Erosion rates of waterfalls in post-volcanic fluvial systems around Aso volcano, southwestern Japan

Estimation of the recession rate of waterfalls is a crucial issue in bedrock river erosion because waterfall recession can cause a major impact on bedrock incision, especially when waterfall recession rates are high. Areas of active volcanoes are often characterized by many waterfalls in the volcanic edifice. This study examines recession rates of waterfalls in welded Aso-1 ignimbrite from the Aso volcano in southwestern Japan using an empirical equation, which comprises a force/resistance index composed of measurable geomorphic parameters. The estimated recession rates are on the order of 0-01-0-07 m a(-1). The estimated rates are then validated by examining the duration and distance of their recession. The duration of waterfall recession is derived from eruptive ages of the Aso ignimbrites, giving waterfall recession distances of approximately 10 km. Although the original locations of the waterfalls suggested by the recession distances exceed the downstream limit of the present Aso-1 ignimbrite remnants along valley floors, features of the surrounding topography are consistent with these localities being where the waterfalls formed. The use of an equation to estimate recession rates is therefore considered to be valid and practical. The contrast between the highly dissected landforms downstream of the present waterfalls and the gentle landscapes upstream of the waterfalls suggests that the rapid recession of the waterfalls is the major cause U of post-eruptive fluvial erosion into ignimbrites. Copyright (c) 2007 John Wiley & Sons, Ltd.