Changes in water corrosivity from 84 river sites were investigated using long-term (1962 – 2016) data sets. Corrosive waters, often having elevated chloride (Cl) concentrations, can lead to metal leaching and promote corrosion in water distribution systems. Two different indices, the Chloride-Sulfate Mass Ratio (CSMR) and the Larson Ratio (LR), are commonly used to indicate the potential to promote corrosion in water distribution systems, with high values indicating an increase in the potential to promote corrosion. Though changes in treatment approaches at drinking water facilities are known to affect the corrosion indices, there is little information regarding how changes in the ionic composition of source waters may affect the corrosion indices. As part of the National Water-Quality Assessment (NAWQA) project of the National Water-Quality Program, long-term water-quality data from multiple sites across the Conterminous United States have been used to analyze seasonal- and flow-related variability and trends in the CSMR and LR indices.