Icebreaking and Winter Navigation on the Great Lakes

Updated 2026-11-10

Icebreaking and Winter Navigation on the Great Lakes

Ice stops most shipping on the Great Lakes for a few weeks or months each year, and it affects the St. Lawrence River both through navigation and through flooding risk. Two coast guards, a federal ice information service and a binational lock system determine how long ships can keep moving. This page describes how icebreaking is organized on the lakes and the St. Lawrence, how the ice season runs, what information is published to mariners, and which vessels are in service or being built.

Why ice matters for lake shipping

Icebreakers on the lakes, the Seaway and elsewhere mainly escort convoys of ships through ice-filled water. On the St. Lawrence they also serve a second purpose: steam icebreakers such as CCGS N.B. McLean (1930) and CCGS D'Iberville (1952) were built partly to keep the river free of ice jams east of Montreal, which prevents flooding.

Lake ice itself varies a great deal from year to year. The 1991 to 2020 climate normals from Environment and Climate Change Canada show a median ice season on Lake Superior from late November to mid-May, with ice present as late as early June. Lakes Michigan, Huron and Erie typically reach their maximum cover in mid-February, and Superior peaks in mid-March. Shallow Lake Erie is often fully covered in mid-winter, while Lake Ontario has the lowest typical maximum cover. The same normals note ridged ice in Whitefish Bay and the Straits of Mackinac, and landfast ice in the St. Marys River and the North Channel of Lake Huron. Variability is wide: maximum cover on Superior has ranged from 10 percent to 98 percent in the record used.

The Canadian Coast Guard icebreaking program

The Canadian Coast Guard was formed on 26 January 1962 and now operates 119 vessels and 23 helicopters. Icebreaking is one of its services, delivered through four Ice Operations Centres: Atlantic, Arctic, Great Lakes and St. Lawrence. The Atlantic, Great Lakes and St. Lawrence centres run from December to May, and the Arctic centre from June to November. The centres work with Marine Communications and Traffic Services to provide ice information, recommend routes and coordinate icebreaker assistance. Icebreakers are in contact with the centres at all times, and the centres plan daily tasks from observed ice conditions and requests.

On the Great Lakes, the Coast Guard describes its season as December to May. Icebreaking starts when temperatures fall and ice forms, and it ends when ice allows unrestricted navigation. The navigation season itself runs from late March to late December. The coverage area runs from the upper Beauharnois Lock to Thunder Bay, including connecting waterways, Georgian Bay and the upper St. Lawrence River, and is coordinated with United States Coast Guard ice navigation centres.

Services and priorities

The 2024 Levels of Service document divides the service into flood prevention, which covers ice jam clearing on the St. Lawrence River, the St. Clair River and the Great Lakes in winter only, and vessel-oriented services such as escorts, help for beset vessels, track maintenance and harbour breakouts. Resources are deployed as appropriate to the advertised level of service and the ice actually observed.

The document sets response targets for icebreaker assistance by water body: 6 hours in the St. Lawrence and Saguenay rivers, 8 hours on Lakes Huron, Erie and Superior and on Lake Ontario when the locks are open, and 12 hours in the Gulf of St. Lawrence. Winter priorities run in this order:

1. distress and emergency situations, including flooding that endangers life and property;
2. Cabot Strait ferry services, then other ferries as the Coast Guard considers appropriate;
3. vessels carrying vulnerable or vital cargo, such as dangerous goods or perishables;
4. commercial traffic and ports, excluding spring breakouts;
5. fishing vessels beset in ice;
6. spring breakouts of commercial and fishing harbours.

Vessels underway rank ahead of vessels secured in port, and the Coast Guard may reprioritize if a situation requires it. Requests from users who did not follow the Coast Guard's ice routing advice receive lower priority. The document also mentions a remission policy that compensates users for delayed service when an icebreaker waits past the agreed rendezvous time.

Ice routing and fees

Ice Operations officers, supported by Ice Service Specialists, prepare recommended routes as waypoints, updated daily or as needed and published on the Coast Guard's e-Navigation Portal. Commercial vessels arriving at or leaving ports in the ice zone during the ice season are subject to an icebreaking service fee. For Zone 2, which covers the Gulf of St. Lawrence, the river to Montreal and the Great Lakes excluding Lake Ontario, the fee period runs from December 21 to April 15.

Canadian and United States cooperation

The two coast guards treat the lakes as one operating area. A Memorandum of Understanding updated on 15 February 2024 covers icebreaking and buoy tending on the Great Lakes, the connecting waterways and the shared stretch of the St. Lawrence River, about 2,400 kilometres from the river to the western end of Lake Superior. Under what the Coast Guard calls a one-fleet approach, vessels work on both sides of the border regardless of nationality or home port.

The United States Coast Guard's main Great Lakes icebreaker is USCGC Mackinaw (WLBB-30), commissioned on 10 June 2006 and based at Cheboygan, Michigan. The 240-foot cutter is described as a heavy icebreaker within Great Lakes terms, though not comparable to a heavy polar icebreaker. It can keep moving through fresh-water ice 32 inches thick at 3 knots, or 14 inches at 10 knots, and can break smooth, continuous ice up to 42 inches thick by riding up on it. It works alongside USCGC Katmai Bay, based at Sault Ste. Marie, Michigan. The Great Lakes Waterway article notes that US Coast Guard icebreakers usually help keep the waterway open for part of the fall and early winter, and that shipping usually stops for two to three months afterwards.

