One of problems that affects the hydro-installation and reduces the useful life of dams, is sedimentation in the reservoir, which is unavoidable. In estimating reservoir sedimentation and accumulation, a number of uncertainties arise. These are related to the quantity of streamflow, sediment load, sediment particle size, and specific weight, trap efficiency, and reservoir operation. To evaluate suspended sediments and bed load in some hydrometric stations, separating the field data into wet and dry time periods and total time periods are used. Harr’s method is used to quantify the uncertainty of accumulated reservoir sedimentation through time. To examine the importance of various factors on the uncertainty of accumulated reservoir sedimentation, sensitivity analysis was conducted. In this study, the effect of each uncertain factor, on the uncertainty of accumulated reservoir sedimentation through time has been examined for Shahar Chai Dam in northwestern Iran. The results show that in Harr’s method, the uncertainty of accumulated reservoir sediment volume is 0.384 in total time periods and 0.244 for wet and dry time periods.