Dispersion and deposition of terrestrial organic matter by flooding on the inner shelf were studied using C/N ratios, δ13C, and Δ14C values of sedimentary organic matter. Surface sediment samples (top 2 cm) were collected from coastal areas near the Saru River in southwestern Hokkaido, northern Japan, 1 yr after a flood event in 2006. Riverine suspended solids were also collected at a fixed station downstream during 2006–2008. Sandy sediments were located at the front of the river mouth and the western part of the sampling area, with the δ13C of organic matter ranging from −23.8‰ to −22.0‰, Δ14C of –655‰ to –388‰, and an organic carbon/total nitrogen (C/N) ratio of 5.9–7.7. On the other hand, silt and clay sediments were distributed in a restricted area 11–16 km from the river mouth, with lighter δ13C (–26.7‰ to −24.1‰) and higher Δ14C (–240‰ to –77‰) of organic matter and C/N ratio (7.8–13.3). From end-member analysis, the apparently younger and less degraded organic matter in the silt and clay sediments consists mainly of terrestrial organic matter released by flood events. They remain in the depression, although most flood deposits were moved to deep-sea environments.