Spatiotemporally varying inter-relationships between mainstem riverbed elevation and tributary sediment supply since the last interglacial in the upper Ara River, central Japan

Abstract

The dynamics of sediment transport processes over millennial time scales in mountain river valleys remain poorly understood, limiting our overall understanding of sediment routing through river basins. Here, we reconstruct changes of the mainstem riverbed elevation, tributary sediment supply processes, and their inter-relationships since the last interglacial in the upper Ara River valley, central Japan. Aggradation of the mainstem river valley began before 100–95 ka and terminated by 41–40 ka. Then, the elevation of the mainstem floodplain remained stable until ~15 ka. Lateral erosion by the mainstem river, and intermittent periods of incision, continued until 5 ka, after which the mainstem river began actively incising. Aggradation of the mainstem riverbed before 41–40 ka elevated tributary junctions, decreasing the slopes of tributaries. Using reconstructed slope changes, and assuming sediment transport by debris flow in tributaries with slopes steeper than 8% and by traction in shallower tributaries, we classified the tributaries into three types: Type-A, with sediment supply by debris flow since the last interglacial; Type-B, with sediment supply by traction during the last glacial period and afterwards by debris flow; and Type-C, with sediment supply by traction since the last interglacial. Based on the relation between relief ratio and drainage area, we subdivided Type-C tributaries into those that plotted along (Type-C1) or markedly below (Type-C2) the regression line of the mainstem river and Type-A and -B tributaries. During aggradation of the mainstem river, sediments supplied by debris flow from Type-A and -B tributaries accumulated around junctions, interfingering with mainstem deposits. During this period, the junctions of Type-C1 tributaries were elevated almost synchronously with mainstem aggradation, while the junctions of Type-C2 tributaries were blocked by mainstem river deposits, forming blocked-valley lakes. Around 30 ka, Type-A and -B tributaries formed fans around their junctions that were subsequently truncated by lateral erosion by the mainstem river during the late glacial period and early Holocene, forming toe-cut terraces. Tributary deposits that had accumulated around the junctions were then reworked while maintaining a dynamic equilibrium state with the mainstem river. We infer that the responses of tributary landforms and sediment supply processes to the changing elevation of the mainstem riverbed during glacial and interglacial cycles varied depending on the drainage area and relief of each tributary. Considering the river basin as a whole, however, tributary deposits stabilized the elevation of the mainstem floodplain.