**6. Conclusions**

The flow conditions, velocity fields, and stability of artificially assembled boulders, in addition to the possibility for upstream migration routes for aquatic animals, were shown experimentally in response to various changes in discharges. It was found that, at down-ward slopes of 1/8.5, 1/12.5, and 1/25, a transverse gradient about 1/10 in the cross-section created a shallow water flow and a gap flow through the assembled boulders with low time-averaged velocity and low turbulence, regardless of the degree of the downward slope, even if discharges changed under normal conditions. The experimental results showed that a shallow water flow and a gap flow of assembled boulders could allow swimming fish, benthic fish, and crustaceans to migrate upstream. The assembled boulders were constructed in such a way to support each other and resist fluid forces, confirming the stability of the assembled boulders during floods. These findings were confirmed from the field construction. In addition, the upstream migration of swimming fishes (e.g. Plecoglossus altivelis, Tribolodon hakonensis, and Oncorhynchus masou ishika-wae) and benthic fishes (e.g. Rhinogobius flumineus and Cottus pollux) were observed at sites where consecutively assembled boulders were constructed in the Ohmu river. This study supports the practical application of assembled boulder installations for river improvement. The installation of assembled boulders can be applied to improve river environments, aquatic habitats such as refugees and aquatic animal migration routes. Research on consecutively assembled boulders is still in its early stages. Practically, six sites were installed in rivers of various sizes, and biological studies have not been carefully conducted. However, it is true that a variety of aquatic animals were observed at the upstream end of the consecutively assembled boulders. Further biological surveys may be needed. In addition, a protruding assemblage of boulders, a pool-type fish passage with assembled boulders, and ground sills of assembled boulders were constructed, but limits to the application of assembled boulders may need to be examined after a systematic investigation in the future.
