Abstract
In the upper reaches of the River Abe, a typical torrential river in Central Japan, there stretches the accumulation terrace which was attributed to the rapid erosion.
The writer has investigated the forming process of the terrace by the exact observation on its topography and deposits, and estimated the velocity of downcutting by correlating these physical aspects with the records of catastrophes described in literature.
The results obtained are as follows:—
1) The accumulation terrace was produced by the surges of erosion on the waste-filled valley which had been formed by recurred debris flows derived from the upper part of Oya River Basin (Oya-kuzure)(Fig. 2 is the topographic map which the writer made by the method of transit-stadia surveying.)
2) The terraces are divided into two groups (as shown in Fig. 3); the higher terrace and the lower one. The former, having the fan-shaped surface, is the depositional surface of debris flow, and the latter is “non-cyclic terrace” which was formed by continued downcutting accompanied by slight lateral erosion. (Cross sections of the valley are shown in Fig. 4)
3) The debris flows repeatedly dammed the tributaries of the River Abe, forming the lakes and ponds Judging from the records in the old manuscripts, the newest damming ocurred in 1702 The higher terrace surface, therefore, seems to have been formed at that time.
4) The longitudinal profiles of the terrace surfaces and the present stream (illustrated in Fig. 5) suggest the amount of downcutting and the mode of revival. The downcutting attains the maximum relief of 70m below the knick point, “Akamizu-fall”.
5) The fall, localized on resistant ledges of sand stone, was produced after the formation of the lower terrace-2 at about the end of the 19th century. This lower terrace-2 is distinguished by the characteristic veneer deposits which contain the exotic gravels derived from the adjacent river basin after the destruction of the dammed lakes.
6) Rejuvenation may have been caused chiefly because of decrease in load.
