Stream thermal regimes are critical to the stability of freshwater habitats. Groundwater plays an important role in controlling stream temperatures in mountain headwaters, where it makes up a considerable portion of discharge. This study investigated the controls on the thermal regime of a headwater stream, and the surrounding groundwater processes, in a catchment in Kananaskis Country, on the eastern slopes of the Canadian Rocky Mountains. Stream temperatures at the site are strongly influenced by the interactions between groundwater and an intermittent lake at higher elevation, with no surface water outflows. During precipitation events, the latent heat flux associated with melting of direct snowfall onto the water surface was responsible for rapid stream cooling. An increase in advective inputs from groundwater and hillslope pathways did not result in observed cooling of stream water during rainfall events. The overburden was determined to be an important conduit of water to the stream. A downstream thinning of the overburden was observed resulting in reduced groundwater contribution. The results from this study will assist water resource and fisheries managers in adapting to stream temperature changes under a warming climate.