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All of the boat traffic that flows into or out of Lake Superior must move through the locks of the St. Mary's River, at Sault Ste. Marie. The St. Marys River is the only water connection between Lake Superior and the other Great Lakes. There is a section of the river known as the St. Marys Rapids where the water falls about 21 feet from the level of Lake Superior to the level of the lower lakes. This natural barrier to navigation made necessary the construction of the locks project known as the St. Marys Falls Canal.
The world-famous Soo Locks form a passage for deep-draft ships around the Rapids ("Sault") in the St. Marys River.  The "Soo Locks" have the distinction of being the busiest locks in the world. 

Here are some facts about the locks:
    Water not flowing through the locks or down the St. Mary's River nearby is diverted into a canal, and the drop in head is used to generate hydroelectric power.   The U.S. Hydroelectric Power Plant located north of the locks generates over 150 million kilowatt hours of electrical power each year. The first priority for the use of the power is for operating machinery at the Soo Locks. The surplus is purchased by a private power company and is distributed to homes and businesses in Sault Ste. Marie and surrounding communities.
    The entire facility at the St. Marys Falls Canal is operated and maintained by the Corps of Engineers, US Army Engineer District, Detroit.
    The Poe Lock has the largest capacity of the four locks. The lock, completed in 1968, took six years to build and is the only lock ever constructed between two operating locks.
    Inspection of the locks themselves, the culverts, and galleries is done periodically to check the structural soundness of these areas. This inspection is normally done during the winter months.
    Many different types of vessels pass through the locks during a year, varying in size from the small passenger vessels and work boats to large ships carrying more than 72,000 tons of freight in a single cargo. In recent years, the number of passages through the locks has averaged about 12,000 per year, down from previous years due to the larger vessels being able to carry more freight.
    The channels through the St. Marys River have been deepened to permit ship loading to a maximum draft of 25.5 feet at low water. When lake levels are above the low water datum, the larger ships load to take advantage of the deeper water at a rate of over 200 tons per inch of additional loading.


How do the locks (and indeed, all locks) work?  View this animation to see.   It's really quite simple--just a matter of closing and opening some gates.  No pumps are involved. 
    After the ship is in the lock, water is allowed to flow in and raise the ship up (if it is travelling to the higher lake--Superior) or water is allowed to flow out and lower the ship (if it is traveling to the lower lake--Huron).  Then the doors are simply opened (see below) and the ship moves on its way.  Traveling through the US side of the locks is free to all ships. 

Ships travelling from Lake Huron to Lake Superior must enter the locks as shown below.   Note that the water level is low in the lock (you can tell this because the walls of the lock are exposed, not filled to the brim with water).  The lock doors are open, and the ship sails in.  You are looking north.
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Source:  Photograph by Randy Schaetzl, Professor of Geography - Michigan State University

    Now, once in the lock, the doors close and water begins to pour into the lock.  Turn around on the ship and see--the image below (looking south) shows the lock doors closing so the ship can be raised up to the level of Lake Superior.  Note again how much of the doors is exposed--a clue that the water is low in the lock and that it is at the level of the lower lake--Huron.
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Source:  Photograph by Randy Schaetzl, Professor of Geography - Michigan State University

Once the water level in the lock has been raised to that of Lake Superior, the doors are opened and the ship sails out, into the higher lake.  The image below is what such a ship would see in that situation.  The view is to the north, into Lake Superior.
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Source:  Photograph by Randy Schaetzl, Professor of Geography - Michigan State University

In the aerial view below, one can see the four sets of locks, and a freighter moving through, into Lake Superior.  Note that, in two of the locks, the water level is lowered, allowing boats from Michigan-Huron to enter.

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History of the Locks
    Before white men came to the area, the Ojibway Indians who lived nearby portaged their canoes around the rapids to reach Lake Superior from the St. Marys River.  The image below is from 1850.

Source: Unknown

The falls (or rapids) of the St. Mary's River were a common "stopping point" for Indians. The falls often served as a trading point, or perhaps a place to meet and exchange stories.  It also was a great fishing spot (especially for Whitefish) for the Native Americans (and still is...see the image below).
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Source: Unknown

The spring catch of Whitefish was crucial to their health and livelihood, as at the end of winter their food supplies were scarce, and their need for quality protein was high.

    Early pioneers arriving in the territory were also forced to carry their canoes around the rapids. When settlement of the northwest Territory brought increased trade and large boats, it became necessary to unload the boats, haul the cargoes around the rapids in wagons, and reload in other boats.
    In 1797, the Northwest Fur Company constructed a navigation lock 38 feet long on the Canadian side of the river for small boats. This lock remained in use until destroyed in the War of 1812. Freight and boats again needed to be portaged around the rapids.
    Congress passed an act in 1852 granting 750,000 acres of public land to the State of Michigan as compensation to the company that would build a lock permitting waterborne commerce between Lake Superior and the other Great Lakes. The Fairbanks Scale Company, which had extensive mining interests in the upper peninsula, undertook this challenging construction project in 1853.
    Building the locks in the 1850's was a major engineering project, since large amounts of solid rock had to be moved.  The images below show part of the construction of the locks, and an early view of the locks.

