The headwater ‘mountain water towers’ in the Eastern Slopes of the Rocky Mountains provide more than 90% of flow in southern Alberta’s rivers. Mounting evidence emphasizes the importance of groundwater dynamics in runoff generation and temporal redistribution of water supplies, challenging our assumptions about what it means for a watershed to be “snowmelt-dominated” and about subsurface storage pathways and residence times that govern water quality and supply in the receiving stream. Although groundwater contributions remain poorly characterized, the Eastern Slopes of the Rockies River (ESRR) project addresses this gap by integrating hydrometric, geochemical and isotopic characterization to evaluate the average age of surface water, delineate major rock aquifer characteristics, and investigate groundwater-surface water interactions among connected alpine lakes, springs, glaciers, and river and groundwater systems. Preliminary findings include a quantification of mean river ages and groundwater contributions to i) a first order stream at Boundary Glacier (5-25y), ii) the Bow River in its mountain reaches (9-12y, ~34-44% GW), and iii) two alpine lakes, Zigadenus and Elbow, (~50-57% GW outflow). This framework clarifies variations in aquifer contributions and flowpath dynamics in the complex mountain block and valley aquifer systems that largely provide streamflow in the Bow River mainstem. Results of this project are invaluable for our understanding of groundwater – surface water interactions in the Eastern Slopes and Alberta’s future water security.