Natural or human-influenced disturbances are important to the health and diversity of forests, which in turn, are important to the water quantity and quality exported from a catchment. However, human-induced disturbances (prescribed fire and harvesting) have been decreasing, and natural disturbances (fires and insects) have been increasing in frequency and severity. One such natural disturbance is the mountain pine beetle (MPB), (Dendroctonus ponderosae) an endemic species. A recent epidemic resulted in the mortality of millions of hectares of lodgepole pine (Pinus contorta) forests in Colorado, USA. Beetle-induced tree mortality brings about changes to the hydrologic cycle, including decreased transpiration and interception with the loss of canopy cover. This study examined the effect of the mountain pine beetle kill on source water contributions to streamflow in snowmelt-dominated headwater catchments using stable isotopes (²H and ¹⁸O) as tracers. Study catchments with varying level of beetle-killed forest area (6% to 97%) were sampled for groundwater, surface water, and precipitation. Streams were sampled to assess whether beetle-killed forests have altered source water contributions to streamflow. Groundwater contributions increased with increasing beetle-killed forest area (p=0.008). Both rain and snow contributions were negatively correlated with beetle-killed forest area (p=0.035 and p=0.011, respectively). As the beetle-killed forest area increases, so does fractional groundwater contribution to streamflow.