Seismic reflection profiling and piston coring identified seismic reflectors in northern Lake Huron and Georgian Bay linked with unconformities caused by at least six reductions in lake level. In ascending stratigraphic order, these lowstands occurred at about 11 200 BP, associated with the Kirkfield outlet from early Lake Algonquin; 10 200 – 9900 BP, associated with the post-Algonquin lake level fall; 9800 – 9050 BP, the most extreme lowstand, associated with the main Stanley – Hough draw down; and 7800 – 7450 BP. The concomitant highstands are Lake Algonquin, from about 11 200 – 10 200 BP; early Lake Mattawa, between 9600 and 9350 BP; the main Mattawa phase, 9050–7800 BP; and the Nipissing highstand, at about 4700 BP. Isotopic and paleoecological data show that all of the lowstands are characterized by cold, dilute, and isotopically very light (< −20‰) waters from the melting Laurentian ice cap. Highstands, on the other hand, are characterized by higher dissolved solid concentrations and a much smaller meltwater component. Oxygen isotope values of the waters in these lakes were −15 to −17‰ in Lake Algonquin, −13 to −14‰ for the early and middle Mattawa stages, −9 to −8‰ for the main Mattawa stage, and −7‰ for modern waters. This association of lowstands and not highstands with isotopically light waters is a new contribution to early Holocene hydrology of the Great Lakes. The Younger Dryas cool episode is coeval with the Lake Algonquin highstand and a younger cool episode is generally coeval with the Mattawa highstand. This supports the hypothesis of C.F.M. Lewis and T.W. Anderson that these large cold lakes were responsible for regional cooling during the early Holocene.