[H-GM03] Geomorphology

[HGM03-05] Initiation factors of an incised meandering river in steep mountainous terrain: a case study of the Higashigouchi River with high-angle bedding plane

*Takuma Watakabe¹, Hikaru Osawa², Noriyuki Arai³, Issei Doi³, Yosuke Yamakawa² (1.Kansai Research Center, Forestry and Forest Products Research Institute, 2.Mountain Science Center, Univ. of Tsukuba, 3. Disaster Prevention Research Institute, Kyoto University)

Keywords:Incised meandering river, Geological structure, Accretionary complex

This study reports conditions of initiation of an incised meandering river running through a steep mountainous terrain in Higashigouchi River in the watershed of Ooi River, Shizuoka prefecture, central Japan. The Higashigouchi River is underlain by Late Cretaceous accretionary complex called by Shimanto Belt, including shale, sandstone, shale-dominated interbedded shale and sandstone, and sandstone-dominated interbedded shale and sandstone. Bedding of these strata trends NE–SW and dips more than 50° to the northwest, which broadly corresponds to the flow direction of the Higashigouch River. Channel meandering develops at 2-6 km in streamwise distance from the confluence with the Ooi River, with a shorter wavelength than that of the Ooi river and without meander cutoff. The type of riverbed in the Higashigouchi River changes in four types: bedrock exposure zone, giant boulder zone, gravel cover zone, and strath terrace covered by thin sediment cover. Channel width in the meandering segment is narrower than the other segments, mostly showing within 30 m. Channel gradient in most of the segment ranges from 0.02 to 0.04. The geological structure shows marked changes along the meandering segment where shale-dominated interbedded shale and sandstone dominate. The streamflow runs alternately through areas with comprehensive geological structure and areas where the bedding trends NW–SE and dips more than 50° to the northeast. Rock strength in the meandering segment is similar to the other segments. Therefore, these results imply that the initiation of incised meander in the steep mountainous terrain with high-angle bedding planes is influenced by internal factors within the catchment that are changes of geological structure and channel gradient.