This study analyzes long-term (1950--1992) stream temperature records over the Lena watershed, in order to understand water temperature regimes and changes induced by reservoir regulations, and by natural variations. Water temperature regimes are similar over the Aldan and Upper Lena regions. Stream temperatures at the Lena basin outlet are up to 8¿C lower than the southern subbasins. Reservoir regulations strongly influence regional water temperature regime and change particularly in the regulated subbasins. Because of dam impact, stream temperatures increased (decreased) in the Vilui valley during the early (middle) open water season. Trend analyses show consistent warming across the entire Lena River basin in the early open water season, and mixed results in summer for the unregulated subbasins. For the regulated Vilui tributary, stream temperatures significantly increased in the early and late parts of the warm season due to combined effects of natural changes and reservoir regulations.