Middle Ordovician to Middle Devonian carbonates in the Manitoba lowlands form an extensive aquifer system along the eastern edge of the Williston Basin. The aquifer is divided into fresh and saline portions by a series of topographic lows defined by major river systems and lakes that create a north–south-trending hydraulic divide. East of this divide Ca–Mg–HCO3 fresh waters are derived by active modern-day recharge focused in the Sandilands and Interlake regions. Stable isotope and geochemical data indicate that Na–Cl saline waters west of the divide are a mixture of original basin brines and glacial melt water pushed into the basin during Pleistocene glaciation. High Na/Cl and Cl/Br ratios are consistent with significant salt dissolution by glacial melt water.