In the Takeharagawa River, running through Gero city in Gifu prefecture, consolidation work (for flood control) was built and the goals of riverbed preservation and irrigation were achieved, but the elevation differences this created stopped fish from going upstream. And so, as we performed aging maintenance on the consolidation work and sluice gates, we also built in stone fish ladders on their upper sections .
Within the Takeharagawa River, the elevation differences created by the aforementioned consolidation work and the downriver foot protection blocks, along with the flattened river bottom, eliminated the deep pools needed for habitat and spawning grounds for fish like char and amago trout. For this, we created a step and pool fish ladder based on the repeating riffles and pools seen in mountain rivers, while also setting a plan to manage erosion and sedimentation from flooding. (Left: Before construction, Below: After)
During the Edo Period, the original river, the Hidagawa, was used to carry lumber from the area’s bountiful forests to Edo, Kyoto and Nagoya. In addition, prior to the installation of any flood control or irrigation works, local residents lived close along the Takeharagawa, with the river a part of daily life used for washing clothes and vegetables, fishing, and swimming. This close connection between human life and nature influenced the local culture, and we wanted to try to preserve that. Thus, we considered ways to make the river easier for locals to use by building stairways and embankments as close as possible to residential homes.
The lower bedrock sections of the natural rock banks had been hollowed out by flooding, creating shelter for fish. Since these rocks create the walls of the fish ladder, we adopted Tosazumi piling to use rubble to fill in the gaps. This method of piling, unique to Aichi Prefecture, creates stone overhangs to provide shade, and we brought masons from Aichi to oversee the process.
The skills of these craftsmen are ingrained through physical sense and movement, and so take long years of practice to develop. They hold a deeply spiritual respect toward nature, born of their pursuit of skill and their engagement with the natural world.
In order to eliminate elevation differences between consolidation works and downstream riverbed, we put in two levels of riverbed girdles. These riverbed girdles are formed as down-turned arches which are intended to gather water to erode pools on the downstream side, which then allows sedimentation further downstream and so creating habitat for fish that live between the stones of the Takeharagawa River, like sculpin and catfish.
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.
Prior to implementing the plan, we performed literature searches and field work to check on fish inhabitation. As a result, not only did we find we could eliminate obstructions to fish migration by building a fish ladder, but also that there was a lack of various environmental; elements: deep pools for amago trout and wide stony environments for sculpin; gentle rocky areas for catfish and loach made through sedimentation and erosion and rocky shelter along riverbanks; riffles, steps and pools and more. And so, as described above, we developed a plan to create all of these environments
