Sarnia is a city in Lambton County, Ontario, Canada. It had a 2021 population of 72,047, and is the largest city on Lake Huron. 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 in the Southwestern Ontario region, which forms the Canada–United States border, directly across from Port Huron, Michigan.

Lake Huron is one of the five Great Lakes of North America. It is shared on the north and east by the Canadian province of Ontario and on the south and west by the U.S. state of Michigan. The name of the lake is derived from early French explorers who named it for the indigenous people they knew as Huron (Wyandot) inhabiting the region. Hydrologically, Lake Huron comprises the eastern portion of Lake Michigan–Huron, having the same surface elevation as Lake Michigan, to which it is connected by the 5-mile-wide (8.0 km), 20-fathom-deep Straits of Mackinac. Combined, Lake Michigan–Huron is the largest freshwater lake by area in the world. The Huronian glaciation was named from evidence collected from Lake Huron region. The northern parts of the lake include the North Channel and Georgian Bay. Saginaw Bay is located in the southwest corner of the lake. The main inlet is the St. Marys River, and the main outlet is through the St. Clair River to Lake Erie. Lake Huron has a fairly large drainage basin covering parts of Michigan and Ontario. Water flows through Lake Huron faster than the other Great Lakes with a retention time of only 22 years.