Yuichi S. HAYAKAWA1, Shozo YOKOYAMA2 and Yukinori MATSUKURA3
1 Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Tokyo 113-0033
2 Faculty of Education, Kumamoto University, Kumamoto 860-8555
3 Geoenvironmental Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572
Recession rates of two waterfalls, Ayugaeri Falls and Sugaru Falls, which are associated with the gorge formation of in the Tateno Canyon in the western part of the Aso caldera, are examined on the basis of the empirical equation after Hayakawa and Matsukura (2003). The waterfalls are postulated to have originated at a location out of the caldera as a waterfall which was formed around the front of the Akase lava erupted from the inside of the caldera. The initial waterfall receded about 2.8 km upstream forming the gorges to the present site of the confluence of the Shirakawa River and the Kurokawa River in the caldera. At the confluence, the waterfall bifurcated into two waterfalls which continued recession along the Shirakawa River and the Kurokawa River to the present locations of the Ayugaeri and Sugaru falls. The estimated recession rates from the equation are 0.086 m/y for Ayugaeri Falls and 0.14 m/y for Sugaru Falls. The calculated time taken for the recession of both waterfalls from the confluence to the present sites of the individual waterfalls is nearly equal: ca. 13,000 y, indicating that the bifurcation of the waterfall occurred about 13,000 years ago. Rough estimation suggests that the time for the initial waterfall to recede the distance of 2.8 km is 14,000-28,000 y. The total time of the waterfall recession from the start to the present location is thus 27,000-41,000 y, which is consistent with the age of the Akase lava flow (ca. 30,000 y).
Key words: waterfall, erosion rate, bedrock erosion, rock control, Aso volcano