The Tamarai River is a Class A river belonging to the Ōno River system. It flows through Kumamoto and Oita prefectures, with its headwaters in the Aso-Kuju National Park, an area known for its rich natural environment and distinctive volcanic landscape.
From its source region around Senomoto Plateau in Kumamoto Prefecture, the river passes through the countryside of central Kyushu, collecting water from numerous tributaries and springs along the way. Its clear streams, deep valleys, and connections with famous natural springs make it an important part of the region’s landscape.
The Tamarai River begins around Senomoto Plateau and flows along the western side of the hilly pastureland of Ueda-jiri Farm. Along its course, it joins streams descending from the eastern side of the surrounding terrain.
Further downstream, the river receives a substantial supply of water from Ikeyama Spring, a celebrated spring in Ubuyama Village that has been selected as one of Japan’s 100 Remarkable Waters. Numerous small mountain streams flow into the river, increasing its volume as it passes through the landscape.
The Tamarai River eventually joins the Ōso River, which flows through Ubuyama Village in Aso District. The combined waters continue toward the border between Kumamoto and Oita prefectures.
After entering Oita Prefecture, the Tamarai River cuts through the landscape to form a V-shaped valley. At Uotsurido Falls, it joins the Hakeai River, adding to the complex network of waterways in this mountainous region.
Farther downstream, it receives water from rivers originating in the Taketa Spring Group, a collection of natural springs in and around Taketa City. These tributaries contribute to the area’s varied topography as the river continues toward Taketa.
Eventually, the Tamarai River joins the Ōno River. Its waters also form part of the artificial lake created by the Taketa Adjustment Reservoir Weir, associated with Taketa Dam, also known as Uozumi Dam.
Within Ubuyama Village in Kumamoto Prefecture, the Tamarai River was traditionally known as the Yamaga River, or Yamagagawa. The local name was even included in the school song of a junior high school in the village, reflecting its deep roots in the community.
In 1966, when the river was designated a Class A river, its name was standardized as Tamarai River, following the name used downstream in Taketa City, Oita Prefecture.
In 2018, junior high school students in Ubuyama raised the issue at the village’s children’s assembly and proposed restoring the traditional name, Yamaga River. The proposal led to consideration of whether the river’s name could be changed back within the village, preserving the local name and its historical significance.
Like many rivers in mountainous areas, the Tamarai River has repeatedly flooded and caused serious damage to surrounding communities. Major flood events affecting the Hōhi region included the 1953 West Japan Flood, the heavy rainfall of July 1982, and the prolonged seasonal rain-front flooding of 1990.
The Hōhi flood disaster was particularly severe. In Taketa City, it caused four deaths and 33 injuries, with total damage estimated at approximately 27.5 billion yen. These disasters highlighted the challenges of managing river water in a region characterized by steep valleys and heavy rainfall.
In response to repeated flooding, the Taketa Emergency Flood-Control Dam Construction Project began in 1991. The plan called for the construction of Inaba Dam on the Inaba River and Tamarai Dam on the main stream of the Tamarai River.
The location and design of Tamarai Dam were determined in 2009. However, following a change of national government, the project was reviewed in 2010. Its continuation was ultimately approved in 2011.
The original schedule had anticipated completion of the dam in 2017. The project became a major subject of discussion because of the relationship between flood prevention, public spending, and the need to protect communities along the river.
During the Northern Kyushu heavy rainfall disaster of 2012, areas where dams had already been completed did not experience the same kind of damage. In Taketa City, however, the Tamarai River overflowed and caused extensive destruction.
Bridges collapsed and houses were swept away. The delay in the dam project, which had been affected by government project reviews, became a subject of public criticism.
However, investigations by specialists indicated that the accumulation of driftwood was a major factor in raising the water level. The disaster demonstrated the complexity of flood management in steep river valleys, where debris can obstruct water flow and worsen flooding.
Ikeyama Spring is located in Ubuyama Village, Aso District, Kumamoto Prefecture. It is a celebrated natural spring selected in 1985 as one of Japan’s 100 Remarkable Waters, a national selection recognizing notable sources of high-quality water.
The spring maintains a water temperature of approximately 13.5°C throughout the year. It produces a plentiful supply of intermediate-hardness water, with approximately 30 metric tons flowing out every minute.
The spring is an important natural resource for the surrounding community. In recent years, the sale of spring water has also become part of local revitalization efforts, helping promote the village and its natural environment.
The abundant water from Ikeyama Spring contributes to the Tamarai River’s flow, illustrating the close connection between the region’s groundwater resources, mountain streams, and larger river systems.
From Miyaji Station on the JR Kyushu Hōhi Main Line, the Tamarai River area is approximately 40 minutes away by taxi.
The area is approximately 58 kilometers from the Kumamoto Interchange on the Kyushu Expressway. One convenient route is to use the junction near Ueda-jiri Farm along Yamanami Highway.
Because the Tamarai River extends across a wide area and passes through both Kumamoto and Oita prefectures, travel times depend on the specific section of the river being visited. Check local road conditions and choose the destination point in advance.
The Tamarai River flows through the following municipalities:
Kumamoto Prefecture: Minamioguni Town and Ubuyama Village in Aso District.
Oita Prefecture: Taketa City.
The Tamarai River receives water from numerous tributaries, including the following rivers:
Ōso River: A river flowing through Ubuyama Village that joins the Tamarai River system.
Hakeai River: A tributary that joins the Tamarai River near Uotsurido Falls in Oita Prefecture.
Takimizu River: One of the tributaries contributing to the river network.
Yamanokuchi River: A tributary of the Tamarai River system.
Fujito River: Another stream that joins the wider river network.
Yagura River: A tributary within the Tamarai River system.
The Tamarai River is an important part of the natural landscape of central Kyushu. Originating around Senomoto Plateau, it gathers water from mountain streams and the celebrated Ikeyama Spring before passing through deep valleys in Oita Prefecture and joining the Ōno River.
The river also tells a story of the relationship between people and nature. Its traditional local name, repeated flood disasters, and the long-running efforts to improve flood control reflect the challenges faced by communities living alongside mountain rivers.
For visitors exploring the Aso and Ubuyama areas, the Tamarai River offers a way to appreciate the region’s clear waters, forested valleys, and interconnected springs and streams, while learning about the natural forces that have shaped local history.