Tributary chloride loading into Lake Michigan

Anthropogenic salt sources have contributed to rising salinities in the Laurentian Great Lakes. In Lake Michigan, chloride concentrations have risen from ~ 1–2 mg L−1 in the 1800s to > 15 mg L−1 in 2020. The watersheds of the approximately 300 tributaries of Lake Michigan vary in size and represent a wide range of land use, from undeveloped forested watersheds to urbanized and agricultural areas. The spatial variability in both size and land cover among Lake Michigan's tributaries contributes to enormous variation in chloride concentrations and loads. We performed a spatial assessment of Lake Michigan tributaries to calculate total annual salt loading, infer future conditions based on current patterns, evaluate the use of synoptic sampling, and identify watershed characteristics that drive high chloride concentrations. We found that the tributary load to Lake Michigan is 1.08 Tg yr−1 of chloride, and that chloride concentrations in Lake Michigan will likely continue to slowly rise in the coming decades.