The Soo Locks (sometimes spelled Sault Locks but pronounced "soo") are two sets of locks operated and kept up by the United States Army Corps of Engineers, Detroit District. These locks allow ships to move between Lake Superior and the lower Great Lakes. They are located on the St. Marys River, which connects Lake Superior and Lake Huron. This river lies between the Upper Peninsula of Michigan and the Canadian province of Ontario. The locks help ships avoid the rapids of the river, where the water drops 21 feet (6.4 meters). On average, about 10,000 ships pass through the locks each year. However, the locks are closed during winter, from January to March, because ice stops shipping on the Great Lakes. During this time, workers inspect and repair the locks.

The locks share their name (often shortened to "Soo") with the two cities called Sault Ste. Marie, one in Ontario and one in Michigan. These cities are located on opposite sides of the St. Marys River. A bridge called the Sault Ste. Marie International Bridge connects the United States and Canada and crosses over the locks. Another bridge, used by trains, crosses the St. Marys River just above the highway bridge.

The first locks were completed in 1855. Along with the Erie Canal, built in 1824 in central New York, they were major engineering projects in the United States before the Civil War. The Soo Locks were named a National Historic Landmark in 1966.

United States locks

The U.S. locks are part of a 1.6-mile (2.6-kilometer) canal officially called the St. Marys Falls Canal. The entire canal, including the locks, is owned and maintained by the United States Army Corps of Engineers. This organization allows free passage through the locks. The first version of the U.S. Soo Locks was completed in May 1855. It was operated by the state of Michigan until it was transferred to the U.S. Army in 1881.

The system includes two parallel lock chambers. Starting at the Michigan shoreline and moving north toward Ontario, these are:

• The MacArthur Lock, built in 1943. It is 800 feet (240 meters) long, 80 feet (24 meters) wide, and 29.5 feet (9.0 meters) deep. This size allows ocean-going vessels ("salties") to pass through, including those that also use the smaller locks in the Welland Canal. The first vessel to pass through was the SS Carl D. Bradley. According to 33 CFR § 207.440 (v), "The maximum overall dimensions of vessels that can pass through MacArthur Lock are 730 feet in length and 75 feet in width, except as provided in paragraph (v)(1) of this section." The U.S. Army Corps of Engineers, Sault St. Marie, states that the ship length is limited to 730 feet due to the alignment of the southwest wall when entering and exiting the lock.

• The Poe Lock, built in 1896. The first vessel to pass through was the U.S. Army Corps of Engineers tug USS Hancock. The original Poe Lock was designed by Orlando Poe and, at 800 feet (240 meters) long and 100 feet (30 meters) wide, was the largest lock in the world when completed in 1896. It was rebuilt in 1968 to accommodate larger ships after the Saint Lawrence Seaway opened, allowing such ships to reach the Great Lakes. The rebuilt Poe Lock is now 1,200 feet (370 meters) long, 110 feet (34 meters) wide, and 32 feet (9.8 meters) deep. It can carry ships with 72,000 short tons (65,000 tons) of cargo. The Poe Lock is the only lock that can handle the large lake freighters used on the Upper Lakes. The first passage after the rebuild was by the Phillip R. Clarke in 1969.

• The State Lock, built between 1853 and 1855. The federal government gave land to the state of Michigan to build a lock that would allow faster transit of copper and iron ore discovered near Lake Superior. The lock had two chambers back-to-back to manage the water level difference. Each chamber was 350 feet (110 meters) long, 70 feet (21 meters) wide at the top of its walls and 61.5 feet (18.7 meters) at the bottom, and 12 feet (3.7 meters) deep. The State Lock was replaced by the original Poe Lock in 1896.

• The Weitzel Lock, built between 1873 and 1881 directly south of the State Lock. It was the first lock operated by the federal government. At 515 feet (157 meters) long, 80 feet (24 meters) wide, and 17 feet (5.2 meters) deep, it was the longest lock in the world when completed. It was decommissioned in 1919 and replaced by the MacArthur Lock in 1943.

• The Davis Lock, built in 1914. When completed, the Davis Lock was the longest lock in the world at 1,350 feet (410 meters) long, and it was also 80 feet (24 meters) wide and 23 feet (7.0 meters) deep. It was officially decommissioned in 2010.

• The Sabin Lock, built in 1919. It was constructed as a twin lock to the Davis Lock and named after Louis Carlton Sabin (1867–1950), the designer of both locks. Sabin served as General Superintendent of the ship canal at The Soo from 1906 to 1925. The Sabin Lock was officially decommissioned in 2010, at the same time as the Davis Lock.

A new lock is currently under construction and is planned to be finished by 2030. Groundbreaking for the new lock project took place on June 30, 2009. The lock will be the same size as the Poe Lock and will provide additional space for large lake freighters. The new lock will replace the Davis Lock and Sabin Lock, which are outdated and rarely used. In May 2020, construction on Phase One of the Sabin Lock replacement began.

North of the new lock is an additional channel with a small hydroelectric plant that provides electricity for the lock complex.

The U.S. Army Corps of Engineers, Detroit District, operates the Soo Locks Visitors Center and viewing deck for the public. On the last Friday of every June, the public is allowed to enter behind the security fence and cross the lock gates of the U.S. Soo Locks for the annual Engineers Day Open House. During this event, visitors can get close enough to touch ships passing through the two regularly operating locks. Except on that day, because the locks are U.S. federal property managed by the U.S. Army Corps of Engineers, unauthorized people are not allowed on the locks. Violators may face fines or imprisonment for trespassing.

Canadian lock

The first lock built in the St. Marys River was on the Canadian side in 1798 by the Northwest Fur Company to help move furs. It was destroyed by American soldiers in 1814 during the War of 1812 to stop British trade. Today, a small lock is used on the Canadian side of the Soo. This lock was opened in 1895 and rebuilt in 1987. It is 77 meters (253 feet) long, 15.4 meters (51 feet) wide, and 13.5 meters (44 feet) deep. The Canadian lock is used by boats for fun and tours, while large ships use the U.S. locks.

Gallery

• The first Soo Locks were built in the 1800s
• An Anchor Line steamer is shown in the Soo Locks around the year 1900
• A whaleback ship passes through the Poe Lock around 1910
• The Canadian Lock is located in Sault Ste. Marie, Ontario
• The final ship of the 2013 season passes through the Poe Lock
• USCGC Mackinaw passes through the Soo Locks