Ota River Headwaters, Hiroshima Prefecture: The Complete Picture of Clear Streams Flowing from Mount Kanmuri to Hiroshima Bay and the Appeal of the Headwater Region
The Ota River, a primary river representing Hiroshima Prefecture, has been the lifeline supporting the development of Hiroshima City. Its headwaters originate from Mount Kanmuri (elevation 1,339 m) in the Kannon district of Hatsukaichi City, Hiroshima Prefecture, and after a journey of approximately 103 km, flow into Hiroshima Bay in the Seto Inland Sea. This article provides a comprehensive explanation covering the geographical characteristics of the Ota River headwaters, water source forest conservation activities, and access methods to the headwater region.
Basic Information about the Ota River and the Location of Its Headwaters
The Ota River is a primary river with a basin in western Hiroshima Prefecture, with a total length of 103 km and a basin area of approximately 1,710 km². The headwaters of the Ota River flowing through Hiroshima City originate from Mount Kanmuri (kanmurieyama) in the Yoshiwa district of Hatsukaichi City, Hiroshima Prefecture.
Mount Kanmuri is a mountain with an elevation of 1,339 m, located near the tristate border of Hiroshima, Yamaguchi, and Shimane Prefectures. Clear water gushing from the northeastern slope of this mountain becomes the headwaters of the Ota River and begins to flow as the Yakeyama River. The headwater region is covered in deep forests, and a rich natural environment is maintained.
From the headwaters downstream, the Ota River successively merges with tributaries such as the Shibaki River, Tsutsuga River, Takiyama River, and Mizuuchi River, gradually increasing its water volume. Near Kabe, the Oomoji River (oomojigawa), Misasa River, and Neya River converge, and further below the Takase Weir, it separates and rejoins with the Furukawa, an old channel, before reaching the center of Hiroshima City.
Geographical Characteristics of the Ota River Headwater Region
Topography and Geology around Mount Kanmuri
The Ota River headwater region is located at the western end of the Chugoku Mountain Range, forming a steep mountainous terrain. The headwater region centered on Mount Kanmuri has a geological structure based on granite bedrock, characterized by deep V-shaped valleys formed through long-term erosion.
The geological layers of the Ota River basin are dominated by faults running from northeast to southwest, and this geological structure has greatly influenced the formation of the river’s course. The forests in the headwater region, under the combined influence of this geology and climate conditions, form deciduous broadleaf forest zones centered on beech and Mizunara oak.
Climate and Precipitation in the Headwater Region
The headwater region around Mount Kanmuri is influenced by both the Seto Inland Sea climate and the Japan Sea climate. Annual precipitation is approximately 2,000 mm, which is higher than that of Hiroshima City proper (approximately 1,500 mm), with particularly heavy concentration during the rainy season and typhoon season.
Winter snowfall is also abundant, and this snowmelt water also serves as an important water source for the Ota River. The combination of abundant precipitation in the headwater region and the water-retaining function of forests enables the Ota River to maintain stable water volume throughout the year.
The Forest of the Ota River Headwaters: Hiroshima City’s Water Source Conservation Project
Overview of the “Forest of the Ota River Headwaters” Project
The Hiroshima City Water Bureau commenced the “Forest of the Ota River Headwaters” project in fiscal year 1998 (Heisei 10) to commemorate the 100th anniversary of the founding of Hiroshima’s water system. This project aims to acquire forests in the headwater region where Mount Kanmuri, the source of the Ota River, is located, and develop them as a model water source forest, while also utilizing them as sites for forest conservation activities and forest learning.
The water source conservation forest acquired by Hiroshima City is located around Mount Kanmuri in the Yoshiwa district of Hatsukaichi City, with an area of approximately 400 hectares. This forest is positioned as an important asset for protecting the clear stream of the Ota River and passing it on to the next generation.
Role of Water Source Conservation Forests
A water source conservation forest is a forest that infiltrates precipitation into the ground and gradually releases it into rivers, thereby adjusting water volume and purifying water quality. The Forest of the Ota River Headwaters performs the following important roles:
- Water Volume Regulation Function: Forest soil stores rainwater, mitigates floods, and supplies stable water volume during dry seasons
- Water Quality Purification Function: The filtering effect of forest soil produces clean water
- Prevention of Sediment Runoff: Tree roots retain soil and prevent sediment disasters
- Biodiversity Conservation: Functions as habitat for diverse flora and fauna
Approximately 80% of Hiroshima City’s tap water is drawn from the Ota River, making water source forest conservation in the headwater region a critically important undertaking directly connected to citizens’ lives.
