St. Clair County is a county located in the U.S. state of Michigan and bordering the west bank of the St. Clair River. As of the 2020 census, the population was 160,383. It is the 13th-most populous county in the state. The county seat is Port Huron, located at the north end of the St. Clair River at Lake Huron. The county was created September 10, 1820, and its government was organized in 1821. This area is sometimes dubbed the Blue Water Area.
Etymology
French explorer René-Robert Cavelier, Sieur de La Salle led an expedition to this area on August 12, 1679. They named the lake as ', because it was the feast day of Saint Clare of Assisi, whom they venerated. English mapmakers adopted the French name, identifying the lake feature as Saint Clare on maps dated as early as 1710. By the Mitchell Map of 1755, the spelling was given as St. Clair, which later became the current version in 1924. Located along the western shores of Lake St. Clair and the St. Clair River, the county was named for them by European-American settlers.
The name is sometimes mistakenly attributed to honoring Arthur St. Clair, an American Revolutionary War general and governor of the Northwest Territory, but it was established long before he was considered a notable figure. The earlier spelling of the lake's name may have been conflated with English practice and the name of the general, as several political jurisdictions near the lake and the river, such as St. Clair County, St. Clair Township, and the cities of St. Clair and St. Clair Shores, share this spelling (see List of Michigan county name etymologies).
The name has sometimes been mistakenly attributed to honoring Patrick Sinclair, a British officer who purchased land on the St. Clair River at the mouth of the Pine River. In 1764, he built Fort Sinclair there, which was in use for nearly 20 years before being abandoned. As noted, the name was established before he was active in the area.
Geography
thumb|left|St. Clair County is also called the Blue Water Area.
According to the U.S. Census Bureau, the county has a total area of , of which is land and (14%) is water. St. Clair County is one of five counties that form the peninsula, known as the Thumb, that projects into Lake Huron. St. Clair County is closely connected in terms of economy with its neighbors, Metro Detroit and Sanilac County in Michigan, and Lambton County across the river in Ontario, Canada. Saint Clair County is the principal county in The Blue Water Area, a sub-region of the Thumb.
Adjacent counties
- Sanilac (north)
- Lapeer (west)
- Macomb (south)
- Lambton, Ontario (east)
Major highways
- enters the county from the west, coming from Lansing and Flint, terminating at the approach to the Blue Water Bridge in Port Huron. (Once fully completed, the mainline of I-69 will span from Brownsville, Texas to Port Huron, Michigan.)
- enters St. Clair County from the southwest, having traversed the entire Metro Detroit region, and terminates at the approach to the Blue Water Bridge in Port Huron. On the Canadian side of the border, in Sarnia, Ontario, the route heads easterly, designated as Highway 402.
- follows the Lake Huron–Saginaw Bay shoreline, beginning in Bay City and ending at a junction with |I-94/|I-69, and BL I-94/BL I-69 on the north side of Port Huron.
- serves Harsens Island, in Lake St. Clair.