Why are the mountains upstream of the Jinzu River prone to collapse?

Volcanic Eruptions Repeated over Centuries
Soil of volcanic ash likely to run down
Fragile and crumbly lava
Heavy rain pushing crumbled rocks

Origins of the Jinzu Sabo（1910 - 1920）

The History of the National Government's Erosion Control Dates Back 100 Years

Flood disasters occurred at various parts of the country in 1910, and serious damage was also done in the Jinzu River basin. These disasters led to the national government's decision to start erosion control (sabo) works in 1911. The Jinzu River was selected as one of the rivers where sabo works were to be conducted. In August 1914, an unprecedented flood occurred. Devastating debris flow hit Sakashimo village (currently part of Hida city) and caused disastrous damage to the village including the loss of 36 people's lives. Sediment ran downstream and was deposited in the river. It raised the riverbed and made the river more likely to flood. The sediment that ran downstream also raised the depth of the water at the Toyama Port and rendered the port almost unable to maintain its port functions. A large-scale sabo project was then necessary in the upstream reaches of the Jinzu River to prevent such a situation. In 1919, the national government decided to conduct the work, and the Sakashimo Sabo Plant was set up as a field organ under the Niigata Construction Branch Office of the Home Ministry.Heavy rain pushing crumbled rocks downstream.

Early Sabo Works（1921 - 1944）

The sabo works of 90 years ago were built by manual labor.
The sabo works in the upstream reaches of the Jinzu River were constucted by the national government from 1919 to 1931. Sabo works were constructed on the Miya River at Azukisawa, Kuwatani, and Takidani, and on the Takahara River at Rokurodani.
The village of Kamitakara (currently part of Takayama city), Gifu Prefecture, made a petition in 1929, and the municipalities in Yoshiki-gun, Gifu Prefecture, jointly submitted a petition to implement sabo works projects in the upstream area of the Jinzu River in 1930. In the same year, 1930, the Governor of Toyama Prefecture submitted an opinion to apply for national treasury disbursement to implement sabo projects for the Jinzu River. In response, work started on the Gamada River at Ashiaraidani and on the Hirayu River at Iwatsubodani in 1932.
In those days, although construction machinery had been introduced, the majority of work was done by manual labor. It was manual labor that cut out roads to transport materials, and quarried and carried the stone to use as materials. Sabo dams in those days were constructed by stone masonry, by breaking up large rocks into rocks of about 1m in diameter and then stacking them up.

Sabo Works Get into Full Swing（1945 - 1969）

Small-sized Machines Came Into Use About 60 Years Ago.

Post-war sabo works on the Jinzu River began in 1949. The first facility constructed was the Hikage No. 1 Sabo Dam. In 1952, the Kamisaka Sabo Dam was damaged by a flood. Furthermore, a large-scale landslide occurred on the mountain side on the left bank side in the upstream reaches of the Sodegadani stream. The risk of large amount of sediments flowing down increased, and the sabo plan was revised.
In 1954, the Jinzu River System Sabo Work Office (currently Jinzu River Sabo Office) was set up to cope with the increasing number of sabo projects. The sabo works were expanded to the main channel of the Gamada River, the Ashiaraidani stream, the Sodegadani stream, the Konabedani stream, the main channel of the Hirayu River, the Iwatsubodani stream, and the Esakakedani stream.
Before the middle of the 1950s, stone masonry was the mainstream method of sabo dam construction. This method was replaced by concrete construction and small machines were actively introduced into sabo works.

Greenery Restored to Rokurodani

Rokurodani is a small stream in front of the Kamioka Kozan Station. Gold and silver were mined in the hillside of Rokurodani in the Edo Period. Water used to clean ores was discharged into the Rokurodani stream, which eventually made the stream dirty and barren. In 1872, a large landslide occurred in Rokurodani. In 1899, heavy rainfall caused sediment run off to dam the Takahara River. Hida city (formerly Kamioka town) was often damaged by such disasters. As a solution, sabo works were finally conducted from 1920 to 1930, including construction of sabo dams, ground sill works, and hillside works. As a result, the valley was stabilized, and a zinc refinery was constructed in the alluvial plain part. The greenery of the past is now restored to Rokurodani.

Greenery Created at Fukuji

Typhoon No.11 that hit the Okuhida area from July 25 to 26 in 1958 caused debris flows as high as 20m, temporarily stopping the flow of the Hirayu River.
The impounded water was finally breached but the riverbed rose by about 3m. It completely changed the look of the river. Construction of sabo facilities including sabo dams, groundsill works, and revetment works was completed in 1960 after about 25 years' work. A rich green environment was created at the Fukuji Hot Spa area under the protection of these facilities. Today the hot spa enjoys prosperity as a hot spring community that features a sedate mountain village environment.

Restoration after the Horadani Disaster

Horadani is a mountain stream that flows through the Tochio Hot Spa community (Okuhida Hot Spa Resort, Takayama city; formerly Kamitakara village).
Record-breaking heavy rainfall with an hourly rainfall of 83 mm occurred in Okuhida area due to Typhoon No. 11 that hit the area from August 21 to 22, 1979. Debris flow was triggered by a landslide in the headwaters upstream of Horadani at around 7:50 on August 22. Large amounts of sediment including gigantic rocks instaneously hit the Tochio Hot Spa community. The debris flow washed away bridges, destroyed a junior high school, covered the roads with an 8m-thick layer of sediment, and buried buildings. The victims included two persons killed, one person missing, and 52 houses destroyed.

Shigeo Kashiwagi's Story(resident of Tochio)
The rain was so heavy we could not run away although we desperately wanted to.
Water dashed into the first floor of my house up to a level of 1m. We could do nothing but stay put. But fortunately the water began to recede. Whenever a disaster occurs, I strongly feel what is important is the power of many people helping each other. I really thank all those who helped me. I am so happy and thankful that a very good revetment has now been constructed.

Horadani area, where the townscape was restored as a result of the completion of sabo facilities.