In this study the lateral migration of an alluvial river, which occurred between 2015 and 2021 at the Gopyeong site of the Naeseong Stream was quantitatively analyzed and the relationship between the hydraulic characteristics of the floods and bank erosion was examined. Bank erosion began in the upstream part of the study reach and then was concentrated on the downstream part beyond the bend apex, showing lateral and downstream migration. Average annual migration rate is 5.6 m/yr. Annual change of the crosssection shows that the lateral migration is accompanied by the retreat of the outer bank, formation of a pointbar at the inner bank together with the incision of the low water channel bed that appeared throughout the river. Volumetric sediment relocation brought forth by these processes was evaluated. Bank erosion is controlled mainly with floods, and eroded area is proportional to the duration of inundation. The results of this study imply that a series of drone/aerial images are useful for the quantitative analysis on the lateral migration of alluvial rivers.