The Gulf of Gorgan is the largest Gulf in the Caspian Sea. It has been created due to the expansion of Mian Kaleh peninsula in southeast of the Caspian Sea. The Gulf of Gorgan covers an area of about 400 km. and its length is about 70 km. in an east-west direction. Its maximum depth is 4 meters and this depth increases from west to east till the southern side of Ashouradeh peninsula. In the western part its salinity is more than salinity of the Caspian Sea because of its reducing depth and increase of evaporation. Due to the presence of sweet water of the gushing rivers like Qarah Soo, the salinity of water is reduced and the water becomes muddier. Generally the water entrance capacity is less than its exit and its water shortage is supplemented by the Caspian Sea.