Effects of Debris Flows on In-channel Sediment Deposits and Habitat Structures in Steep Headwater Streams

Abstract

奈良県十津川村の人工林施業域において、土石流発生履歴の異なる42の源頭渓流を対象に、河道内土砂量、ハビタット要素の多さ、ミクロハビタット構造を調査した。土石流発生履歴と河道内土砂の関係は見られなかったが、土石流によって倒木、落葉枝ダムや堆積卓越型ミクロハビタットが減少することを明らかにした。底生動物群集を規定するミクロハビタットの多様性は土石流発生の履歴とともに変動することを示した。

Disturbance plays key roles maintaining species diversity and ecosystem function in wide range of ecosystems. We examined relationships between debris flow history and stream habitat structure (in-channel sediment deposits, abundance of streambed elements, occurrence of different types of microhabitats) using 42 headwater streams with different histories of debris flows in a managed forest watershed, Totsukawa, Nara. In-channel sediment deposit was highly correlated with channel slope and valley width, and related little to debris flow history. Number of large woody pieces and woody debris dams were more in streams with older occurrence of debris flow. Erosive microhabitats such as bedrock, splash zone, moss mat were common in all streams, while depositional microhabitats such as mud and leaf pack occurred only in streams with old (>30 yrs) debris flow, and sand was absent in streams with very old (>50 yrs) debris flow. These results indicate that stream habitat diversity is dynamically maintained in a repeated manner through debris flow history.