Systematic modes of spatial and temporal variation in a 13-y record of stream sulfate from a nationwide network of headwater sampling stations are defined using principal components. Based on the undisturbed nature of the sampling network, it is suggested that these modes of stream sulfate variability are analogues for variations in acid deposition. Three statistically significant components, accounting for approximately 50% of the total stream sulfate variance, are identified. Analysis of component loadings and scores indicates that a major transition occurred in the early 1970s when stream sulfate concentrations in the northeast changed from persistently above mean levels to persistently below. At the same time concentrations of sulfate in Gulf and Southeast Atlantic coast streams shifted from persistently below to persistently above mean concentrations. Significantly, these changes occurred contemporaneously with regional trends in sulfate emissions which can generally be characterized as decreasing in the northeast and increasing in the southeast.Systematic modes of spatial and temporal variation in a 13-y record of stream sulfate from a nationwide network of headwater sampling stations are defined using principal components. Based on the undisturbed nature of the sampling network, it is suggested that these modes of stream sulfate variability are analogues for variations in acid deposition. Three statistically significant components, accounting for approximately 50% of the total stream sulfate variance, are identified. Analysis of component loadings and scores indicates that a major transition occurred in the early 1970s when stream sulfate concentrations in the northeast changed from persistently above mean levels to persistently below. At the same time concentrations of sulfate in Gulf and Southeast Atlantic coast streams shifted from persistently below to persistently above mean concentrations.