Ministry of Environment Environmental Stewardship Division Region 2 – Lower Mainland Ecosystems SectionPrepared by

Two different dredging techniques in similar channelized stream reaches of the same watershed were compared in a field trial in the lower Fraser Valley, near Chilliwack, British Columbia, during the late summer to early autumn of 2003 and 2004. The objective of this study was to assess the within-season and the one-year post impacts of drainage-channel maintenance on juvenile salmonid habitat. A reach of Marblehill Creek experienced a “hard ” bank-to-bank dredging with little-to-no utilization of fish habitat mitigation measures. The second treatment reach, in Big Ditch, experienced a “softer ” form of dredging where fish habitat structures, in the form of rip-rap rock weirs, were restored as the dredging was undertaken. The key variables used to determine effects through comparison between the treatments included: 1. an assessment of the relative abundance of juvenile fish using minnow trapping; 2. measurements of angular canopy density (ACD) along the stream channels (a proxy measurement for stream-channel shading); 3. an assessment of thalweg-velocity variability; and 4. the calculation of channel-roughness coefficients. The fish-use comparisons support four important conclusions regarding the juvenile salmonid habitat impacts relating to these dredging activities: 1. unmitigated channel dredging had a severe detrimental effect on the within-season relative abundance of over-summer rearing coho salmon (Oncorhynchus kisutch) (up to-94%) and coastal cutthroat trout (O. clarki clarki) (up to-