Detailed vertical profiles of dD and d¹⁸O in porewaters were obtained from four, thick (typically 60 to 80 m thick), clay-rich till sequences located across Saskatchewan, Canada. The isotopic analyses were performed on porewater samples collected using squeezing techniques on core samples and from piezometers as well as using new direct equilibration technique. Interpretation of these profiles allowed the hydrogeology of the vertical profiles through very simple to very complex tills to be characterized. Specifically, the dD and d¹⁸O data were used to define the distribution and thickness of zones dominated by advective transport and those that dominated by diffusive transport. In the zones dominated by diffusive transport, these data were used to constrain the vertical solute transport rates through the unit. The dD and d¹⁸O data also provided information on the timing of the deposition of the till units and paleoclimatic conditions. In some cases, the profiles also raised questions as to the validity of the current classification of individual till units. The implications of the dD and d¹⁸O profiles were supported by classical hydrogeologic investigations (e.g., lab and field hydraulic testing, water level responses, and gradient measurements) and ³H, ¹⁴C, ³⁶Cl, and major ion analyses.