This paper discusses the development of a coastal sedimentary prism in recent 100 years that has taken place on an initially rocky coast (Teradomari-Nozumi beaches) facing the Japan Sea. The interesting case history actually began in 1922 when the Ohkouzu diversion channel started operation for safely conveying large quantities of flood water of the Shinano River into the ocean. Based on field studies, the discussion explores the fate and delivery of the fluvial sediment in waters up to forty meters and beyond, thereby emphasizing the importance of muddy flood layers in deciphering the complex coastal sedimentary architecture. The approach and outcome should facilitate insights into the nature of a sediment routing system, including the likely evolution of the catchment area over a prolonged period of time.