Forest Conservation Activities and Citizen Participation
The Hiroshima City Water Bureau regularly holds events such as “Let’s Go to the Forest of the Ota River Headwaters!” providing citizens with opportunities to visit the headwater region and participate in forest conservation activities. At these events, tree-planting activities, thinning experience, and nature observation in the headwater region are conducted, allowing participants to experientially learn about the importance of water sources.
Such initiatives serve an important role not merely in forest management, but also in environmental education for the next generation and raising awareness about the importance of water resources.
Tributaries and Water System of the Ota River
The Ota River water system is composed of the main stream of the Ota River and numerous tributaries. Major tributaries that converge from the headwater region to Hiroshima Bay include the following:
Major Tributaries in the Upper Reaches
Shibaki River: One of the major tributaries in the upper reaches of the Ota River, flowing through Aki-Takata Town.
Tsutsuga River: With a basin in the Tsutsuga district of Aki-Takata Town, it converges into the main stream of the Ota River.
Takiyama River: An important tributary forming the Sandankyo Gorge, it carries the clear water of Sandankyo, designated as a National Special Scenic Site, to the Ota River.
Mizuuchi River: Flowing through the Mizuuchi district of Aki-Takata Town, it is a tributary that enriches the water volume of the Ota River.
Tributaries in the Middle Reaches
Oomoji River (oomojigawa): Flowing through Asa-Kita Ward in Hiroshima City, it converges into the main stream of the Ota River near Kabe.
Misasa River: A major tributary flowing through Asa-Kita and Asa-Minami Wards in Hiroshima City, converging near Kabe.
Neya River: One of the tributaries that flows into the Ota River near Kabe.
Through the convergence of these tributaries, the Ota River becomes a great river with abundant water volume by the time it reaches Hiroshima City proper.
The Ota River and Hiroshima’s History
Formation of the Hiroshima Delta
The lower reaches of the Ota River form one of Japan’s most prominent deltaic regions, with sediment flowing from upstream accumulating over long periods to form the Hiroshima Plain. This delta became the foundation for Hiroshima City’s development.
The sediment deposition caused by the Ota River continues to the present day, with continuous topographical changes visible in the river mouth area. This natural process is deeply intertwined with the very origin of the city of Hiroshima.
Terumoto Mori and Hiroshima Castle
The origin of Hiroshima as a city began when Terumoto Mori built Hiroshima Castle on the Ota River delta in 1589 (Tensho 17). A delta previously characterized by marshland and wetlands was organized into a castle town through water management and land development.
The abundant water volume of the Ota River was utilized for the castle’s moats and daily water needs, and as a vital conduit for goods transport by water transport, it supported Hiroshima’s development. The starting point of today’s city of Hiroshima, with a population of approximately one million, can be said to lie precisely in the utilization of river resources like the Ota River.
Utilization and Development since Modern Times
Since the Meiji Period, the Ota River has been utilized more systematically as industrial water, agricultural water, and the source of tap water supply. Hiroshima City’s water system was established in 1898 (Meiji 31), and a system for supplying Ota River water to citizens’ lives was put in place.
Even since the Heisei Period, water management and utilization projects on the Ota River have continued to advance, with dam construction and river modification being implemented. These projects aim to achieve both flood prevention and stable water supply.
River Facilities and Dams in the Ota River Water System
The Ota River water system has multiple dams and weirs installed for purposes of flood control and water utilization.
Major Dam Facilities
Nukui Dam: A multipurpose dam constructed on the Takiyama River, a tributary of the Ota River, serving functions including flood control, water supply for tap water, and power generation.
Tarubai Dam: A dam constructed on the main stream of the Ota River, aimed at flood control and securing tap water supply.
These dams play important roles in efficiently managing Ota River water resources and realizing both the safety of downstream areas and stable water supply.
Takase Weir and Diversion Facilities
The Takase Weir is located in the lower reaches of the Ota River, where the old channel Furukawa separates and rejoins. This facility serves as a crucial point for water management and water utilization in Hiroshima City proper.
Sandankyo: A Scenic Site of the Ota River Tributary
The Sandankyo Gorge, formed by the Takiyama River, a tributary of the Ota River, is a scenic site boasting one of Japan’s finest gorge landscapes, designated as a National Special Scenic Site. Along the approximately 16 km-long gorge, scenic views including the “Five Great Vistas”—Kurobuchi, Sarutobi, Nidannotaki, Sandantaki, and Mitsutaki—dot the landscape.
The clear water of Sandankyo, like the Ota River headwater region, is nurtured by abundant forests. This clear water, converging into the main stream of the Ota River, also contributes to improving the water quality of the Ota River.
Sandankyo is also popular as a tourist destination, offering beautiful scenery for all seasons—fresh greenery in spring, coolness in summer, autumn leaves, and snowy landscapes in winter.
