Reflectance patterns apparently from calcium carbonate (CaCO3) precipitation have been mapped in the Great Lakes using satellite multispectral imagery. The milky water phenomenon (“whiting”) occurred regularly in summer and fall during the period studied, 1972–1975, in Lakes Ontario, Erie, and Michigan but not in Superior and Huron. In situ data provide nearly irrefutable evidence that these whitings are calcareous. They are attributed to supersaturation of CaCO3 during periods of thermal stratification and are most intense in the warmer areas of the lakes. The whitings are maximal several meters below the surface and are undoubtedly significant with respect to light transmission, affecting the euphotic zone and thereby photosynthetic production. They may serve as lakewide markers in synoptic analysis of large‐scale epilimnial horizontal motions.