Lakes and streams exhibit short-term episodic decreases in acid neutralizing capacity (ANC), and often also pH, usually in response to hydrological events, such as snowmelt or rainfall. Periods of episodic acidification may last for hours to weeks, and sometimes result in depletion of ANC to negative values with concurrent increases in potentially toxic inorganic Al in solution. Many of the characteristics that predispose aquatic systems to chronic acidification from acidic deposition also predispose aquatic systems to episodic acidification. The single most important factor governing the sensitivity of a given watershed to episodic acidification is hydrology. Episodic acidification is nearly ubiquitous in drainage waters. Since preparation of the National Acid Precipitation Assessment Program 1990 Integrated Assessment, the Environmental Protection Agency has completed the Episodic Response Project, an integrated evaluation of episodic acidification of surface waters during high-discharge periods in portions of the eastern U.S.