The Dez River originates from the Cezar and Bakhtiari tributaries in the Zagros Mountains, in southwestern Iran. Bakhtiari River has more sediment annually than the Cezar tributary. The Dam was built on the Dez River in 1960 for electricity generation and agricultural purposes, with a volume of 3,300,000,000m3. After 65 years, the dam lake was occupied by layers of accumulated sediment with a volume of nearly 1,000,000,000m3. The study of the density current reveals a new theory that can be interpreted mathematically by the Control Volume -CV- and Momentum Differential Equation -MDE-. TSI can investigate the complex behavior of the density current in the catchment. It can also prove why the useful volume of the reservoir has suddenly decreased in the first decade. It can be concluded that the predicted dead volume will occupy the lake before one third of the operational life of the dam. It is necessary to solve the MDE for the density current in the basin. A longitudinal profile should be drawn at least every five years through the lake hydrography to show the reservoir delta. In fact, the delta sedimentation is the total sediment displacement during one third of the nominal life of the dam. By solving the MDE, parameters such as the annual sedimentation rate behind the dam and the flood periods will be used as auxiliary indicators. A matrix of possible data is generated using the TSI and the hydrological characteristics, then the best answers are selected for use in the density current.