The Siak River Basin (DAS) including its special downstream river basin, where the decrease of river flow and water quality is triggered by increased activity along the river flow that is dominated by plantation activities. This study aims to analyze the load pollution load of BOD, COD and TSS in Siak downstream watershed using WASP 7.3 method; and determine the amount of pollution load reduction. Siak River Basin downstream is divided into 14 segments then simulated load pollution parameters BOD, COD and TSS in each segment. In this simulation, the minimum discharge of 151 m3 / dt is followed by reducing the pollution load by 75%, 50% and 25% until in accordance with the standard water quality of class II. In this simulation, the minimum discharge of 151 m3 / dt is followed by reducing the pollution load by 75%, 50% and 25% until in accordance with the standard water quality of class II. WASP7.3 Modeling Results Siak Siege Load Capacity of Siak Siak downstream of Siak District at minimum discharge respectively for BOD parameters of -68,877.82 kg / day, COD of -300.242,11 kg / day and TSS of -169,065,78 kg / day, where the minus sign (-) indicates that the pollutant load exceeds the capacity of the river. The BOD parameter is the highest parameter of reduction because all segments of Siak DAS downstream of Siak regency exceed the standard class for class II. Siak River Basin capacity downstream of Siak district to accommodate pollutant load based on standard quality for class II after the reduction of each pollutant parameters is 75% BOD 12,134.95 kg / day, COD 50% of 12,958.94 kg / day, and TSS 25% of 36,280.66 kg / day.