Climate change is altering physical environments and biotic communities globally. High-elevation mountain streams are particularly at risk because rising air temperatures can reduce snowpack and extend the duration of summer low flow, consequently altering a variety of abiotic variables. In turn, populations and communities exposed to environmental change can undergo shifts in phenology, fitness, and behavior–altering the ecosystem processes that these biota control. In this dissertation, I examined how climate change-induced low flows are impacting stream water temperature, invertebrate communities, and the mechanistic pathways through which low flow acts on communities. I achieved this with three complementary approaches. First, I tested the effects of earlier low flows on organismal phenology, community composition, and resulting ecosystem processes via a mesocosm experiment that simulated flow regimes expected under end-of-the-century climate projections. Second, I assessed spatiotemporal variation in thermal vulnerability to climate change in a mid-elevation stream network in the Sierra Nevada over an extreme drought year (2020–2021). Lastly, I investigated the abiotic and biotic pathways whereby drought alters invertebrate community composition and structure in a California Sierra Nevada watershed across nested spatio-temporal scales–from microhabitat to watershed, and over two decades. I found that extended low flows will likely have diverse abiotic and biotic ramifications on stream ecosystems, but the mechanisms behind these changes are complex and require deep understanding of the ecosystem context.In the summer of 2019, I experimentally examined how earlier snowmelt will alter the phenology of mountain stream organisms and ecosystem processes via outdoor mesocosm stream channels in the Eastern Sierra Nevada, California. Channels were assigned to three hydrograph treatments that simulated the current flow regime or a 3 to 6 week earlier return to summer baseflow conditions projected under regional climate change scenarios. I measured discharge, water temperature, primary production, benthic macroinvertebrate secondary production and phenology, macroinvertebrate emergence, and predatory behavior of a riparian bird. Water temperature increased under advanced low flow conditions, which may have played a role in biofilm production to respiration ratios declining by 32%. Additionally, the majority of the benthic and emergent invertebrate species explaining community dissimilarity changed in phenology as a consequence of the early low-flow treatment. Emergent flux pulses of the dominant insect group (Chironomidae) also nearly doubled in magnitude, benefitting riparian predators. One such riparian predator, the Brewer’s Blackbird, gained access to feed on benthic macroinvertebrates under the 6 week early low flow treatment that aligned with their nesting period. Changes in both invertebrate community structure (composition) and ecosystem processes were mostly fine-scale, and response diversity at the community level stabilized seasonally aggregated responses. My findings illustrate how climate change in mountain streams at the rain-to-snow transition is poised to alter the dynamics of stream food webs via fine-scale changes in phenology even when community structure and ecosystem processes appear stable over longer time periods. In 2020-2021, I deployed a nested array of high-frequency sensors and used advances in time-series models to examine spatiotemporal variation in thermal vulnerability. This work took place in Bull Creek, one of the Kings River Experimental Watersheds (KREW). Stream thermal sensitivity to atmospheric warming fluctuated strongly over the year and peaked in spring and summer. I found that spatially, the reach scale (~50 m) best captured variation in summer thermal regimes. Spatial variation in summer water temperature was driven firstly by upstream water temperature, with elevation, discharge, and conductivity as local correlates. Lastly, I combined the estimated summer thermal sensitivity and downscaled projections of summer air temperature to forecast end-of-the-century stream warming. I found that 25.5% of cold-water habitat may be lost under high-emissions scenario RCP 8.5 (or 7.9% under mitigated RCP 4.5). This estimated reduction suggests that up to 27.2% of stream macroinvertebrate biodiversity could be stressed or threatened in what was previously cold‑water habitat. My results support that thermal vulnerability in montane stream networks may be highly variable over space and time. Taking spatiotemporal variation into account is critical to understand how climate change will impact high mountain stream ecosystems through rising temperatures and shifts in precipitation. Stream low flows can alter communities via multiple environmental and biological mechanisms across time and space, but support is mixed as to which mechanisms are paramount and how spatial and temporal context determines their relative importance. I investigated the mechanisms whereby low flow alters stream invertebrate community composition and structure in high-mountain streams–across space and over time. To this end, I sampled aquatic macroinvertebrates from the same 60 sites in Bull Creek where temperature sensors were deployed in 2020, using a nested sampling design. Additionally, long-term data in four reaches were sampled 11 times from 2002 to 2023. The inspected abiotic mechanisms of drought (temperature, water velocity, and fine sediment) all explained variation in a similar percentage of taxa in the community (36.8% - 47.4%), but effects differed when examined spatially vs. temporally. Total spatial variance explained by abiotic mechanisms for each species had no relationship with its temporal counterpart. Biological mechanisms also differed across space and time; community dissimilarity across space was driven by differences in fine sediment causing species turnover (i.e., sensitive species being replaced by tolerant ones), while temporal dissimilarity was driven by differences in temperature and water velocity causing reordering (i.e., shifts in relative abundance). These results challenge the key assumption of ‘space-for-time’ substitution that underpins abundant research on climate change ecology. I contend that space-for-time substitution approaches may be inappropriate in mountain river studies because of their hierarchical structure, high temporal variability, and mechanisms operating distinctly across space and time. This dissertation provides evidence that climate change-induced low flows will alter Sierra Nevada stream ecosystems in a variety of ways. High elevation mountain streams will increasingly be affected by climate change, a threat that is not well understood despite extensive research efforts. My findings demonstrate that climate change effects are highly context dependent and examining them at the appropriate spatiotemporal scale is necessary to properly assess their impact. Notably, changing abiotic conditions due to extended low flows may benefit some ecosystem processes and taxa at the upper edge of their elevation range, potentially at the cost of coldwater specialists adapted to harsh high mountain streams. High response diversity of species in this dissertation ensured that ecosystem processes often remained stable even if many individual species responded to changing flow regimes. Advances in ecological models and methodology enable finer assessment of environments and communities than ever before, but my research shows that extensive data collection and knowledge of local natural history are necessary for these methods to be effective. This dissertation expands the field of drought ecology, which is of the utmost importance in a quickly changing world.