Streams and springs in and near surface coal mines located on the eastern slopes of the Canadian Rocky Mountains were sampled between 1972 and 1978. Streams unaffected by mining activity characteristically have calcium, magnesium, and bicarbonate as the dominant ions. Total dissolved solids concentration usually ranges between 125 and 231 mg/L. Sulfate is usually about 20% of the anions and has a concentration less than 75 mg/L. Relative amounts of sodium, potassium, chloride, and nitrate usually are less than 6% of the ions. Values of pH commonly range between 7.6 and 8.2.The effects of mining activity on water quality vary in intensity but follow a consistent pattern. The relative proportion of the various cations does not change; however, their concentrations may rise to four times background values. The concentration of anions also increases and there is a shift in the relative proportions from bicarbonate to sulfate and, at the more strongly affected sites, to nitrate. The distribution of pH values is not significantly different from unaffected sites.Enhanced oxidation of sulfide minerals, which account for less than about 0.07% of the loose, unsaturated spoil piles, probably is responsible for the increase in sulfate ions. The hydrogen ion produced during the oxidation of the sulfide minerals is consumed in the dissolution of the abundant carbonate minerals thus releasing calcium and magnesium. Elevated nitrate concentrations result from the use of ammonium nitrate for blasting.The estimated chemical denudation rate at sites unaffected by mining activity ranges from 4–18 mg/(s∙ha), while at affected sites it varies from 4–55 mg/(s∙ha).