04/11/2025
Koajiro no Mori (Forest of Koajiro)
Yuji Kishi, Ph.D., Professor Emeritus, Keio University
President, Koajiro NPO Coordinating Field Activities
[email protected]
Overview
Koajiro no Mori, located in Miura City, Kanagawa Prefecture, is a rare and precious natural conservation area covering about 70 hectares. It is the only site in the Kanto region where an entire watershed ecosystem—from forest and wetlands to tidal flats and the sea—remains almost completely intact and free from artificial structures such as roads or housing developments. The forest centers on the 浦の川 (Ura-no-kawa) River, which flows through the valley and connects diverse habitats into a continuous natural environment.
Biodiversity and Symbolic Species
This rich ecosystem supports more than 2,000 species of plants and animals. Among them, the red-clawed crab (Chiromantes haematocheir), locally called アカテガニ (Akategani), symbolizes the harmony between forest, river, and sea. Adults live on land within the forest, while their larvae spend about a month in the sea each summer before returning inland after several molts. Fireflies and various dragonfly species have also reappeared, thanks to ongoing conservation efforts that maintain clean river water and thriving wetland environments.
Visiting the Forest
Visitors can explore the forest along a narrow wooden boardwalk designed to protect the fragile soil and the underlying narrow stream while allowing an immersive experience of its vegetation and wildlife. The one-hour route leads to the 白髭神社 (Shirahige Shrine) and the Koajiro Fishermen’s Port, offering scenic and educational encounters with the ecosystem.
Collaborative Conservation System
Koajiro no Mori is jointly protected by Kanagawa Prefecture, Miura City, the local NPO (Koajiro NPO Coordinating Field Activities), and the Kanagawa Trust Midori Zaidan, a semi-official environmental foundation. These four organizations cooperate to preserve biodiversity by controlling invasive species, maintaining stable river flow, rejuvenating wetland vegetation, and managing the health of trees and forests to sustain ecological diversity.
Environmental Education and Research Value
Recognized for its importance in environmental education, Koajiro no Mori is conserved under the joint supervision of the Ministry of Land, Infrastructure, Transport and Tourism, Kanagawa Prefecture, Miura City, the Kanagawa Trust Green Foundation, and the Koajiro NPO. As the entire watershed—including the Ura-no-kawa River and its tributary basins—is conserved and continuously managed by scientific initiative of NPO, the area provides a unique opportunity to observe the origins of river systems and understand the stepwise landscape changes caused by erosion, transport, and sedimentation. At the river mouth, the marsh and tidal flat reveal traces of ancient tidal processes from about 6,500 years ago, providing valuable insight into geological and ecological evolution.
Conclusion and Reference
Koajiro no Mori stands as a rare sanctuary that embodies the complete watershed ecosystem of the Kanto region. It offers visitors and learners a chance to experience Japan’s natural heritage through direct contact with its flora, fauna, and landscape history.
For further reading, see my book 奇跡の自然の守り方 (How to Protect a Miraculous Nature) by Yuji Kishi and Yanase, published by Chikuma Primer Shinsho.
For inquiries or guided visits, contact Yuji Kishi, Ph.D., Professor Emeritus, Keio University.
(Nov, 5, 2025)