Ota River and the Hundred Select Waters
The middle reaches of the Ota River were selected in 1985 (Showa 60) as one of the “Hundred Select Waters” designated by the Ministry of the Environment (then the Environmental Agency). This selection recognized both the good water quality of the Ota River and the region’s efforts in water environment conservation.
The middle reaches of the Ota River selected for the Hundred Select Waters are characterized by maintaining a water environment that inherits the clear water from the headwater region while coexisting with people’s lives. This selection demonstrates that the Ota River possesses cultural and environmental value beyond being merely a water resource.
Access to the Ota River Headwater Region
How to Reach Mount Kanmuri and the Headwater Region
The area around Mount Kanmuri, where the Ota River’s headwaters are located, can be accessed via the following routes:
By Car:
- Head north from Hiroshima City center on National Route 186 toward the Yoshiwa district of Hatsukaichi City
- Travel time is approximately 1.5 to 2 hours
- From the Yoshiwa district, forest roads can be used to approach the headwater region
By Public Transportation:
- From Hiroshima Station on the JR Sanyo Line to Miyauchi Kushido Station, then by bus or taxi toward Yoshiwa
- However, access to the deepest part of the headwater region requires a vehicle
Precautions When Visiting
The headwater region is a deep mountain forest area, and visiting requires adequate preparation and caution:
- Season and Weather: Forest roads may be closed during winter due to snow
- Equipment: Prepare hiking boots, rain gear, maps, compass, and other basic equipment
- Safety Management: Avoid traveling alone and inform family or friends of your itinerary
- Environmental Conservation: Always take trash with you and exercise care not to damage nature
By participating in events such as “Let’s Go to the Forest of the Ota River Headwaters!” hosted by the Hiroshima City Water Bureau, you can safely visit the headwater region with a guide.
Water Quality and Environmental Conservation of the Ota River
Current Water Quality Conditions
The water quality of the Ota River remains extremely good from the headwater region through the upper reaches. In environmental standards classification, much of the upper reaches is designated as AA class (the strictest standards), maintaining clean water quality.
The water quality of the Ota River, also utilized as a water source for Hiroshima City’s tap water, is managed through regular monitoring, demonstrating favorable values in indicators such as coliform bacteria count, BOD (biochemical oxygen demand), and turbidity.
Efforts for Water Quality Conservation
Various initiatives are undertaken in the basin to protect the water quality of the Ota River:
Forest Conservation: Forest management in the headwater region maintains natural purification functions
Sewerage System Development: Increased sewerage penetration in basin municipalities prevents contamination from domestic wastewater
Factory Wastewater Regulation: Strict adherence to wastewater standards among businesses in the basin
Public Awareness Activities: Raising awareness through river cleanup activities and environmental education
Through these comprehensive efforts, the Ota River maintains relatively good water quality despite being an urban river.
Ecosystem of the Ota River
Flora and Fauna in the Headwater Region
The rich forests of the Ota River headwater region are inhabited by diverse flora and fauna.
Plants: Deciduous broadleaf trees including beech, Mizunara oak, and maple species are predominant, with ferns and alpine plants abundant on the forest floor.
Animals: Large mammals such as Asiatic black bears, Japanese sika deer, and wild boars inhabit the region, along with wild fowl including Japanese pheasants and blue-and-white flycatchers.
Aquatic Life: From the headwater region through the upper reaches, stream fish such as white-spotted char and land-locked masu salmon inhabit the waters, with abundant larvae of stoneflies and mayflies, indicator organisms of clear streams, frequently observed.
Activities of the Ota River Fisheries Cooperative
The Ota River Fisheries Cooperative works to conserve the river environment and manage aquatic resources. Through the release of sweetfish, yamame salmon, carp, and other fish species, development of spawning grounds, and monitoring of illegal fishing, it promotes sustainable river utilization.
For fishing enthusiasts, the Ota River is known as a high-quality field for stream and sweetfish fishing, with angling available upon purchase of a fishing license.
Disaster History and Flood Control of the Ota River
Past Flooding Disasters
The Ota River basin, owing to its topography and climate conditions, has historically experienced flood damage. Particularly during the rainy season and typhoon season, concentrated heavy rainfall frequently causes inundation.
Recent memory includes the August 2014 (Heisei 26) torrential rainfall disaster in northern Hiroshima City, during which major sediment disasters occurred in the tributary basin of the Ota River. Learning from this disaster, improvements to sediment control facilities and emergency evacuation systems have progressed.
History of Flood Control Measures
Flood control measures on the Ota River have been undertaken since the Edo Period. Since the Meiji Period, modern river modifications have begun, with staged implementation of dike construction, river channel expansion, and discharge channel construction.
