Abstarct: Sedimentation and its deposition due to the reduction of the reservoir active capacity, instability of dam and economic losses is one of the issues that has always been investigated by researchers and engineers. In addition, seepage in earth dam is an essential and important issue regarding the high hydraulic gradient, porosity of the body of the dam foundation and also economic issues. In order to prevent water seepage from earth dam foundation, the area below the body of the dam is sealed or almost sealed in different ways which although it reduces a large amount of foundation seepage, it imposes significant costs which sometimes does not seem economical. The reduction of the permeability of the reservoir floor due to the gradual increase of sediments is scientifically-experimentally proven. This means that sediments deposited in the reservoir, gradually accumulate on each other and while increasing the density of the previous sediments, they affect the thickness of sedimentation and decrease the permeability of the reservoir floor, so that it can be expected that the permeability of the reservoir floor will be very limited practically after a few years, which means no other foundation sealant systems will have a role in reducing the water seepage from the dam. In this research, modeling was done by seep/w for the Ardak earth dam baxsed on the sediments deposited in each semester, for the first 5 years of operation of the dam, and an analysis was carried out for three modes namely presence of a cutoff wall with sediment, a cutoff wall without sediment, and sediment without a cutoff wall. Then they were compared with each other. Analyzing and measuring the amount of seepage in two states, the presence of a cutoff wall with sediment and the presence of a cutoff wall without sediment, it was found that the deposition of sediments behind the dam caused 58% reduction of seepage in a state of a cutoff wall with sediment compared to state without sediment. Furthermore, considering the results obtained from the software, the costs of building a cutoff wall and the amount of water seepage from the dam foundation, assuming the loss of all water leaked from the foundation in the state of without a cutoff wall with sediment, water rate of 1000 tomans per cubic meter and the comparison of costs with each other, it was determined that around 4.918 billion tomans is saved by removing the cutoff wall.