At 8 a.m. on August 29, fifteen international geomorphologists plus their international luggage squeezed into a standard issue Chuo University vehicle outside the internationally recognised Suidobashi Grand Hotel in downtown Tokyo. The group included five Japanese, two Americans, two Canadians, and one each from Australia, Austria, Belgium, England, Scotland and Singapore. Two other Japanese colleagues joined us on August 31 at the Aichi Experimental Forest.
The leader of the field trip was Dr. Yuichi Onda of the University of Tsukuba. Co-leaders were Drs. Takashi Oguchi (University of Tokyo), Maki Tsujimura (University Tsukuba) and Hiroaki Sugimori (University of Tokyo). The organisation of the event was meticulous and the scientific quality of the research was excellent.
1. The geomorphological setting (selected paragraphs from Onda et al, 2001)
a) The Japanese Alps
The Japanese Alps, the highest non-volcanic ranges in Japan, consist of three separate ranges called the Northern, Central, and Southern Japanese Alps (hereafter they are referred to as NJA, CJA, and SJA). The length of each range is about 80 to 130 km, and the width is about 30 to 50 km. Their summit altitudes are about 3,000 m. The three ranges are underlain mostly by rocks of the Paleozoic and Mesozoic periods. CJA and the northern half of NJA are characterized by granite rocks; whereas, SJA and the southern half of NJA are characterized by sedimentary rocks. The higher parts of the mountains were glaciated during the Last Glacial age. The extent of glaciers was greater during Isotope Stage 4 than during Isotope Stage 2, reflecting greater snowfall in Stage 4. Some small cirque glaciers also advanced during the Younger Dryas period. The orographic snowline at Stage 2 was located around 2,600 m in most of the mountains. However, the northwestern mountains tended to have lower snowline altitudes, reflecting greater snowfall due to the moisture supply from the Japan Sea. Most cirques occur on the eastern side of the mountains, reflecting snow transportation over the ridge by strong westerly winds.
b) The Central Japanese Alps
CJA is characterized by low-angle thrust faults with high slip rates. The range began to uplift around 0.5 Ma, indicating that the average uplift rate of the mountains is at least 4 mm/yr. The horizontal slip rate of the main fault below the range is even greater, resulting in more than 4-km horizontal shortening between the range and the Ina Valley at the eastern foot of the range. Late Quaternary alluvial-fan surfaces in the Ina Valley have been displaced by the faults. The rapid uplift of fan toes induced by the thrust faults led to wide exposure of deeply incised Pleistocene surfaces in the Ina Valley, especially those formed during Isotope Stage 4. In contrast, alluvial-fan surfaces formed in the Pleistocene are often covered with Holocene gravel at the foot of NJA and SJA because of abundant postglacial sediment supply.
c) The Southern Japanese Alps
SJA has been uplifting since the early Quaternary due to tectonic processes. These examples show that the period of orogeny differs even among adjacent mountains. Most of the Japanese Islands have been subjected to compressive stress resulted from the subduction of the Pacific Plate along the Japan Trench, the Sagami Trough and the Nankai Trough.
d) Erosion and sedimentation
Analysis of dam sedimentation rates in the Japanese Alps has revealed that erosion rates in the ranges often exceed 2,000 m3/km2/yr, which is one of the world highest erosion levels. Slope failures caused by storms associated with typhoons and the Polar front are mainly responsible for the rapid erosion. In central Japan, heavy rainfall with an intensity of 50 mm/hr or 200 mm/day occurs once in ten years. This amount of rainfall is normally sufficient to cause many slope failures in steep watersheds despite the fact that most hillslopes are covered with vegetation.
2. The morning of August 29
The morning of August 29 was spent in driving to the Central Japanese Alps, where we made an ascent of Mt. Hoken-dake to 2500 m.a.s.l. by gondola and onwards to 2900 m on foot. The site of interest was the so-called Senjojiki cirque, a south-eastward facing cirque in which moraines assigned to Isotope Stage 4, the Last Glacial Maximum and the Younger Dryas have recently been described (Aoki, 2000). Tephrochronology, the beryllium-10 exposure method and weathering rind thickness of the moraine gravels are the methods used. International geomorphologists, who had been locked up in lectures in Tokyo during the previous five days, showed impressive form in climbing the steep headwall of the cirque and were able to note the more modest development of cirques on three other sides of the mountain and absence of depositional forms.
