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The chemical composition during baseflow was used to elucidate the fundamental processes controlling longitudinal and seasonal patterns of stream acidity in Yellow Creek, a chronically acidic headwater (pH range 3.7-4.2) on the Appalachian Plateau in northeastern West Virginia. Sulfate concentrations controlled the variability of stream acidity within the Yellow Creek catchment. Decreases in stream free H+ acidity with decreasing elevation likely resulted from SO42- retention in riparian wetland areas as well as spatial variation in dominant tree species. Seasonal variations in free H+ and inorganic monomeric aluminum (Aln+) concentrations appeared related to seasonal fluctuations in baseflow discharge which was controlled by vegetative activity. Baseflow streamdischarge, as well as H+ and Aln+ acidity, gradually declined during the growing season (June through October), likely reflecting microbial SO42- reduction in saturated anaerobic environments within riparian wetlands. A marked pulse of stream H+ Aln+, and SO42- coincided with an abrupt increase in baseflow discharge resulting from the cessation of transpiration after leaf-fall in November. This seasonal pattern suggests that autumn may be a critical period for eastern brook trout in streams draining wetlands on the AppalachianPlateau. [ABSTRACT FROM AUTHOR]