Are you fascinated by engineering marvels that solve seemingly impossible problems? The Upper Peninsula is home to several of the most amazing human-made solutions in the world. One is in Sault Ste. Marie, Michigan. The Soo Locks are among the most significant pieces of infrastructure in North America. The locks allow vessels to travel between Lake Superior and Lake Huron by adjusting for a 21-foot drop in the St. Marys River!
More than 7,000 vessels complete this passage each year from mid-March through mid-January, including tall ships, cruise liners, military vessels and massive freighters. An estimated 80–86 million tons of cargo move through the locks each year, including most of the United States’ iron ore, valued at more than $500 billion annually.
Watch: Behind-the-Scenes Videos about the Soo Locks
For those who want to dive deeper into how the Soo Locks work — and why they matter — the videos included in this blog from Alexis Dahl offer a clear, engaging look behind the scenes. Alexis is a science communicator and Michigan-based storyteller who’s spent years turning complex topics into accessible content for channels like SciShow and Crash Course. In her series, she explores the overlooked science and history of the Great Lakes region.
One of the videos focuses on the region’s geology and the engineering behind how the locks work. The other goes behind the security fence with the U.S. Army Corps of Engineers to show the new lock construction. Filmed on location, both videos include commentary from lock operators, engineers and historians. They are a great visual companion to the engineering, geology and history shared below.
The Geology Beneath the Soo Locks
Beneath the bustling engineering of the Soo Locks lies a geologic foundation that has shaped the entire region’s landscape and economy. Sault Ste. Marie sits on a thick layer of Jacobsville Sandstone, a reddish-brown sedimentary rock that dates back over one billion years to the Precambrian era. This sandstone, found across much of the southern Lake Superior shoreline, is more than 1,000 feet thick in places and provides the bedrock on which the locks are built.
What makes the geology of the Sault Ste. Marie region significant is its ongoing role in shaping the land. The red Jacobsville Sandstone bedrock beneath the St. Marys River is still slowly rising — a process known as isostatic rebound. After the last Ice Age, glaciers that once covered the Great Lakes region with miles-thick ice compressed the Earth’s crust and as the ice melted and the weight lifted, it began to rebound, slowly lifting the land back up.
This geologic rebound wasn’t uniform. The Lake Superior basin rose faster than areas farther south, gradually tipping the region. Around 10,000 years ago, early Lake Superior was held in by a natural barrier. But as meltwater poured in from the north and west, that barrier eroded. Geologists believe this led to the separation of Lake Superior and Lake Huron levels and the formation of the St. Marys Rapids — the very obstacle that made the Soo Locks essential for navigation.
Why the Soo Locks Exist
Long before engineers got involved, the Ojibwe people portaged canoes around the rapids of the St. Marys River, calling the place Bawating, or “place of the rapids.” The first permanent lock was built by the Fairbanks Scale Company and completed in 1855. Known as the State Lock, it allowed ships to bypass the falls for the first time. The State Lock was replaced in 1896 by the Poe Lock and in 1881, the U.S. Army Corps of Engineers assumed full federal control of the canal system. Since then, passage through the locks has been free of charge.
How the Locks Work
The Soo Locks move boats using only gravity and precision engineering. No pumps. Just smart infrastructure and water pressure.
- Ships enter a chamber and are sealed in by massive miter gates.
- Valves are opened to let water in or out, raising or lowering the ship 21 feet.
- The largest lock, the Poe Lock, requires up to 22 million gallons of water per cycle.
- The process takes 15–20 minutes per lockage.
A full trip through the St. Marys River system — from Lake Superior to Lake Huron — takes about nine hours for a freighter.
Structural Design and Expansion
The locks are anchored in a 1,000-foot-thick layer of reddish sandstone. This stable bedrock supports the 1.6-mile St. Marys Falls Canal, operated by the U.S. Army Corps of Engineers, Detroit District.
Active Locks:
- Poe Lock: 1,200 ft long, 110 ft wide, 32 ft deep. Rebuilt in 1968, it handles all 1,000-foot freighters on the Great Lakes — including the largest, the Paul R. Tregurtha, at 1,013.5 ft.
