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INSIGHTi
Line 5 Pipeline: Replacement/Tunnel Project
Updated July 16, 2024
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.
Figure 1. Enbridge Line 5 Pipeline
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 Line 5 to shut down. Whether
the pipeline should continue operating is the subject of litigation, federal oversight, and a U.S.-Canada
treaty dispute.
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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 11, 2018, state statute creating the Mackinac Straits Corridor Authority, which
subsequently authorized assignment of easement rights for tunnel construction.
On June 27, 2019, 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.” On November 13, 2020, under a new
gubernatorial administration, state officials notified Enbridge that the 1953 easement was being revoked,
requiring the Line 5 segment across the straits to cease operating by May 2021. On November 24, 2020,
Enbridge filed a challenge in federal court to the state’s jurisdiction, which resulted in the litigation being
moved to federal court. Most recently, on June 17, 2024, a U.S. district court remanded the state’s case
back to state court. 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 tunnel for the proposed Line 5 pipeline crossing. The Corps has permitting
authority over the proposed tunnel pursuant to Section 404 of the Clean Water Act (33 U.S.C. §1344),
under which the Corps permits activities that may discharge dredge or fill material into waters of the
United States, and Section 10 of the Rivers and Harbors Act of 1899 (33 U.S.C. §403), under which the
Corps permits structures and work in or affecting navigable waters.
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.) among other statutory requirements. On June 23, 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.” On June 28, 2023, the Corps stated that its environmental review
would be “limited to the proposed crossing of the Straits of Mackinac, including the construction
activities associated with the crossing.” 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.” According to its NEPA timeline, the Corps anticipates
publishing a draft EIS for the tunnel project in Spring 2025.
On January 29, 2021, EGLE announced that it had approved Enbridge’s application for permits required
to build the tunnel. EGLE stated that the existing pipeline “violates the Public Trust Doctrine and poses
an unacceptable risk to the Great Lakes,” but nevertheless concluded that the proposed project could
comply with state environmental laws. On December 1, 2023, the Michigan Public Service Commission
approved the siting application for the tunnel, concluding that “without the pipeline’s operation, suppliers
would need to use higher-risk and costlier alternative fuel supply sources and transportation for Michigan
customers.”
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U.S.-Canada Pipeline Treaty
The Canadian government has ardently 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.” On October 4, 2021, Canada
announced that it was formally invoking the dispute settlement provision of the treaty regarding Line 5.
On November 9, 2021, the White House Principal Deputy Press Secretary stated that “both the U.S. and
Canada will engage constructively” in Line 5 negotiations and that shutting down the existing pipeline “is
something that we’re not going to do.” Details about the ongoing treaty dispute resolution process have
not been publicly released.
Litigation unrelated to the tunnel project complicates matters. On September 7, 2022, a U.S. district court
ruled that a segment of Line 5 was trespassing on tribal lands in Wisconsin. On June 16, 2023, the court
ordered Enbridge to reroute Line 5 around the tribal land—or shut it down—within three years. Enbridge
has begun the process for the reroute but has expressed concerns about securing the necessary permits in
time to meet the three-year deadline. Both the tribe and Enbridge have appealed the ruling to the U.S.
Court of Appeals for the 7th Circuit.
Author Information
Paul W. Parfomak
Specialist in Energy Policy
Disclaimer
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan shared staff
to congressional committees and Members of Congress. It operates solely at the behest of and under the direction of
Congress. Information in a CRS Report should not be relied upon for purposes other than public understanding of
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IN11798 · VERSION 5 · UPDATED