Pronounced decadal shifts in major-ion chemistry indicate increasing annual groundwater contribution in rivers of the Eastern Slopes of the Canadian Rocky Mountains . We evaluated long-term geochemical records and discharge at three sites using Modified Seasonal Mann–Kendall trend analysis and a geologically explicit five-end-member mixing framework that resolves low-solute water (LSW) and distinct lithologic/geochemical groundwater flowpath domains. Over ~25y only slight change occurred in Q at Elbow River at Bragg Creek (–0.06 m3/s/year), and the upstream Bow River at Lake Louise (0.04 m3/s/year), with no change for Bow River at Canmore. Conversely, concentrations of carbonate-associated solutes (SO₄, Ca, Mg) increased at all sites, while flux trends varied. Seasonal partitioning points to shifting seasonal flux distributions; the baseflow (Nov–Apr) contribution of carbonate-flux increased by ~10% at BowLL and ~20% at the lower elevation stations, BowCanmore and Elbow, reflecting changes in the timing of groundwater domain (end-member) contributions. End-member mixing indicates that groundwater contributions have also become more seasonally differentiated but less variable, consistent with increased subsurface storage and routing under altered seasonal inputs. Across sites, fractional contribution of low-solute water declined: ~5-10% at BowLL and ~10–15% at lower elevations. At BowLL two groundwater domains increase during recession and baseflow; Elbow a single carbonate groundwater domain year round; complex structural geology at BowCanmore integrates seasonally intricate but increasing inputs from four groundwater domains within the valley aquifer. These patterns highlight the critical response of increased subsurface storage and supply to altered recharge timing in mountain streamflow over decadal timescales.