Sarnia is a city in Southwestern Ontario located on the eastern bank of the junction between the Upper and Lower Great Lakes where Lake Huron flows into the St. Clair River, which forms the Canada-United States border, directly across from Port Huron, Michigan.. It is the largest city on Lake Huron. The city's natural harbor first attracted the French explorer LaSalle, who named the site "The Rapids" when he had horses and men pull his forty-five-ton...