In modern times, comprehensive flood control measures are implemented, including dam-based flood control, river modification, and installation of internal water drainage facilities. Additionally, soft measures such as hazard map creation and evacuation system development are emphasized.
Culture and History of the Ota River Headwater Region
History of the Yoshiwa District
The Yoshiwa district of Hatsukaichi City, where the Ota River’s headwaters are located, has developed historically as a mountain village engaged in forestry and agriculture. The abundant forest resources around Mount Kanmuri have been utilized as sites for timber production, supporting the regional economy.
Additionally, the Yoshiwa district has long been situated on an important route, serving as a traffic hub connecting Hiroshima with Yamaguchi and Shimane Prefectures. Currently, with tourist resources such as ski resorts and hot spring facilities developed, it has become a place of recreation for Hiroshima City residents.
Consciousness as a Water Source Village
The people of the Yoshiwa district are acutely aware that their region is a water source for Hiroshima City residents and have undertaken forest conservation and environmental protection efforts. The “Forest of the Ota River Headwaters” project through cooperation with Hiroshima City is realized through such regional consciousness and cooperation.
The conservation of the headwater region is understood as an issue to be addressed on a basin-wide scale, not merely a Hiroshima City problem.
The Ota River and the Lives of Hiroshima Citizens
Importance as a Tap Water Source
Approximately 80% of Hiroshima City’s tap water is drawn from the Ota River, making it an indispensable water resource for citizens’ lives. The Ota River water that sustains the life of approximately 1.2 million residents of Hiroshima City is protected through the abundant forests of the headwater region and water quality conservation efforts throughout the basin.
The Hiroshima City Water Bureau operates multiple water intake facilities on the Ota River, processing water at purification plants before distributing it throughout the city. Water source conservation forest development in the headwater region can be said to be the most upstream endeavor of this water supply system.
Recreation and Tourism
The Ota River and its tributary regions also serve as important recreation sites for citizens. Various activities are enjoyed in the river bed areas, including walking, cycling, fishing, and canoeing.
Particularly the upper basin scenic sites, including Sandankyo, hold high value as tourist resources, attracting numerous tourists from within and outside the prefecture. The natural environment of the headwater region is also utilized as a site for ecotourism and environmental education.
Issues and Prospects in Ota River Headwater Conservation
Current Challenges
Several challenges exist in the conservation of the Ota River headwater region:
Aging and Management of Forests: The aging of planted forests advances, necessitating appropriate thinning and renewal.
Wildlife Damage Control: Increasing forest damage from deer and wild boar requires countermeasures.
Shortage of Successors: Forest management succession faces challenges due to declining numbers of forestry workers.
Climate Change Impacts: Adaptation to climate change effects such as increased concentrated rainfall is necessary.
Future-Oriented Efforts
To achieve sustainable headwater region conservation, the following efforts are being advanced:
Participation of Diverse Stakeholders: Creation of frameworks for cooperation among government, corporations, civic organizations, and local residents
Promotion of Environmental Education: Transmission of water source conservation consciousness to the next generation
Scientific Monitoring: Continuous monitoring of forest conditions and water quality, implementing management based on data
Basin-Wide Cooperation: Integrated conservation activities from the headwater region to the river mouth
Through such efforts, the goal is to pass on the rich natural environment of the Ota River headwater region to future generations.
Conclusion: The Value of the Ota River Headwaters and Our Role
The Ota River, originating from Mount Kanmuri in the Yoshiwa district of Hatsukaichi City, Hiroshima Prefecture, completes a journey of 103 km to flow into Hiroshima Bay, serving as a primary river representative of Hiroshima Prefecture. Its headwater region, covered in abundant forests as a water source forest, plays an important role in supporting the lives of Hiroshima City residents.
The value of the Ota River headwater region extends beyond merely supplying water resources. It possesses multifaceted functions including biodiversity conservation, sediment disaster prevention, landscape preservation, and provision of recreation sites.
Conservation activities, including the Hiroshima City Water Bureau’s “Forest of the Ota River Headwaters” project, represent efforts to protect the value of the headwater region and pass it on to the next generation. However, headwater region conservation cannot be realized by government alone.
It is important for each of us to know where the source of the tap water we use daily originates, how that water is protected, use water carefully, participate in environmental conservation activities, and convey its importance to the next generation.
Visiting the Ota River headwater region, seeing the source of clear water with your own eyes, and experiencing its importance firsthand can be a first step toward conservation. Participating in headwater region tours hosted by the Hiroshima City Water Bureau is also recommended to learn about the current state of water source forests.
The Ota River is a river of life that nurtured Hiroshima’s history, supports our present lives, and continues to flow into the future. Protecting the clear water of Mount Kanmuri, its source, is both a responsibility of those of us living in the basin and a gift to the future.
