Groundwater discharges in the western Canadian oil sands region impact river water quality. Mapping groundwater discharges into rivers in the oil sands region is important to ensure wastewater and steam injections remain sequestered, rather than eventually resurfacing. Saline springs comprised of Pleistocene-aged glacial meltwater enter regional rivers, but their spatial distribution has not been mapped comprehensively. Here we show substantial increases in salinity along three major rivers as they flow through the Athabasca Oil Sands Region adjacent to many active oil sands projects. Major ion concentrations and isotope (2H/1H, 18O/16O, 87Sr/86Sr) compositions suggest that increases in river water salinities are caused by saline groundwater discharges from Cretaceous or Devonian aquifers. These regional subsurface-to-surface connections signify that injected wastewater or steam may potentially resurface in the future, emphasizing the critical import of mapping groundwater flows to understand present-day streamflow quality and to predict potential for injected fluids to resurface.