A combination of synoptic surveys and detailed studies was used to assess the effects of logging and mass wasting on salmonid spawning habitats in steepland streams on the Queen Charlotte Islands. Gravel composition was determined at 69 sites in 22 streams using a McNeil gravel sampler to collect 40 Kg samples of gravel at each site for sieve analysis, while gravel stability was measured at 96 sites in 12 streams using vertical erosion monitors. To estimate potential egg losses due to scouring, gravel scour depths were further compared to the vertical distribution of chum and coho salmon eggs in recently completed redds.
Declines in coho egg to fry survival due to logging and/or mass wasting related increases in fine sediment levels are estimated at 15 to 20%. Logging alone accounted for as much of the decline in gravel quality as mass wasting upstream. This indicates that the cumulative impacts of other sediment sources associated with timber harvesting may be at least as great as those caused by a single conspicuous event like a debris torrent upstream. Where logging and mass wasting upstream occurred together there were only slight additional increases in the amount of fine sediments present in spawning gravels. Further increases tended to be obscured by increased gravel scour resulting from large-scale changes in channel morphology. In logged stream reaches directly affected by mass wasting, or in streams where bedloads appeared to be excessive because of mass wasting events upstream, estimated egg losses due to scouring ranged from 66-86% for chum salmon and 45-70% for coho salmon. In other logged but otherwise stable stream reaches with no recent mass wasting, scour related egg losses ranged from 2-14% for chum salmon and 0-4% for coho salmon.
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Updated December 16, 2008