Evaluation of river restoration techniques using woody debris and boulders naturally generated from flood events

A stream that was severely damaged by heavy rainfall was restored using large woody debris and boulders from the site. The experimental site was a natural stream of the Horonai River system in northern Hokkaido, where intense rainfall triggered debris flow and flooding in 1998. Several types of woody structures were installed to stabilize sediment and gravel. A series of low check dams (log dams) constructed from large woody debris was installed along the main channel to prevent movement of new debris deposits. Complex grids of woody structures (debris catcher) were also constructed along alternating sides of the shore to capture drifting wood. Large boulders were selected, attached to steel wires, placed along the bottom of the channel, and buried (boulder cluster). Construction was carried out in 2000, and changes in the geomorphological characteristics, debris movement, vegetation recovery, and influence on stream-dwelling fishes were monitored until 2008. Most of the log dams were functional, and although evidence of minor lateral scouring was observed, no serious damage had occurred. The log dams were effective at creating deep plunge pools while maintaining a variety of water currents and substrates. The debris catcher captured drifting logs as well as provided rearing habitat for juvenile masusalmon (Oncorhynchus masou). The boulder clusters stabilized the stream channel, enabling tree seedlings to establish. Significantly more juvenile masu salmon were seen in June and October at the log dam section and in October at the debris catcher compared to the reference section. A similar result was observed for sculpin (Cottus nozawae) except in October. Generally, a large concrete dam is constructed after a flooding disaster, which seriously damages stream ecosystems. Given our results, we suggest that a combination of wood and boulder structures is an attractive alternative to concrete dams from ecological, economical, and landscape perspectives.