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Abstract

Influence of woody structures on stream morphology and masu salmon habitat was monitored for four years in a suburban stream, in Tobetsu River of Ishikari River system. Three types of woody structures, log dams, wedge dams and deflector logs, were constructed to create deep pools and prevent lateral erosions, and an abandoned secondary channel was also excavated. After log placement, the channel environment such as water depth, mean current velocity and substrate composition was significantly changed. The deep plunge pools were formed on downstream of the wedge dams and rapid current were appeared along deflector logs. The fine sand and gravels were accumulated behind the woody structures. The reference and control sections were set on downstream of the improved sections, and the ecological influence of woody structures was evaluated by comparing the density of released masu salmon Oncorhynchus masou among the three sections. The juvenile masu salmons released from eyed embryo were abundant in June in the reference and improved sections where slower current such as secondary channel were created. During the August, the juvenile masu salmons were the most abundant in the reference section, and significant difference was not observed between the improved and control sections. The created pool unit should be important habitat for juvenile masu salmon. However, their abundance was remarkably decreased in October, which may largely due to angling. Although small portion of log exposed above water were rotted and flown away, whole structures were maintained during the four years despite of large flooding. Considering ecological and geomorphological effects, these structures should be introduced as one method to restore stream environment in Japan.