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 is a method of streamflow augmentation increasingly used to offset the impacts of anthropogenic flow alteration. However, to date, no studies have evaluated the effects of these small-scale flow augmentations on salmonids. 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, flow augmentation significantly improved dissolved oxygen and habitat connectivity at sites > 1.5 km downstream from the point of augmentation and had a marginal warming effect on stream temperature. During the wet year, both dissolved oxygen and water temperature effects were negligible. In both years, augmentation had a small but positive effect on invertebrate drift. Inter-pool movement of juvenile steelhead (Oncorhynchus mykiss) and stocked Coho Salmon (O. kisutch) increased due to augmentation during the dry summer. Flow augmentation also increased the survival probability for salmonids, with a larger effect during the dry summer (24% higher survival for Coho 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 and timed 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.

These data were generated to investigate the effect of flow augmentation into a small, intermittent stream: Porter Creek in Healdsburg, California, USA. The flow augmentation experiment released 13.9 L/s of water from an off-stream pond into the stream channel for one month in the summers of 2019 and 2020. Our objectives were (1) to evaluate the spatial effects of flow augmentation on habitat connectivity and (2) to measure the response of physical and biotic variables that influence the rearing profitability and fitness of summer-rearing salmonids to a flow manipulation of known proximity and magnitude. To address the first objective, we measured the extent of the wetted channel from the point of augmentation downstream to the creek outlet. Wetted channel extent below the augmentation release point was compared to an unaltered, control reach extending approximately 2 km upstream of the augmentation. To address the second objective, we estimated how flow augmentation affected dissolved oxygen, water temperature, hydraulic habitat, invertebrate drift, and inter-pool movement, growth, and survival of salmonids.