Features of Nature (Stream Ecosystem)

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A Green Valley Produced by
Water Account Stability

Oirase is known for its scenery with features like a natural Bonsai  amongst many dotted mossy boulders. Most rivers rise when it rains heavily, and mosses never stay on rocks as being washed by torrents. When boulders in rivers get mossy, it means that the stream has a stable water volume. The source of Oirase is a huge “natural dam” called Lake Towada. When it rains a lot, rainwater is received by the lake, which enables it to reduce the amount of outflow water into the stream. Additional factor to prevent floods is a gentle slope. Around the stable stream develops a forest. Oirase has hardly ever had repair works and is thickly covered with vegetation even to the river limit by a stable water amount, which is the most suitable green valley for us to learn the relation between forests and rivers, and how rivers should be. A natural river with accessibility for humans is now very precious.

Discover the Mechanism of a Stream


Please try to pay attention to the water flow. You will notice that a river runs not just simply, directly, and evenly, but in fact it runs through a very complicated system with various types of flow, including the side stream, which flows from the center and runs in different directions, the line of stream, and a back current to swirl around both sides of the stream.

A Forest that Nurtures Water



Almost all waterfalls in Oirase stem from tributaries, except Choshi Otaki Falls  (Choshi Otaki ), the only falls on the main current of the stream. Water from the falls is mostly spring water brought into and coming out of the beech forest. The basis of a forest that nurtures water lies on fallen leaves. Rainwater collects on every tree trunk, drips from canopies, and flows down to reach the forest floor. Piled up with fallen leaves with a layer of humus under it, it acts as a dam to store water. Water and snow that fall on forests penetrates through a bed of fallen leaves, is cultivated, and slowly flows through tributaries and waterfalls to a stream.


The Utility of Fallen Trees

Trees falling in urban rivers are removed, without exception, to prevent hydrological problems. Since Oirase Stream is located in the special protection area of the national park, fallen trees are basically left as they naturally are, unless they do not impede footpaths and roadways. Thus, Oirase has become a precious area where you can observe up close what happens when fallen trees are left as they are.