The Toyohira River has formed an alluvial fan during the Holocene. The author studied the Toyohira River alluvial fan and distal floodplain from the viewpoint of the fan-delta complex deposited through a river system. A series of lobes was found on the distal part of the Toyohira River alluvial fan. These lobes consisted of bed load gravels and natural levee deposits. The stratigraphy of the fan deposit and marginal alluvium was studied based on ¹⁴C datings and geological sections.
The Holocene evolution of the Toyohira River alluvial fan and its distal floodplain can be summarized as follows (Fig. 7).
During the latest Pleistocene, the Toyohira River formed a fan-delta system at the lower sea level of the last glacial age, As the sea level rose, in the early Holocene, the Toyohira River formed a confined fan delta in the western part of the bay mouth, dissecting the Pleistocene fan in the upper part of the fan-delta system. In the late Holocene, after the sea level had risen to the present stage, the Toyohira River shifted its channel gradually eastwards, and buried a lagoon confined by the Momijiyama barrier. This channel shift was accompanied by lateral erosion at the fan head, and with the formation of lobes distal to the fan. Such alluvial fan construction means that the Holocene fan surface of the Toyohira River was formed by two separate processes, erosion and deposition, independently (Fig. S). During the late Holocene, the Toyohira River alluvial fan has developed not continuously, but episodically, as is suggested by an extraordinarily large-scale lobe of ca. 3, 500y.P. P. The geomorphic process controlling such episodic fan development is not clarified.

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