Riverbed is consisted of various habitat types, e.g., riffles, pools, side pools, back waters, etc. These habitats are changing in time and space by deposition and erosion processes caused by floods. These processes should be appropriately considered in the ecological evaluation. This article discusses riverbed characteristics of spawning redds for Ayu, in relation to changing patterns of habitats in the Tenryu River. Firstly, we conducted field surveys over November to December 2013 and found two spawning redds at small riffles in channels flowing from the spring welling up in a side pool. Physical characteristics of the spawning redds were identified based on the DO concentration and riverbed softness measured at the various habitats in the field. Spatial and temporal changing patterns of the habitats suitable for spawning redds were also investigated using the images of interval-recording- cameras installed at the study site for analyzing habitat scale changes and aerial photos for reach scale changes.

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 Spawning redds of Ayu fish (Plecoglossus altivelis) have been degraded by decrease in sediment supply and increase in turbidity derived from reservoir sedimentation in the lower reaches of the Tenryu River, Japan. In order to restore the riverbed for their suitable spawning redds, both newly deposited gravel riverbed and clear water flow are required. We conducted a series of habitat surveys in the two reaches of the Tenryu River to seek the spawning redds and suitable physical conditions. Then, we found the spawning redds of the fish in the upwelling-zones of hyporheic water in the secondary channels. Based on the results we conducted a sediment replenishment work in the secondary channel to create a new riffle and upwelling zone of hyporheic water. The work resulted in successful creation of upwelling zone but it lasted only a month and amount of hyporheic discharge decreased within three month. Results of the work also indicated that effective distance from the sediment replenishment site to the target riffle is very short less than 100m.

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