Tandem V-shaped gabion weirs for improving spawning habitat for pink (Oncorhynchus gorbuscha) and chum salmon (0. keta) in landslide-damaged streams were installed and examined in Sachs Creek, Queen Charlotte Islands. Spawner use and egg survival, as well as stability and quality of gravels entrapped by the structures were compared to that of natural control sites. Two gabion sites at 1% slope gradient attracted moderate spawner usage within weeks of installation; a third gabion site at 3% slope gradient was beyond the upstream limit of pink and chum salmon but was used by coho (O. kisutch) spawners. Pink salmon egg survival appeared high in the first autumn and winter seasons after installation (approximately 11% to the "eyed" stage), and improvement is anticipated as the gabion sites undergo natural stabilization of streambed configurations. Gravel stability at all three gabion sites was poor over the first winter, and excessive scour threatened the integrity of the upstream, steeper slope gabion site within 1 year. However, the two gabion sites at 1% slope gradient successfully stabilized gravel over their second season of stormflows through local reductions in streambed slope gradients. The effects of gabions on gravel composition and intragravel permeability were variable. Demonstrated improvements in intragravel dissolved oxygen probably resulted from channel "roughness" added by the gabions. Intragravel dissolved oxygen had a significant (p < .05) positive correlation with egg survival. High juvenile coho and steelhead (Salmo gairdneri) densities at the gabion sites (average 1.6/m2 compared to 0.9/m2 at control sites) indicated high quality rearing habitat as an additional benefit. Achievement of cost effectiveness (recovery of construction costs through increases in returning adult pink salmon) was attainable in gabion sites well-used by spawners. The stability in streambed configurations provided by gabions in their second year would be useful in rehabilitating low-gradient reaches of streams damaged by landslides. Recommendations for gabion installation include their restriction to downstream reaches of less than 2% slope gradient, where moderate to heavy spawner usage would occur.
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Updated December 16, 2008