Aspects of streamflow and reproductive success of Harlequin Ducks (Histrionicus histrionicus) were measured to explore how variation in streamflow impacts reproduction, and to consider how climate change might influence these parameters in the future. A 24-year data set (1990-2013) of Harlequin Duck breeding season surveys conducted on Upper McDonald Creek in Glacier National Park (GNP), Montana, USA was used to assess how annual variation in the proportion of broods to pairs (reproductive success) relates to streamflow. Between 1990 and 2013, GNP staff and volunteers conducted 102 spring surveys and 112 brood surveys counting 896 total ducks, 212 pairs, 56 broods, and 278 ducklings. Four streamflow metrics (pre-incubation streamflow - corresponding with nutrient acquisition and nest site selection, hydrographic peaks – corresponding with nest site selection and availability, value of the greatest single hydrographic peak post average peak flow - corresponding with risk of a nest washing out, and average streamflow during incubation - corresponding with foraging condition for an incubating female) were all negatively related to reproductive success. The first three of these metrics are predicted to become more extreme with climate change, with potential negative effects on breeding Harlequin Ducks.