Effects of Engineered Dam Releases on Benthic Assemblages During Drought and Comparison With Unregulated Reaches

River regulation following damming is often associated with deleterious downstream effects, in large part due to reduced total discharge and disruption of seasonal flooding. These effects would be expected to be exacerbated by drought, particularly extended drought. River regulation also enables flow engineering that has the potential to partially mitigate downstream effects. We examined Tuolumne River benthos, in the reach below the iconic Hetch Hetchy Reservoir and within Yosemite National Park (California, USA), in comparison with two reference reaches, above the reservoir and on the Merced River, across 5 years (2015–2019) during the extreme California drought of the 2010s. One wet year (2017) facilitated large engineered floods that contrasted strongly with antecedent flows. We were thus able to assess conditions across reaches before, during, and after the large releases, allowing comparison of overall differences by reach as well as immediate effects of releases and the durability of those effects. We examined assemblage structure of benthic invertebrates, periphyton biomass, and other variables across reaches and time periods. Our results suggest that river regulation has resulted in increased nuisance periphyton and negative effects on the benthic macroinvertebrate assemblage, but also that the large releases immediately improved river condition as measured across most variables, despite the antecedent drought conditions. Some variables indicated transitory effects, whereas others showed more durable, positive effects, such as reduced periphyton biomass and increased faunal richness and diversity as well as proportion and species richness of mayflies, stoneflies, and caddisflies.