On Aug. 12, 1679, Lake St. Clair was discovered by an expedition led by René Robert Cavelier, Sieur de La Salle who had sailed across Lake Erie and reached the Detroit River in early August. They named Lake St. Clair “Lac Sainte Clair” in honor of Sainte Claire of Assisi, whose feast fell on that day. The spelling was later changed by mapmakers or government officials.
The Windsor Border Region: Canada’s Southernmost Frontier (A Collection of Documents), Ernest J. Lajeunesse, 1960. Tornoto: The Champlaign Society, 1960, p. XXXV. Alternate Description: The Publications of the Champlain Society, Ontario Series, vol. IV.
Michigan Every Day and Wikipedia.