In Mediterranean-climate California and other arid regions of the Western United States, high human water demands are depleting streamflow and threatening freshwater biodiversity. To address these impacts, conservation organizations have explored strategies for protecting instream flows by reducing water diversions and, recently, augmenting dry season flows with water released from off-stream storage. Though limited in scale, such augmentation projects offer a unique to quantify flow-ecology relationships using a controlled experimental design, which are difficult to implement in natural settings. Here, we conducted a before-after-control-impact (BACI) experiment to quantify the effects of dry season flow augmentation on habitat connectivity, water quality, invertebrate drift, and juvenile salmonid movement, growth, and survival. Our two-year 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, but had a marginal warming effect on stream temperature. During the wet year, 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 coho Salmon (O. kisutch) increased following augmentation during the dry summer. Flow augmentation also increased survival probability for salmonids, with a larger effect during the dry summer than during the wet summer. This study indicates that appropriately designed and timed flow augmentation can improve habitat conditions for rearing salmonids, particularly during moderately dry years. It also provides empirical evidence that efforts to restore flow in small streams can yield significant ecological benefits.