This study compares the morphology of coastal, gravel-bed streams in two logged, one logged and debris torrented, and two unlogged watersheds in the Queen Charlotte Islands. The influence of both logging and direct mass wasting events on channel morphology and, consequently, on the in-stream physical habitat of salmonids is quantified. The comparison also provides a basis for determining habitat rehabilitation criteria for damaged channels. Pools and riffles are discussed in detail.
Longitudinal profiles of long channel segments indicated that channels in watersheds either logged to the channel banks by old techniques (but not torrented), or logged by more recent methods (but which have experienced direct mass wastage events) result in reduced pool-to-pool spacings, increased riffle heights, and altered pool depths. Increases in stored sediment volumes produce proportionally larger riffles and smaller pools, thus reducing available rearing havitat. No significant differences in pool and riffle characteristics were found between unlogged watersheds and those logged by contemporary techniques and not torrented.
Large organic debris (LOD) appeared to be a major influence on stream morphology. A comparison of reaches indicated that LOD characteristics were altered in older logged and torrented channels, including a shift in the LOD size distribution, with smaller in-channel material being more prevalent. Orientation of material was also altered: more LOD lay parallel to the flow direction, as opposed to the more common diagonal orientation found in unlogged and non-torrented channels. This shift in orientation was responsible for a reduction in channel width and depth variability, reduced sediment texture variability, fewer cut banks, smaller pool areas, and decreased channel stability. These changes resulted in reduced habitat diversity and quality. No morphological differences were detected between unlogged and recently logged (non-mass wasted) reaches.
Results of this study suggest that the architecture of unlogged channels can be duplicated to help rehabilitate streams damaged by mass wasting. LOD should play a major role in rehabilitation, and several recommendations for channel restoration are made.
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Updated December 16, 2008