Geomorphic development and its controlling factors of a fluvial fan in a humid region over the past 200,000 years: The case of the Kinugawa fluvial fan, Japan

We aim to elucidate the Quaternary geomorphic development of fluvial fans in a humid region, focusing on the Kinugawa fluvial fan located in the central Japanese Islands. The study area is situated within a humid temperate climatic zone, receiving 1500–2000 mm of annual precipitation, and is strongly influenced by the East Asian Summer Monsoon. We classified the terraces based on their elevations, outcrop observations, and borehole data. The formation ages of the terraces were estimated based on tephra analysis and aeolian deposit accumulation rates, and variations in sediment supply were evaluated through gravel lithology analysis. The fluvial terraces in the Kinugawa fluvial fan were classified into ten terraces (T1–T10), with detailed analysis conducted for T3–T10. The findings indicate that fluctuations in precipitation were a significant factor in terrace formation, except for T4. Specifically, aggradation during periods of weakened monsoon activity and reduced precipitation likely caused river channel instability, leading to frequent avulsions and lateral expansion of active zones. Conversely, increased precipitation facilitated degradation, resulting in channel stabilization. The formation of T4 is inferred to have been influenced by an increase in sediment supply triggered by the sector collapse of Mt. Nyoho. These findings indicate that the geomorphic development of fluvial fans in humid regions is strongly influenced by variations in precipitation, leading to avulsions and terrace formation. • Study clarifies terrace formation factors in humid fluvial fans over the past 200,000 years. • Most terraces are formed by precipitation shifts; some reflect changes in sediment supply. • Lower precipitation caused aggradation, more avulsion, and expansion of the active depositional area. • When precipitation increased, degradation occurred, and terraces were formed.