the spatial variability of stream temperature is an important component of habitat within streams providing optimal temperatures for foraging and thermal refugia for sensitive species such as brook trout. riparian shading and lateral contributions of groundwater through the hyporheic zone are the main contributors to spatial variability in stream temperature. the first two objectives of this study were to quantify thermal variability in stream systems through extensive mapping of streambed temperatures and to evaluate the influence of thermal variability on the stream fish distribution and community structure. the third objective was to examine associations between thermal variability and environmental variables at reach, riparian and catchment scales to identify features that may be used to characterize thermally important stream reaches. a total of 55 sample sites were surveyed during the warmest and driest season for northwestern ontario (mid-july - september) in streams from 4 catchments size classes; 1, 3, 5 and 10 km [squared]. streambed thermal variability occurred on a sub-metre scale with temperature fluctuating up to 5.8 °c across a transect perpendicular to stream flow. the maximum variability found was 10.1 °c within a 50 m reach and 12.0 °c within a 300 m survey. thermal variability was driven by cold streambed temperatures; 44 of 55 reaches had larger deviations below the mean streambed temperature than above the mean, which is an indication of cool groundwater entering the streambed. fish species diversity and brook trout abundance was significantly higher in reaches with high thermal variability, while rainbow trout abundance was significantly lower. fish species richness within a reach could be predicted as low (<5) or high (>5), with thermal variability as an independent variable using logistic regression. high (>0.10) or low (<0.10) rainbow trout abundance (fish/m[squared]) could also be predicted using thermal variability. fish size was not found to be associated to thermal variability. furthermore, thermal variability was correlated with terrestrial variables associated with groundwater movement, including the amount of adjacent land contributing surface and subsurface runoff to the stream, also known as reach contributing area (rca). reaches with large rcas had significantly higher levels of thermal variability compared to reaches with small rcas. however, the relationship between thermal variability and rca was only found for reaches in the two largest stream catchment size classes (5 and 10 km[squared]) due to the dominance of groundwater during base flow of the two smallest catchments (1 and 3 km[squared]). areas of low and high thermal variability differed in landscape topography, terrestrial surface roughness, landform geology and streambed permeability, which are all related to groundwater flow. in regions such as northwestern ontario, where hydrologic pathways are related to topographic features it is possible to use environmental features, such as rca, to locate lateral groundwater inputs into streams. this predictive ability allows for identification, management and protection of valued ecosystem components important for the maintenance of ecological integrity of streams. study area : nipigon bay basin, northwestern ontario.