As the final closure of the world's longest sea dike of 33 km, the use of the Saemangeum reclaimed land becomes an issue in Korea. The Korean government has proclaimed that the Saemangeum Reclamation Project will be handled in an environmentally friendly manner but its effect on the water quality of reservoirs has always been controversial. This study was conducted to estimate the water quality of the Saemangeum reservoir using WASP5 according to the new land use plan adopted in 2007. Predictions on water quality shows that Dongjin reservoir would meet the standards for COD, T-P, and Chl-a if the wastewater from the Dongjin region was properly managed. However, T-P and Chl-a in Mangyeong reservoir would exceed the standards even without releasing the treated wastewater into the reservoir. With further reductions of 20% for T-P and Chl-a from the mouth of Mangyeong river, the water quality standards in the reservoir were achieved. This means that additional schemes, as well as water quality management programs established in the Government Master Plan in 2001, should be considered. Although the Saemangeum reservoir would manage to achieve the standards, it will enter a eutrophic state due to the high concentration of nutrients.