Isotopic composition of snow cover and streamflow was determined in a snow-dominated, forested watershed to quantify the spatial variability and processes that alter stable isotope (oxygen-18, 18O and deuterium, 2H) composition under different forest canopy conditions (clear-cut, partial-cut (thinned), and unimpacted forest). Snow sampling was carried out on 4 days in late winter and early spring 2006. Meteorological data, precipitation, and streamflow were continuously monitored during the study. Isotope analyses of precipitation samples were conducted weekly through the 2005–2006 snow season. Values of δ18O varied between − 22·0 and − 9·5‰ , and δ2H varied between − 170 and − 76‰ . Isotope concentrations from snowpack samples varied between − 17·5 and − 13·8‰ for δ18O, and between − 129 and − 102‰ for δ2H. These ranges reflect differences in precipitation, accumulation, sublimation, and melting of the snow cover. Streamflow samples were collected during the snowmelt season from two locations every alternate day from the beginning of April until the end of May. Streamflow and snow from a partial-cut and an uncut forest were enriched in the heavy isotopes (18O and 2H) relative to streamflow and snow from a clear-cut forest. Based on the low water contents of the snowpack under dense canopies, we infer that the isotope enrichment resulted primarily from sublimation of snow intercepted by the canopy, with more enrichment in denser canopies. There was no significant correlation between snowpack isotope concentration and altitude. Results indicate that variations in canopy structure can alter snow isotope composition. This finding will provide a useful index of snowpack sublimation, and thus, improved parameterization of distributed hydrological models. Copyright © 2008 John Wiley & Sons, Ltd.