In order to clarify the characteristics of stream floodplain which has been formed in moutainousarea, floodplain microtopography was surveyed and process of channel course fluctuation was researched.By reading the chronological indexes of woods stands and aerial photographs taken in the past, processof channel course was recognized for last 30-40 years. Two types of channes coure fluctuation were recognized: continuous transfer to outsides because ofbank erosion at curved places and abrupt fluctuation of it. In the latter case bed load depositionoccured at the fluctuating place burying the channel crosssection and brought about the abruptfluctuation. Although the scale of the bed load movement is small (deposit volume is almost less than1000m3 and moving distance of the bed load less than 100m), cchannes course fluctuation is active. Thecharacteristics of channel course fluctuation is controlled by the frequency of the bed load movement, and the characteristics of stream floodplain is closely influenced by the characteristics of the channelcourse f uctuation.
This report deals with the accumulative mechanism of the unstable debris and the vegetationalinvasion on the Iandslide scars in the upper basin of the Oi River. The main results are as follows: 1) Itis pointed out that rock fall, debris fall, dry fragments flow, debris avalanche and debris floware main processes of the movement and deposition of rock fragments on the scars. The unstabledebris on the scars are accumulated by these processes. Jupging from the characteristics of eachprocess, it is concluded that rock fall is the most important process. 2) Four vegetation types are recogniged on the scars, these are Fujiazami (Cirusium purpuratun)type, Kusakoakaso (Boehmeia tricuspis var. unicuspis) type. Susuki (Miscanthus sinensis) type andWoody plant type. 3) The relation shown in Fig.9. between the movement and deposition of rock fargments and the vegetational invasion s obtained. It is clear from Fif.9. that rock fall plays an important role forthe vegetational invason, and debris avalanche or debris flow plays an important role for thedestruction of vegetat on.