GEOMORPHIC STUDY OF AVALANCHE CHUTE IN THE UPPER DRAINAGE BASIN OF THE TADAMI RIVER

The ground avalanche mainly occurring in early spring constructs both erosional and depositional landform; the avalanche chute (Mathes, 1938) and the avalanche boulder tongue. (Rape, 1959) In the upper drainage basin of the Tadami River and its surroundings (Fig. 1), most of the avalanche debris are accumulated in the narrow valley floor where the work of running water is so predominant as to carry away all of them in snow thawing season. Therefore, the avalanche boulder tongues are hardly formed in the narrow valley floor. But avalanche chutes are widely distributed on the steep slopes of the studied area (Photo. 1). In this paper, the author mainly studied on the factors determining the formation of the avalanche chute, the morphological descriptions of which have been scarcely found in the Japanese literatures. Avalanche chutes widely distributed in this area have shallow open U-shaped cross profiless which can be attributed not to the work of running water but to that of ground avalanche repeatedly occurred in every early spring (Photo. 2). Grooves cut by the rill erosion intermittently occurred at the time of snow thawing or heavy rainfall are often found on the floor of the lower stretch of large avalanche chute. But they are not so large as to change the cross profile of avalanche chute. The longitudinal profile of avalanche chute is straight or slightly concave. The heads of avalanche chutes are stretched up to the ridge on the lee side slopes where snow cornice and/or snow drifts tend to be frequently formed. Thiss implies that the ground avalanche has removed weathered materials from the convex slope of the crest where only sheet flow of water occurs to the down slope, the original convex slope has been changed into straight or slightly concave slope. Smoothly polished bed rock surface with parallel and cross striations on it is developed in the avalanche chutes where ground avalanches are frequently occurred. On the other hand, it has generally low relief controlled by the distribution of joints in it. Protruded portion of bed rock surface is easily abraded into rounded form by the strong impact of ground avalanche. Plucked fresh rockfaces are seen here and there on some meshes of joint networks. This fact implies that the erosional process of ground avalanche is carried on in two ways; avalanche polishing and avalanche plucking. In the area studied, avalanche chute is 10 SO m in width and 150_? _700m in length (Fig. 4). The compound avalanche chute composed of more than two adjoining avalanche chutes attains up to 700m in width. The inclination of most avalanche chutes ranges from 350 to 50°. According to the papers so far pubished, the angle of slopes on which avalanche have occurred has a wider range between 20° and 75° (Fig. 5). It is interpreted that ground avalanches are too scarcely occurred to form avalanche chute on the slopes less than 35° On the other hand, it is difficult for snow cover to reach enough depth for causing destructive avalanche on the slopes more than 500. The distribution of avalanche chutes in this area is mainly mapped by means of air-photo interpretation (Fig. 6). Density of avalanche chutes is high in the northern part of the area, gradually becoming low toward south. Factors determining the distribution of avalanche chute are summerized as follows. 1. The cause that avalanche chutes are abundantly distributed in the northern part lies in the extensive exposure of Neogene sediments and rhyolite on which avalanche chutes are easily formed (Fig. 10).