EFFECTS OF EXPERIMENTAL REMOVAL OF WOODY DEBRIS ON THE CHANNEL MORPHOLOGY OF A FOREST, GRAVEL-BED STREAM

Experimental removal of woody debris from a small, gravel-bed stream i n a forested basin resulted in dramatic redistribution of bed sediment and changes in bed topography. Removal of debris changed the primary flow path, thereby altering the size and location of bars and pools an d causing local bank erosion and channel widening. Marked bed adjustme nts occurred almost immediately following experimental treatment in Ma y 1987 and continued through to the end of the study period in 1991. I ncreased bed material mobility was attributable to destabilization of sediment storage sites by removal of debris but-tresses, elimination o f low-energy, backwater environments related to debris, and an inferre d increase in boundary shear stress resulting from the removal of debr is-related flow resistance. In contrast to these changes, which favore d sediment mobilization, deposition was favored by the elimination of debris-related scouring turbulence and by increased flow resistance fr om a developing sequence of alternate bars. A more regularly spaced se quence of alternate bars replaced the pretreatment bar sequence, whose location, size, and shape had been strongly influenced by large woody debris as well as by bank projections and channel curvature. Followin g initial readjustment of the stream bed during the first posttreatmen t year, loss of scouring turbulence and increased flow resistance from alternate bars resulted in deposition of approximately 44 m3 of sedim ent within the 96 m study reach. The loss of 5.2 m3 to bank erosion le ft a net increase in sediment storage of 39 m3. Mean spacing of thalwe g cross-overs and pools did not change measurably following debris rem oval, although variability of spacing between thalweg cross-overs tend ed to decrease with time as the location of bars stabilized. No consis tent pattern of change in mean residual depth of pools or in distribut ion of depths occurred within the first 4 years following debris remov al.