倒流木の除去が河川地形および魚類生息場所におよぼす影響 [in Japanese] Effects of experimental removal of woody debris on channel morphology and fish habitats [in Japanese]
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Experimental removal of coarse woody debris (CWD) was conducted in a low-gradient, second-order stream covered by deciduous riparian forest in northern Hokkaido, Japan. We examined channel morphology, habitat variables and fish densities in 2 study reaches (cleaned and control, 100 m length) during autumn (Sept.-Oct.) of 1993, spring (May - Jun.) and autumn (Sept. - Oct.) of 1994. Immediately after the first survey, CWD and other small organic materials within the cleaned reach were removed by hand or hand-winch. Aggraded areas in the cross-sections of the cleaned reach were larger than those of the control reach during the autumn of 1993 - spring 1994. Cross-sectional changes tended to correlate with channel variables, such as curvature and width, in the cleaned reach, and with CWD amounts in the control reach. After CWD removal, small pools associated with CWD disappeared and pool number decreased, but pool volume did not vary. Cover area decreased, but depth, velocity, and substrate did not change in the cleaned reach. Density of juvenile masu salmon (Oncorhynchus masou) declined rapidly, but that of stone loach (Noemacheilus barbatulus toni), a benthic fish, did not show any significant changes.<BR>Deposition of sediment after CWD removal was associated with development of gravel bars at channel bends or wider sections. Pools were recreated by lateral scouring at the opposite sides of banks where gravel bars developed. We concluded that CWD pieces play an important role in forming pools and controlling pool number, although they are not effective in increasing pool volume in low-gradient meandering streams. Thus, a decline in masu salmon population could be attributed to a decrease in cover and pool number in this stream.
ECE 2(2), 179-190, 1999-11-19
Ecology and Civil Engineering Society