There the pipeline divides into two separate 20-inch (51 cm) lines, which reunite when they reach McGulpin Point on the southern side of the straits.

Volumes destined for Michigan and Ohio refineries such as BP Husky and Marathon Detroit are offloaded there,[4] and transferred to a Sunoco pipeline that runs from Marysville to Toledo.

The MSCA approved the transfer of a property right, allowing Enbridge to construct the new tunnel in the bedrock beneath the Straits of Mackinac.

[17] In late June 2019, the state of Michigan filed a lawsuit asking the Ingham County Court to compel the decommissioning of the segment of Line 5 that runs under the Straits of Mackinac.

A Reuters news report defined Line 5 as "a critical part of Enbridge's Mainline network, which delivers the bulk of Canadian crude exports to the United States".

The basis of the suit is the claim that the pipeline is a public nuisance and violates the Michigan Environmental Protection Act since it may become a major source of pollution.

[19] Attorney General Dana Nessel commented: "We cannot prevent accidental or emergency anchor deployments in one of the busiest shipping channels in the Great Lakes.

It is estimated that replacing the propane lost by closing the line would require an additional 30,000 truckloads and 9,600 rail cars annually".

[17] In May 2021, citing a 1977 treaty with the United States on the uninterrupted flow of oil and gas across the border, the Canadian government asked a U.S. court to stop Michigan from shutting down the pipeline, thus triggering a bilateral negotiation with Washington.

[27] According to a 2016 study published by the University of Michigan, in partnership with the National Oceanic and Atmospheric Administration, a leak in Enbridge 5 near the Straits of Mackinac could affect roughly 700 miles of shoreline.

[28] Key areas recognized for the highest potential risk include Mackinac Island, Bois Blanc Island, Mackinaw City, and areas on the northern shore and southern shore of the Straits with 3,528 square miles (15%) of Lake Michigan's open water and roughly 13,611 square miles (60%) of Lake Huron's open water possibly impacted by visible oil.

[30][31][28] Due to these issues, expert reports concluded that Line 5 should be shut down in the Straits pending a full review under state public trust law.

[34] The 369-page first volume of the document almost immediately drew sharp criticism from environmental advocates and tribal leadership of the Bad River Band of the Lake Superior Tribe of Chippewa Indians[35] whose land is intersected by the pipeline.

Congressman Tom Tiffany spoke in favor of the pipeline expansion and former Bad River Band chairman Mike Wiggins Jr. was one of many speaking against it.

[38] In August 2022, the Canadian government again invoked a 1977 treaty with the United States in order to prevent the shutdown of the pipeline in Wisconsin, citing concerns about jobs, energy security, and the economy.

Canada, which backs Enbridge, invoked the treaty for the first time in 2021 when the state of Michigan attempted to shut down Line 5 under the Straits of Mackinac due to environmental concerns.