Stream bed configuration and stability following gabion weir placement to enhance salmonid production in a logged watershed subject to debris torrents

Abstract

Tandem V-shaped gabion weirs for improving spawning habitat for salmon (Oncorhynchus spp.) were installed to replace large organic debris at three sites below the terminus of a debris torrent in Sachs Creek, Queen Charlotte Islands. Stream conditions were compared between gabion and nearby control sites. The stability of added and entrapped gravel at all gabion sites was poor over the first winter and excessive scour threatened the integrity of the upstream steeper (3%) slope gabion site. However, the two gabion sites at a lower (1%) slope successfully stabilized spawning gravel in the 2nd year after installation, probably through a reduction in the local slope gradient and self-armouring of the high flow channels. Higher summer densities of juvenile coho salmon (Oncorhynchuskisutch (Walbaum)) and steelhead trout (Salmogairdneri Richardson) were recorded at the gabion sites (means, 1.2 and 0.33/m2, respectively) compared with the control sites (means, 0.89 and 0.10/m2). Underyearling coho fry were also significantly larger (p < 0.05) at gabion sites (mean, 50 mm) than at control sites (mean, 45 mm). Improved rearing habitat was created for coho juveniles by the gabions, a result of increased pool area and cover.