Understanding sediment flux from the inflowing rivers to Lake Shihwa is crucial for the integrated management of the nearby coastal areas. This study simulated the sediment yield using the Soil and Water Assessment Tool (SWAT) from 1975 to 2010 to understand the temporal variation of sediment input into the lake. The average sediment yield during the period was 69.34ton/ha/year, and it was found that precipitation conditions had a greater impact on sediment yield than land cover status. The Banwolcheon and Donghwacheon watersheds contributed to approximately 60% of the total sediment load. From 1994 to 2010, before the operation of a tidal power plant, Hwajeongcheon, Ansancheon, Banwolcheon, Donghwacheon, and Namjeoncheon contributed to sediment deposition in the lake at rates ranging from 0.78 to 1.93cm/year. However, this result is interpreted as reflecting uncertainties in channel deposition in the downstream reach, as well as limitations of the SWAT model in capturing water level fluctuations due to the operation of a watergate. Despite the limitation of not undergoing calibration and validation due to insufficient continuous data for use, this study has significance in discussing the major source area of sediment by considering environmental changes within the watershed, with different land cover maps being used as input data for each period.