Abstract
The Satsunai-Gawa Valley of Mts. Hidaka, one of the avalanche hazard areas in Hokkaido, was investigated on May and October of 1969. The following became clear after these twice observation. 1. As shown in Fig. 1, snow debris were widely distributed and most of them were those of channelled avalanche. The large one (airborne-powder type) cut off many tree trunks and eroded the gorge wall and it was rough estimated as more than 150,000 tons (Fig.2.). 2. The two damaged forests along the bottom of the valley were surveyed from the view point of tree-indicator of avalanches, like the plant-indicator of land-slide slopes. The two directions of trunk-breakage were apparently shown in (the site II) Fig. 6. and Photo. 2. and the twice breakage form of a trunk could be found there (Photo. 3.). This indicator seems to be the strongest proof of such a double action in the same season as in Fig. 8. 3. Willow plants of the steps-type (Fig. 4.) were found at the site I (Fig. 3.). The analyses of annual rings of the breaking parts of these vertically elongated branches (Fig. 5.) made it possible to estimate the past avalanched seasons : both '65/'66 and '68/'69 winter seasons. These two also agree very well with the seasons of the maximum snow depths and the second of the latest 10 years at the nearest meteorological station. 4. The tree forms of the avalanche hazard valley were similar to those of the heavy snow area as shown in Fig. 8. Natural functions as avalanche recorder in woody plant were partlyproved by our research.In order to get more accurate information of the avalanche, further field study is requiring.
