Isotopic and major-ion analyses of 130 fresh and brackish groundwater samples reveal a strikingly consistent pattern of variation over 28,000 km 2 of the western Canada sedimentary basin. Hydrodynamic interpretations based on drill-stem tests and piezometric data reveal a pattern of broad, regional flow from south to north. However, there is some evidence to suggest that patterns of groundwater flow have been influenced in the past by Wisconsin glaciation. All the permeable units appear to be recharged by meteoric water in the south where they outcrop or come close to the surface. Samples of groundwater collected down dip in each of six major sandstone units are progressively enriched in D, 18O, Na + and Cl −. For example, the deepest waters of the artesian Milk River aquifer are enriched by up to 70‰ and 15‰ with respect to the δD and δ 18O of the modern recharge waters. The pattern of chemical evolution is strongly related to the hydraulic characteristics of individual units. The isotopic composition is determined by the extent to which hydraulic conductivity has facilitated meteoric water flushing of connate water. Thus, more permeable units and permeable zones within a unit tend to be isotopically lighter because of a more complete and more rapid invasion of meteoric water. Interestingly, the largest conductivity values are found in some of the deepest units (1500 m). Consequently, water in these deeper units have δD- and δ 18O-values which approach that of the recharge. The major-ion chemistry is controlled both by this process and a complex set of rock-water interactions. In addition to providing information about the chemical evolution of groundwater, this study can begin to quantify the complex pattern of flushing in a large sedimentary basin.