To clarify the extent of sensitivity of surface waters to acidic deposition in Minnesota, Wisconsin, and Michigan, lake and stream patterns were mapped for alkalinity. Data was analyzed from approximately 14,000 lakes and streams and evaluated for the apparent spatial associations between these data and macro-scale factors thought to influence alkalinity (such as land use, land forms, hydrology, geology, and soils). The level to which each factor is important in influencing alkalinity varies within the region. The regional map compiled in the study helps meet the urgent need to understand the relative potential sensitivity of surface waters in the Upper Midwest, provides a tool for selecting geographic areas for more detailed studies, and allows more accurate regional assessments of effects of acidic deposition on aquatic resources.