The biodiversity of freshwater fishes has declined rapidly across North America over the past 50 years. At higher latitudes (>60°N) knowledge of freshwater fish biodiversity is incomplete, hampering our abilities to understand how species may respond as climate changes. Bull trout (Salvelinus confluentus) is a societally important salmonid that is considered a sentinel species for monitoring impacts of climate change due to its requirement for cold water. In this thesis, I describe the thermal ecology and assess potential effects of climate warming on north-temperate montane bull trout populations. In Chapter 2, I used juvenile distributional data to evaluate design considerations for detecting watershed-scale population trends. Detectability was not uniform, and imperfect detection affected accuracy of assessments most in fringe habitats near distributional boundaries. Detecting a 30% change in watershed-level occupancy ≥78% of the time is possible, but requires three repeat surveys (i.e., temporal replicates) and increased sampling intensity of fringe habitats. Additional sampling effort in fringe sites could be offset by sampling fewer sites in core habitats, while still minimizing risk of non-detection. In Chapter 3, I use full-year stream temperature records to describe thermal regimes that populations experience across a broad latitudinal gradient. All streams showed high thermal stability and similar insensitivity to the warming effects of air temperature. However, fish distribution at higher latitudes was constrained by cold-limiting streams in both the summer and winter (i.e., do not freeze); which is opposite to lower latitudes, where distributional patterns are governed by warm-limiting streams. In Chapter 4, I examined how climatic and geomorphic factors influence the distribution of juveniles in a northern watershed and forecast effects of climate warming on the distribution of suitable habitat. Juvenile distribution is driven by cold-limiting streams, prevalence of perennial groundwater, and stream size. Suitable habitat, based on both climatic and geomorphic factors, is projected to decline across all warming scenarios. Conversely, availability of thermally suitable habitat, which does not consider other habitat factors, is projected to increase. The dichotomy in these projections illustrates the importance of considering broader dimensions of the ecological niche for climate change vulnerability assessments of northern stream fishes.