Kozagawa Prefectural Natural Park stretches mainly across Kozagawa Town in Higashimuro District, Wakayama Prefecture. The park is known for its beautiful river valleys, enormous rock formations, rugged mountain landscapes, and rich forests.
Along the Kozagawa River and its tributaries are a remarkable collection of unusual natural features, including the enormous Ichimai-iwa Monolith, the towering Tenchū-iwa Rock, and the unique rocky riverbed of Takinohai.
The landscape was strongly shaped by large-scale volcanic activity that occurred approximately 15 million years ago. Over an immense period of time, weathering and erosion gradually transformed the volcanic rocks into the spectacular cliffs, giant boulders, and strange rock formations seen today.
The park is also important for its cultural heritage. Local legends, traditional festivals, and ways of life closely connected with the river have been passed down through generations.
With scenery that changes dramatically from season to season, Kozagawa Prefectural Natural Park attracts travelers, hikers, nature lovers, and photographers who want to experience the wilder side of the Kii Peninsula.
One of the most important geological features of the region is the Kozagawa Arc-Shaped Dike.
This enormous geological structure extends for approximately 22 kilometers in an arc from the Kozagawa River area toward Taiji Town. It was formed when magma rose from deep underground during ancient volcanic activity and gradually cooled and solidified.
Approximately 15 million years ago, the southern Kii Peninsula experienced enormous volcanic activity associated with a large caldera system.
The area known as the Kumano Caldera is thought to have once extended across a vast area between present-day Kushimoto and Hongu. The original caldera landscape has been greatly altered by erosion, but parts of its underground geological structure are still visible at the surface today.
The Kozagawa Arc-Shaped Dike is one of the most important examples of this ancient volcanic structure.
The dike consists of geological materials including rhyolitic pyroclastic rocks and Kumano granitic porphyry. These rocks have produced the region’s distinctive formations, including Ichimai-iwa, Tenchū-iwa, and Mushikui-iwa.
Because of its outstanding geological value, the Kozagawa Arc-Shaped Dike has been selected as one of Japan’s 100 Important Geological Sites, together with the famous Hashigui-iwa rock formations.
One of the most famous sights in Kozagawa is Ichimai-iwa, a giant rock wall designated as a National Natural Monument of Japan.
The enormous monolith rises approximately 100 meters high and stretches about 500 meters wide, making it one of the largest single rock formations in Japan.
Seen from the banks of the Kozagawa River, the immense rock face is overwhelming in scale. Its appearance changes with the seasons: cherry blossoms add color in spring, fresh green foliage appears in early summer, and autumn leaves decorate the surrounding landscape later in the year.
Ichimai-iwa is also associated with a fascinating local legend.
According to the story, a monster that ate rocks once tried to bite into the enormous monolith. However, it was chased away by a dog before it could finish. The long vertical grooves visible on the surface of the rock are said to be the monster’s bite marks.
Today, Michi-no-Eki Ichimai-iwa Monolith stands in front of the rock, providing parking and rest facilities. The area is also popular with families who come to enjoy camping and playing in the river.
Further upstream from Ichimai-iwa stands Tenchū-iwa, a dramatic rock formation that rises almost vertically from beside the Kozagawa River.
Its towering shape looks like a gigantic stone pillar reaching toward the sky. It is also known as Yakeni-iwa and is considered one of the representative geological formations of the Kozagawa Arc-Shaped Dike.
Surrounded by lush green mountains, the sharp rock tower creates an impressive contrast between the rugged power of the earth and the softness of the surrounding forest.
Takinohai, located along the Ogawa River, a tributary of the Kozagawa, is one of the most unusual natural landscapes in Wakayama Prefecture.
For approximately 200 meters, the riverbed is covered with countless round holes known as potholes.
These formations, called ōketsu in Japanese, were created over a very long period as flowing water caused small stones to rotate against the bedrock, gradually grinding circular depressions into the rock.
The countless holes of different sizes create a landscape that looks almost like a work of natural art.
At the center of the area is a waterfall approximately 8 meters high, where clear water rushes through the rocks.
During summer, ayu sweetfish gather below the waterfall, and local fishermen practice a traditional fishing technique known as tonton-zuri. Around July, visitors may also be lucky enough to see bonzuhaze and Yoshinobori gobies climbing the rocky surfaces of the river.
