Line 5 Pipeline: Relocation and Permitting
The Line 5 Pipeline is a 30-inch diameter, 645-mile pipeline owned by Enbridge carrying up to 540,000 barrels per day of crude oil and natural gas liquids from Superior, WI, to Sarnia, Ontario (Figure 1). A key segment of Line 5 is an underwater crossing at the Straits of Mackinac—between Michigan's upper and lower peninsulas—where it runs for approximately four miles across the lakebed. This crossing has been an environmental concern due to the risk of a "worst case" oil spill into the Great Lakes.
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Source: CRS using data from Platts, and Esri Data & Maps. |
Under the terms of various state agreements, Enbridge plans to replace this pipeline segment with a new one constructed through a tunnel beneath the lakebed—which requires federal approval. However, for different reasons, the State of Michigan and a federal court have ordered that Line 5 be shut down. Whether the pipeline should continue operating is the subject of litigation, federal oversight, and a U.S.-Canada treaty dispute.
Michigan Easement Challenges
Line 5 was constructed in 1953 as part of a pipeline system linking oil fields in Alberta, Canada, to refinery markets in the Great Lakes region. At the time, Michigan granted Enbridge's predecessor—Lakehead Pipe Line Company—an easement to cross the Straits of Mackinac. In October 2018, Michigan and Enbridge entered an agreement whereby Enbridge would construct a tunnel under the straits and replace the existing Line 5 segment with a new segment through the tunnel. The agreement was followed by a December 2018 state statute creating the Mackinac Straits Corridor Authority, which subsequently authorized assignment of easement rights for tunnel construction.
In 2019, under a new gubernatorial administration, the Michigan Attorney General filed a complaint in state court seeking to shut down Line 5 as a "public nuisance" that was "likely to cause pollution." In 2020, state officials notified Enbridge that the 1953 easement was being revoked, requiring the Line 5 segment across the straits to cease operating.
Since that time, there has been ongoing litigation in both state and federal courts regarding the easement and the pipeline. Most recently, on April 2, 2025, tribal groups filed an appeal with the Michigan Supreme Court seeking to overturn a lower court's decision upholding state regulators' approval of the Line 5 relocation project. Separately, on April 23, 2025, a federal appeals court upheld a lower court's ruling that Enbridge could sue Michigan's governor for revoking the easement. The pipeline remains in operation as litigation continues.
Army Corps Tunnel Permit
In April 2020, prior to the revocation of the easement, Enbridge filed a joint application with the U.S. Army Corps of Engineers (Corps) and the Michigan Department of Environment, Great Lakes, and Energy (EGLE) to build the Line 5 tunnel. The Corps has permitting authority over the proposed tunnel pursuant to Section 404 of the Clean Water Act (33 U.S.C. §1344) and Section 10 of the Rivers and Harbors Act of 1899 (33 U.S.C. §403). In carrying out its authorities, the Corps must consider the environmental impacts of its proposed actions (e.g., issuing a tunnel permit) pursuant to the National Environmental Policy Act (NEPA; 42 U.S.C. §4321 et seq.).
In January 2021, EGLE announced that it had approved Enbridge's application for permits required to build the tunnel. EGLE stated that the existing pipeline "poses an unacceptable risk to the Great Lakes," but nevertheless concluded that the project could comply with state environmental laws.
In June 2021, the Corps announced that it would be preparing an Environmental Impact Statement (EIS), to "ensure all potential impacts and reasonable alternatives ... are thoroughly analyzed." In June 2023, the Corps stated that its environmental review would be "limited to the proposed crossing of the Straits of Mackinac," including associated construction activities. Some advocates have expressed concerns about the greenhouse gas emissions associated with the fossil fuels passing through the pipeline. The agency stated that "these factors are not within the Corps' scope of analysis," a view that appears to be consistent with a recent U.S. Supreme Court decision in an unrelated case.
On April 15, 2025, the Corps announced that, in accordance with President Trump's Executive Order 14156 ("Declaring a National Energy Emergency"), the tunnel project was "subject to special emergency permitting procedures." On May 30, 2025, the Corps published its draft EIS for the tunnel project, initiating a 30-day public comment period.
U.S.-Canada Pipeline Treaty
The Canadian government has strongly supported the continued operation of Line 5. Canada has sought to intervene though public statements and court filings, citing a 1977 pipeline treaty with the United States that, among other provisions, prohibits a "public authority in the territory of either" from instituting "any measures … which are intended to, or which would have the effect of, impeding, diverting, redirecting or interfering with in any way the transmission of hydrocarbon in transit." In October 2021, Canada announced that it was formally invoking the dispute settlement provision of the treaty regarding Line 5.
In November 2021, the White House Press Secretary reportedly stated that the Biden Administration was studying the potential impact of the proposed Line 5 replacement/tunnel project, but was not considering shutting down the pipeline. Canadian officials have been engaged with U.S. officials regarding the treaty dispute, but details about the dispute resolution process have not been publicly released.
Litigation unrelated to the tunnel project complicates matters. In September 2022, a U.S. district court ruled that a segment of Line 5 was trespassing on tribal lands in Wisconsin. In June 2023, the court ordered Enbridge to reroute Line 5 around the tribal land—or shut it down—within three years. Both the tribe and Enbridge have appealed the ruling to the U.S. Court of Appeals for the 7th Circuit. In November 2024, the Wisconsin Department of Natural Resources issued state permits required for the reroute. In May 2025, the Corps held a public hearing on Enbridge's Section 404 permit application for the project.