The Straits of Mackinac is a major shipping lane, providing passage for raw materials and finished goods and connecting, for instance, the iron mines of Minnesota to the steel mills of Gary, Indiana. Before the railroads reached Chicago from the east, most immigrants arrived in the Midwest and Great Plains by ships on the Great Lakes. The straits is five miles (8 km) wide at its narrowest point, where it is spanned by the Mackinac Bridge. Before the bridge was built, car ferries transported vehicles across the straits. Today passenger-only ferries carry people to Mackinac Island, which does not permit cars. Visitors can take their vehicles on a car ferry to Bois Blanc Island.
Islands in the Straits of Mackinac include the two populated islands, Bois Blanc and Mackinac, and two that are uninhabited: Round and St. Helena islands. At 11 miles (18 km) in length, Bois Blanc is the largest island in the straits.
Satellite photograph of icebreaker paths through the ice in the straits. The Mackinac Bridge is the vertical line in the center, connecting the landmass of the Upper Peninsula above to lower Michigan below. The icebreaker paths run right-to-left, connecting the open water of Lake Michigan with the open water of Lake Huron between Mackinac Island and Round Island.
The Straits of Mackinac, spanned by the Mackinac Bridge, seen from the southern shore
The straits are shallow and narrow enough to freeze over in the winter. Navigation is ensured for year-round shipping to the Lower Great Lakes by the use of icebreakers.
The straits were an important Native American and fur trade route. Located on the southern side of the straits is the town of Mackinaw City, the site of Fort Michilimackinac, a reconstructed French fort founded in 1715, and on the northern side is St. Ignace, site of a French Catholic mission to the Indians, founded in 1671. The eastern end of the straits was controlled by Fort Mackinac on Mackinac Island, a British colonial and early American military base and fur trade center, founded in 1781.
The straits are patrolled by a detachment of the United States Coast Guard based at Graham Point, St. Ignace. A shipping channel through the winter ice is maintained by the Coast Guard's Great Lakes icebreaker, USCGC Mackinaw, based in Cheboygan near the eastern edge of the Straits. This new vessel went into service during the 2005/06 ice season.
Most of the Straits have been set aside by the state of Michigan as the Straits of Mackinac Shipwreck Preserve, a riparian public space dedicated to those personnel who were lost aboard the boats and ships that sank in these dangerous shipping lanes.