Unpolluted rivers and streams that drain marine shales show an excess of sodium compared to chloride and a deficiency of calcium and magnesium compared to sulfate and alkalinity. This is due in part to cation exchange of sodium for divalent cations on clay minerals. Consideration of the global weathering budget suggests that up to 34% of the sodium in the total dissolved stream load may be due to cation exchange rather than sodium production via silicate dissolution weathering reactions. These results suggest that the weathering budgets for sodium and calcium are in need of revision because of the inclusion of cation-exchange processes in the weathering cycle. This implies that silicate dissolution is less important in determining the composition of global river water than was previously thought.