Decades of hydrological connectivity work challenge us to consider groundwater flowpaths as spectra: integrating reaction along varying lithologies; complicating quantification of groundwater contributions to streamflow. End-member mixing models implicitly assume homogeneous groundwater sources, but in structurally complex mountain systems with sharp geological boundaries, this approach can obscure meaningful differences in groundwater origin. To address pathway variability, we conducted targeted sampling of mountain groundwater sources that provided the basis for a multi-ion end-member mixing framework (EMMF), which we combined with long-term river geochemistry to resolve relative end-member contributions to streamflow. Our transferable, uncertainty-aware approach resolves Main and Front range groundwater contributions of the Eastern Slopes of the Canadian Rocky Mountains. Geological/geochemical contrasts between these regions apportion streamflow between five end-members: low-solute water (LSW; nominally interflow), and distinct siliciclastic and carbonate groundwaters from each. End-member compositions are anchored to groundwater and spring geochemistry. Groundwater contributions to streamflow are quantified using linear inversion; ion-specific, globally-constrained uncertainty floors ensure solution stability. Application to ~20 years of monthly sampling at four stations on the Bow and Elbow Rivers reveals coherent spatial and seasonal patterns. Main Range reaches are dominated by LSW (~68%) and buffered groundwater composition . Front Range reaches exhibit less LSW contribution (~56%) and mixing from both Main and Front Range carbonate and siliciclastic pathway domains, with pronounced seasonal patterns. Explicitly resolving groundwater sources associated with contrasting geological domains is necessary to represent groundwater–surface water interactions in mountain systems and provides a robust foundation for further analyses of seasonal dynamics and longer-term change.