How local geomorphic and hydrologic features mediate the sensitivity of stream thermal regimesto variation in climatic conditions remains a critical uncertainty in understanding aquatic ecosystem responsesto climate change.We used stable isotopes of hydrogen and oxygen to estimate contributions of snow and rainfallto 80 boreal streams and show that differences in snow contribution are controlled by watershed topography.Time series analysis of streamthermal regimes revealed that streams in rain-dominated, low-elevation watershedswere 5–8 times more sensitive to variation in summer air temperature compared to streams draining steepertopography whose flows were dominated by snowmelt. This effect wasmore pronounced across the landscape inearly summer and less distinct in late summer. Thus, the impact of climatewarming on freshwater thermal regimeswill be spatially heterogeneous across river basins as controlled by geomorphic features. However, thermalheterogeneity may be lost with reduced snowpack and increased ratios of rain to snow in stream discharge.