The ice season and the Seaway

Shipping season on the system is set by two things: the lock operators and the ice. At the Soo Locks between Lake Huron and Lake Superior, the locks close for winter, from January through March, which is also the period used for inspection and maintenance. The Poe Lock, 1,200 feet long, 110 feet wide and 32 feet deep, is the only lock able to handle the largest lake freighters operating on the upper lakes. The locks pass about 10,000 ships a year.

On the St. Lawrence side, the client requirements document records that the Seaway closing date is currently December 31, and that industry anticipates alignment on January 7 in future. Industry representatives in the same document expect longer seasons as ice coverage has trended lower. The proposals were to move the end of the Lake Huron icebreaking season from March 15 to March 30 and to end the Thunder Bay commitment on April 15, with service dates aligned with the Seaway and the Soo Locks.

In March 2026 the Coast Guard reported that CCGS Martha L. Black would break ice between Cardinal, Ontario and Saint Lambert, Quebec, in support of the safe reopening of the shipping channel and to help prevent ice jams and flooding. The release warns that being on the ice near icebreakers or hovercraft is dangerous and that ice movement can endanger people after a vessel has left.

Vessels in service

The Coast Guard has acknowledged that its icebreakers are old. Two vessels matter most for the lakes and river.

CCGS Samuel Risley is a light icebreaker and buoy tender, ordered in 1983 and completed on 4 April 1985, with an Arctic Class 2 ice rating and a length of 69.7 metres. It is based at Parry Sound, Ontario, and is responsible for keeping an ice-free passage between Port Colborne and Thunder Bay. Reported tasks include helping free ten commercial vessels near Whitefish Point, Michigan, in April 2015 and a bulk carrier near Buffalo in January 2025.

CCGS Martha L. Black is an 83-metre light icebreaker, launched on 6 September 1985 and certified Arctic Class 2. Its home port is Quebec City. It tends navigation aids on the St. Lawrence River and Seaway and the Saguenay River and also breaks ice there.

The client requirements document counts two Coast Guard icebreakers in the Great Lakes region and notes that stakeholders consider that insufficient. Stakeholders also raised the age and condition of those vessels and the decline in the number of icebreakers serving the lakes since the 1970s. In the 2024 deployment tables, no Great Lakes zone had more than one asset scheduled in any quarter, while the Seaway zone had up to two assets in the spring quarters. The same document records a Lake Carriers Association view that the Canadian and American service metrics are not aligned and that the US tiered system amounts to a reduction in service, and the Coast Guard noting that a new heavy icebreaker is under construction.

New vessels and the polar program

Icebreaker procurement is part of the National Shipbuilding Strategy. Two polar icebreakers are the largest items. A contract of $3.15 billion went to Seaspan's Vancouver Shipyards on 7 March 2025 for a 158-metre ship with delivery expected in 2032, and a contract of $3.25 billion went to Chantier Davie on 8 March 2025 for a 139-metre Polar Class 2 ship expected in 2030, partly built at Helsinki Shipyard in Finland. The ships were named CCGS Imnaryuaq and CCGS Arpatuuq in August 2024. The Seaspan ship's keel was laid on 14 October 2026, and the Coast Guard calls it the largest vessel ever built for it, able to operate year-round in the high Arctic.

The program has a long history. A Polar 8 project of the late 1980s was cancelled on 19 February 1990 after cost increases. The Louis S. St-Laurent, delivered in 1969 and 120 metres long, was modernized and CCGS Terry Fox was bought as a stopgap. The polar project was announced in 2008 at C$720 million to replace the St-Laurent. These polar ships are intended mainly for the Arctic, not the Great Lakes.

Ice information services

The Canadian Ice Service is a division of the Meteorological Service of Canada within Environment and Climate Change Canada, with about 60 staff at its headquarters in Ottawa. It describes itself as the leading authority for information about ice in Canada's navigable waters, with two main objectives: warning of hazardous ice to protect people, property and the environment, and building knowledge of the ice environment.

Its information on ice conditions is one input to the Coast Guard's ice centres, which plan daily icebreaker tasks from observed conditions.

Frequently asked questions

When does icebreaking take place on the Great Lakes?

The Canadian Coast Guard defines the Great Lakes icebreaking season as December to May. It begins when ice forms and ends when ice allows unrestricted navigation. Flood-prevention icebreaking is a winter service on the St. Lawrence, St. Clair and Great Lakes.

How quickly is an icebreaker expected to respond?

Targets in the 2024 Levels of Service are 6 hours on the St. Lawrence and Saguenay rivers, 8 hours on Lakes Huron, Erie, Superior and Ontario (the last when the locks are open), and 12 hours in the Gulf of St. Lawrence. Requests are ranked by priority, with distress situations first.

Which coast guard operates on the lakes?

Both. The Canadian and United States coast guards run a joint program under a Memorandum of Understanding updated in February 2024. Vessels may operate on either side of the border, over a shared area of about 2,400 kilometres.

When do the Soo Locks close for winter?

The locks close from January through March, a period also used for inspection and maintenance.

What is the main US icebreaker on the Great Lakes?

USCGC Mackinaw, commissioned in 2006 and based at Cheboygan, Michigan. It is 240 feet long, can break smooth ice up to 42 inches thick and is supported by USCGC Katmai Bay at Sault Ste. Marie.

What are the new Canadian polar icebreakers?

CCGS Imnaryuaq, built by Seaspan in Vancouver, and CCGS Arpatuuq, built by Davie in Lévis with Helsinki Shipyard. Their contracts were awarded in March 2025, with expected deliveries in 2032 and 2030. They are intended for Arctic service.

Who publishes ice charts for the lakes?

The Canadian Ice Service, a division of the Meteorological Service of Canada within Environment and Climate Change Canada. It describes itself as the leading authority for information about ice in Canada's navigable waters.