The Soo Locks (sometimes spelled Sault Locks but pronounced "soo") are two sets of locks managed and kept in good condition 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. The locks are between the Upper Peninsula of Michigan and the Canadian province of Ontario. They help ships avoid the rapids of the river, where the water drops 21 feet (6.4 meters). On average, 10,000 ships pass through the locks each year. The locks are closed from January to March during winter 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 two cities called Sault Ste. Marie, one in Ontario and one in Michigan. These cities are on opposite sides of the St. Marys River. The Sault Ste. Marie International Bridge connects the United States and Canada and crosses over the locks. A railroad bridge also 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 named the St. Marys Falls Canal. The entire canal, including the locks, is owned and maintained by the United States Army Corps of Engineers, which allows free passage for ships. The first version of the U.S. Soo Locks was completed in May 1855. It was managed by the state of Michigan until it was transferred to the U.S. Army in 1881.

The canal has two parallel lock chambers. Starting from 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 meters) deep. It is large enough to allow ocean-going vessels ("salties") to pass through, including those that also travel through the smaller locks in the Welland Canal. The first vessel to pass through was the SS Carl D. Bradley. According to U.S. Army Corps of Engineers, Sault St. Marie, the maximum length of a ship allowed in the MacArthur Lock is 730 feet due to the alignment of the lock walls.

• 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. It was rebuilt in 1968 to allow larger ships to pass through after the Saint Lawrence Seaway opened. 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 up to 72,000 short tons (65,000 metric tons) of cargo. The Poe Lock is the only lock that can handle the large lake freighters used on the Upper Lakes. The first ship to pass through after the rebuild was 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 movement of copper and iron ore from the Lake Superior region. The lock had two chambers connected 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, 61.5 feet (18.7 meters) wide 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, it was the longest lock in the world at 1,350 feet (410 meters) long, 80 feet (24 meters) wide, and 23 feet (7 meters) deep. It was officially decommissioned in 2010.

• The Sabin Lock, built in 1919. It was built as a twin lock to the Davis Lock and named after Louis Carlton Sabin, the designer of both locks. It was officially decommissioned in 2010 at the same time as the Davis Lock.

A new lock is currently being built and is expected to be completed by 2030. Groundbreaking for the project took place on June 30, 2009. The new lock will be the same size as the Poe Lock and will increase capacity for large lake freighters. It will replace the Davis and Sabin Locks, which were outdated and rarely used. In May 2020, construction began on Phase One of replacing the Sabin Lock.

North of the new lock is a 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 walk behind the security fence and cross the lock gates of the U.S. Soo Locks during 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 for this day, the locks are U.S. federal property managed by the U.S. Army Corps of Engineers. Unauthorized people or civilians who enter the locks without permission may face fines or be arrested for trespassing.

Canadian lock

The first lock on the St. Marys River was built on the Canadian side in 1798 by the Northwest Fur Company to help move fur during trade. In 1814, during the War of 1812, American forces damaged it to stop British trade. Today, a single small lock operates on the Canadian side of the Soo. This lock was first 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 small boats for recreation and tours, while large ships use the U.S. locks.

Gallery

• The first Soo Locks were built in the 19th century
• An Anchor Line steamship passes through the Soo Locks around 1900
• A whaleback ship moves 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