Land degradation due to soil erosion makes out of reach a large part of available water in addition of soil losses. There are several ways to study soil erosion. A new method is flow accumulation which emphases on stream power. The stream power can mathematically be derived from hydraulic shear stress and flow velocity. The study catchment is located in the western part of the Zanjan River valley, an area below the dam watershed of Sefidrood. An operator which given the drainage direction matrix and a weight matrix, determines a resulting matrix such that each element represents the sum of the weights of all elements in the matrix which drain to that element. Flow accumulation procedure for creating the cumulative streampower was done by using Digital Elevation Model (DEM), the SWFM program, the drainage network, C-factor map, slope map and GIS. The results of applying the streampower approach with the SWFM in this study showed low erosion on relatively large areas, even in bare units, except where flow concentrated in drainage networks and the main river.