The response of snow- and glacier-fed mountain streams and lakes to climate warming is of growing concern. A full understanding of these systems must include the role of groundwater, but this is poorly understood, especially for high-elevation lakes. This study addresses the role of groundwater in an alpine watershed, located at the continental divide of North America, with a focus on quantifying the groundwater exchange with Lake O'Hara in British Columbia, Canada. This is facilitated using a water balance approach and measurements of electrical conductivity in inflowing streams. The water balance indicates that groundwater inflow is substantial, as it was equivalent to at least 30--67% and 35--74% of the total outflow for the 2004 and 2005 field seasons, respectively. Hydrological and chemical data also suggest contributions from both deep and shallow groundwater flow paths.