Why Koshibu Dam Was Built
A Major Challenge in Flood Control
In 1959, the former Ministry of Construction completed Miwa Dam on the Mibugawa River in the upper reaches of the Tenryu River system. Although this improved flood control upstream, serious flooding continued downstream, particularly around the area where the Koshibu River joins the Tenryu River.
The Koshibu River rises near Mount Akaishi in the Southern Alps and flows rapidly through an area prone to landslides. Large quantities of sediment were carried into the Tenryu River, making flood management in the lower reaches particularly challenging.
The 1961 “Sannroku Disaster”
A decisive turning point came in 1961, when torrential rainfall known as the “Sannroku Disaster” caused the Tenryu River to overflow in the Ina Basin, resulting in severe damage in areas including Iida City.
In response, the national government began full-scale planning for a dam on the Koshibu River as a flood-control measure. Construction began in the same year, and after eight years of work, Koshibu Dam was completed in 1969.
It was the first concrete arch dam in the Tenryu River system. Since its completion, the dam has played an important role in protecting communities downstream from flooding.
Restoring the River Environment
Bringing Water Back to the Downstream River
After Koshibu Dam was completed, a 5.1-kilometer section downstream became a long-term “waterless section,” where there was little or no regular river flow.
To address this issue, the Koshibu Dam Water Environment Improvement Project began in April 2000. A maintenance flow of 0.7 cubic meters per second from Ikuta Dam and 0.72 cubic meters per second from Koshibu Dam was released into the river.
As a result, water once again flowed through the river channel, and fish and aquatic insects were confirmed to be living there. The project helped restore the natural river environment while also creating a more pleasant setting for local residents and visitors.
Managing Sediment: An Engineering Challenge
A River Carrying Large Quantities of Sediment
The Koshibu River is known as the river with one of the highest rates of sediment transport in the Tenryu River system. At Koshibu Dam, sediment accumulated faster than originally anticipated, creating a major challenge for the long-term operation of the reservoir.
To address the problem, the Koshibu Dam Facility Improvement Project began in 2000. A 4-kilometer-long sediment bypass tunnel was constructed along the left bank of the dam.
Construction of the tunnel began in 2009 and was completed in 2016. The facility allows sediment to bypass the reservoir, helping maintain the dam’s functions over the long term.
Testing, Repairs, and Full Operation
During prolonged operation in the rainy season of 2020, abrasion and scour occurred inside the bypass tunnel and around the water intake facilities. As a result, reinforcement and repair work became necessary.
After approximately two years of reinforcement work, full-scale operation resumed in June 2023. These improvements have strengthened the dam’s long-term operational safety while helping preserve its flood-control capabilities.
Lake Koshibu and Its Natural Beauty
The reservoir created by Koshibu Dam, Lake Koshibu, was designated as part of the Tenryu-Koshobusuikei Prefectural Natural Park in 1970.
The lake is open to the public and is particularly popular with anglers. Different types of fish can be caught depending on the season, allowing visitors to enjoy a quiet outdoor experience surrounded by mountain scenery.
The calm lake surface, forests, and mountains of the Southern Alps create a peaceful atmosphere that changes beautifully throughout the year. Autumn is especially attractive, when the surrounding mountains become covered in brilliant foliage.
Learning About the Dam
Near the lakeshore is the Koshibu Dam Information Center, where visitors can learn about the history, construction, and structure of the dam.
The dam crest is accessible for viewing, and observation areas provide expansive views of the enormous concrete structure and Lake Koshibu. Standing above the valley, visitors can appreciate both the scale of the engineering project and the dramatic mountain environment in which it was built.
Access and Nearby Hot Springs
By car, Koshibu Dam can be reached from the Matsukawa Interchange on the Chuo Expressway. From the interchange, follow Prefectural Route 59 toward Oshika Village.
The surrounding area also offers opportunities to relax in traditional hot spring resorts. Nearby attractions include Koshibu-yu Onsen in Nakagawa Village and Kashio Onsen in Oshika Village, making it possible to combine a visit to the dam with a relaxing mountain journey.
The surrounding mountains are also known as an autumn foliage destination, and the area becomes particularly attractive when the forests turn red, orange, and yellow.
A Dam Supporting Safety, Nature, and the Community
In 2012, a helipad was established near the dam to prepare for accidents and disasters in the surrounding area. It was subsequently registered as a designated landing site for the Nagano Prefecture Doctor-Heli emergency medical helicopter.
This illustrates how Koshibu Dam serves a role extending beyond its original functions. It is an important part of the region’s infrastructure, supporting disaster prevention, water management, environmental restoration, and tourism.
A Powerful Landmark in the Southern Alps
Koshibu Dam is an impressive example of how engineering and the natural environment can exist side by side. Built amid the rugged landscape of the Southern Alps, the 105-meter-high arch dam continues to perform essential roles in flood control, irrigation, power generation, and environmental management.
At the same time, Lake Koshibu offers a tranquil landscape far removed from the image of a large infrastructure facility. The mountains are reflected in the water, forests change color with the seasons, and the quiet lakeshore provides a place to enjoy fishing and nature.
A visit to Koshibu Dam offers more than a scenic view. It provides an opportunity to understand the challenges of living alongside powerful mountain rivers, the history of flood-control efforts in the Ina Valley, and the continuing efforts to balance infrastructure with the surrounding natural environment.
For travelers exploring the southern part of Nagano Prefecture, Koshibu Dam and Lake Koshibu are a rewarding destination where spectacular mountain scenery and the history of Japanese civil engineering come together.