Decadal changes in river geomorphology, and its effect on chum salmon spawning habitat, Toyohira River, Japan

Natural spawning of Chum salmon has recovered in Toyohira River, an urban river in Sapporo, Japan, from the 1980s, a time when this species had almost disappeared from this river because of water pollution. Channelization has altered river geomorphology, which may have influenced spawning habitat. We divide salmon spawning habitat in Toyohira River into three sections and measure changes in riverbed morphology using transverse cross-sectional profiles from 1988-2015. The relationships between 8 topographic parameters, number of spawning redds and their decadal changes are examined. Principal component analysis reveals the strengths of relationships between these topographic parameters and numbers of spawning redds per kilometer post. Upper-and middle riverbed sections were significantly deeper, but spawning redds occurred in localized groundwater springs where water levels were higher than the riverbed. Significant lower river section changes included increased elevation gaps between lowest and highest riverbed of the low-water channel, narrowing of the water surface during spawning season, and decreased thalweg migration distance. Of two principal components, PC1 correlated with changes in riverbed elevation, elevation gaps between the lowest and highest riverbed in the low-water channel, and water surface width during spawning season, which represent "susceptibility to flood disturbance." PC2 correlated with groundwater level and streambed particle size, possibly indicative of upwelling water volume. The number of chum salmon spawning redds was significantly, positively correlated with PC1 and PC2. Because the coefficient of PC1 is decreasing with time, we surmise that appropriate habitat for spawning condition in Toyohira River has deteriorated over 27 years. As Chum salmon is a symbolic species in Toyohira River, it is important to keep dynamic feature of thalweg and gravel bed for natural spawning.