AbstractThermal and mineral waters from the Cascade Mountains can be divided, on a chemical basis, into three groups. Chemically homogeneous waters that issue from the geothermally attractive Central Cascades comprise Group 1. The individuality of Group 1 is exhibited by a neutral pH, total dissolved solids averaging 2650 parts per million (ppm), dominant Cl and Na ions, and less significant amounts of Ca, SO4, SiO2, HCO3, K, B, and Mg, listed in order of decreasing abundance. The chemistry and uniformity of these waters suggests that many of the dissolved constituents were derived through prolonged contact with Eocene marine sediments deposited in a Mesozoic structural embayment prior to volcanic events.Acid‐sulfate waters from the vents of Mt. Lassen, Newberry Crater, and Mt. Hood represent Group 2. These fumarolic waters have a chemistry that results when H2S in rising volcanic steam oxidizes forming sulfuric acid. Basic brines discharge at the base of Mt. Lassen and Mt. Hood; these may be in hydraulic connection with the fumaroles.The majority of Cascadian waters are included within Group 3, and are characterized by chemical inhomogeneity between individual points of discharge. Total dissolved solids average 1620 ppm, with dominant Cl, Na, HCO3, and SO4ions, and less significant concentrations of Ca, SiO2, K, Mg, and B; the pH ranges from 6.2 to 9.7. The Cascade Province possesses complex volcanic stratigraphy and varying degrees of hydrothermal alteration. In addition, the thermal waters have been subjected to differing degrees of mixing and proximity to heat, thus chemical inhomogeneity might be expected.Isotopic evidence indicates that all Cascadian thermal and mineral waters were oringinally meteoric. Wateres from Groups 1 and 3 individually show linear relationships between cl and Na; for both groups, cl/Na is similar to that of SMOW, suggesting some recycling of the dominant ion