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Abstract Changing climate conditions are expected to cause increases in the frequency and severity of drought conditions in many areas around the world, including the Pacific Northwest region of North America. While drought impacts manifest across the landscape, headwater streams are particularly susceptible to droughts due to limited deep‐water habitats and low water volumes that allow for substantial increases in water temperature. While low volumes of water and increased stream temperature will likely affect all aquatic species to some degree, the response of different taxa to these impacts is expected to vary with differences in physiological needs and habitat preferences among species. Using a before–after control‐impact (BACI) experimental design, this study investigates how reduced streamflow and increased stream temperature affect the two dominant apex predators in headwater streams of the Pacific Northwest, coastal cutthroat trout (Oncorhynchus clarkii clarkii) and coastal giant salamander (Dicamptodon tenebrosus). In a second‐order stream in the H.J. Andrews Experimental Forest in OR, USA, experimental flow diversions created decoupled drought conditions of reduced streamflow and elevated temperatures. Low‐flow conditions were created by diverting water around a 100‐m stream reach and this diverted water was passively warmed before re‐entering a downstream channel to create an increased temperature reach. We compared fish and salamander abundances and stream habitat in an upstream unmanipulated reference reach to the two experimental reaches. Relative increases in temperature ranged between 0.41 and 0.63°C, reflecting realistic stream warming in this region during drought events. Trout responded positively to increased temperatures, showing an increase in abundance, biomass, condition factor, and growth, whereas salamanders responded negatively in all metrics except condition. The low‐flow reach diverted approximately 50% of the flow, resulting in a relative pool area reduction of about 20%. Relative to the reference reach, salamanders displayed a net positive abundance response while trout declined in the low‐flow reach. The contrasting responses of these populations to decoupled drought conditions suggest that interactions of flow and temperature changes together will influence drought responses of the vertebrate communities of headwater streams. 

Many headwaters across temperate North America have uniform mid‐succession riparian forests recovering from historic land clearing. These young riparian stands contrast with late‐succession forests, which have complex structural characteristics including canopy gaps. Canopy gaps provide structural diversity that can be important for terrestrial species, and they are also hypothesized to be important features for aquatic environments. The light patches below gaps create productivity hotspots in streams and therefore create potential for increased stream apex predator abundances through bottom‐up food web drivers. However, increasing light may also affect stream temperature, a consideration for coldwater fish (salmonids). We established an experimental before‐after control‐impact study to explicitly assess how creating canopy gaps in the riparian forest affects the abundance and biomass of coastal cutthroat trout (Oncorhynchus clarkii clarkii) and Pacific giant salamanders (Dicamptodon tenebrosus) in paired reference and treatment reaches at five replicate streams. Gaps were designed to resemble those in old‐growth forests in the treatment reach of each system although wood was explicitly left out of the stream. At four of five sites, we found small and generally consistent positive responses in adult cutthroat trout and total vertebrate biomass to localized increases in light but only 2 years after treatment. Results suggest that opening riparian canopies adjacent to streams via gaps is a viable tool to enhance structural complexity of riparian forests without negatively impacting stream vertebrates; however, a single gap alone had small effects on aquatic vertebrates. More or larger gaps would likely be needed to notably enhance aquatic apex predators.