Restoration of Stream Channel and Masu Salmon Habitat by Woody Structures in a Suburban Stream, Central Hokkaido, Northern Japan

27 Mar Restoration of Stream Channel and Masu Salmon Habitat by Woody Structures in a Suburban Stream, Central Hokkaido, Northern Japan

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Abstract

北海道札幌市近郊の渓流において木製構造物を造成し，渓流の物理環境構造とサクラマス現存量の変化を4年間にわたって調査した. 設置構造物の種類は，カラマツ間伐材とヤチダモ天然木を使ったログダム，ウエッジダム，およびデフレクターログであり，緩やかな流れと落ち込み淵を造り，渓岸侵食を防ぐことを目的とした. 施工の結果，施工前に比べて水深，流速そして渓床基質は著しく変化した. 落ち込み淵はウエッジダムの下流側に形成され，デフレクターの下流には緩やかな流れ，先端に速い流れが形成された. 施工以前の渓床は大礫が卓越していたが，施行後小礫や砂などが木製構造物の上下流に堆積した. この構造物がサクラマスに与える影響を，下流に標準区，対照区を設定し発眼卵と稚魚を放流してその密度を比較した. この結果，6月には当歳魚は標準区で最も高く，ついで施工区，対照区の順となった. とくに流れの緩い二次流路や水際のくぼ地区間に多く分布していた. また7，8月の密度は標準区が最も高かったが，施工区と対照区の差は殆どなく，施工区内においては落ち込み淵が形成された区間で，サクラマス当歳魚が多く見られた. しかし，10月には釣りの影響と考えられるサクラマス当歳魚の密度低下が生じた. 木製構造物の一部は腐朽して流出したものも見られたが，大部分は渓床を維持·固定していた. このような木製構造物が渓流に与える生態的，地形的な影響を総合的に考慮すると，悪化した渓流環境を改善する一つの工法となると考えられた.

English 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.