Dam construction is one of the measures that is inevitable in many cases and must be done to supply drinking water, agricultural uses and electricity generation. There are many challenges to a successful dam project, and the managers of each project must consider the appropriate solutions for them. One of the studies that is done in dam design is sedimentation in dam reservoirs. The experimental area-reduction method is a very common technique that obtains the sediment distribution in depth and longitudinal profile. This technique shows that sediment accumulation is not limited to the bottom reservoirs. Sediment accumulation in a reservoir is usually distributed below the top of the protection reservoir or normal water level. In this study, the distribution of sediment in the reservoir of Karun Dam after a period of 65 years has been done using the experimental area-reduction method. Elevation–volume and elevation–area curves of the dam reservoir are obtained after the useful life of the dam and sediment deposition. The results showed that after 65 years, 106.47·10 ⁶ m ³ of sediment is deposited in the reservoir of the dam and the useful volume of the reservoir is significantly reduced. Also, up to a height of 36.4 m, the dam reservoir is filled with sediment. Therefore, no valve should be placed up to this height.