From March 12–14, 1993, up to 1 m of snow fell on the northeast United States, with central Pennsylvania receiving some of the largest accumulations. The rain-on-snow events that followed provided a unique opportunity to monitor episodic acidification and fluctuations in dissolved aluminum concentrations during extreme flows in three forested Appalachian Plateau basins in north-central Pennsylvania. These events produced peak flows 2–5 times greater than previously monitored flows in the basins we studied, and in the Susquehanna River Basin, March discharge was the highest sustained flow on record.
High flows during the event were accompanied by greater streamflow dilution by rain and meltwater and shifts to surface and near-surface hydrologic flow paths, which reduced contact of water with the soil matrix. In the three basins we monitored, these extreme flows caused little water quality degradation relative to earlier events of lesser magnitude. High flows resulted in stream acidification due to base cation dilution rather than increased acid anion concentrations as found in previously monitored events. Total dissolved aluminum concentrations peaked and then remained nearly constant as flow increased during the event; however, aluminum concentrations were elevated above levels toxic to native trout populations in these streams for an extended period of time.