This study presents results of a human impact investigation of large-scale construction projects on sedimentation trend and morphologic changes in the Ganghwa tidal flats over 10 years using time-series waterlines from Landsat thematic mapper/enhanced thematic mapper. Waterlines were extracted from remotely sensed images. These images are useful for studying changes in coastlines and tidal flat topography. A reference digital elevation model (DEM) was constructed; then actual waterlines from satellite images were compared with modeled waterlines from the reference DEM. Systematic comparison with respect to horizontal differences between the two waterlines provided information on local conditions of sediment trends within the study area. Seaward or landward migration of waterlines was a clear indication of the change in sedimentation pattern. Deposition has been dominant at the main channel between the southern Ganghwa and the Yeongjong tidal flats, whereas erosion has been dominant at the eastern lower tidal flat. These sedimentation patterns complied with field observations along two survey lines and with the result from depth-sounding data. Deposition narrowed the channel and created a slightly more meandering shape. A hydrodynamic model anticipated a significant change of ocean current as a result of the coastal construction projects, and the modeling result matched well in terms of current conditions with the present sedimentation pattern analyzed by waterline comparison. A series of coastal construction projects in this area clearly affected the local hydrodynamics of tide and currents to a large extent and resulted in the changes of sedimentation trends during a decade, and it is necessary to keep the monitoring area to understand long-term impacts initiated by human activities within a relatively short period. Although it was not possible to estimate the total volume of sediment transportation with this method, trends of sedimentation processes within tidal flats can be effectively deduced.