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

Abstract In the Western United States, juvenile salmon and steelhead 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, juvenile salmonid movement and survival. Our study took place in a Northern California stream and included an unusually wet summer (2019) and a more typical dry summer (2020). We found that differences in ambient streamflows between the two years mediated the physical and ecological effects of a 13.9 L/s augmentation treatment. In the dry year, habitat connectivity and dissolved oxygen markedly increased at sites > 1.5 km downstream from the point of augmentation, whereas during the wet year effects on those variables were negligible. In both years, invertebrate drift marginally increased following augmentation. Inter‐pool movement of wild juvenile steelhead ( Oncorhynchus mykiss ) and stocked Coho Salmon ( O. kisutch ) increased following augmentation during the dry summer but not 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 for steelhead), than during the wet summer (when no effect was observed for steelhead survival and Coho Salmon survival increased by 11%). 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.