Line 5 is a 30-inch diameter, 645-mile pipeline owned by Enbridge carrying 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 across the lakebed. This crossing has been an environmental concern due to the risk of a "worst case" oil spill into the Great Lakes. Another key segment crosses the Bad River Reservation in northern Wisconsin, where there are also environmental concerns.
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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 the Michigan pipeline segment with a new one constructed through a tunnel beneath the lakebed, and it plans to reroute the Wisconsin segment around the reservation. Both plans require federal approval. However, the State of Michigan and tribal groups, for different reasons, are seeking to permanently shut down Line 5. 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 refineries 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.
Notwithstanding Enbridge's prior state agreements, in 2019, a new gubernatorial administration filed a complaint in state court seeking to shut down the lakebed segment of Line 5 due to it being a "public nuisance" that was "likely to cause pollution." On November 13, 2020, Michigan officials notified Enbridge that the 1953 easement was being revoked, requiring the Line 5 lakebed segment to cease operating within 180 days—which would effectively shut down the entire pipeline. Since that time, Line 5 has continued operating, but there has been ongoing litigation in both federal and state courts regarding the easement and the pipeline. Most recently, on January 6, 2026, Michigan filed an appeal of a federal court ruling that the state lacked the authority to order a Line 5 shutdown because the pipeline is under exclusive federal jurisdiction. 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. On June 30, 2025, the U.S. Supreme Court agreed to consider an aspect of the litigation.
Tunnel Permit Requirements
In April 2020, prior to the revocation of the Michigan easement, Enbridge filed a joint application with the Michigan Department of Environment, Great Lakes, and Energy (EGLE) and the U.S. Army Corps of Engineers (Corps) to build the Line 5 tunnel. 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. On December 1, 2023, the Michigan Public Service Commission (MPSC) separately approved Enbridge's siting application to the commission for the tunnel project, concluding that "without the pipeline's operation, suppliers would need to use higher-risk and costlier alternative fuel supply sources." On April 2, 2025, tribal groups filed an appeal with the Michigan Supreme Court seeking to overturn a lower court's decision upholding the MPSC's approval.
The Corps has permitting authority over the proposed tunnel project 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 June 2023, the Corps stated that its environmental review of Enbridge's permit applications 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."
In April 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 November 13, 2025, the Corps issued a Supplemental Draft Environmental Impact Statement for the tunnel project, initiating a comment period ending December 5, 2025.
Bad River Reservation Easement
The Bureau of Indian Affairs granted the original easement for Line 5 across the Bad River Reservation in 1953. The easement expired in 2013. Subsequently, the Bad River Band of Lake Superior Chippewa (tribe) has been seeking the removal of the pipeline from its reservation due to concerns about a potential oil spill. In June 2023, a federal court ordered Enbridge to reroute Line 5 around the tribal land in Wisconsin—or shut it down—within three years. Both the tribe and Enbridge have appealed this ruling to the U.S. Court of Appeals for the 7th Circuit. The tribe opposes a project to reroute the pipeline outside the reservation but still within its watershed. In November 2024, the Wisconsin Department of Natural Resources issued state permits required for the reroute. Most recently, on December 16, 2025, the tribe filed a complaint in federal court challenging a Corps permit for the reroute project.
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. Canadian and U.S. officials have been "engaged in ongoing negotiations" regarding the treaty dispute, but details have not been publicly released.