 Sarnia is a city in Southwestern Ontario , Canada , and had a 2011 population of 72 @,@ 366 . It is the largest city on Lake Huron and in Lambton County . Sarnia is 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 . The city 's natural harbour first attracted the French explorer La Salle , who named the site " The Rapids " when he had horses and men pull his 45 tonnes ( 50 short tons ; 44 long tons ) barque " Le <unk> " up the almost four @-@ knot current of the St. Clair River on 23 August 1679 . 
