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

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.