Many factors control morphological and structural features of alluvial fans. The author assessed quantitatively by statistical analyses the degree of importance of factors influencing the features of fans (Table 9 and Fig. 3).
The dominating factors that exert an influence upon volumes of alluvial fan deposits are in order of importance as follows; the stage when fans were formed, drainage basin area, vertical displacement during the Quaternary, a sedimentary environment, existence of an intermontane basin, and presence of volcanoes (Table 9). Because the stage is the most important factor influencing the volumes, it is demonstrated that the interpretation of the differences of volumes in each stage is very important. Furthermore, all the dominant factors, with the exception of sedimentary environment, control the debris production. Consequently, the volumes are determined strongly by conditions of debris production.
Concerning the alluvial fan area, the dominating factors are drainage basin area and stage (Table 9). The size of fans during specific stages are in good agreement with volumes of fan deposits (Table 8). These phenomena are attributed to great fluctuations of debris from one stage to another. Incidentally, under suitable conditions of other factors the areas are not necessarily large in spite of the great volumes of fan deposits. Rather the areas are very much related to the thickness of the fan deposits.
Mean slopes of alluvial fans are controlled primarily by relief ratio and drainage basin area, and secondly by geology (rock type) of drainage basin and climatic conditions. Alluvial fans are steep in drainage basins underlain by tuff, agglomerate, and volcanic rocks, whereas in Southwest Japan where the precipitation is intense they are gentle.
The factors controlling the depths of stream downcutting in alluvial fans are in order of significance as follows; the time elapsed after the cessation of fan formation, stage, variations in climatic conditions, existence of volcanoes, vertical displacement, These factors are concerned with climatic changes, crustal movement, and volcanic activity which produce changes of transportation load. Of the factors, it is demonstrated that the interpretation of the differences of downcutting for each stage is the highly important. On the contrary, such factors as drainage basin area, existence of an intermontane basin, relief ratio, and geology of drainage basin, which cause changes of transportation load to a lesser extent, have little influence upon the depth of downcutting.