link to page 1 
INSIGHTi
Line 5 Pipeline: Replacement/Tunnel Project
Updated December 6, 2023
The Line 5 Pipeline is a 30-inch, 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 it poses 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 2019.
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. Enbridge plans to
continue operating the existing pipeline in the meantime. 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.
Congressional Research Service
https://crsreports.congress.gov
IN11798
CRS INSIGHT
Prepared for Members and
Committees of Congress
Congressional Research Service
2
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. 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 pipeline segment through the tunnel. The agreement was followed by
a December 11, 2018, state statute creating the Mackinac Straits Corridor Authority, which approved
tunnel construction on December 19, 2018.
On November 13, 2020, under a new gubernatorial administration, state officials notified Enbridge that
the 1953 easement was being revoked on safety grounds, 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 pipeline safety jurisdiction and shutdown order, initiating ongoing litigation. Most recently, as of
On September 28, 2023, the state filed a brief on appeal of an unfavorable U.S. district court ruling in
order to return its case to state court. The pipeline continues operating in the meantime.
Army Corps Tunnel Permit
In 2020, 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 new
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 associated with this project are thoroughly analyzed.” The Corps states that its environmental
review “is limited to the proposed crossing of the Straits of Mackinac and adjacent wetlands.” However, it
is not clear to what extent the Corps’s NEPA review may consider the greenhouse gas emissions
associated with the fossil fuels passing through pipeline segment, as called for by some advocates. On
March 23, 2023, the Corps announced its intent to publish an 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. Although EGLE “acknowledged public concerns about the existing oil pipeline and
affirmed … that the current pipeline … poses an unacceptable risk to the Great Lakes,” the agency
concluded that “the proposed tunnel could comply with state environmental laws.” On December 1, 2023,
the Michigan Public Service Commission approved the siting application for the tunnel, concluding that
“there is a public need for the replacement section of Line 5,” and that “without the pipeline’s operation,
suppliers would need to use higher-risk and costlier alternative fuel supply sources and transportation for
Michigan customers.”
Congressional Research Service
3
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 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 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.” However, 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
information that has been provided by CRS to Members of Congress in connection with CRS’s institutional role.
CRS Reports, as a work of the United States Government, are not subject to copyright protection in the United
States. Any CRS Report may be reproduced and distributed in its entirety without permission from CRS. However,
as a CRS Report may include copyrighted images or material from a third party, you may need to obtain the
permission of the copyright holder if you wish to copy or otherwise use copyrighted material.
IN11798 · VERSION 4 · UPDATED