The deposits and erosions were actually investigated in a volcanic torrential river which flowed down from Tokachi Volcano in central Hokkaido. The relation between distribution of depositional space and channel width statistically analyzed, and the matter relative to the scale of bed load movement was discussed. The results are as follows:
1) In the upper reaches of Furano River in which debris flows have occurred frequently, the depositional spaces defined as the superior section to deposition were formed in the extensive place of channel width (Fig. 1, 2).
2) Estimating the age of deposits with the tree rings' analysis for the forest on deposits, the depositional spaces were distinguished.
3) Distribution of deposits volume and changes of channel width by deposition and erosion were measured at each segment by surveying the two flows' traces of 1980, 1981 (Fig. 3-6).
4) The autocorrelation coeffecient and the power spectral density of deposits and erosions volume were calculated to the distribution along longitudinal distance (Fig. 7). It was considered that the power spectral density was a good indicator to express the scale of bed load movement and the larger the scale, the more superior the component of long wave length (Fig. 8).
5) The moving translation ratio of channel width by means of lag length that related to the scale was calculated and discussed corresponding to deposits and erosions volume for each scale (Fig. 9-11).
6) The depositional spaces varies with each scale. At the planning for disaster prevention, therefore, it is nessesary to take out the morphological element as the plane shape to each scale (Fig. 12-45).

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