Minoh River Dam is a prefectural dam built across the Minoh River in Minoh City, Osaka Prefecture. The Minoh River is a tributary of the Yodo River system, flowing through the northern part of Osaka before eventually joining the Inagawa River.
The dam is a 47-meter-high rockfill dam managed by Osaka Prefecture. Its primary purposes are to control flooding along the Minoh and Inagawa river systems and to maintain an appropriate flow of water in the river.
The reservoir created by the dam is known as Yuyu Lake. The surrounding area forms part of Meiji no Mori Minoh Quasi-National Park, giving the dam a particularly scenic setting of forested mountains and seasonal greenery.
The Minoh River is a relatively small river that eventually flows into the Inagawa River, which joins the Kanzaki River, one of the major tributaries of the Yodo River system.
The river rises around Mount Aketao and Mount Hachibuse in the Hokusetsu hills. From its mountainous upper reaches, it flows through Minoh, Ikeda, and Toyonaka in Osaka Prefecture before continuing into Itami in Hyogo Prefecture and joining the Inagawa River.
The river is approximately 15 kilometers long and has a drainage basin of approximately 23.6 square kilometers. More than half of the watershed consists of mountainous terrain.
The famous Minoh Falls is also located along the Minoh River. The waterfall and surrounding valley have long been popular destinations for residents and visitors from throughout Osaka.
The upper Minoh River runs through steep mountain valleys, while its lower reaches pass through increasingly urbanized areas. After World War II, population growth in the lower basin increased the potential impact of flooding, particularly during typhoons and periods of intense rainfall.
A major turning point came with the Hokusetsu Torrential Rain of 1967. Heavy rainfall caused serious flooding across northern Osaka, including areas around Senri New Town.
Following the disaster, Osaka Prefecture substantially revised its flood-control plans for the Inagawa River basin. Particular attention was given to the Minoh River and Aigawa River, where conventional river improvements were difficult because of the surrounding urban development and terrain.
As a result, a dam was considered an effective way to control flooding on the lower Minoh River, and the construction plan for Minoh River Dam was formulated in 1968.
The dam was designed using the rainfall and flooding conditions experienced during the 1967 Hokusetsu Torrential Rain as an important reference.
The planned flood discharge is 125 cubic meters per second. At the dam site, the dam is designed to reduce this flow by 109 cubic meters per second, allowing only approximately 16 cubic meters per second to be released downstream during the design flood.
This is expected to reduce the flood discharge at Ishibashi from approximately 400 cubic meters per second to 315 cubic meters per second, helping to protect more heavily populated areas downstream.
Minoh River Dam also has a role in maintaining the river’s flow during normal conditions. A controlled amount of water, approximately 0.08 to 0.14 cubic meters per second, is released to maintain downstream flow.
This function is important not only for water management but also for the aquatic environment and the ecosystems that depend on the river.
Minoh River Dam is primarily a flood-control dam, rather than a multipurpose dam designed for water supply or hydroelectric power generation.
It is classified as a subsidized flood-control dam, a type of dam constructed with government support to address flood-control needs. Minoh River Dam is regarded as one of the early examples of this type of flood-control facility in Japan.
Although there were no homes or agricultural fields requiring relocation at the construction site, environmental conservation presented a major challenge.
The area had already been designated as Meiji no Mori Minoh Quasi-National Park in 1967, making the protection of the surrounding forest and wildlife an important consideration during construction.
Osaka Prefecture conducted extensive environmental surveys and incorporated measures to protect the area’s natural environment. Particular attention was given to maintaining the water quality of Minoh Falls, restoring and greening forest areas, and protecting the habitat of the Japanese macaque, one of the characteristic animals of Minoh.
The artificial lake created by Minoh River Dam is called Yuyu Lake. The name was officially given in 1988 and combines the ideas represented by the Japanese characters for “play” and “tranquility.”
The name expresses the hope that the lake would become a welcoming place where people of all ages could relax and enjoy the natural surroundings.
Yuyu Lake is also notable for its unusual name. It is described as the only lake in Japan whose name is written entirely in katakana, giving it a distinctive and approachable character.
Minoh River Dam and Yuyu Lake lie within the natural environment of Meiji no Mori Minoh Quasi-National Park. The landscape changes dramatically with the seasons.
Fresh green foliage in spring creates a vibrant mountain landscape, while autumn foliage transforms the surrounding forest into a rich display of seasonal color. The combination of the reservoir, dam, and wooded mountains makes the area particularly attractive for walking and sightseeing.
Walking routes are available around the dam and through the surrounding forest. Trails extend from the lower reaches of the dam toward the Minoh Gorge, allowing visitors to experience the landscape at a slower pace.
Rather than being simply an engineering structure, Minoh River Dam can therefore be appreciated as part of the wider natural landscape of Minoh, where forests, streams, waterfalls, and mountain paths come together.
The Minoh mountains are home to populations of Japanese macaques. Visitors may occasionally encounter these wild animals around roads and forested areas.
Feeding the monkeys is prohibited, and visitors are asked to follow local regulations designed to protect both the animals and people. Drivers should also remain alert when traveling through the area, particularly where monkeys may appear near the road.
Minoh River Dam has continued to play a role in Osaka’s flood-control system while conservation measures have been maintained in the surrounding natural environment.
These environmental efforts were recognized in 1993, when the dam received an Environmental Award from the Environment Agency of Japan, the predecessor of today’s Ministry of the Environment.
Forest restoration and ecological conservation measures have continued around the dam, reflecting the importance of balancing flood-control infrastructure with the protection of Minoh’s natural landscape.
By public transportation, visitors can travel from Kita-Senri Station on the Hankyu Senri Line or Senri-Chuo Station on the Kita-Osaka Kyuko Line and take Hankyu Bus Route 29 toward Katsuoji. From the Katsuoji bus stop, the dam area can be reached on foot.
By car, the area can be approached via Osaka Prefectural Routes 43 and 4, following the road toward Katsuoji.
Traffic congestion can occur around the Minoh and Katsuoji area during busy holiday periods, particularly around New Year and the summer Obon holiday. Visitors traveling by car should allow extra time.
Minoh River Dam offers a fascinating combination of engineering, flood-control history, and natural scenery. Its 47-meter-high rockfill structure and Yuyu Lake form an impressive landscape within the forested mountains of Minoh.
At the same time, the dam has an important practical role in protecting downstream communities from flooding and maintaining the flow of the Minoh River. Its construction also demonstrates how infrastructure can be developed alongside efforts to protect a designated natural environment.
For travelers exploring Minoh, a visit to the dam and Yuyu Lake provides a different perspective on the region, complementing attractions such as Minoh Falls, the forest trails, historic temples, and the area’s abundant wildlife.