It is said that in the Kanto 関東 and Tokai 東海 Regions of Honshu Island 本州 the Koajiro Forest is the only remaining environment where the entire watershed is preserved with the minimum invasion of human construction. The area for the entrance to the forest from Route 134, with ASL 47.9m, is covered by a forest whose underground water system is the source for Uranokawa River 浦の川, a tiny 1.3km stream but the backbone of the ecosystem of drainage basin. From the entrance, a boardwalk goes down to Koajiro Bay. The wooden deck is the only artificial structure in the forest and does NOT nudge visitors to stray from the route. The path does not have much detour, and this is the key for the conservation of the area. It’s a familiar story ill-behaved tourists stomping precious natural resources and destroying nature. Stop such overtourism is important. That’s easy to understand, isn't it? 😉 Ho ho ho. The floor has another mission. It is a culvert preventing the area from aridification. Very important thing, and the source of uniqueness of Koajiro Forest.
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 I’ve been walking on boardwalk |
2025 is the 20th anniversary for Koajiro Forest for obtaining legal protection as a nature reserve. This year is also the 10th anniversary for the Forest to open to the public as a nature park. For celebrating the occasion, last February there was a symposium in which the players for protecting Koajiro Forest gathered and talked the “development” (oh, yes) of the place as a nature reserve. At that time, one of the players, Prof. Hiroichi Yanase 柳瀬博一 of Institute of Science Tokyo, showed us the backstage of Koajiro Forest. And he revealed to us the importance of boardwalk. To understand it, it’s better to start with the history of the place.
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| Route
of Koajiro Forest is like this: occasional trekking road with boardwalk. Here, we can see the next beginnings of the boardwalk over there. |
Koajiro Forest is located in the west shore of Miura Peninsula 三浦半島. The peninsula was created by plate tectonics of Eurasian and Philippines plates. The sediments accumulated in the deep ancient sea was upthrusted by humongous push by Philippines Plate to Eurasian one. Eruptions of undersea volcanos frequented and provided scoria that is sandwiched between deposits. Piling up sediments, scoria from volcanos, piling up, then pushed upward … The process is continuing for millions of years and has formed coastal terrace, which is Miura Peninsula in this case. For Koajiro Forest, until 120 thousand years ago, the place was undersea. Then about 100 thousand years ago, the rising occurred. This means, the geological base structure for Koajiro Forest is fragile sandstone, mudstone and occasional scoria. After it became land, loam layer fell from many volcanos, including Mt. Fuji, for ONLY 100 thousand years. The topsoil of Koajiro Forest, and surrounding area of Miura Peninsula, is not thick, 15-30cm at best, and water retention of the soil is not good. That’s the reason even today, the agriculture of Miura Peninsula is mainly vegetable cultivation, not rice needing lots of water during growing season.
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| Koajiro
Bay. It remains under water after the tectonic thrust, but very shallow sea. |
When you walk Miura Alps (; my post for April 29, 2016), you may notice lots of trees can grow large due to warm climate, but their roots are almost exposed, clinging to sandy and steep slopes along the hiking course. Do you think they can stand for hundreds of years as such? Nope. It’s easy for trees in the area falling. Still, humans established their villages for millennia. People cultivated the place for veggies and rice when it was possible. In Koajiro Forest, for more than 1000 years local fishermen raised rice using Uranokawa River and in hillside planted jolcham oaks for fuel, chestnuts for preserved food, pines for constructing boats, and cherries for comfort. The size of rice harvest was small, and large trees could collapse easily. Not an easy life. When in the 1960s locals became possible to live a modern life of petrol, and did not have to depend on the forest and Uranokawa River, they quickly left the area. The forest was abandoned.
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| A familiar forest scenery in Koajiro Forest |
Abandoned forests were ubiquitous in Miura 三浦 -Hayama 葉山 -Zushi 逗子 area during the 1960s to the 1980s. They often spread along a small river just like Koajiro. But complete neglect includes forgetting water flows. As the topsoil is very shallow for the area, water quickly washed away the basin. The soil from the ridge soon filled up the remaining wetlands downstream. The small valleys were worn deeper and the entire water basin covered by dark forests. The climate of Miura Peninsula could reach its climax with broadleaved evergreens, i.e. dark forest. Such forests and shadowy streams lose their biodiversity quickly. People did not want to enter such places. In contrast, Koajiro was not forgotten completely. After private owners left the place, Keikyu Corporation 京急 bought the Forest to develop housing and resort facilities. Also, the area is warm and has lots of rain. Rainwater steadily goes underground and flows out to Uranokawa River. The water system of Koajiro Forest has a certain level of dependability. The municipality which traditionally had water supply problem maintained the water station at the ridge of Koajiro Forest near Route 134. These new arrangements for land ownership did a kind of “good thing.” Municipal water control prevented Uranokawa River from sweeping away the shallow topsoil of the forest to the downstream. Keikyu patrolled the place to plan their business. The wetland started where the slope from Route 134 ended, i.e. the place the soil from the higher place to be deposited, was survived, though not as it is for today. Let me tell you what happened to Koajiro Forest after that next week. Please stay tuned. 😊
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| Ridge
Entrance for Koajiro Forest. This particular point is not in the Special Conservation Zone. When we start descending from here, we enter the nature reserve. Then, what is this place? Could you see a fence in this photo? It’s the fence surrounding the water station of Miura City, standing next to the entrance signboard of the Forest. Though practically defunct these days, the city office maintains the facility just in case. |
If you find environmental issues in Koajiro Forest, please make a contact with
Greenery Section, Environment Division
Yokosuka-Miura Region Prefectural Administration Center
横須賀三浦地域県政総合センター環境部みどり課
2-9-19 Hinode-machi, Yokosuka 238-0006
〒238-0006 横須賀市日の出町２-９-19
Phone: 046-823-0381
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