Reductions in acid deposition have led to rapid and substantial recovery from episodic acidification associated with spring snow melt in circumpolar waters including 80,000 lakes and 1,000,000 km of watercourses in northern Sweden. Our results are based on the correlation between SO42- concentration in snow and the anthropogenic component of spring flood ANC decline. An empirical model of spring flood response to acid deposition built around that correlation predicts that the 65% reduction in sulphur deposition between 1970 and 1990 has reduced the area of seriously acidified spring floods across 250,000 km2 of northern Sweden by 75%. The study suggests that much of what has been achieved so far by the large and costly emission reductions in Europe and North America the last decades lies not primarily in chronically acidified regions but rather in more marginally impacted areas where episodic acidification has been the primary chemical impact of SO42- deposition.