Factors affecting the magnitude of post-glacial hillslope incision in Japanese mountains

In Japanese mountains, increased post-glacial rainfall triggered gullying in bedrock and thousands of shallow failures in regolith, resulting in widespread hillslope incision. To identify factors influencing the areal extent of the post-glacial incision, fifteen rugged river basins in central and northeast Japan were surveyed. Topographical and geological properties of hillslopes were measured on geomorphological maps, digital elevation data, and geological maps. Analyses of ca. 3,000 0.5 × 0.5 km morphometric samples have shown that the magnitude of regolith failure depends mainly on slope inclination and geology, and subordinately on local storm intensity. The magnitude of bedrock gullying also depends on these three factors, but with greater dominance of storm intensity. These observations reflect the differential stability of hillslopes with varying inclination, erodibility of bedrock, and sensitivity of regolith and bedrock to change in storm intensity. The three factors also exert control on alluvial-fan development in the lower reaches, because they affect the amount of hillslope sediment supply and in turn the type of fluvial processes.