天竜川における副流路の湧水環境創出によるアユ産卵床の造成実験

Abstract

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.