The quantitative distinction of the natural and anthropogenic components of episodic pH decline is important but problematic. This paper presents the Boreal Dilution Model as a means of separating the natural effects of acid neutralizing capacity (ANC) dilution and organic acids from those of acid deposition on stream pH during spring flood or other flow episodes in areas that are not chronically acidified. Key model assumptions are that base flow ANC reflects preindustrial ANC, that the contemporary dissolved organic carbon is natural, and that dilution of base cations during spring flood is a measure of natural ANC dilution. The model could help elucidate the natural variation of spring flood chemistry, as well as the response to changes in acidification pressures. An example of the model's application to one catchment is presented. This example, together with the correlation between winter SO42- deposition and modeled anthropogenic ANC depression during spring flood in some 27 episodes from 18 catchments, is presented to assess the model's plausibility. ¿ 2000 American Geophysical Union