The topography of the sea floor and textural characteristics of surface sediments are documented in areas off sectors 1,2, and 4 of the Saemangeum dyke and inside the dyke. These were investigated during the years 2002 to 2005, when the dyke construction almost came to an end, and were compared with natural topographic analogs before the dyke construction from the maritime maps of 1982 and 1994. Along and across the dyke are a number of erosional troughs formed by intensified currents during the dyke construction. The sea floor off sector 4 has undergone gradual accumulation of sands moving from the north by currents varying in direction from normal to parallel to the dyke. This is in a strong contrast with a slight erosional environment prevailing over the sea floor before dyke construction. off sectors 1 and 2, a topography with an alterative pattern of erosional troughs and sand shoals develops normal to the dyke. Eroded materials from the troughs seem to have added to the sand shoals. As a result, the troughs, former natural tidal channels, have become increasingly deeper, whereas the sand shoals have become somewhat shallower. The sea floor inside the dyke has also been remarkably shallower with expanding tidal flats due to trapping by the dyke of continuous sediment input from the Dongjin and Mangyeong rivers. Sands, all of which are considered to have originated originally from the two rivers, dominate the entire Saemangeum area.