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Source: Unknown

    In spite of adverse conditions, Fairbanks completed a system of two locks, in tandem, each 350 feet long, within the 2 year deadline set by the State of Michigan. On May 31, 1855, the locks were turned over to the state and designated as the State Lock.  The canal had opened on schedule in June 1855, but its cost ran three times the estimate, at just under $1,000,000.

An important date:  JUNE 22, 1855. The passage of the steamer Illinois through the locks at Sault Ste. Marie marks the opening of unobstructed shipping between Lakes Superior and Huron. Ships are no longer forced to stop at Sault Ste. Marie and portage their cargoes around the rapids of the St. Mary's River, which drops twelve feet from Lake Superior to Lake Huron. The canal is the result of a long-sought 1852 grant by Congress to Michigan of 750,000 acres of public land. Construction, begun in mid-1853, has progressed despite cost overruns, food shortages, a hostile climate and a cholera epidemic. The mile-long canal and two 350-foot locks arranged in tandem have been completed in two years. The Sault locks provide new impetus to Michigan's fledgling mining industry. Copper mining on the Keweenaw Peninsula began in the early 1840s, and Michigan led the nation in copper production for many years. In 1844 surveyor William A. Burt discovered iron ore deposits near Negaunee. Iron ore mining expanded gradually, but by the late nineteenth century Michigan produced more iron ore than any other state. Michigan also produced significant amounts of salt, gypsum, oil and natural gas.

    Boats which passed through the State Lock (picture below) were required to pay a toll of four cents per ton, until 1877, when the toll was reduced to three cents. 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 US Army Corps of Engineers. The Corps has operated the locks, toll free, since that time.

    The opening of the canal, named the Michigan State Locks, but eventually called simply the Soo Locks, perfectly coincided with the increased demand for iron as railroads expanded westward. Between 1850 and 1860, the railroad network in the United States nearly quadrupled in size. The new locks expedited the shipment of much-needed iron ore to steel mills in the southern Great Lakes. In return, investment capital flowed north to the Lake Superior region, expanding mining operations and improving transportation arteries.
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    The first load of iron ore through the Soo Locks amounted to 132 tons in August 1855. Total ore tonnage that year was 1,447. During the 1860 shipping season 120,000 tons of ore passed through the Soo Locks. One century later the total volume of downbound iron recorded at the Soo exceeded 100 million tons.

The falls of the St. Mary's River, where today exist the Soo Locks seen above, are present because of a resistant sill (layer) of rock within the river channel. The map below shows that area of the river and its main islands in 1938.

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The Soo Locks during World War II

During World War II, the fear of axis attacks on the Soo Locks was extremely prevalent throughout the United States. Although far from the European and Pacific theater, the Soo Locks played an important role during World War II, due to the large amounts of iron ore being transported on large barges through the locks. 90% of America’s iron ore during this time was shipped through the Soo Locks to be transported to steel mills and then factories to build war machinery, such as tanks, planes, and munition.

The fear of a homeland attack in America grew as the war waged on, and with the growing importance of the Soo Locks and iron for the war effort, military protection of the Soo Locks was critical. By 1942, large numbers of troops were sent to Sault Sainte Marie to protect the Soo Locks, making it one of the most guarded places in the United States during World War II. The militarization of the Soo Locks was so large that Fort Brady-a military fort built in Sault Sainte Marie to guard against British invasion from Canada- was so full, there was no room to house all the military personnel.

The military defense tactics were very extensive. Defenses included land, air, and sea to protect the Soo Locks from foreign attacks. Air defenses including searchlights to patrol the sky were added, and defenses against possible torpedo attacks were put in place.

During 1942 an order for construction of a new lock was put into place. This lock would allow larger barges to transport larger amounts of iron ore through the Soo Locks. The new MacArthur lock was completed soon after orders in 1943, allowing larger freighters to pass through. The name of the new lock which replaced the previous Weitzel Lock was named after Douglas MacArthur, a five-star general in the Pacific theatre of World War II.

Due to the lessening threat to the Soo Locks by Axis powers during the war, military security was lessened in 1943. The Soo Locks were never attacked during World War II despite the fear of foreign threat.

Today, the Soo Locks are still responsible for 90% of America’s iron ore transportation. The Soo Locks are still known as a military target, but security for the Soo Locks has decreased significantly since World War II.

This material has been compiled for educational use only, and may not be reproduced without permission.  One copy may be printed for personal use.  Please contact Randall Schaetzl ( for more information or permissions.