Episodic acidification occurs when the pH and ANC of surface waters temporarily decrease during rain events and snowmelt. The principal drivers of episodic acidification are increases in sulfuric acid, nitric acid, organic acids, and dilution of base cations. In regions where surface waters are sensitive to acid deposition, episodic acidification may result in several days to weeks when ANC values may approach or decline below 0 µeq/L, a threshold below which increases in monomeric aluminum concentrations become evident, which may result in deleterious effects to sensitive aquatic biota. The Adirondack Mountains of New York is a region with abundant streams and lakes, many of which are highly sensitive to the effects of acid deposition. Long-term monitoring data indicate that pH and ANC in regional surface waters are increasing in response to decreases in the acidity of atmospheric deposition that result from decreasing SO₂ and NO_x emissions as the Clean Air Act and its ancillary rules and amendments have been implemented. Most surface-water monitoring focuses on low-flow and broad seasonal patterns, and less is known about how episodic acidification has responded to emissions decreases. We are exploring spatial and temporal patterns in episodic acidification through analysis of stream chemistry from surveys that target varying flow conditions as well as data from a few long-term intensively sampled stream monitoring sites. Each stream sample is assigned a flow percentile value based on a resident or nearby gage, and a statistical relation between ANC values and flow percentile is developed. The magnitude of episodic decreases in ANC increases as low-flow ANC increases, a pattern that likely results from an increasing influence of dilution, especially evident when low-flow ANC values exceed 100 µeq/L. Chronically acidic streams with low-flow ANC near 0 µeq/L show little episodic acidification, whereas streams with low-flow ANC values of about 50 µeq/L generally show ANC decreases to less than 0 µeq/L at high flow. Preliminary analysis of a 24-yr data set (1991-2014) at Buck Creek near Inlet, NY indicates that high-flow ANC has recovered twice as much as low-flow ANC, and values generally no longer decline below 0 µeq/L at the highest flows, which typically occur during spring snowmelt. Further analyses will explore how the drivers of episodic acidification vary across the region with low-flow ANC and whether clear trends are evident in the influence of these drivers over the past few decades.