抄録
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 2.8km upstream forming the gorge 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 recession rates estimated from the equation are 0.086m/y for Ayugaeri Falls and 0.143m/y for Sugaru Falls. The calculated time taken for the recession from the confluence to the present sites of the individual waterfalls is nearly equal : ca. 13,000y, indicating that the bifurcation of the waterfalls occurred about 13,000 years ago. Rough estimation suggests that the time for the initial waterfall to recede the distance of 2.8km is 14,000-28,000y. The total time of the waterfall recession from the start to the present location is thus 27,000-41,000y, which is consistent with the age of the Akase lava flow (ca. 30,000y).
