The Bengawan Solo River is the largest river on the island of Java and flows water from the river basin area (DPS) covering an area of ± 16,100 km2 starting from the Sewu Mountains in the west-south of Surakarta to the Java Sea north of Surabaya through a ± 600 km long channel. Wonogiri Reservoir as a reservoir in the upstream area of the Bengawan Solo River with a watershed (DAS) covering an area of 1350 km2 is still operating well since 1981 as a flood control raw water supply hydroelectric power river maintenance tourism and fisheries. With a river maintenance discharge from the reservoir of 2 m3/s the amount of river pollution load in the downstream area is still quite large with the highest amount of pollutant load found in the Kajangan segmentt in September 2019 with a BOD of 103 mg / L and COD of 441.7 mg / L.-133.254.59 kg / day for BOD parameters and -555.039.07 kg / day for COD parameters. After simulating the addition of discharge to the upstream different results are obtained. The most optimal dilution discharge was found to reduce the pollution load in the downstream area to comply with class II water quality standards with the lowest discharge of 23 m3 / s for April 2019 and the highest discharge of > 150 m3 / s for September 2019 to December 2019. Efforts to rely on the addition of upstream river discharge as a diluent to reduce pollutant loads in the downstream need to pay attention to the quality of water contained in the water discharge. If the water quality at the discharge used as a diluent is not too good it will not have a big effect in reducing the pollution load