Geological controls on dispersal and deposition of river flood sediments on the Hidaka shelf, Northern Japan

The distribution and characteristics of marine surface sediments are a basic marine geological information. Large river floods are a frequent natural hazard that transport substantial terrigenous sediments into the marine environment. In August 2003, TY ETAU (0310) caused heavy rainfall in the southern coast of Hokkaido, north Japan, where some mountainous rivers in the Hidaka region flooded. Two deposition modes for the 2003 flood sediments can be identified by comparing the pre- and post-flood surface sediment distribution. Shore-normal shallow depressions off the mouth of the Saru and Atsubetsu rivers served as channels for the discharged floodwater preventing dispersion and maintaining the necessary water density to transport the materials as density bottom currents. This action also promoted long-distance transport of flood materials across the continental shelf. Absence of depression on the inner shelf off the mouth of the Niikappu and Shizunai rivers may have dispersed floodwaters near the river mouth and deposited the flood materials close to the shore. Marine geological mapping suggests that the differences in submarine topography (the presence or absence of shallow depressions) are closely related to the regional geological structure. Thus, submarine geology is a controlling factor of the seafloor environments influenced by the river flood.