Glaciers make significant contributions to streamflow in western North America. However, glaciers throughout the region are losing mass and retreating, leading to concerns about summer streamflow, particularly during extended spells of warm, dry weather. An empirical analysis of climatic and streamflow trends in the Canadian portion of the Columbia River basin found a regional climate-driven decline in August streamflow, which was augmented in glacier-fed catchments by glacier retreat. The objective of this study was to use a hydrological model to quantify the relative roles of glacier retreat and climatic trends on streamflow. We applied a modified version of HBV-EC within the Raven modelling framework to Illecillewaet River, located in the Columbia River headwaters, from 1977 to 2020. The model was calibrated using daily streamflow, snow water equivalent from snow pillows, geodetic glacier mass change, and glacier snow cover extracted from Landsat images. The model was run for 1977-2020 with two scenarios: (a) declining glacier cover based on historical imagery and (b) constant glacier cover. Key conclusions are: (1) glacier melt contributed on average 27% of the streamflow in August; (2) glacier retreat accounted for up to a 15% decline in August flow compared to the constant-glacier scenario; and (3) temperature and precipitation trends also contributed to summer streamflow decline in the catchment.