 Photo 1 - August 29, Senjojiki cirque headwall (photo by Yuichi Onda) |
 Photo 2 - August 29, Senjojiki cirque viewed from the headwall (photo by Yuichi Onda) |
 Photo 3 - August 29, Senjojiki cirque viewed from the headwall (photo by Christine Embleton-Hamann) |
August 30 concluded the context setting field sites with visits to a large exposure of alluvial fan gravels, including Isotope Stage 6 and 4 gravels capped by tephra (Ono, 1990), alongside the Yotagiri River (Photos 4 and 5), and a spectacular low-angle thrust fault in the displaced fan gravels and tephra of the Nakatagiri alluvial fan (Photo 6). Both sites emphasised the active fluvial environment during glacial stages and the tectonic instability of the landscape. There followed a series of stops that illustrated the active hillslope hydrology, massive debris flows and landslides in the Central Japanese Alps, specifically associated with intensive field work in the Yotagiri watershed (granite) and in the Southern Japanese Alps, specifically in the Koshibu watershed (shale) (the latter were to have been visited on August 31, but because of an intense rainstorm overnight, access was prohibited) (Photos 7, 8, 9, 10 and 11). Radical differences in the hillslope hydrology were said to be reflected in the contrasting geomorphology of the two basins. The granite watersheds were shown to respond rapidly to precipitation input with a lag time of 0.6 hours versus 10.1 hours for the shale basins. Stream hydrographs, tensiometric responses and ionic concentrations in the run off are consistent with these contrasts. The geomorphic response shows high drainage density and high debris flow incidence in the granite watersheds and lower drainage density and deep-seated landslides in the shale watersheds.
 Photo 4 - Uplifted and dissected alluvial fans at the base of the Central Alps. Vertical displacement during Quaternary of up to 150m. (photo by Yuichi Onda) |
 Photo 5 - August 30, Exposure of alluvial fan gravels alongside the Yotagiri River (photo by Yuichi Onda) |
 Photo 6 - August 30, Outcrop of a low-angle thrust fault in the fan gravels of the Nakatagiri alluvial fan (photo by Alan Dykes) |
 Photo 7 - August 30, Debris torrent deposits in the upper reaches of the Yotagiri River (photo by Maki Tsujimura) |
 Photo 8 - August 30, Check dam in the upper reaches of the Yotagiri River (photo by Alan Dykes) |
 Photo 9 - August 30, Concrete reinforcing lattice at slope failure source in the Central Japanese Alps (photo by Maki Tsujimura) |
 Photo 10 - August 30, Ohnishiyama landslide, Southern Japanese Alps (photo by Maki Tsujimura) |
 Photo 11 - August 31, Concrete reinforcing structure on a recently failed slope in the Southern Japanese Alps (photo by Arthur Conacher) |
Overnight August 30 was spent at the Akaishi Sou Hot Springs Hotel. This was a unique experience for western participants including traditional hot springs, traditional tatamis as bedding and a typical Japanese cuisine including crispy grasshoppers for breakfast.
4. August 31
August 31 was the climax of the field trip when we visited the Obara region, near Nagoya, in the foothills of the Southern Japanese Alps. This is the scene of intensive field work by Onda (1992; 1994) in zero order basins developed in granite and granodiorite lithologies. His essential findings are that water storage capacity in the regolith controls hydrological characteristics, slope processes and slope form. Granodiorite slopes have low landslide density, convex profiles and thick regolith with large storage capacity whereas the granite slopes have high landslide density, concave profiles and thin regolith with small storage capacity. Consequently, zero order basins developed in the granodiorite are amphitheatre shaped valleys with steep slopes and flat bottoms; granites generate v-shaped valleys with no valley bottom development.
September 1 was spent in the University of Tokyo's Aichi Experimental Forest (Photo 12). The previous evening, after celebrating Professor Thomas' retirement from active service, the group had been regaled with a detailed account of the history of hydrologic monitoring in the forest by Dr. Hirofumi Shibano. So we were well prepared for the field. Within walking distance of the hostel, we were able to see a wide variety of geomorphic responses to the previous year's typhoon (Photo 13) and were introduced to new modelling approaches to the impact of vegetation cover changes on erosional processes. The availability of such a long time series of hydroclimatic data is crucial to the success of the modelling.
 Photo 12 - September 1, Hydrological monitoring in the University of Tokyo's Aichi Experimental Forest (photo by Yuichi Onda) |
 Photo 13 - September 1, International geomorphologists examining geomorphic response to a previous (2000) year's typhoon. (photo by Yuichi Onda) |
6. Conclusion
International geomorphologists were dropped off at various points between Nagoya and Narita. One brief vision of Mt. Fuji; a tortured transect of the city of Tokyo and a celebratory dinner for the retiring IAG executive members in Narita marked the end of a splendid field trip and the end of a superb 5th ICG. With our united thanks to Japanese colleagues for their fine scientific leadership.
Reference
Onda, Y., Oguchi, T., Tsujimura, M. and Sugimori, H., 2001. Mountain environments, field experiments in hydro-geomorphology and rock control in central Japan. Field Trip B3 Guide Book, 5th International Conference on Geomorphology: Tokyo.
© 2002 International Association of Geomorphologists