Boreal forests hold significant soil carbon stores in a region experiencing rapid climate change. How these carbon reservoirs respond will depend in part on hydrology and resulting terrestrial-to-aquatic (T-A) export. Exploring 5-7 years of air and stream temperature in 9 boreal forest headwater streams, with similar channel width, located across a climate transect we assessed variations in warm season air-stream water regression slope and intercepts. Variation in geomorphic and climatic factors across sites and years was used to evaluate their influences on slope and intercept and thus sources of streamflow generation. The warmest region catchments were smallest (0.4-3.5 km2), and exhibited the highest median stream gradients. These characteristics resulted in the lowest air-water regression slopes (0.4-0.7) and highest regression intercepts (2.4-4.1), indicating groundwater sources not supplied by summer precipitation. The two colder regions, with larger (4-12 km2), lower relief catchments exhibit higher regression slopes (0.7-1) and lower intercepts (0.1-2.1) consistent with closer connections between warm season precipitation and streamflow. Results suggest groundwater sources and stream temperature vary as a function of the interaction between geomorphology and regional climate variability. Warm region catchments experience higher, more variable water availability, where increased precipitation during the late summer and fall results in more localized groundwater contributions to streamflow. In contrast, only stream temperature is impacted by MAT in the cold regions. Thus, regional climate change projections of higher precipitation is likely to increase shallow subsurface contributions to streamflow, enhancing T-A export and reducing infiltration, but only in the steep, small warm region catchments.