How the Locks Work
Sault Ste Marie MI

The locks connect Lake Superior
to the remaining Great Lakes


The locks are necessary as the only connection between Lake Superior and the rest of the Great Lakes is the St. Mary's Canal. The St. Mary's Canal, at the Lake Superior end, has a 21 foot waterfall between Lake Superior and the lower lakes.

The locks permit deep draft ships to travel around the St. Mary's Waterfall and manage the 21 foot difference in water levels. Lake Superior is 21 feet higher than the other Great Lakes.

The locks raise and lower the vessels easily because the water will seek its own level as they open the gates. The locks move the water by gravity.

soo locks, lake superior, upper peninsula

As a boat locks in from Lake Superior, the gates at each end of the locks are closed and a valve is open to let the Lake Superior water already in the lock flow out to the lower water level of the St Mary's Canal and Lakes Huron and Michigan. When the water has dropped to the lower level, the lock gate at the south end is opened and the boat proceeds out of the locks into the St Mary's Canal. The north gate remains closed, holding back the waters of Lake Superior.

sault ste marie, mi
sault st marie, michigan

PAUL R. TREGURTHA inching its way into the Poe Lock. It took about 1 hour for the ship to enter the lock, and prepare to drop the 21' to the lower Great Lakes.

The Paul R. Tregurtha is 1013' 6" overall, 105' wide and just barely fits in the Poe Lock that is 1200' long and 110' wide.

sault st. marie michigan
soo  locks at lake superior

She is the longest vessel on the Great Lakes and has held this honor since she was built in 1981.

The Paul R. Tregurtha within the Poe Lock and starting to lock down.


The Paul R. Tregurtha down the 21 feet to the level of the lower Great Lakes and moving out of the lock. She is carrying iron ore (taconite pellets). Her cargo capacity is 68,000 tons.

When a boat locks in from the St. Mary's Canal at the lower level of Lakes Huron and Michigan, the gates at both ends are closed and the filling valve at the north end is opened to permit the Lake Superior water to flow into the lock. When the water in the lock has risen to that of Lake Superior, the lock gate at the north end is opened and the boat locks out into Lake Superior.

Before the locks were built, early pioneers portaged their canoes, small boats and then larger boats around the waterfall and refloated them in Lake Superior. Early shipwreck history has repeated mentions of small boats wrecked at Sault Ste. Marie.

In 1797, the Northwest Fur Company built a lock 38 feet long on the Canadian side of the river for small boats. This lock remained in use until it was destroyed in the War of 1812. Freight and boats were again portaged around the rapids until another lock was built.

Congress passed an act in 1852 for the construction of 2 new locks which were completed in 1855.

Within a few years, commerce through the canal had grown to national importance, and the need for new locks became clear. The funds required exceeded the state's capabilities, and thus, in 1881 the locks were transferred to the United States government under the jurisdiction of the U.S. Army Corps of Engineers. The Corps has operated the locks, toll free, since that time.



Check out what is happening right now at the locks with the Soo Locks Live Web Cam
Maybe one of the 1000' freighters is locking through.  

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Exploring Sault Ste. Marie, Michigan

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Photos: Vivian Wood