The hydrogeomorphic approach, which assesses a wetland's geomorphic setting, water sources, and hydrodynamics, has proven to be a useful framework for characterizing wetlands. Yet, estimating wetland water sources quantitatively across more than a few study sites remains impractical for larger comparative studies due to the time-intensive measurements required. In this research we tested the efficacy of using dissolved magnesium (Mg² ) as a semiconservative tracer of wetland water sources in southern Lower Michigan. Because concentrations of magnesium in groundwater are high due to equilibrium with dolomite in the glacial deposits, dissolved magnesium distinguished the relative importance of groundwater vs. precipitation as wetland water sources. We examined a set of 24 wetlands to consider relationships between a wetland's water sources and its geomorphic setting, water level variation, and biogeochemistry. Fens and swamps were generally groundwater-dominated, whereas wetlands dominated by Sphagnum mosses (“bogs”) were all strongly precipitation-dominated; few wetlands exhibited relatively equitable contributions of groundwater and precipitation. In terms of geomorphic setting, precipitation-dominated acidic bogs were restricted to isolated basins that were positioned higher on the landscape. In contrast, groundwater-dominated circumneutral fens and swamps occurred in basins that spanned the range of landscape position and were associated with surface inlets and outlets. Mean and maximum water levels were lower in more groundwater-dominated wetlands associated with outflow streams. Mean nutrient (ammonium and phosphate) availability was generally higher in more groundwater-dominated wetlands, whereas all precipitation-dominated wetlands had low nutrient availability. We conclude in the southern Great Lakes region: (1) dissolved magnesium can be used to characterize wetland water sources, and (2) the relative importance of groundwater vs. precipitation is strongly linked to a wetland's geomorphic setting, water level variation, and biogeochemistry.