In the Azumagawa river, the washing away of riverbed blocks downstream from riverbed girdles created a 1m drop. In order to decrease the elevation difference and allow fish to climb without destroying the existing girdles, we added 3 new stonework girdles with a step of about 30cm each.
These riverbed girdles are formed as down-turned arches which are intended to gather water to erode pools on the downstream side, and also soften the ground further downstream for sedimentation and so creating habitat for fish that live between the stones, like freshwater goby. .
In addition, since no concrete was used when building the girdles, the sand accumulated around them can also help filter water going into downstream pools, so we hope that it can help improve water quality and spawning grounds as well.
When building the girdles for the Azumagawa River fish ladder, we used heavy stones in the arch-shaped stone pilings. In consideration of the river’s strength, we used massive stones of about 1.8m in length.
These stones were placed with their upstream ends low, so as not to impede the river flows. The stones were piled using a rock piling technique known as Tosa piling, which stresses the matching of stone on stone, and results in a stable structure without the need of concrete. To deal with turbulent water during floods, we also added anchors and wire. All this was done under the guidance of master masons.
