Abstarct: Abstract Karun River is the longest river in Iran that flows into the Persian Gulf. One of the most important factors and challenges affecting the quality of the water resources of the Karun River are the branches and channels of natural salt. The natural tidal branches of the Karun route include: Shurandika, Shur Lali, Shurband Qir and Dasht-e-Bozarg. Upper Gotvand Dam is the largest gravel dam in the country and the last dam on the Karun River in the north of Khuzestan province. The presence of salt gypsum formations in the lake of this dam causes the water entering the lake to increase. Several reports have been presented regarding the role of Gotvand Dam in the salinization of Karun River. Those who know some studies that the cause of the Karun River are related to other sources of pollution. In order to determine water quality, 17 stations including 5 stations upstream and 12 stations downstream of the dam were selected for sampling. The results of the analysis of the samples showed that the quality of the water coming out of the Gotvand Dam is in the C1S1 group and is very suitable for agricultural purposes. The output from the dam lake is significantly better compared to the water quality of the nearby plains (especially the Mianab Shushtar plain and the northern Ahvaz plain). Comparing the quality of the water coming out of the dam lake with the water of Karun in the north of Ahvaz shows an increase of more than two times the salinity of the river water, which is due to the input of the branches with higher salinity (such as Dasht-e-Bozarg branch) and underground waters with higher relative salinity. It is the Karun River.