Among the countries in the North Pacific where salmon (Chum salmon, Oncorhynchus keta) is distributed, Korea is located in the southernmost region. In Korea, Gangwon Province has the largest distribution of chum salmon that migrate to spawn. The east coast of Korea is the boundary where the cold water mass flowing from north to south and the warm water mass flowing from south to north meet; therefore, the coastal environment changes as per the changes in the two water masses. Rivers show environmental changes based on various geographical features and sediments within a narrow range. This study investigated when and how chum salmon migrates from the coast to river in this environment. Chum salmon usually migrates to the river between early October and early December. The number of chum salmon entering the river increased significantly when the coastal surface-water temperature fell to <18°C and reached a peak when the surface and bottom layers were actively mixing. The number of salmon sharply decreased when temperatures of the surface and bottom layers became uniform and the surface-water temperature fell to <14°C. The time of the chum salmon entering the river was classified into three types on the basis of when such environment was formed: Type 1 (late October), Type 2 (early November), and Type 3 (mid-November). The salmon showed active vertical movement within the surface and bottom layers of the coast and moved from the bottom to the surface layer along with the rising tide level as the temperature of the coastal surface layer and the distance to the river decreased. The salmon moved more slowly in regions where the riverbeds comprised gravel that was suitable for spawning than in regions where the riverbeds comprised sand and mud. During the day, they stayed in deep and shaded areas, whereas during the night, they moved rapidly upstream along the deep depth zone.