Mushikui-iwa, located in Ikenoyama, Kozagawa Town, is a massive rock covered with countless holes.
Years of weathering and erosion have created a honeycomb-like surface across the rock. Because the formation appears to have been “eaten by insects,” it became known as Mushikui-iwa, literally “Insect-Eaten Rock.”
The unusual formation is also recognized as a geosite within the Nanki-Kumano Geopark, allowing visitors to see the effects of geological processes that have unfolded over immense periods of time.
Botan-iwa, located in the Tsukinose area, is another fascinating rock formation.
Numerous holes and patterns appear across its surface, creating shapes reminiscent of peony flowers. The rock was formed from pyroclastic material, including volcanic ash, that gradually weathered over time.
Its unusual patterns demonstrate how erosion can transform volcanic rock into surprisingly delicate and beautiful forms.
Shōjohō is a striking rock peak rising beside the Kozagawa River. Its reflection on the calm surface of the river creates a particularly beautiful scene.
Parking facilities are available nearby, and the area becomes popular during summer when visitors come to enjoy river activities and barbecues.
Kozagawa Prefectural Natural Park protects extensive areas of natural forest representative of the southern Kii Peninsula.
Around Oashidani, Mount Ōmori, and Mount Shōgun, forests contain a mixture of trees such as zelkova, fir, hemlock, Nothotsuga longibracteata, and Japanese cypress.
These forests provide important habitats for a wide variety of plants and animals and contribute to the health of the clear rivers flowing through the region.
The Ogawa River and its surrounding wetlands are also home to rare insects, including the Hatchō-tonbo, one of the smallest dragonflies in the world.
The presence of such species is closely connected with the area’s clean water, healthy forests, and relatively undisturbed natural environment.
For nature lovers, the park offers an opportunity to observe not only spectacular landscapes but also the smaller creatures that depend on these ecosystems.
Nature is not the only attraction of the Kozagawa region. The river landscape is closely connected with a rich cultural and spiritual heritage.
Kochijima Island, which lies in the Kozagawa River, is regarded as a sacred place where the island itself is worshipped as a divine object.
There is no shrine building or torii gate on the island. Instead, the island itself represents an ancient form of nature worship in which the natural landscape is considered sacred.
Every year, the Kōchi Festival is held around Kochijima. Traditional boats travel along the Kozagawa River, while the Koza-style lion dance is performed on the riverbank.
The festival has been designated an Important Intangible Folk Cultural Property of Japan and remains one of the most important expressions of the traditional culture of the Kozagawa region.
The sight of decorated boats moving along the clear river, accompanied by traditional music and performances, demonstrates how closely nature, spirituality, and community life have been connected in this part of the Kii Peninsula.
Kozagawa Prefectural Natural Park forms an important part of the Nanki-Kumano Geopark, a vast geological area extending across southern Wakayama Prefecture and into Totsukawa Village in Nara Prefecture.
The Nanki-Kumano Geopark was recognized as a Japanese Geopark in 2014. Its landscapes reveal the effects of plate subduction, volcanic activity, uplift, weathering, and erosion that have shaped the Kii Peninsula over geological time.
Throughout the region, visitors can encounter dramatic geological landmarks such as Ichimai-iwa, Hashigui-iwa, and Nachi Falls.
In this sense, the Kozagawa River basin can be viewed as a vast open-air museum of the Earth’s history.
Kozagawa Prefectural Natural Park is much more than a collection of scenic viewpoints. Its giant rock formations, clear rivers, forests, wildlife, legends, and traditional festivals are all connected by the long geological and cultural history of the Kii Peninsula.
The enormous Ichimai-iwa, the towering Tenchū-iwa, the strange potholes of Takinohai, and the honeycomb-like Mushikui-iwa all reveal different chapters in the story of the earth.
At the same time, Kochijima and the Kōchi Festival show how local people have developed spiritual traditions and community customs alongside the natural environment.
For travelers exploring the Kumano region, Kozagawa Prefectural Natural Park offers a rare opportunity to experience the immense power of nature while also discovering the culture and beliefs that have grown from it.
With its giant rocks, clear mountain rivers, deep forests, unusual geological formations, and living traditions, Kozagawa is a place where the natural and human histories of the Kii Peninsula come together in a truly remarkable landscape.