This thesis aimed to provide improved understanding of the spatial and temporal variability of stream temperature, the influence of riparian woodlands on thermal regimes and examples of the ecological consequences of different thermal regimes.  First, the spatial and temporal variability of temperature within a large montane river basin at nested spatial scales was established.  It was from this analysis that (i) variation that can occur at all spatial scales was observed, (ii) different physical factors are controlling these variations at all scales, and (iii) greatest differences occur between different tributaries.  The extent of riparian forestry proved to be in important factor in determining river temperatures.  The results suggest that broadleaved tree species within the riparian zone are more able to reduced incoming solar radiation and consequently control stream temperature.  This highlights the potential of riparian plantings in mitigation efforts against high temperatures under future climate scenarios.  To complement these empirical assessments, the ecological significance of observed thermal variability was assessed in a laboratory study of the growth of juvenile Atlantic salmon.  The effects of the temperature regime on growth were in the range of about 3% reduction in final length in the variable regime compared to the constant one.  Relations between invertebrate growth, adult emergence patterns and thermal regime differences related to riparian cover were assessed in relation to the mayfly Baetis rhodani.  Statistically significant differences in mean size and size distribution of Baetis were found in the Girnock, between all sites separated by no more than 5 km.