The two to nine categories of eleven factors leading to the existence and distribution of alluvial fans are listed in large drainage basins of areas over 200km² (Table 1). Discriminant function analysis shows that favourable categories for a fan formation are the relief ratio greater than 39.8‰, Quaternary vertical displacement over 1, 000m, alluvial plain, basin, the Coast of the Sea of Japan, the Pacific side of Japan, and so on (Fig. 3). The ratio of each factor to form a fan is drawn in each drainage basin in Figs. 4 and 5.
The high frequencies of the dominant factors, such as the sedimentary environment, climatic condition, relief ratio, and vertical displacement, explain the distribution of fans to a considerable degree (Fig. 7). The factor of the sedimentary environment is also remarkably dependent on the factors, such as the relief ratio, geology of the drainage basin, and vertical displacement (Fig. 8).
Now, the debris supply is abundant in drainage basins with great relief ratio or large vertical displacement, where fans and alluvial plains develope. In the Coast of the Sea of Japan and the Pacific side of Japan where the periglacial areas spread widely during the glacial age, a great deal of debris was produced in the source area of the mountain. Contrary to this, the debris productions were small in the Southwest Japan where the periglacial areas did not spread at the time. In Hokkaido, the debris supplies were small under the low preciptation.
These factors determined by the above analyses shall provide clues to explain the reason of each alluvial fan development.