Abstract
In Japan, where uplift and denudation have been active during the Quaternary, thf growing stage of mountains has been divided into the earliest, early, younger-middle, older middle, and later substages. In this study, relationships have been discussed quantitatively between the development stage of mountains and alluvial-fan existence and size in tectonically active Japan, Taiwan, and the Philippines. Among relief ratio, drainage-basin area, tempera tore, and precipitation, relief ratio was determined to be the most important variable foi predicting the existence of alluvial fans for rivers with source areas greater than 100 km2. As mountains pass through their development stages from earliest to later, their relief ratios increase and the number and size of alluvial fans increase, especially from the older-middh substage. In drainage basins over 100 km2, the relief ratio of 30‰ is estimated to be at important threshold for the existence of an alluvial fan. In the older-middle substage almost all rivers have high relief ratios greater than 30‰, while only 63% of the rivers show such higr relief ratios in the younger-middle substage. In addition, the size of alluvial fans in drainage basins with relief ratios greater than 60‰ is larger than the size with relief ratios lower than 60‰. The percentage of the rivers with relief ratios over 60‰ in the older-middle substage is markedly higher than that in the younger-middle substage. In this way, the increase in relies ratios (steeper riverbeds) beginning in the older-middle substage provides good conditions for the development and increased size of alluvial fans. Furthermore, the tropical climate in the Philippines, which seems to be disadvantageous for the development of alluvial fans in general does not necessarily result in smaller alluvial fans when alluvial fans do develop.
