| Research Abstract |
There are 129 rivers in Philippines, 71 ones in Tiawan, 490 ones in Japan, 2 ones in southern Kuril Islands, 66 ones in Poland, that have fans of over 2km^2. The ratios of rivers with fans to rivers whose drainage basins are more than 100km^2 are 20.3% in Philippines, 58.0% in Taiwan, and 25.9% in Japan, whereas the ratios in area over 200km^2 are 9.4% in Philippines, 70.4% in Taiwan, 24.5% in Japan, and 20.0% in Poland.
The development stages of moutains were estimated in Taiwan and Philippines, based on the method of Ohmori(1978) in Japanese mountains. The ratios of rivers with fans to rivers whose drainage basins are more than 100km^2 are 0.0% in the earliest and early substage, 5.7% in younger middle one, 22.8% in older middle one, and 47.6% in later ones. Relief ratios with the advance of the develoment stage of mountains. Drainage basins with high relief ratios have steeper river-bed gradients, and are apt to make fans to carry larger gravel around valley mouths.
Based on the great
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generalized weight and the high frequency in the categoty of climatic conditions, it is concluded that the dominating factor for the distribution of alluvial fans indrainage basins larger than 200km^2 is the climatic conditions. Sedimentary environments and relief ratios are also important factors. The fact that West Philippines has more suitable conditions than those in Hokkaido and West Japan os presumed that the climatic conditions with dry and wet seasons are advantage for fans. East Philippines has most unfavorable conditions due to poor coarse-material production under tropical climate and perennial rainfall.
Alluvial fan areas increase with the advance of the development stage of mountains, specially from younger middle substage to older one, and from older one to later substage in Philippines, Taiwan, and Japan.
There are 44 rivers in Philippines, 48 ones in Taiwan, and 37 ones in Japan, that have formed fans in the plains at the front of the mountains whose development substage are older middle or later. The correlation coefficients between drainage basin areas and fan areas are 0.855 in Taiwan, 0.680 in Japan, and 0.734 in Philippines. In Taiwan and Philippines, the temperature and amount of precipitation are considered to have been changed through the Glacial stage and the Interglacial stage as well as in Japan, but the quantative are not supposed to have drastically affected that the amount of debris supplies. The fan areas in Philippines are 110 to 120% and those in Taiwan are 120 to 160% as large as those in Japan. The fans in tropical Philippines are small due to poor production of coarse materials as compared with those in Taiwan, but are large owing to abundant precipitation and high discharge in comparison with Japan. Less
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