Hydrogen ion concentration, [H+], of discharge water from Pardee reservoir in the central Sierra Nevada, California was greater than expected in years of El Nino occurrence over the period 1954–86. This pattern is in addition to the general increase in [H+] over the same period attributed to acidic atmospheric deposition. Monthly means of [H+] also show differences between El Nino and non-El Nino years. Total annual runoff does not seem to be a controlling factor; the source and timing of storms are probably more important. Storms are usually from the west or northwest, but during El Nino years tropical-like storms from a more-southerly direction appear to carry acidic pollutants to the central Sierra Nevada.