Because of importance of rivers in water supply, the knowledge of surface runoff and sediment rate can help us to future programming of water supply. Nowadays, usage of systemic methods such as mathematical improvement and simulation, are effective to hydrologic studies. Also, use of this methods because of their ability to analysis data in short time, is increasing. In this study, SWAT2009 was applied to simulate runoff and sediment load and to consider Effect of Construction of Dams on runoff and sediment load in Mazandaran watershed with an area of 29666.92 Km 2 .Calibration, uncertainty analysis and validation were performed with SWAT-CUP software and SUFI-2 program. The model was run for period of 2000-2010 and data of 57 rain gages, 6 synoptic gages and 12 Hydrometry stations were used for running of model. P-factor, r-factor, R 2 and Nash-Sutcliffe values were used to assess the ability of SWAT model. The results of the model in simulating daily runoff and sediment of Hydrometry stations were not satisfactory at calibration and validation stages. For calibration of daily runoff, Ns, r-factor, p-factor and R 2 values were -0.07 to -1.77, 0.04 to 0.33, zero to 44 percent and zero to 28 percent , respectively for selected stations. For calibration of daily sediment load, Ns, r-factor, p-factor and R 2 values were -0.07 to -6.11, 0.08 to 6.28, 3 to 41 percent and zero to 25 percent, respectively. Also, for validation of daily runoff and sediment load in mazandaran watershed, these values were likely the values in calibration. The results showed that SWAT can not simulate runoff and sediment load very good in mazandaran watershed. Since there is no enough data for Dams such as storage volume, reservoir area and etc. Weak simulation at stations in downstream of Dams obtains. Other reasons for weak simulation in model were variety in soil and land use. In other hands, flow of water in most rivers was provided from snow melt and springs. Weak simulation of SWAT in these stations showed that SWAT could not predict snow melt and springs water very well. Finally, weak simulation in model arising from unsuitable of flow net, DEM and land use maps, weak estimation of snow melt and inadequate information for dams in watershed. Keywords : Systematic methods, Runoff and sediment load, SWAT2009 model, SUFI-2 program.
