Fluvial geomorphological development associated with MIS 5e–4 sea-level fluctuations in the lower reaches of the Kinu River, Japan

Understanding fluvial geomorphological responses to sea-level fluctuations during Marine Isotope Stage (MIS) 5 is essential for predicting future evolution of coastal lowlands under sustained high sea-level conditions. Although downstream river development since the Last Glacial Maximum (LGM) has been extensively studied, geomorphic processes during MIS 5 remain poorly constrained because pre-LGM deposits are commonly buried, eroded, and difficult to date. In this study, we reconstruct depositional environments and longitudinal river-profile evolution from MIS 5e to MIS 5a in the lower reaches of the Kinu River, central Japan, where Late Pleistocene landforms and sediments are exceptionally well preserved due to regional tectonic uplift. Through detailed terrace mapping, sedimentary facies analysis, grain-size analysis, and post-infrared infrared stimulated luminescence (pIRIR) dating, we establish a geomorphic framework resolved at the scale of individual MIS 5 substages. The results indicate that widespread aggradation occurred during MIS 5e and MIS 5c–5a, whereas degradation occurred during MIS 5d and MIS 4. Aggradation phases correspond to high or relatively stable sea-level conditions, while degradation phases coincide with rapid sea-level falls. Tectonically corrected longitudinal profiles suggest that, especially in inland reaches, riverbed elevations during MIS 5c–5a were rebuilt to levels close to those attained during MIS 5e through sustained fluvial aggradation. These findings suggest that, in the lower Kinu River system, autogenic floodplain aggradation under prolonged high or relatively stable base-level conditions can raise riverbed elevations over broad inland areas. The results provide a field-based geomorphological perspective for evaluating fluvial responses to sustained highstand conditions, particularly with respect to inland floodplain aggradation and associated avulsion potential.