Living With Mt.Fuji  series 1
−Osawa Collapse and Southwestern Torrent−

 We sing that "It rises above the clouds and looks down on all other mountains...Mt. Fuji is the highest mountain in Japan.” With each season, Mt. Fuji changes its appearance and in so doing reminds us that nature is wonderful but severe.
We each have our own image of Mt. Fuji. As we live in the shadow of Mt.Fuji, it is important that we protect the mountain and thank it for its blessings of nature.Mt. Fuji allows us to appreciate the the importance of nature. We strive to protect, and we aim to improve awareness of, thebeautiful mountain and its surroundings.

We are pleased to express our thoughts and feelings about Mt. Fuji through this page.

Ochudo Area  2,300m above Sea Level Senjochi Area 650m above Sea Level

1. The Majesty of Mt. Fuji: Mt. Fuji is so beautiful that we can hardly imagine it erupting violently. However, its steep slopes and weak soil, and the development of torrents prove that Mt. Fuji is a volcano. People living in the foot of Mt. Fuji have always fought against sediment disaster.

2. Here, people lead their lives in the shadow of Mt.Fuji.

3. There are many valleys at the foot of Mt. Fuji, often referred to as the "Happaku-yazawa".

4. The debris flow produced by strong rain, melting snow and typhoons has caused many problems in the downstream area.

5. Early residents of the Mt. Fuji area struggled to dig irrigation canals for their farms out of the washed-out earth and sand.

6. Since it was essential to keep irrigation canals from being blocked, their rice fields were used to place the outwash. Even today, "Harubori", or spring excavation, is carried out every spring. Where does this silt come from?

7. The Osawa Collapse is the largest area, beginning right below the summit of Mt. Fuji and reaching about 2,200 meters above sea level. It is 2.1kilometers long, 500 meters at its maximum width, and 150 meters deep. To date, the amount of sediment washed out is estimated to be 75,000,000 cubic meters. In recent years, about 160,000 cubic meters of earth and sand is washed out each year.


What is happening in the Osawa Collapse?

 The soil of the valley head is basaltic
lava and pyroclastic rock. Repeated freezing
and thawing erode the soil, resulting in frequent
collapse of earth and sand. The result is pyro-
clastic rock formation, or the soft accumulation
of layers of volcanic waste. First, both sides
of the valley collapse to make a cavity.
Thereafter, the hard basaltic lava collapses
under its own weight.
Osawa Collapse
2,400m above Sea Level

9. The bottom of the valley is scoured out by receding waterfalls.

Waterfall No. 2 At the Bottom of the Valley
2,100m above Sea Level

10. In this way, the evolution of the Osawa Collapse is an endless process. The affected area continues to expanded year after year. Pyroclastic rock is called scoria, which includes volcanic ejection such as volcanic ash. The geology of the Osawa Collapse is an interaction of soft scoria and basaltic lava. Repeated sedimentation of different materials in this fashion is referred to as “Alternated Layers”. Collapsed rocks and sand are deposited in the valley bottom of Osawa River and are discharged to the lower reaches at the time of flooding.