in lime and gypsum exist in Gachsaran and Mishan formations. The concentrations of strontium in 150, 200, and 250 m predicted to be at 4.7, 3.8, and 1.8 mg/L due to gypsum solution and 4.2, 4.5, and 1.8 mg/L due to solution of lime.