What Was the Tenmei Mudflow?
In 1783, Mount Asama, located near the border of present-day Gunma and Nagano prefectures, experienced one of the largest volcanic eruptions in Japan’s recorded history.
The event is known as the Tenmei Asama Eruption. A massive debris avalanche was generated on the northern side of the volcano and struck the village of Kambara.
Huge quantities of earth, rocks, and other volcanic material then entered the Agatsuma River. The material transformed into a massive mudflow that traveled downstream, devastating villages along the river, including communities in the Yamba area.
Historical records indicate that more than 1,500 people lost their lives and more than 2,000 houses were affected or destroyed in the disaster.
The Tenmei Mudflow remains an important example of the destructive power of volcanic disasters and is remembered as a major event in the history of the Agatsuma region.
Twenty-Six Years of Archaeological Research
One of the museum’s greatest strengths is its extensive collection of archaeological discoveries made during the construction of Yamba Dam.
From 1994 to 2019, large-scale archaeological excavations were conducted along the Agatsuma River by the Gunma Prefectural Museum of History and Archaeology and the Naganohara Town Board of Education.
The investigations covered an enormous area of approximately 1 million square meters. Archaeologists discovered sites and artifacts dating from the earliest Jōmon period, around 12,000 years ago, through the late Edo period.
Among the most remarkable discoveries were remains of Edo-period villages buried beneath thick layers of Tenmei Mudflow deposits.
Houses, fields, farming implements, household objects, and other remains were preserved beneath the mud. These discoveries provide an unusually detailed picture of how people lived immediately before the disaster.
Tenmei Mudflow Immersive Theater
One of the museum’s most impressive attractions is the Tenmei Mudflow Immersive Theater.
The theater recreates the sequence of events from the eruption of Mount Asama to the arrival of the devastating mudflow using large-scale video, sound, and synchronized lighting.
A screen approximately 7 meters wide and 4 meters high is combined with surround-sound speakers and lighting effects that respond to the images, creating an experience similar to a movie theater.
The approximately 10-minute presentation begins by introducing everyday life in the Yamba area during the late Edo period. It then depicts the eruption of Mount Asama and the subsequent mudflow as it swept through the surrounding communities.
The presentation helps visitors understand both the physical scale of the disaster and the fear experienced by the people who witnessed it.
Tenmei Mudflow Exhibition Room
The Tenmei Mudflow Exhibition Room uses archaeological discoveries and historical materials to explain the disaster and the lives of the people who experienced it.
The exhibition is organized into four main sections.
Guidance
The Guidance section introduces the Yamba area as it existed in 1783, along with the social and historical background of the late Edo period.
Everyday Life Brought Back to Life
Everyday Life Brought Back to Life focuses on household objects, farming tools, and other excavated materials.
These artifacts reveal how people worked, prepared food, farmed the land, and lived their daily lives in a mountain community more than 200 years ago.
A Day When Everyday Life Was Swept Away
The A Day When Everyday Life Was Swept Away section explains the sequence of the disaster and the immense force of the mudflow that struck the villages.
The archaeological evidence makes the disaster particularly tangible, showing how an ordinary community was suddenly buried beneath enormous quantities of volcanic material.
Memories of Disaster
Memories of Disaster introduces historical records and accounts connected with the eruption and mudflow.
By bringing together archaeological evidence and written records, the exhibition preserves the lessons of the disaster for future generations.
Theme Exhibition Room — The Long History of Yamba
The Theme Exhibition Room looks beyond the Tenmei disaster to explore the much longer history of the Yamba area.
Archaeological excavations uncovered numerous sites dating from the Jōmon period through the Edo period. Pottery, stone tools, and other artifacts illustrate how communities developed and adapted to the mountainous environment over thousands of years.
The exhibits provide insight into the distinctive lifestyles and culture that developed in this part of Gunma and show that the history of Yamba extends far beyond the events of 1783.
The Former Naganohara First Elementary School Building
Part of the museum grounds is also home to the relocated former Naganohara First Elementary School building.
Constructed in 1911, the wooden schoolhouse is one of the older surviving school buildings in Gunma Prefecture. It originally stood in an area that was scheduled to be submerged by the Yamba Dam reservoir.
Local residents and former students worked to preserve the historic building, and part of the schoolhouse was relocated to the museum site.
Inside, visitors can see materials related to Masahiko Urano, who is associated with the creation of Jōmō Karuta, Gunma’s famous regional karuta card game, along with other historical materials from the local community.
The traditional wooden architecture and old-fashioned school atmosphere offer a different perspective on the region’s history and culture.
Access
Yamba Tenmei Mudflow Museum is conveniently located near several major attractions around Yamba Dam.
It is approximately a 10-minute walk from Roadside Station Yamba Furusato-kan and about a 10-minute drive from Naganohara-Kusatsuguchi Station on the JR Agatsuma Line.
By car, the museum is approximately 60 minutes from the Shibukawa-Ikaho Interchange on the Kan-Etsu Expressway and approximately 75 minutes from the Usui-Karuizawa Interchange on the Joshin-Etsu Expressway.
Understanding the Past to Prepare for the Future
Yamba Tenmei Mudflow Museum offers an important opportunity to learn about one of the most devastating volcanic disasters in the history of the Agatsuma region.
Rather than presenting the Tenmei Mudflow simply as a historical catastrophe, the museum also focuses on the people who lived through that period. Excavated houses, farming tools, household objects, historical records, and immersive video bring their everyday world back to life.
The museum also shows how modern archaeological research conducted during the Yamba Dam project has revealed layers of history stretching back thousands of years.
For travelers exploring Mount Asama, Yamba Dam, and the surrounding mountain region, a visit to the museum provides valuable context for understanding the landscape and the communities that have lived here for generations.
By learning about the Tenmei Eruption and the people affected by it, visitors can gain a deeper appreciation of both the power of nature and the importance of preserving memories of past disasters for the future.