Relative role of rock erodibility and sediment load in setting channel slope of mountain rivers

Rock strength influences channel slope by altering substrate erodibility and the size of sediments supplied to the channels. Although the frequent presence of knickpoints at lithological boundaries indicates that rock erodibility significantly determines channel morphology, a growing body of field evidence suggests that the coarse sediment supply from less erodible rock units is a primary factor in channel steepening. To assess the relative effects of rock erodibility and imposed sediment load on channel slope, I studied five rivers in Tsugaru, northern Japan. These rivers flow through alternating volcanic rock and sedimentary rock. The minimum channel slope required to transport both in situ sediments and those supplied from upstream was calculated using slope component analysis. The findings suggest that sediment effects largely account for the observed variations in channel slope across both volcanic and sedimentary rocks. The proportion of channel slope not explained by the imposed sediment load was slightly higher in volcanic rock reaches than in sedimentary rock reaches, which can be attributed to the lower erodibility of volcanic rock. Based on the grain size distributions of volcanic and sedimentary rock particles and the calculated impacts of sediment load, I conclude that the coarse sediment supply from volcanic rock is the primary cause of the difference in channel steepness between the rock types in Tsugaru. Although this conclusion holds generally true across Tsugaru, certain reaches with locally high channel steepness exhibit more extensive bedrock exposure than adjacent gentler reaches, suggesting that contrasts in erodibility also play a significant role in determining the channel slope. Therefore, examining what factors alter the relative significance of rock erodibility and sediment load can enhance our understanding of how rock properties influence longitudinal stream profiles.