- MacArthur Lock: Built in 1943, 800 ft long, handling smaller freighters. Named after General Douglas MacArthur.
Retired Locks:
- State Lock (built 1855)
- Weitzel Lock (built 1881)
- Davis Lock (built 1914)
- Sabin Lock (built 1919)
New Construction:
A new lock equal in size to the Poe is under construction, replacing Davis and Sabin. Initially approved in 2009, construction officially began in 2019 and is expected to finish by 2030. When completed, the new lock will increase reliability, reduce bottlenecks and secure the national supply chain.
International Reach and Global Trade
The Soo Locks are part of the Great Lakes–St. Lawrence Seaway system which connects Duluth, Minnesota to the Atlantic Ocean via 2,342 miles of inland waterways. This vital shipping route allows vessels from around the world to pass through Sault Ste. Marie. By cargo tonnage, it’s the busiest lock system in the world.
Soo Locks Park and Visitor Experience
Each year, more than 500,000 people visit Soo Locks Park, drawn by the chance to watch freighters pass just feet away. Self-proclaimed “Boat Nerds” return season after season to witness the drama of massive ships navigating the narrow chambers.
The park includes:
- Observation platforms
- Gardens with walking paths and trees representing the entire U.P.
- A lighted fountain
- Japanese archway
- The 1899 U.S. Weather Bureau, now home to the Great Lakes Shipwreck Historical Society, features both exhibits and a gift shop
- The Soo Locks Visitor Center offers interactive displays, historical exhibits and the daily freighter schedule
One unique display features a propeller from the steamer Independence, which exploded near the locks. A crewman reportedly survived the disaster by riding a hay bale down the rapids.
Engineer’s Day – June 27, 2025
One of the most anticipated events of the year is Engineer’s Day, when the locks open to public foot traffic for one day only.
- Next event: Friday, June 27, 2025
Visitors can walk across the MacArthur Lock, view freighters up close and visit 30+ vendor booths along W. Portage Ave. - The park and observation deck are open during the event.
The U.S. Army Corps of Engineers, U.S. Coast Guard and the City of Sault Ste. Marie will host this event.
National Recognition
The Soo Locks are part of the St. Marys Falls Canal and are federally protected under multiple historic designations:
- Michigan State Historic Site (1959)
- National Historic Landmark (1966)
- National Register of Historic Places (1966)
They are considered one of the most important infrastructure accomplishments in U.S. history, alongside the Erie Canal.
Why the Soo Locks Matter
The locks are known as the “Linchpin of the Great Lakes” and with good reason. According to the U.S. Department of Homeland Security, an unexpected shutdown of the Poe Lock for six months would cost the U.S. economy over $1 trillion. Thanks to a well-trained team, ongoing investment and precise engineering, the Soo Locks continue to serve as the gateway to American steel production, agriculture, energy and trade.
Plan Your Visit
Make your visit to the Soo Locks more than just a day trip — turn it into a full-on Upper Peninsula getaway. There is no need to stand on land to watch the freighters — you can go through the locks on either the Original Soo Locks Boat Tours or the Famous Soo Locks Boat Tours. Both offer memorable maritime adventures you won’t forget!
Extend your time exploring Michigan’s oldest city by booking a room in Sault Ste. Marie or nearby lodgings, where you’re just minutes from freighter action, riverside strolls and small-town charm. After a day of watching 1,000-foot ships squeeze through the locks with inches to spare, reward yourself with a meal at a local restaurant. Whether you're into fresh-caught whitefish, wild game or something sweet with a strong cup of coffee, you’ll find it downtown.
Speaking of downtown — don’t skip the shops. Sault Ste. Marie’s historic city center is packed with local boutiques, galleries and gift stores that offer everything from handmade U.P. souvenirs to Great Lakes-inspired art.
And while the Soo Locks are a bucket-list stop, make them just one part of your Upper Peninsula road trip. From here, it’s easy to travel to Tahquamenon Falls, Whitefish Point and Drummond Island — all within a couple hours’ drive. Make your base camp in Sault Ste. Marie, and launch your next great U.P. adventure.