Rocky Mountain snowpack chemistry relative to El niño effects of water years 1993-97

Snowmelt-dominated runoff is correlated with El Nino-related Southern Oscillation Index events in the Rocky Mountain region. Generally. during El Nino winters, the Rocky Mountains are drier in the north and wetter in the south. Annual snow sampling by the U.S. Geological Survey during water years 1993-97 at 52 sites throughout the Rocky Mountain region near a variety of regional anthropogenic emissions sources revealed strong inverse relations between snow depth and chemical concentrations in the snowpack. The Southern Oscillation Index (SOl) indicates that 1995 was the strongest El Nino year affecting the Rocky Mountains during water years 1993-97. However, neither greater snowfall amounts nor correspondingly lower snowpack concentrations of ammonium, nitrate, or sulfate occurred that year. During water year 1997, the year with the least negative SOl and the weakest El Nino effect, the greatest variance in snow depths among the study sites was observed (p<0.005) relative to the other 4 years. Snow depths as a group in water year 1997 were significantly greater (p<O.06) than any other year (1993-96) with numerous record depths. Variance in snowpack chemistry among sampling years relative to water year 1995 correlates poorly to expected effects of El Nino. Differences in snow depths or snowpack chemistry have not occurred contemporaneously with El niño events, indicating that recent El Nino years may have little effect on snowpack chemistry in the Rocky Mountains.