A Dam Built for Flood Control and Water Management
Yokokawa Dam was constructed primarily for flood control and stable water use. During periods of heavy rain, it helps reduce the impact of flooding downstream, while during dry periods it contributes to a stable supply of water for daily life and agriculture.
The dam uses a gravity-type concrete structure, in which the weight of the concrete itself resists the pressure of the stored water. Behind its quiet and solid appearance is engineering designed to protect the region and support the lives of local residents.
From Flood Disasters to the Construction of the Dam
The Yokokawa River originates on Mount Kyogatake in the Central Alps, also known as the Kiso Mountains, before eventually flowing into the Tenryu River. In the past, the river basin lacked sufficient infrastructure for irrigation and flood prevention, and heavy rainfall frequently caused flooding.
One of the most serious disasters occurred during torrential rains in 1963, when severe flooding struck the area and claimed the lives of 10 people. In response, Nagano Prefecture planned a comprehensive flood-control project for the Yokokawa River and decided to construct a dam in its upper reaches.
Investigations began in 1972, followed by many years of planning and construction. Yokokawa Dam was completed in 1986. In the same year, local residents were invited to suggest a name for the reservoir, and the friendly name Yokokawa Lake was selected.
Yokokawa Gorge and the Surrounding Natural Landscape
Upstream from Yokokawa Dam lies the beautiful Yokokawa Gorge, an area of forests, clear streams, and striking geological formations. One of its highlights is San-kyu-no-taki Falls, where clear mountain water cascades over exposed rock.
Another important attraction is Yokokawa no Jaishi, a remarkable geological formation designated a Natural Monument of Japan. In autumn, the mountains surrounding the gorge turn brilliant shades of red, orange, and yellow. The contrast between the colorful forests and the tranquil waters of the reservoir is particularly beautiful from the walking paths along the gorge.
Yokokawa no Jaishi: A Remarkable Geological Formation
Yokokawa no Jaishi was designated a Natural Monument of Japan in 1940. It is a rare geological formation created where a dike of diorite intruded into slate and became exposed along the riverbed. Quartz veins run across the dike at regular intervals, creating a distinctive pattern.
The winding appearance of the rock resembles the body of a snake, which is why it is known as Jaishi, or “Snake Stone.” Its unusual structure provides an opportunity to observe the results of geological processes over immense periods of time and is one of the most fascinating natural features in the Yokokawa area.
A Gateway to Nature, Recreation, and Local Culture
Following the completion of the dam, the surrounding area was gradually developed as a destination for recreation and tourism. A campground near the dam attracts families and outdoor enthusiasts, particularly during the summer.
Nearby, Furusato Noson Park Kayabuki-no-Yakata is a traditional-style experience facility where visitors can enjoy activities such as soba-making and pottery. The facility offers a chance to experience local culture while surrounded by the natural scenery of the mountains.
Yokokawa Dam therefore serves a role beyond flood control and water management. Together with the gorge, forests, lake, and cultural facilities, it has become part of the area’s tourism resources and a place where visitors can experience the close relationship between nature and local life.
Yokokawa Jaishi Hydroelectric Power Station
Immediately below the dam is the Yokokawa Jaishi Hydroelectric Power Station, a dam-based hydroelectric facility operated by the Nagano Prefectural Enterprise Bureau. It uses the natural flow of water to generate renewable electricity.
The facility is equipped with a horizontal-shaft Francis turbine generator. It can take in a maximum flow of 1.40 cubic meters of water per second, using a maximum effective head of 17.89 meters to generate up to 199 kilowatts of electricity.
Annual electricity generation reaches approximately 1.51 million kilowatt-hours. Construction began in December 2018, operations started in April 2020, and the completion ceremony was held on October 27 of the same year. The facility demonstrates how the region’s water resources can be used to produce renewable energy while contributing to a more sustainable local community.
Exploring Yokokawa Dam and the Surrounding Area
Yokokawa Dam can be reached by car from Ono Station on the Tatsuno Branch Line of the JR Chuo Main Line. By car, it is approximately 30 minutes from either the Ina-Kita Interchange on the Chuo Expressway or the Shiojiri Interchange on the Nagano Expressway.
Walking paths and viewpoints around the reservoir allow visitors to enjoy the changing scenery throughout the year. The area is particularly attractive in autumn, when the surrounding mountains are covered in colorful foliage.
Other attractions nearby include Shidareguri Forest Park, the Former Ono Family Residence, and Ono Shrine and Yahiko Shrine, important places associated with the history and religious traditions of the region. Yokokawa Dam can therefore be combined with a broader sightseeing trip around Tatsuno Town and neighboring Shiojiri.
A Place Where Nature and Human Ingenuity Meet
Yokokawa Dam is more than an infrastructure project designed to protect communities from flooding. Its reservoir, surrounding gorge, geological heritage, and hydroelectric power station illustrate how people have worked with the natural environment to support life in the mountains of Nagano.
In spring, fresh greenery surrounds the lake; summer brings clear mountain streams and refreshing forests; autumn transforms the gorge with colorful foliage; and winter creates a quiet landscape of snow and stillness. With its combination of peaceful scenery, geological wonders, and local history, Yokokawa Dam offers a distinctive glimpse into the natural and cultural character of Tatsuno Town.