Geomorphic Dynamics and Sediment Budget in the Southern Japanese Alps: Recent Studies and Prospects

Abstract

This review paper synthesizes geomorphic dynamics, sediment transport and resulting natural hazards in mountains of the southern Japanese Alps and their drainage basins, where climatic and geological situations produce highly active landform dynamics. In alpine areas above the timber line, shallow diurnal freeze-thaw action operating in the thin topsoil produces smallscale periglacial forms, and gravitational spreading leads to numerous sackung features where snow-melt and heavy rain in places promote rockslides. In subalpine and montane areas, deepseated landslides originate from fractured sedimentary rocks, deep V-shaped valleys, and heavy rain, while shallow landslides continue with historical forest clearance. Continuous slope failures prevent vegetation recovery and maintain debris input to valleys. Steep valleys contribute to high-density debris flows. Frequent or repetitive occurrences of these mass movements promote continuous denudation of slopes, rockfall accidents along hiking trails, and sedimentation at artificial dams. They occasionally cause significant hazards to villages further downstream. Predicting and mitigating slope hazards require distinguishing among annual, low-magnitude processes, episodic high-magnitude processes and geomorphic changes associated with long-term climate change.

Micro-landform Structure and Tree Distribution in Subalpine Riparian Area of V-shaped Valley, Minami Alps, Central Japan

“…These findings suggest that clear terrace components of our study are common for V-shaped valleys in mountainous regions, irrespective of the climatic zone; besides, terrace segments were more conspicuous, which was caused by scarps of terraces being detectable as clear units. Rocky debris assumed to have been supplied by infrequent deep-seated landslides as noted to be relevant to Minami Alps geology (Matsuoka et al 2013) may reinforce this morphological attribute. Tsukamoto (1998) also remarked on the contribution of debris flow to the emergence of terrace segments in mountain valleys.…”

“…Because of the geological conditions and abundant rainfall, deep valleys have developed. Deep-seated large landslides adding shallow and rapid slope failures frequently occur as one of the most active mass-movement areas in Japan (MLIT Japan 2012b; Matsuoka et al 2013). Basements of valleys are sometimes covered with thick deposits originating from the deep-seated landslides (Matsuoka et al 2013); however, exposed bedrock is continuously observed in this study site, namely, bedrock in 60-90% of the total length of the stream segments studied, indicating a "bedrock channel", a major type of topographical feature for mountain streams (Tsukamoto 1998).…”

“…Because of the geological conditions in Japan, the subalpine zones of Honshu are usually located in areas of steep relief, where stream topography is basically regulated by excavating processes that prevail over debris accumulation, which latter is recognized as the collateral process (Tsukamoto 1998). This causes a typical landscape of well-dissected V-shaped valleys in the subalpine areas of Honshu (Yonekura et al 2001), including Minami Alps (Matsuoka et al 2013). Little is known about riparian forests in such typical subalpine valleys, such as species composition and forest structure, or about the composition and arrangement of micro-landform units.…”

Photoreceptor genes in a trechine beetle,Trechiama kuznetsovi, living in the upper hypogean zone

“…In contrast, the upper hypogean zone, in which T. kuznetsovi adults live, is a few or some dozen centimeters below the slope surface of a v-shaped valley. Trechiama kuznetsovi adults would occasionally be exposed to the surface due to landslides occurring as a result of precipitation or earthquakes [48][49][50]. This temporary light stimulus could function as a selective pressure to retain the function of opsin genes and other phototransduction genes.…”