Abstract: The effects of a giant offshore dyke, Saemangeum, have been of special interest on both scientific and environmental aspects. Without any rival worldwide with regard to the ample scale and estuarine setting, the Saemangeum dyke and its vicinity (Saemangeum Area) should be an unprecedented experimental site for monitoring the consequence of the estuarine developments with dykes. In order to unravel geologic changes caused by the Saemangeum dyke, we collected a number of surface sediment samples and bathymetric profiles from a full coverage of the Saemangeum Area, and compared topography and sediment phase on the inter-annual (2002–2006, post-construction) and decadal (1982–2002, between pre- and post-construction) timescales. The dyke construction induced vigorous erosion along the dyke, especially across a few open gaps between dyke segments. These open gaps led to forming deep erosional troughs and the associated channels to the river mouth. At the same time, mild accumulation of sediments occurred on the broad seafloor offshore the erosional zone of dyke sector IV and on shoals between the erosional troughs off dyke sectors I–II. Inside the dyke, tidal flats have become expanded and aggraded with continuously discharged riverine sediments. Sands have been newly introduced into the offshore area of sector IV owing to tidal currents with an orientation forced to be parallel to sector IV (NE–SW), reinforced by high waves during winter. By contrast, the expanded sandy area off sectors I–II resulted from redistribution of pre-existing sands without any new input from offshore. As a whole, the concentrated erosion and sandy-seafloor expansion offshore represent the man-made influences triggered by the Saemangeum dyke. [Copyright &y& Elsevier]