Flow augmentation from off-channel storage improves salmonid habitat and survival

Abstract

Objective: In the western United States, juvenile salmon Oncorhynchus spp. and steelhead O. mykiss are especially vulnerable to streamflow depletion in the dry season. Releasing water from off-channel storage into small streams is a novel restoration strategy to offset impacts from anthropogenic flow alteration on salmonid fishes. To date, no studies have evaluated the ecological effects of small-scale flow augmentations. Here, we quantify the effects of one such augmentation project on habitat connectivity, water quality, invertebrate drift, and juvenile salmonid movement and survival. Methods: Our study took place in a northern California stream and included an unusually wet summer (2019) and a more typical dry summer (2020). We used categorical and time-series analyses in a before–after, control–impact (BACI) design, along with capture–mark–recapture methods to evaluate the ecological impacts of a 13.9-L/s flow augmentation. Result: We found that differences in ambient streamflows between the two years mediated the physical and ecological effects of the flow augmentation treatment. In the dry year, habitat connectivity and dissolved oxygen markedly increased at sites over 1.5 km downstream from the point of augmentation, whereas during the wet year, the effects on those variables were negligible. In both years, invertebrate drift marginally increased after augmentation. Interpool movement of wild juvenile steelhead and stocked Coho Salmon O. kisutch increased after augmentation during the dry summer but not during the wet summer. Flow augmentation increased the survival probability for salmonids, with a larger effect during the dry summer (24% higher survival for Coho Salmon and 20% higher survival for steelhead) than during the wet summer (no effect was observed for steelhead survival, and Coho Salmon survival increased by 11%). Conclusion: This study indicates that appropriately designed small-scale flow augmentations can improve conditions for rearing salmonids in small streams, particularly during dry years. More broadly, it provides empirical evidence that efforts to restore summer streamflow in small, salmon-bearing streams can yield significant ecological benefits.