Activity ratios of the Bow River, southern Alberta, Canada, are constant, both seasonally and spatially, and are distinctly different from precipitation. Shallow ground water in the basin has activity ratios similar to those of the river. Thermodynamic modeling suggests that equilibrium exchange on smectite has a significant influence on the river chemistry. X-ray diffraction analyses indicate that smectite is present in both the soil and ground water zone, suggesting that precipitation must pass through these zones before reaching the river. This implies that the inverse relationship between total dissolved solids and discharge, common to many rivers, may reflect the residence time of water in the ground before entering the river, rather than simple mixing of ground water with surface runoff. The influence of smectite-exchange reactions on river chemistry indicates that water chemistry cannot be used as a simple proxy for the stoichiometry of primary mineral weathering in the basin.