Based on a sedimentological analysis and tephrochronology, development of the Ara River terrace is reconstructed focusing on the Nagatoro Gorge between the Chichibu Basin and the Arakawa Fan. Terrace treads along the Nagatoro Gorge can be classified into Oy1, Oy2 and Hg. Oy1 comprises eroded remnants of the fill terrace probably formed in Marine oxygen Isotope Stage (MIS) 6. Oy2 and Hg are toe-cut terraces formed by tributaries from MIS 3 to MIS 2 and early Holocene, respectively. The possible range of the elevation of the Ara River floodplain in the Nagatoro Gorge during MIS 3 and MIS 2 is reconstructed by extending the cross-sectional profiles of Oy2 terrace treads. The longitudinal profile of Ara River in the Nagatoro Gorge during MIS 3 and MIS 2 continues to that of paired terraces formed in the last glacial period: Kagemori Terrace in the Chichibu Basin and Miizugahara-1 Terrace in the Arakawa Fan. The profile of the Ohnohara Terrace of the Chichibu Basin and the Nagatoro Gorge continues to that of the Hanazono Terrace of the Arakawa Fan. As a result of continued lateral erosion of the Ara River during the two periods, wide floodplains formed throughout the three segments of the Chichibu Basin, the Nagatoro Gorge and the Arakawa Fan in the last glacial period and the early Holocene. During these periods, most sediments passed through the Nagatoro Gorge and were discharged onto the Kanto Plain. The river began to incise in the Holocene throughout all three segments.
