About 93 percent of the Kiso drainage basin is occupied by mountainous areas including the Central Japan Alps where are developed wide fossil periglacial slopes covered with coarse debris. Occurrence of such slopes that a large amount of the debris and gravels have been produced under cold climatic conditions the Last Glacial Age. The sediments underlying the river terraces formed during the late Pleistocene and those underlying alluvial fans in the Kiso drainage basin mainly consist of these debris and gravels.
Alluvial fans have, however, become small in size during the Holocene or the Post Glacial Age because the supply of coarse debris and gravels has relatively decreased. By contrast, a large amount of finer floating materials have been deposied in the western subsiding part of the Nobi Plain, being carried by the rivers. As the lower reaches of the Kiso “Three” Rivers, the Kiso, the Nagara and the lbi, have flowed and gathered to the western part of the lowland, their river beds have been successively raised in such depositional environment. The river bed of the Kiso is the highest and that of the lbi is the lowest among three. The inundation have, therefore, concentrated to the lowest lbi and their river banks were broken by the attacks of river flow at inundation. The flood control to the Kiso “Three” Rivers in historical time has been carried out in such fluvial conditions.
